Electronic spectroscopy of N- and C-substituted chlorocarbazoles in homogeneous media and in solid matrix

Journal of Luminescence 97 (2002) 83–101

Electronic spectroscopy of N- and C-substitutedchlorocarbazoles in homogeneous media and in solid matrix

Sergio M. Bonesi, Rosa Erra-Balsells*

Departamento de Qu!ımica Org !anica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, c.c.74-suc. 30,

1430 Buenos Aires, Argentina

Received 14 February 2001; received in revised form 20 July 2001; accepted 11 August 2001

Abstract

Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of chlorocarbazoles 1a–e, 2a–b,3a, 4a–b, 5a, 6a, 7a, 8a–c, 9a and 9b in acetonitrile and in solid matrix have been recorded at 298 and 77K. Thedynamic properties of the lowest excited singlet and triplet states in term of fluorescence and phosphorescence lifetime,tf and tp; fluorescence and phosphorescence quantum yield, ff and fp have been measured in the same experimentalconditions and from these data the radiative and the radiationless rate constants (k0f ; kisc; k0f ð77Þ; kiscð77Þ; k0p and k0nr)and the intersystem crossing quantum yield, fisc; were derived.The intramolecular heavy atom effect (HAE) on the spectroscopic data and photophysical rate constant was analyzed

and the incorporation of chlorine atoms to the carbazole moiety proved their ability to quench the fluorescenceemission by spin-orbital coupling.The values of the HOMO and LUMO energy, the oscillator strength ( f ) and the lmaxðabsÞ associated to the

electronic transitions, the heat of formation of the chlorocarbazoles and the corresponding radical cation (DHf andDHf ðRCÞ) and the adiabatic ionization potential (Ia) were also calculated by using the semiempirical PM3 method afterHF/3-21G geometrical optimization, and were compared with the spectroscopic and photophysical data as well as with

the one electron oxidation potential data (Ep=2). r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Chlorocarbazoles; Electronic spectra at 298K; Electronic spectra at 77K; PM3 chlorocarbazole electronic spectra

calculation; PM3 chlorocarbazole molecular orbital calculations

1. Introduction

The effect of heavy atoms on the photophysicalproperties of aromatic compounds has been ofgreat interest [1]. A halogen with high atomicnumber (heavy atom) attached directly to anaromatic compound can reduce significantly the

fluorescence quantum yield [2–6]. The spin-orbitalcoupling mechanism of the heavy atom enhancesthe rate of intersystem crossing to the triplet state[3,6,7]. This is called the intramolecular heavyatom effect (HAE) [1]. Solvents that contain heavyatoms can also enhance intersystem crossing,giving rise to an intermolecular HAE [8]. Themechanism for the solvent heavy atom effect iscomplex and may involve the formation ofweak charge-transfer complexes and/or exciplexes.

*Corresponding author. Tel./Fax: 54-11-4576-3346.

E-mail address: [email protected] (R. Erra-Balsells).

0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 2 4 0 - X

Historically, in order to study both intra-molecular and intermolecular HAE bromine andiodine as heavy atoms have been preferentiallyused [1–7].In the absence of photochemical reactions the

fluorescence quantum yield ff of an emittingsubstance is given by the ratio of rate constantsshown in Eq. (1):

ff¼kf=½kf þ kisc þ kIC� ¼ kftf ¼ tf=t0f ; ð1Þ

where the subscripts denote respectively theprocesses of fluorescence ( f ), intersystem crossing(ISC) and internal conversion (IC). The parametertf and t0f represent respectively the fluorescenceand radiative lifetime. In the presence of a heavyatom such as halogen, typically bromine or iodine,the rate constant kisc is increased, and henceboth tf and ff are reduced significantly, as kiscgrows larger. As it is known [9] the HAE onabsorption spectra is to strongly enhanceeðS0-T1Þ but not eðS0-S1Þ: Because of therelationship between eðS0-T1Þ and k0p and be-tween eðS0-S1Þ and k0f ðk

0e ¼ 1=t0Þ it is expected

that k0p but not k0f will be influenced by heavy-atom

perturbation. Thus, not only is it necessary tostudy the HAE on the fluorescence and phosphor-escence spectra and the corresponding ff and fpbut also the comparison of the k0f and k0p isessential to understand the HAE. The much highervalues of fp reflect both a greater efficiency ofpopulation of T1 (kisc is enhanced) and a greaterefficiency of emission from T1 (k

0p is enhanced

more than kst).According to our experience bromocarbazole

and iodocarbazole are extremly photoreactive [10].As chlorocarbazoles are shown to be more stable[10,11] we decided to study the intramolecularHAE of chlorine on the photophysical propertiesof several chlorocarbazole derivatives. Besides, thephotostability shown by chloroderivatives ofcarbazole together with their still highly efficientfluorescence makes them more suitable to be usedin the synthesis of organic polymers and biologicalphotosensors [12].In a previous paper we described the intermo-

lecular HAE of chlorine using dichloromethane assolvent and we observed that it produces a very

mild solvent effect [12] since the ff measuredin this medium is similar to those obtained inethyl acetate and in air saturated (AS) acetonitrile,ethanol and cyclohexane solutions. We concludedin this previous study that a similar externalmild spin-orbital coupling effect was producedby dichloromethane, molecular oxygen andthe oxygen of ethyl acetate molecule. We alsodescribed elsewhere [13] the quenching of thefluorescence of several carbazoles by polychloro(trichloromethane and tetrachloromethane) andpolybromomethanes (dibromomethane, tri-bromomethane and tetrabromomethane) aswell as the photoinduced single electrontransfer processes that occur in these cases[13,14].In the present study the effect of chloro as

substituent on the fluorescence (ff ; tf ; t0f ; k0f ),phosphorescence (fp; tp; k0p) and intersystemcrossing (kisc; fisc) parameters of several chlor-ocarbazoles is investigated in acetonitrile and iniso-propanol; ethyl ether (1 : 1; v : v), at 298 and at77K, respectively. The chlorocarbazoles studiedare: 3-chloro- (1a), 1,6-dichloro- (1b), 3,6-di-chloro- (1c), 1,3,6-trichloro- (1d), 1,3,6,8-tetra-chloro- (1e), 3-chloro-2-hydroxy- (2a), 1,3-dichloro-2-hydroxy- (2b), 3,6-dichloro-2-acetyloxy(3a), 3-chloro-2-methoxy-N-methyl- (4a), 3,6-di-chloro-2-methoxy-N-methyl- (4b), 1,3,6-trichloro-2-methoxy-N-methyl- (4c), 3-bromo-6-chloro- (5a),6-chloro-3-nitro- (6a), 3-chloro-N-phenyl- (7a),1,3-dichloro-N-methyl- (8a), 3,6-dichloro-N-methyl- (8b), 1,3,6,8-tetrachloro-N-methyl- (8c),3-chloro-N-acetyl- (9a) and 3,6-dichloro-N-acetyl-(9b) (Scheme 1). All our experiments were carriedout both (i) at room temperature in acetonitrilesolution and (ii) at 77K in iso-propanol; ethylether (1 : 1; v : v) by using time correlated singlephoton counting technique and phosphorescencelifetime spectroscopy.In order to compare the effect of a light atom as

substituent (light substituent=H) and a heavyatom as substituent (heavy substituent=Cl) in thesame carbazole moiety, the spectroscopic data forcarbazole (1), 2-hydroxycarbazole (2), 2-acetyl-oxycarbazole (3), N-methyl-2-methoxycarbazole(4), 3-bromocarbazole (5), 3-nitrocarbazole (6),N-phenylcarbazole (7), N-methylcarbazole (8)

S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 97 (2002) 83–10184

and N-acetylcarbazole (9) are also included inthe Tables 1–5 and described briefly in thepresent paper. For each carbazole studied theHOMO and LUMO energies and the ionizationpotential (Ia) were calculated (PM3 method)and related to its one electron oxidation potential(Ep=2). Besides, theoretical absorption spectrawere calculated (f ðSn2S0Þ; lmaxðabsÞ; semiempiri-cal PM3 method) and compared with the experi-mental.

2. Experimental

2.1. Materials

Carbazole (1), N-methylcarbazole (8), N-phe-nylcarbazole (7), 2-hydroxycarbazole (2) and 3-bromocarbazole (5) were purchased from AldrichChemical Co. Chloroderivatives of carbazole (1)(3-chlorocarbazole (1a); 3,6-dichlorocarbazole(1c); 1,6-dichlorocarbazole (1b), 1,3,6-trichloro-carbazole (1d) and 1,3,6,8-tetrachlorocarbazole(1e)), 3-bromocarbazole (5) (3-bromo-6-chlorocar-bazole (5a)), 3-nitrocarbazole (6) (6-chloro-3-nitrocarbazole (6a)), 2-hydroxycarbazole (2) (3-chloro-2-hydroxycarbazole (2a) and 1,3-dichloro-2-hydroxycarbazole (2b)) and 2-acetyloxycarba-zole (3) (3,6-dichloro-2-acetyloxycarbazole (3a))were synthesized according to procedures de-scribed elsewhere [15]. Chlorination of N-methyl-carbazole (8) (1,6-dichlorocarbazole (8a), 3,6-dichlorocarbazole (8b), 1,3,6-trichlorocarbazole(8c)), N-phenylcarbazole (7) (3-chloro-N-phenyl-carbazole (7a)), 2-metoxy-N-methylcarbazole (4)(3-chloro-2-methoxy-N-methylcarbazole (4a);3,6-dichloro-2-methoxy-N-methylcarbazole (4b)and 1,3,6-trichloro-2-methoxy-N-methylcarbazole(4c)) and N-acetylcarbazole (9) (3-chloro-N-acetylcarbazole (9a) and 3,6-dichloro-N-acetylcar-bazole (9b)) were performed following protocolsthat we previously described [16]. The chloroder-ivatives were characterized by their m.p. 1H-nmr,13C-nmr and MS [15,16].Acetonitrile, ethyl ether and iso-propanol

HPLC grade (Merck) were used as purchasedwithout any further purification. Water of MilliQgrade, perchloric acid and sulfuric acid of analy-tical grade were used. Rhodamine B, Quininesulfate and p-terphenyl were used as purchasedfrom Aldrich Chemical Co. and Sigma.

2.2. Equipment

The absorption measurements were performedwith a spectrophotometer Hewlett Packard HP5.The spectrophotometer employed in this study wasa Hitachi F-500. The quantum yields at roomtemperature were determined relatively to thequantum yield of Quinine sulfate in HClO4 0.1N

Compound R1 R2 R3 R4 R5 R6

1 H H H H H H

1a H H H Cl H H

1b H Cl H H Cl H

1c H H H Cl Cl H

1d H Cl H Cl Cl H

1e H Cl H Cl Cl Cl

2 H H OH H H H

2a H H OH Cl H H

2b H Cl OH Cl H H

3 H H AcO H H H

3a H H AcO Cl Cl H

4 Me H MeO H H H

4a Me H MeO Cl H H

4b Me H MeO Cl Cl H

4c Me Cl MeO Cl Cl H

5 H H H Br H H

5a H H H Br Cl H

6 H H H NO2 H H

6a H H H NO2 Cl H

7 Ph H H H H H

7a Ph H H Cl H H

8 Me H H H H H

8a Me Cl H H Cl H

8b Me H H Cl Cl H

8c Me Cl H Cl Cl Cl

9 Ac H H H H H

9a Ac H H Cl H H

9b Ac H H Cl Cl H

Scheme 1. Structure of the carbazoles studied.

S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 97 (2002) 83–101 85

(QS). The general method followed for themeasurement has been previously described [12].The fluorescence lifetimes were measured by the

time correlated single photon counting techniqueusing an Edinburgh 0B 900 ns fluorescence spec-trometer. The resulting decay curves were analyzedby deconvoluting a single exponential with theLamp function as it was described elsewhere [12].The resulting decay curves were analyzed byconvoluting a single exponential with the Lampfunction. The statistical parameters for lifetimeanalysis are, in average, w2 ¼ 1:037 and Durbin–Watson parameter=1.824. The excitation and theemission wavelength were set as the lmaxðabsÞ andlmaxðfluoÞ respectively depending on the values

selected on the compounds studied. Total lumines-cence emission spectra (fluorescence and/or phos-phorescence) were measured in the samespectrophotometer at 77K. The conventional flashapparatus was used in order to measure thephosphorescence lifetimes. The fluorescence quan-tum yields at 298 and at 77K and the phosphor-escence quantum yields at 77K were determinedaccording to the procedure previously described indetail [12].

2.3. Theoretical calculations

The ground state geometries were optimized byab initio calculations (HF level; 3-21G basis set;

Table 1

Spectroscopic data of carbazoles in acetonitrile at 298K under inert atmosphere (N2) together with calculated spectroscopic data by

PM3 after HF/3-21G geometrical optimization

Carbazoles lmaxðabsÞ/nm lmaxðfluoÞ/nm f ðS12S0Þa Dm/Db f ðS12S0Þ

c Dm/Dc lmaxðabsÞ/nmc

f ðS22S0Þc Dm/Dc lmaxðabsÞ/

nmc

1 292 334 F 341 355 0.071 5.63 0.004 0.51 325 0.048 1.81 323

1a 296 330 343 350 364 0.060 F 0.025 1.33 330 0.052 1.89 325

1b 296 330 344 351 364 0.059 F 0.034 1.57 335 0.079 2.64 327

1c 300 336 350 361 364 0.039 F 0.062 2.09 333 0.047 1.82 329

1d 298 338 354 362 376 0.030 F 0.178 3.64 349 0.049 1.91 345

1e 298 340 356 365 378 0.037 F 0.170 3.51 341 0.066 2.17 336

2 302 318 F 332 345 0.048 6.26 0.029 1.46 329 0.086 0.77 325

2a 304 322 334 345 352 0.045 F 0.019 1.15 335 0.014 1.00 326

2b 302 320 334 344 355 0.048 F 0.040 1.70 342 0.015 1.04 334

3 294 318 332 340 357 0.059 5.88 0.001 0.30 327 0.024 1.28 322

3a 298 320 338 346 362 0.050 F 0.036 1.58 335 0.038 1.64 331

4 302 324 336 343 358 0.040 4.71 0.004 0.52 329 0.016 1.10 326

4a 306 328 342 353 364 0.027 F 0.014 0.99 336 0.039 1.63 326

4b 312 330 348 362 373 0.046 F 0.029 1.45 337 0.019 1.17 329

4c 312 344 358 366 382 0.029 F 0.040 1.71 341 0.074 2.31 338

5 298 330 346 340 357 0.057 5.04 0.043 1.72 324 0.008 0.74 322

5a 302 336 350 358 373 0.014 F 0.042 1.71 328 0.0004 0.17 315

6 F 0.100 2.68 336 0.008 0.71 323

6a F 0.111 2.82 339 0.026 1.33 326

7 292 328 340 347 360 0.050 3.81 0.001 0.313 341 0.048 1.79 320

7a 292 338 348 352 369 0.042 F 0.193 3.70 335 0.003 0.46 317

8 294 332 346 352 366 0.059 5.36 0.044 1.78 338 0.056 2.01 337

8a 298 340 354 362 375 0.051 F 0.033 1.64 336 0.084 2.42 329

8b 300 346 362 370 385 0.038 F 0.128 3.07 346 0.052 1.95 344

8c 300 352 368 379 395 0.051 F 0.141 3.19 339 0.078 2.35 336

9 288 302 314 318 329 0.010 6.06 0.085 2.41 320 0.015 1.01 315

9a 298 328 356 364 379 0.045 F 0.083 2.38 324 0.011 0.85 317

9b 298 328 356 364 378 0.053 F 0.076 2.29 325 0.023 1.26 321

aCalculated according to Eq. (4) (see text).bFrom Ref. [12].cCalculated by using the semiempirical PM3 method after HF/3-21G geometrical optimization.


Gaussian 98W [17]). Heat of formation ofcarbazoles were calculated by using the semiempi-rical parameterized PM3 method as implementedin version of the HyperChem Suite 5.1 [18]program, which has proved to be effective instudies on molecules containing heteroatoms,compared with other methods such as MINDO/3or MNDO. The geometries of the radical cationswere optimized using the unrestricted Hartree–Fock (UHF) formalism. The adiabatic ionizationpotentials (Ia) were calculated as the difference inDHf of the radical cation, calculated by usingRHF formalism from the optimized structuresusing the UHF formalism and DHf of the neutral

form with the optimized geometry. Qualitativestructure activity relationships (QSAR) propertieswere calculated as implemented in ChemPlus:Extensions for Hyperchem Suite 5.1 [18] UVvisible spectroscopic transitions and the corre-sponding oscillator strength (f ) were calculated byusing PM3 method as it is parameterized inHyperChem Suite [18].

3. Results

The absorption and fluorescence emission spec-tra of carbazoles 1–9 and chlorocarbazoles 1a–e,

Table 2

Spectroscopic data of carbazoles in solid matrix (iso-propanol : ethyl ether; (1 : 1; v : v)) at 77K together with calculated spectroscopic

data by PM3 after HF/3-21G geometrical optimization

Carbazoles lmaxðfluoÞ/nm l0;0ðphospÞ/nm

f ðS12S0Þa f ðT12S0Þ

ð�109ÞbDEðS12S0Þ/eV

DEðT12S0Þ/eV

DEðS12T1Þ/eVc

f ðS12S0Þd f ðS22S0Þ

d

1 341 356 406 0.051 1.36 3.47 3.04 �0.43 0.004 0.048

1a 342 356 410 0.061 2.54 3.47 2.97 �0.501b 340 349 414 0.073 9.77 3.54 2.99 �0.55 0.034 0.079

1c 348 357 414 0.052 37.87 3.46 2.99 �0.47 0.062 0.047

1d F 359 420 0.019 67.76 3.44 2.94 �0.50 0.178 0.049

1e F 362 420 0.051 78.06 3.40 2.95 �0.46 0.170 0.066

2 331 346 419 0.061 2.55 3.57 2.94 �0.62 0.024 0.086

2a 330 346 421 0.132 7.23 3.57 2.86 �0.63 0.005 0.014

2b 339 354 434 0.271 26.53 3.49 3.02 �0.64 0.006 0.015

3 338 353 409 0.057 2.11 3.50 3.01 �0.48 0.001 0.024

3a 339 353 411 0.093 18.04 3.50 3.00 �0.49 0.036 0.038

4 344 350 412 0.054 2.47 3.53 2.98 �0.53 0.004 0.016

4a 355 360 415 0.099 7.13 3.43 2.91 �0.45 0.014 0.039

4b 350 367 424 0.101 11.62 3.37 2.90 �0.46 0.029 0.019

4c 360 378 426 0.757 58.25 3.27 3.00 �0.37 0.040 0.074

5 340 360 412 0.049 25.33 3.48 3.00 �0.48 0.043 0.008

5a F F 412 F 619.48 F 3.00 F 0.042 0.0004

6 F F 471 F 73.21 F 2.62 F 0.100 0.008

6a F F 470 F 159.37 F 2.62 F 0.111 0.026

7 343 358 407 0.049 1.74 3.45 3.04 �0.41 0.001 0.048

7a 343 360 417 0.056 18.36 3.43 2.96 �0.47 0.193 0.003

8 348 364 408 0.058 1.32 3.40 3.03 �0.37 0.044 0.056

8a 346 357 418 0.075 7.86 3.46 2.96 �0.50 0.033 0.084

8b F 364 415 0.058 25.84 3.40 2.98 �0.42 0.128 0.052

8c F 375 444 0.221 163.23 3.30 2.78 �0.52 0.141 0.078

9 314 326 414 0.010 1.52 3.79 2.99 �0.80 0.085 0.015

9a F 362 421 0.093 72.85 3.41 2.94 �0.47 0.083 0.011

9b F 361 427 0.078 88.34 3.42 2.89 �0.53 0.076 0.023

aCalculated according to Eq. (4) (see text).bCalculated according to the following equation: f ðT12S0Þ: 1:5� k0p n

�2:cDEST ¼ DEðS1Þ2DEðT1Þ:dCalculated using the semiempirical PM3 method after HF/3-21G geometrical optimization.


2a–b, 3a, 4a–b, 5a, 6a, 7a, 8a–c, 9a and 9b inacetonitrile at 298K were obtained. The absorp-tion spectra were recorded in the 200–800 nmregion and generally three bands were observed.The first and the second band are located atapproximately 260 and 298 nm while the thirdbroad band shows a clearly overlapping of twobands with lmax at approximately 335 and 355 nm.The position and oscillator strength of the secondand third bands of chlorocarbazoles were com-pared with that of carbazole [19] and we assignedthem as 1Lað

1S221S0Þ and 1Lbð

1S121S0Þ electronic

transitions, respectively. Furthermore, the positionand oscillator strength of S12S0 and S22S0 werealso verified by quantum chemical calculations(PM3) (Tables 1 and 2). In Table 1 are shown themeasured lmax of absorption (lmaxðabsÞ) and

fluorescence emission (lmaxðfluoÞ) together withthe oscillator strength (f ) associated to themeasured absorption bands as well as the valuesobtained by semiempirical calculations (PM3).The chlorocarbazole derivatives exhibit struc-

tured fluorescence spectra in acetonitrile at 298Kwith excitation into the S1 level. The fluorescenceand absorption spectra at room temperature arefound to display in general excellent mirrorsymmetry. Fig. 1 shows the absorption and thefluorescence emission spectra of 3,6-dichlorocar-bazole (1c) in acetonitrile at 298K. The chlor-ocarbazoles shows structured fluorescence andphosphorescence spectra in iso-propanol-ethylether (1 : 1; v : v) glass matrix at 77K withelectronic excitation into the S1 level. Fig. 2 showsthe fluorescence and the phosphorescence emission

Table 3

Photophysical depletion rate parameters and intersystem crossing quantum yield of carbazoles in acetonitrile at 298K

Carbazoles ff tf /ns kf ð�107Þ/s�1 a k0f (� 10

7)/s�1 b kisc(� 107)/s�1 c fisc

d

1 0.62 15.1 6.62 4.10 2.52 0.38

1a 0.033 1.04 96.2 3.17 93.04 0.97

1b 0.049 1.50 66.7 3.27 63.50 0.95

1c 0.021 0.98 102.0 2.14 99.86 0.98

1d 0.014 0.89 112.4 1.57 110.87 0.99

1e 0.010 0.51 196.1 1.96 194.14 0.99

2 0.36 12.3 8.13 2.89 5.24 0.64

2a 0.126 2.47 40.5 5.10 35.4 0.87

2b 0.002 0.70 142.9 2.86 140.1 0.98

3 0.45 13.2 7.58 3.41 4.17 0.55

3a 0.021 0.98 102.0 2.14 99.9 0.98

4 0.29 14.3 6.99 2.27 4.72 0.68

4a 0.050 2.13 46.9 2.35 44.6 0.95

4b 0.032 1.26 39.4 2.54 36.9 0.94

4c 0.002 >0.1 1000 1.50 999.5 0.98

5 0.038 1.20 83.3 3.17 80.3 0.96

5a 0.002 0.26 384.6 0.77 383.8 0.99

7 0.31 11.35 8.81 2.73 6.08 0.69

7a 0.024 1.04 96.15 2.31 93.84 0.98

8 0.47 14.7 6.80 3.19 3.61 0.53

8a 0.030 1.11 90.9 2.70 87.4 0.97

8b 0.013 0.68 147.06 1.91 145.15 0.99

8c 0.003 0.12 833.3 2.50 830.8 1

9 0.003 0.38 263.2 0.66 262.5 0.99

9a 0.010 0.42 238.1 2.38 235.7 0.99

9b 0.009 0.32 312.5 2.81 309.7 0.99

akf ¼ t�1f :bk0f ¼ ff kf :ckisc ¼ kf2k0f :dfisc=1–ff.


spectra of carbazole (1), 3,6-dichlorocarbazole(1c), 2-hydroxycarbazole (2) and 3-chloro-2-hydroxycarbazole (2a) measured in solid matrixat 77K. It is noteworthy to mention that there isno noticeable change of lmaxðfluoÞ on going from298 to 77K.The experimentally obtained lowest radiative

triplet transition values for chlorocarbazoles areamong 410 and 444 nm. From the spectra theoscillator strengths, f ðS12S0Þ and f ðT12S0Þ werealso calculated together with the energy gap ðDE �ðS1�T1ÞÞ between the singlet (S12S0) and thelowest triplet (T12S0) excited states (see Table 2).

The photophysical parameters of the lowestexcited singlet and triplet states such as, fluores-cence lifetime tf ; phosphorescence lifetime tp;fluorescence quantum yield ff and phosphores-cence quantum yield fp for the carbazoles andchlorocarbazoles in acetonitrile at 298K and iso-propanol: ethyl ether (1 : 1;v : v) at 77K are shownin Tables 3 and 4.From these spectroscopic data we could calcu-

late different photophysical rate constants such askf ; k0f ; kisc and fisc at 298K and kf ; k0f ; kisc; kp; k0p;knr and fisc at 77K and these parameters are alsoshown in Tables 3 and 4. These photophysical rate

Table 4

Photophysical depletion rate parameters and intersystem crossing quantum yield of carbazoles in iso-propanol-ethyl ether (1 : 1;v : v)

at 77K

Carbazoles ff kf (� 107)/s�1 a k0f (� 10

7)/s�1 b kisc(� 107)/s�1 c fisc

d fp tp/ns k0p/s�1 e knr/s

�1 f

1 0.44 6.62 2.91 3.71 0.56 0.24 7.73 0.055 0.074

1a 0.059 66.7 3.94 62.83 0.94 0.66 1.85 0.380 0.161

1b 0.026 102.4 2.66 99.78 0.97 0.70 0.49 1.475 0.568

1c 0.013 112.4 1.46 110.98 0.99 0.71 0.28 2.561 1.010

1d 0.013 196.1 2.55 193.56 0.99 0.73 0.25 2.950 1.050

1e >0.001 384.6 0.38 384.29 1 0.73 0.03 24.33 9.00

2 0.46 8.03 3.69 4.33 0.54 0.22 4.19 0.097 0.142

2a 0.20 40.5 8.10 32.4 0.80 0.50 2.30 0.272 0.163

2b 0.11 142.9 15.7 127.2 0.89 0.61 0.73 0.939 0.431

3 0.44 7.58 3.34 4.24 0.56 0.24 5.13 0.084 0.111

3a 0.053 102.0 5.41 95.6 0.95 0.67 0.99 0.712 0.298

4 0.45 6.81 3.06 3.75 0.55 0.23 4.32 0.097 0.134

4a 0.112 46.9 5.26 41.64 0.79 0.60 2.75 0.272 0.088

4b 0.046 79.4 3.65 75.79 0.95 0.68 1.66 0.431 0.171

4c 0.039 1000 39 961.0 0.96 0.74 0.36 2.14 0.637

5 0.030 83.3 3.17 80.3 0.96 0.54 0.34 0.995 0.759

5a F F F F 1 0.73 0.03 24.33 9.0

6 F F F F 1 0.35 0.16 2.20 4.09

6a F F F F 1 0.75 0.16 4.81 1.60

7 0.340 8.77 2.79 5.48 0.62 0.35 7.57 0.070 0.062

7a 0.033 96.15 3.17 92.98 0.97 0.69 1.01 0.704 0.286

8 0.430 6.80 3.19 3.61 0.53 0.24 7.88 0.053 0.074

8a 0.046 90.91 4.18 86.72 0.95 0.68 1.59 0.450 0.179

8b 0.020 147.06 2.94 141.12 0.96 0.70 0.47 1.55 0.578

8c 0.013 83.33 10.8 72.44 0.87 0.72 0.15 5.52 1.149

9 0.003 263.2 0.66 262.5 0.99 0.40 6.93 0.059 0.085

9a 0.020 238.10 4.76 237.34 1 0.70 0.25 2.74 1.261

9b 0.013 312.5 4.05 307.45 1 0.71 0.22 3.23 1.315

akf ¼ t�1f :bk0f ¼ ff kf :ckisc ¼ kf2k0f :dfisc ¼ 12ff :ek0p ¼ fp kp=fisc:fknr ¼ kp2ko

p; where kp ¼ t�1p :


constants were calculated under the assumptionthat the radiationless internal conversion (IC) isneglected owing to the planarity and the rigidity ofthe carbazole structure [12,20,21]. Taking intoaccount this assumption and considering that theintroduction of one to four chlorine atoms onthe carbazole moiety there is no change in thegeometry and planarity of the carbazoles struc-tures, then the intersystem crossing quantum yields(fisc) were calculated according to the followingequation:

fisc ¼ 1� ff ¼ kisc=kf : ð2Þ

Comparing the photophysical rate constants ofchlorocarbazole derivatives (compounds 1a–e,

2a,b, 3a, 4a,b, 5a, 6a, 7a, 8a–c, 9a and 9b) withthat of the corresponding non-chlorinated carba-zoles (compounds 1–9) an important intramolecu-lar HAE was observed at room temperature and at77K. When one heavy atom, such as chlorine, wasattached to the carbazole moiety a dramaticdecrease of the ff and the tf values was observedas the kisc value increases significantly while theintramolecular heavy atom effect on the radiativefluorescence rate constant (k0f ) occurs in a lesserextent. Nevertheless, when additional chlorinesubstituents were attached to the carbazole moi-ety, a low HAE on the photophysical rateconstants was observed. Also, an importantincrease of k0p was observed on going from one

Table 5

Thermodynamic and electrochemical parameters of carbazoles

Carbazoles EHOMO/eVa ELUMO/eVa DE/eVb Ia/eVc Ep=2/V vs SCE

d

1 �8.49 �0.30 8.19 7.87 1.16

1a �8.51 �0.46 8.05 7.88 1.27

1b �8.54 �0.61 7.93 7.99 F1c �8.55 �0.61 7.94 7.99 1.38

1d �8.36 �0.67 7.69 7.97 F1e �8.60 �0.88 7.72 F2 �8.53 �0.27 8.26 7.90 0.85

2a �8.58 �0.41 8.16 8.06 1.01

2b �8.56 �0.53 8.02 8.00 F3 �8.53 �0.36 8.17 7.89 1.14

3a �8.57 �0.69 7.89 7.89 1.34

4 �8.41 �0.23 8.18 7.78 0.90

4a �8.39 �0.36 8.02 7.79 1.05

4b �8.45 �0.51 7.94 7.87 F4c �8.53 �0.78 7.75 7.90 F5 �8.64 �0.51 8.13 8.01 1.26

5a �8.64 �0.65 7.99 8.06 1.46

6 �9.07 �1.06 8.01 8.44 1.51

6a �9.03 �1.15 7.87 8.41 1.71

7 �8.24 �0.39 7.85 7.60 1.21

7a �8.29 �0.54 7.75 7.68 1.38

8 �8.11 �0.18 7.93 7.75 1.09

8a �8.47 �0.60 7.88 7.85 1.28

8b �8.23 �0.50 7.73 8.32 F8c �8.58 �0.88 7.71 7.93 F9 �8.65 �0.49 8.08 8.16 1.64

9a �8.67 �0.64 8.03 8.13 1.75

9b �8.70 �0.78 7.92 8.20 F

aCalculated by using the semiempirical PM3 method.bDE ¼ ELUMO2EHOMO:cCalculated according to Eq. (3) (see text).dData from Refs. [22,23].


to four chlorine atoms attached to the carbazolemoiety when the experiments were carried out at77K.Besides, the ground state geometry, the heat of

formation of chlorocarbazoles and their radicalcation, (DHf and DHf ðRCÞ; respectively), werecalculated by using the semiempirical PM3 meth-od. The adiabatic ionization potentials (Ia) of thecarbazole derivatives were calculated according tofollowing equation [22]:

Ia ¼ DHf ðRCÞ � DHf : ð3Þ

Also, the HOMO and LUMO energy (EHOMO

and ELUMO) and the LUMO-HOMO energydifference (DE) of the carbazole derivatives werecalculated by using the PM3 semiempiricalmethod. These thermodynamic parameters arelisted in Table 5 and two examples of theHOMO 3D isosurfaces calculated are shown inScheme 2. The EHOMO and the DE values arerelated to the 1Lb(

1S1’1S0) energy transition of

the chlorocarbazole, which is denoted as DE �ðS12S0Þ and are obtained from their absorptionspectrum recorded in acetonitrile (see Table 1).

Fig. 2. Electronic fluorescence and phosphorescence emission spectra of carbazole (1), 3,6-dichlorocarbazole (1c), 2-hydroxycarbazole

(2) and 3-chloro-2-hydroxycarbazole (2a) in solid matrix at 77K. (a) Solid line: (1); dotted line: (1c). (b) Solid line: (2); dotted line: (2a).

Fig. 1. Electronic absorption spectra and fluorescence emission

spectra of 3,6-dichlorocarbazole (1c) in acetonitrile at 298K

under nitrogen atmosphere.


Good linear correlation was obtained betweenboth EHOMO and DE and DEðS12S0Þ (see Fig. 4).Finally for carbazoles and chlorocarbazole

derivatives we attempted to correlate the oneelectron oxidation potential (Ep=2) [23,24] to theadiabatic ionization potential (Ia) and a goodlinear correlation was obtained for some chloro-carbazoles (see Fig. 5).

4. Discussion

4.1. Absorption spectra

The absorption spectra of carbazoles andchlorocarbazoles were recorded in acetonitrile at298K and generally they showed three bands at260, 298 and 350 nm (Table 1). Previously spectro-scopic studies and theoretical calculations havebeen carried out [25–27] allowing for unambiguousassignment of the lowest-lying excited electronicstates of carbazole as 1La and

1Lb in the C2v

symmetry point group with the short (z) axis andlong (y) axis, respectively in the plane of themolecule. Comparing the position and oscillatorstrength of the bands located in the 298 and

350 nm region for carbazole derivatives (see Table1) with that of carbazole, it is possible to assignthem as 1Lb ðS12S0Þ and

1La ðS22S0Þ electronictransitions, respectively. Recently, we have ana-lyzed [12] the solvent effect on the absorptionspectrum of carbazole, N-substituted and C-substituted carbazoles where a small bathochromicshift was observed on going from non-polar topolar solvents and we concluded that the nature ofthe lowest singlet excited state is most likely to bep;p : Furthermore, in the same study [12] weobserved that 3-chloro, 3-bromo and 3,6-dibro-mocarbazole showed a small red shift of the 1Lbelectronic transition ranging from 6 to 10 nm asthe solvent polarity was increased which suggeststhat the nature of the singlet excited state of thesecompounds is a p;p electronic transition too.Therefore, it is expected that the introduction ofone to four chlorine atoms as substituents in thecarbazole moiety will produce no change on thenature of the excited singlet state as it is shownthrough the k0f ; kisc and fisc values (see latter in thetext).The absorption spectra of the chlorocarbazoles

recorded in acetonitrile at room temperatureexhibit a noticeable bathochromic shift of the1Lb band ranging between 10 and 22 nm comparedto that of the carbazoles 1–9. This large red shiftaccounts for the in-plane conjugation of thechlorine substituents and it can be interpreted bythe mesomeric effect induced by the chlorine atomsbonded to the carbazole moiety.In general, the absorption spectrum of chlor-

ocarbazoles show a small bathochromic shift ofthe 1La electronic transition bands with respect tonon-chlorinated carbazoles ranging from 2 to10 nm as the number of chlorine atoms bondedto the carbazole moiety increases (see Table 1).The oscillator strength f ðS12S0Þ associated to

1Lb electronic transition of carbazoles were calcu-lated from the radiative fluorescence rate constant(k0f ) and the wavenumber of the absorption-fluorescence crosspoint in reciprocal centimeters(n) according to Eq. (4) [19,20]

f ðS12S0Þ ¼ 1:5� k0f n�2: ð4Þ

This spectroscopic parameter depends on thenumber of chlorine substituents bonded to the

Scheme 2. Orbital plotting: HOMO 3D isosurfaces calculated

by PM3 method for (a) 1,3,6,8-tetrachlorocarbazole (1e) and

(b) carbazole (1).


carbazole moiety (see Table 1). As can be seen inTable 1, in general the f ðS12S0Þ values of thechlorocarbazoles are similar or lower than that ofthe corresponding non-chloro-substituted carba-zole (1–9). This dependence may be attributed to achange in the charge transfer character of the 1Lbelectronic transition and therefore, it is expectedthat a noticeable change of the transition dipolemoment (Dm) associated to this electronic transi-tion could occur. The f ðS12S0Þ; f ðS22S0Þ valuesand the corresponding (Dm) associated to bothelectronic transitions were calculated from theoptimized ground state structure of carbazole byusing the semiempirical PM3 method. Thesevalues are also shown in Tables 1 and 2.

4.2. Emission spectra

The fluorescence and absorption spectra ofcarbazoles 1–9b recorded in acetonitrile at 298Kdisplay excellent symmetry with exact overlap of0,0 band as is shown in Fig. 1. This indicates thatthe geometry of the molecules is quite similar intheir ground and excited state. As can be seen inTable 1, lmaxðfluoÞ of the chlorocarbazoles showan important bathochromic shift ranging from 10to 24 nm when the number of chlorine atomsattached to the carbazole moiety increases. Thistrend is due to an in-plane conjugation of thechlorine atom with the carbazole moiety.An interesting feature to be pointed out is the

intramolecular quenching of the fluorescenceemission of carbazoles 1–9 as the number of thechlorine atoms bonded to the carbazole moiety isincreased which is evidenced through the relativefluorescence emission intensity. As can be seen inTable 3, the fluorescence quantum yield ofcarbazoles decreases 10–50 times when one tofour chlorine atoms are introduced as substituentsat the aromatic ring. This trend is alsoobserved for the rest of the carbazole deri-vatives studied (see Table 3). Besides, forN-acetylcarbazole and its chloro derivatives (com-pounds 9, 9a and 9b) the trend is opposed and thisbehaviour accounts for a change in the geometryof S0 and S1 as well as for an n;p and p;pmixture character of the excited states (see latter inthe text).

4.3. Fluorescence lifetimes

In acetonitrile solution carbazoles and itschloroderivatives exhibited transients whosemeasured lifetimes could be fitted by a mono-exponential curve. The observed tf values for allcompounds studied decrease significantly when achlorine atom is introduced as substituent at thearomatic ring, and in general tf is ten times minor(see Table 3).Thus, 2-hydroxycarbazole (2) and 3-chloro-2-

hydroxycarbazole (2a) showed tf ¼ 12:3 ns theformer and tf ¼ 2:47 ns the latter. The introduc-tion of a second chlorine as substituent at thearomatic ring, e.g. 1,3-dichloro-2-hydroxycarba-zole (2b) decreases the tf value to 0.70 ns (Table 3).In general the subsequent addition of two chlorinesubstituents in the carbazole moiety modifies the tfvalues in similar way (Table 3, 2-methoxy-N-methyl carbazole (4), 3-chloro-2-methoxy-N-methylcarbazole (4a) and 3,6-dichloro-2-meth-oxy-N-methylcarbazole (4b); tf ¼ 14:3; 2.13 and1.26 ns respectively, and N-phenylcarbazole (7)and 3-chloro-N-phenylcarbazole (7a), tf ¼ 11:35and 1.04 ns, respectively). As can be seen inTable 3 the introduction of two chlorine sub-stituents diminishes the tf values from 11–15 nsto 0.68–1.50 ns. As seen in the same table, theeffect of the introduction of a second, third orfourth chlorine as substituent is milder thanthe effect produced by the first chlorine introducedas substituent. Thus, we can conclude thatthe heavy atom affect (spin-orbital coupling)induced by the first chlorine substituent is onlyslightly modified by additional chlorine substitu-ents (Table 3, carbazole (1) and chlorocarbazoles(1a–e), 2-acetoxycarbazole (3) and 3,6-dichloro-2-acetoxycarbazole (3a), N-methylcarbazole (8)and dichloro- and tetrachloro-N-methylcarbazoles(8a–c)).An interesting result was obtained when the

fluorescence of N-acetylcarbazole (9) was studied.As shown in Table 3, N-acetylcarbazole (9)showed a fð298 KÞ

f lower than that of carbazole(1), 0.003 and 0.62 respectively, and also a lower tf,0.38 and 15.1 ns, respectively. When chlorine wasintroduced as substituent at C-3 (3-chloro-N-acetylcarbazole (9a)) and at both C-3 and C-6


(3,6-dichloro-N-acetylcarbazole (9b)), the tf andthe fð298 KÞ

f values were not drastically affected.It is interesting to point out that for all the

carbazoles studied (Table 3) the introduction ofjust a chlorine atom as substituent in the aromaticring diminishes drastically both the fð298 KÞ

f and thetf ; being the effect of additional chlorine sub-stituents not so drastic while the effect of chlorineas substituent in N-acetylcarbazole is quite mild.The p; p character of the fluorescent emissionstate would account for the important chlorineintramolecular induced HAE [9]. Thus, a simpleintramolecular HAE on a p;p state enhancingthe S1-T1 process would account for theformer result while a mixture of effects (spin-orbital coupling, inductive-mesomeric substituenteffects and an important modification of thegeometry of S0 and S1 states together with an n,p and p;p mixture character of the excitedstates) would account for the latter result. Addi-tional results obtained at 77K, support thesesuggestions.

4.4. Singlet excited state deactivation rateconstants at 298 K

The individual rate constants of the radiativeand radiationless process from S1 (kf ; k0f and kisc)were derived from the measured tf and ff and areshown in Table 3. These data clearly show that thek0f values of chlorocarbazoles decrease slightlywith respect to those of the non-chlorinatedcarbazoles (compounds 1–9) as the number ofchlorine atoms bonded to the carbazole moiety isincreased. The lack of a heavy atom effect on k0f ongoing from non-chlorinated carbazoles to chlori-nated carbazoles can be understood in terms of avery fast inherent fluorescence [9]. However,compounds 1–8 show a moderately fluorescencequantum yield which decreases as the chlorineatoms present in the aromatic ring is increased dueto an important intramolecular heavy atom effect(see Table 3). This behaviour accounts for anefficient deactivation of the singlet excited state ofchlorocarbazoles through the intersystem crossingchannel which is highly favored owing to both thehigh rigidity and planarity of the carbazolestructures [20] and the spin-orbit coupling process

between the singlet and the triplet excited states[9,20]. Therefore, this behaviour may be rationa-lized according to the kisc values shown in Table 3.The larger the spin-orbit coupling perturbation is,the higher the kisc values are. Thus, the kisc valuesincrease dramatically as the number of chlorineatoms attached to the carbazoles moiety increases,showing a trend which is in general opposite tothat of the k0f values trend (for comparison seeTable 3). It is noteworthy to mention that thetrend for the photophysical parameters of N-acetylcarbazole and its chloroderivatives is a bitdifferent. Thus, on going from N-acetylcarbazole(9) to 3-chloro-N-acetylcarbazole (9a) a noticeableincrease in ff and k0f as well as a decrease in kisc isobserved (see Table 3). However, substitution by asecond chlorine atom in compound 9b results in anincrease of both k0f and kisc while no significantchange on the ff value is observed. This ‘‘inverse’’heavy atom effect is explained in terms of theinfluence of chlorine substitution on the positionof T2 [9]. Thus, in compounds 9a and 9b the T2electronic state is lowered in energy so that it fallsbelow S1 and a S12T2 radiationless mechanism forintersystem crossing becomes available. This im-plies that the intramolecular HAE does notdominate and bring about very rapid intersystemcrossing. This behaviour agrees with the modest ffand the high k0f values obtained for compounds 9aand 9b (see Table 3).

4.5. Spectroscopic and photophysical data at 77 K

The total luminescence spectra (fluorescence andphosphorescence spectra) of carbazoles 1–9b weremeasured in iso-propanol-ethyl ether (1 : 1; v : v) at77K. These compounds exhibit structured fluor-escence and phosphorescence emission spectrum inthis conditions with excitation into the S1 level (seeFig. 2). The shape and lmaxðfluoÞ of the fluores-cence emission spectra of carbazoles do not changenoticeably on going from 298 (acetonitrile) to 77K(see Tables 1 and 2) which accounts for the factthat the geometry of the first excited state ofcarbazoles are quite similar at both temperatures.The experimentally obtained lowest radia-

tive triplet transition of chlorocarbazoles arefound to range from 410 to 434 nm while


6-chloro-3-nitrocarbazole (6a) shows a l0;0

(phosph) at 470 nm. This red shift is due to animportant mesomeric effect of the nitro group, astrong electron withdrawing substituent, on thelowest triplet excited state.The phosphorescence lifetimes (tp) of chloro-

carbazoles shown in Table 4 range from 2.75 to0.03 ns and these values are lower than those of thenon-chlorinated carbazoles 1–9. Also, these valuesdecrease significantly as the number of chlorineatoms bonded to the carbazole moiety increasewhereas introduction of a chlorine atom at C-6position of 3-nitrocarbazole has almost no effecton the tp value of compound 6a. These resultssuggest that the nature of the lowest triplet excitedstate for compound 6a is most likely to be an n;pelectronic state because of any important intra-molecular HAE was observed. For the otherchlorocarbazoles studied the intramolecular HAEon tp is important and this behaviour suggests thatthe lowest triplet excited state is a p;p electronicstate [9].Experiments of the temperature dependence of

the fluorescence quantum yield can provide,directly or indirectly, significant information aboutthe behaviour of excited states of organic mole-cules such as the temperature effect on radiation-less transitions and on photochemical reactivity.As it was suggested [28,29] the restraints placedupon geometric modifications of the excitedmolecule by temperature-induced changes in thesolvent cage (variation of site structure) arereflected in varying ff : But, quite disparate caseshave been described, e.g. 9,10-diphenylanthraceneff value shows no temperature dependence whilephenoxazine exhibits a dramatic increase in ffwith decreasing temperature [28].As can be seen in Table 4 for carbazole and its

chloroderivatives a lesser temperature dependencefor ff was observed with the exception of 1,6-dichloro carbazole (1b). As can be seen in Table 4,when chlorine is present at C-1 the ff at 77Kshows a more noticeable modification. For carba-zole substituted at C-positions (2-hydroxy- (2) and2-acetyloxycarbazole (3)) and at N-position(N-methyl- (8), 2-methoxy-N-methyl- (4) and N-phenylcarbazole (7)) the introduction of chlorinemodifies the electronic properties of the molecule

showing a clear ff dependence on temperature as aresult. In general the ff is higher at 77K than at298K and the polysubstituted carbazoles exhibita dramatic increase in ff at 77K (Table 4,3,6-dichloro-2-hydroxycarbazole (2b), 3,6-dichloro-2-acetyloxycarbazole (3a), 1,3,6,8-tetrachloro-N-methylcarbazole (8c), 1,3,6-trichloro-2-methoxy-N-methylcarbazole (4c)). This increase is max-imum when substituents are in vecinal and in orthoposition. Elimination of radiationless deactivationprocess at 77K from the S1 state owing tocombined vibration of ortho substituents wouldaccount for this result.The dynamic properties of the lowest triplet

excited states in terms of phosphorescence lifetimetp and phosphorescence quantum yield fp in iso-propanol: ethyl ether (1 : 1 (v : v)) at 77K forchlorocarbazoles are shown in Table 4. As can beseen in this table, the introduction of the firstchlorine atom increase the fp value while the otherchlorine atoms modify only slightly the fp values,e.g. carbazole (1) 0.24, 3-chlorocarbazole (1a)0.66; 1,6-dichlorocarbazole (1b) 0.70, 1,3,6-tri-chlorocarbazole (1d) 0.73 and 1,3,6,8-tetrachlor-ocarbazole (1e) 0.73; 1,6-dichloro-N-methyl- (8a),3,6-dichloro-N-methyl- (8b) and 1,3,6,8-tetra-chloro-N-methylcarbazole (8c) 0.24, 0.68, 0,70and 0.72, respectively. The much higher values offp reflect both a greater efficiency of populationof T1 (kisc is enhanced) and a greater efficiency ofemission from T1 and thus, it is expected that k0pincreases more than kisc: Simultaneously, a clearheavy atom effect was observed on the tp valuesshowing this effect a direct dependence with thenumber of chlorine substituents, e.g. 2-hydroxy-carbazole (2), 3-chloro-2-hydroxycarbazole (2a)and 3,6-dichloro-2-hydroxycarbazole (2b), 4.19,2.30 and 0.73 ns, respectively.It is interesting to mention that the introduction

of chlorine in the N-acetylcarbazole (9) structure(3-chloro-N-acetylcarbazole (9a)) enhanced thephosphorescence quantum yield and diminishedthe tp: As the phosphorescence (T1-S0) of N-acetylcarbazole is modified by an intramolecularHAE, in some extent a p; p character should beassigned to the T1 excited state [9].The individual rate constants of the radiative

and radiationless processes from S1 and T1 (kf ; k0f ;


kisc; kp; k0p and knr) derived from the measuredquantities listed in Table 4 (ff ; fp; tf and tp) aresummarized in the same table. These data werecalculated with the assumption that only fluores-cence emission and intersystem crossing processesdeactivate the singlet excited state of chlorocarba-zoles because the non-radiative internal conversion(kic) is neglected owing to the planarity and therigidity of the carbazoles structures [19]. Also, thisassumption is good because we know that inacetonitrile solution they did not show anysignificant phototransformation during the presentmeasurement and after longer irradiation time[29]. In general for all the chlorocarbazoles studiedk0f is slightly modified by the introduction of achlorine substituent at the carbazole moiety butessentially remains constant through this seriesbecause the values are in the order of 1� 107 s�1.Simultaneously, kisc increases dramatically and fiscincreases by a factor of about 1.5 (or higher) (seeTable 4) on going from non-chloro- to themonochloroderivative. Thus, the much highervalues of kisc and fisc reflect a greater efficiencyof population of the lowest triplet excited state(T1) through the spin orbit coupling processbetween the S1 and T1 excited states.The k0p values of chlorocarbazoles show an

important increase respect to those of the non-chlorinated carbazoles 1–9 as the number ofchlorine atoms bonded to the carbazole moietyincreases. Also, the knr values show a similar trendbut it is important to note that these values arelower than those of the k0p values for the samechloroderivative. These results show that thelowest triplet excited state (T1) deactivates pre-ferentially through the phosphorescence emissionprocess while the radiationless process occurscompetitively in a lesser extent.As we previously pointed out, 3-chloro-2-

hydroxycarbazole (2a) showed a special beha-viour, high ff (0.126 at 298K and 0.20 at 77K),high tp (2.30 ns) and low k0p (0.272 s

�1). A stereo-electronic interaction between chlorine and hydro-xyl groups in ortho position would account for thisresult.As it was expected the introduction of a chlorine

at C-6 position in 3-bromocarbazole (5), yielding3-bromo-6-chlorocarbazole (5a) with tp 0.030 ns;

k0p ¼ 24:33 s�1; knr ¼ 9:0 s�1 is less effective thanthe introduction of a bromine at C-6 (3,6-dibromocarbazole Ref. [12]; tp 0.018 ns;k0p ¼ 32 s�1; knr ¼ 23:6 s�1). As it is known HAEof bromine is stronger than that of chlorine [9].Finally, it is interesting to point out that when

we compared the photophysical parameters of 3-nitrocarbazole (6) and 6-chloro-3-nitrocarbazole(6a) a quite different behaviour was observed.Thus, for compounds 6a an important increase ofthe fp and k0p values is observed with respect tocompound 6, while knr decreases and tp remainsconstant. As it was pointed out previously [12] thelowest triplet excited state of compound 6 wasassigned to be an 3n; p electronic transition and asis known, the photophysical parameters (fp; tp;k0p; knr) for

3n;p states are not significantlyaffected by changing from a ‘‘light’’ to ‘‘heavy’’atom [9]. This behaviour is due to an inherent spin-orbit coupling in 3n; p states which is strongerthan an external (intermolecular) or internal(intramolecular) HAE can induce. Therefore, theexperimental results obtained for compound 6a(see Table 4) account for the fact that the lowesttriplet excited state is in agreement with a mixing3p;p and 3n; p electronic configuration ratherthan a pure 3n;p electronic state because thephotophysical parameters are largely modified bythe introduction of a chlorine atom at thecarbazole moiety.At 77K the DEðS12S0Þ values were calculated

and the values obtained are shown in Table 2together with the DEðT1S0Þ values measured inthe same experiments. Also, the energy gap(DEðS12T1Þ) between the singlet and the lowesttriplet excited state were calculated for thechlorocarbazoles and these values are shown inthe same table.According to the literature [30,31], there is a

linear relationship between the DEðT1S0Þ and thelogðkiscÞ values. This linear relationship is observedwhen the energy gap (DEðS12T1Þ) between S1 andthe state to which intersystem crossing actuallyoccurs (i.e., T1 or some upper triplet excited state,Tn) becomes lesser than 30 kcalmole

�1. However,when the electronic configuration of the statesundergoing intersystem crossing differ the lineardependence is not observed.


Figs. 3 and 4 show the linear regressionsobtained between the logðkiscÞ values and the DE �ðT12S0Þ values of chlorocarbazoles. As can beseen in Fig. 3 a good linear correlation wasobtained for compounds 1, 2, 4 and 8 and theirchloroderivatives (1a–e, 2a–b, 4a–c and 8a–c)while for N-acetylcarbazole and its chloroderiva-tives no dependence between logðkiscÞ andDEðT12S0Þ was observed. These results indicatethat for the former compounds it appears that S1crosses directly to an excited vibrational level of T1with a favorable small energy gap and conse-quently a favorable Franck–Condon factor existsfor intersystem crossing. Since the energy gap(DEðT12S1Þ) of compounds 9, 9a and 9b is larger,a poor Franck–Condon factor accounts for aconsiderably slow kisc and as a consequence aslower diminishing of ff (see Table 4).We have also calculated the oscillator strength

f ðT12S0Þ of the chorocarbazoles as f ðT12S0Þ ¼1:5� ko

pn�2 with n as the wavenumber of the

maximum of phosphorescence in reciprocal cen-timeters, and the values are shown in Table 2. It is

known that the oscillator strength f ðT12S0Þ valuesranging from 10�5 to 10�9 are characteristic ofspin forbbiden transitions and this type ofelectronic transitions are classified as ‘‘naphtha-lene-like’’ molecules. Also, these kind of moleculeshave the k0p and kisc values ranging from 10 to0.01 s�1 and from 106 to 108 s�1, respectively,which indicates that the radiationless intersystemcrossing process from S1 to T1 is considered a slowprocess relative to benzophenone-like molecules.Therefore, taking into account the f ðT12S0Þ; the

kisc and the k0p values for chlorocarbazoles shownin Tables 2 and 4 we conclude that the lowesttriplet excited state of these carbazoles is mostlikely to be 3p;p : Also, for these compounds,both the radiative (phosphorescence) and radia-tionless intersystem crossing processes between thesinglet and triplet manifolds are spin forbiddenand they are allowed through spin-orbit couplingof electron motion in the presence of nuclearpotential fields. It should be pointed out thatin addition to the dominant electron spin-orbit mechanism, a second order spin-vibronic

Fig. 3. Correlation between logðkiscÞ and DEðT1Þ of carbazoles.Symbols: experimental values. Lines: best linear regression

obtained (m: r2: 0.997; slope: �15.09; K: r2: 0.972; slope:

�18.26; .: r2: 0.954; slope: �19.82; ’: r2: 0.999; slope:

�10.45). Numbers are the carbazole derivatives (see Scheme 1).

Fig. 4. Correlation between HOMO energies and the lower

energy absorption bands (DEðS1Þ) of carbazoles. Symbols:experimental values. Lines: best linear regression obtained (m:

r2: 0.939; slope: 0.49; K: r2: 0.927; slope: 1.71; .: r2: 0.996;

slope: 1.45;E: r2: 0.622; slopeB0). Numbers are the carbazolederivatives (see Scheme 1).


mechanism is expected in this carbazole series as itwas observed for carbazole [32].For 3-nitrocarbazole (6) and 6-chloro-3-nitro-

carbazole (6a) an interesting feature may beconsidered. The kisc values of these carbazoleswere estimated taking tfo0:1 ns and ffo5:0�10�4: Thus, fisc was taken to be approximatelyequal to 1 and the kisc rate constant reaches amaximum value of 1.0� 10�10 s�1. The k0p valuesare very large while the tp values are significantlydecreased (see values in Tables 2 and 4, respec-tively). It is also interesting to mention that theshape and the intensity of the phosphorescencespectrum of the two above mentioned carbazolesare very similar each other showing only threebroad bands which are quite different than thoseobserved in the structured phosphorescence spec-trum of carbazole (1). Taking into account thespectroscopic and photophysical results abovedescribed which resemble to that of benzophe-none-like molecules [9] we can state that the lowesttriplet excited state of 6-chloro-3-nitrocarbazole(6a) derivatives is most likely to be an 3n,p*electronic state.

4.6. Semiempirical calculations of thermodynamicparameters

The HOMO and the LUMO energies werecalculated by using the semiempirical PM3 methodand are shown in Table 5. For the carbazoleseries all of the HOMO’s are located on thecarbazole moiety and consist mostly of p-typeoverlap of the fused rings. From the analysisof the MO 3D isosurfaces plots it can be concludethat there is a noticeable contribution to theHOMO orbital by the substituents where electrondonor and electron acceptor groups attached tothe N- and C-position of the carbazole moietyenhance or diminish the HOMO energy. When thesubstituent is an halogen such as chlorine orbromine, a neat contribution to the HOMO wasalso observed. In Scheme 2 are shown as examplesthe HOMO 3D isosurfaces caculated for 1,3,6,8-tetrachlorocarbazole (1e) and carbazole (1). In thecase of N-acetylcarbazole and its chloroderivatives(compounds 9, 9a and 9b) where the acetyl groupas substituent is forced out-of-plane with a

dihedral angle of 31 degree, the p overlap isdecreased significantly and as a consequence theHOMO energy value is decreased. Thus, aninductive effect on the HOMO energy of thesecompounds is observed due to partial p-over-lapping of the substituents and the carbazolemoiety.The LUMO’s for the chlorocarbazoles

studied show essentially the same trend asthat of the HOMO’s and it is noteworthy tomention that the LUMO energy values of 6-chloro-3-nitro- (6a) and 3-nitrocarbazole (6) de-crease significantly owing to the strong electronacceptor group with p orbitals bonded to thecarbazole moiety.Also the contribution of the substituents to the

HOMO energy is evident in the absorption spec-trum due to a significant red and blue shift observed(Table 1). Fig. 4 shows the linear correlationobtained between the EHOMO energy values andthe 1Lb transition (DEðS12S0Þ) values measured inacetonitrile, where depending on the substituent, abathochromic or an hypsochromic shift of the lmaxin the absorption spectrum is evidenced.In this connection, the substituent contribution

to the HOMO energy is also evident on theoscillator strength value calculated ( f ðS12S0Þ andf ðS22S0Þ) for the

1La and 1Lb electronic transition(compare the f ’s values calculated for carbazolesshown in Table 1). Thus, a significant enhance-ment of the f ’s values of the carbazoles isattributed to an increase of the MO overlap ofthe substituent with the carbazole moiety favoringan increase of the transition dipole moment (Dm;Table 1) through inductive and mesomeric effects.Additionally, we have related the Ia value,

calculated by using the semiempirical PM3 methodaccording to Eq. (3), to the one electron oxidationpotential (Ep=2) of carbazoles [22,23]. Fig. 5 showsthe plot of this parameter versus the Ep=2 values ofcarbazoles and a linear correlation was obtainedwith positive slope values. The trend observed maybe interpreted as an enhancement of the one-electron reductive property of carbazoles whichdepends on the substituent. Thus, electron donorand electron acceptor substituents bonded to theN- and C-atoms of carbazole moiety produce anincrease or a decrease of the Ep=2 value respectively


(see also Table 5). This observation agrees with thefact that the significant change of the Ep=2 valuesof chlorocarbazole derivatives compared to that ofthe Ep=2 value of non-chlorinated carbazoles isconsistent with an inductive and mesomeric effectsof the substituents bonded to the carbazolemoiety.The one-electron oxidation potentials of carba-

zole and its derivatives are irreversible [23] even ifmolecular modeling shows no significant change inthe geometry of carbazoles between the neutraland charged species. However, this electrochemi-cal behaviour is due to follow-up chemicalreactions of the carbazole radical-cation such asdeprotonation follow by N–N and N–C dimeriza-tion of the carbazole radical intermediate formed[23,24]. Thus, there are some carbazoles whoseEp=2 are not measured or are difficult to bemeasured by cyclic voltammetry owing to the highirreversibility and the above linear correlationsmay be used for estimating the Ep=2 values ofcarbazoles from thermodynamic parameters, suchas EHOMO, DE or Ia which are easily calculated bysemiempirical method.

5. Conclusions

In the present study the radiative and radiation-less rate constants k0f ; kisc; k0p and knr and theintersystem crossing quantum yield fisc of chlor-ocarbazoles 1a–e, 2a–b, 3a, 4a–c, 5a, 6a, 7a, 8a–cand 9a–b in acetonitrile at 298K and in solidmatrix at 77K were calculated from the tf ; ff ; tpand fp:For carbazoles in general no noticeable change

of the shape of the spectra was observed on goingfrom 298 to 77K. Likewise, an important bath-ochromic shift of the lmaxðabsÞ; lmaxðfluoÞ andlmaxðphospÞ; ranging from 5 to 20 nm, wasobserved as the number of chlorine atoms attachedto the carbazole moiety was increased. This redshift is due to a combination of inductive andmesomeric effects of the chlorine atoms on the p-carbazolic system. The concrete interaction of thechloro substituents with the p-electrons in theHOMO was predicted by molecular modeling(PM3 method) as is shown in Scheme 2. Also,the incorporation of chlorine atoms to thecarbazole moiety proved their ability to quenchthe fluorescence emission by spin - orbital couplingwhich is evidenced through the lowering of the ffand tf values.The intramolecular HAE on the photophysical

rate constants was studied in detail. A strongheavy atom effect on the tf ; ff and kisc values wasobserved; the two former parameters showed animportant decrease of their values while for thelatter parameter a high increase was observed. Thek0f rate constants of chlorocarbazoles did notshow any significant decrease as the number ofchlorine atoms increased suggesting that a veryfast intrinsic fluorescence takes place in thesespecies.A comparative analysis of the photophysical

rate constants k0p; kisc; and knr; the shape of thephosphorescence emission spectra and the valuesof f ðT12S0Þ let us to conclude that the lowesttriplet excited state of compounds 1–8c is to be a3p;p electronic state while for compounds 9, 9aand 9b the T1 electronic state is to be likely a3n;p :The values of the HOMO and LUMO energy

and the LUMO-HOMO energy difference (DE) of

Fig. 5. Correlation between the adiabatic ionization potential

(Ia) and the one electron oxidation potential (Ep=2) of

carbazoles.K: experimental values. Line: best linear regression

obtained (r2: 0.944; slope: 0.45). Numbers are the carbazole

derivatives (see Scheme 1).


the chlorocarbazoles calculated by using thesemiempirical PM3 method were related to the1Lb electronic transition (DEðS0Þ ¼ EðS12S0Þ)values measured in acetonitrile and good linearregressions were obtained. This fact means that, inagreement with the results obtained by computa-tional calculations (PM3 method, Scheme 2), animportant contribution of the chloro substituentsin the HOMO orbital is operating and, therefore,the red shift observed on the absorption spectrumaccounts for this contribution. Additionally,this fact means that the contribution of thesubstituents to the HOMO orbitals is properlydescribed by the semiempirical method used(PM3), which has the advantage of having thecorresponding parameters for halogen substituentsincluded.It is interesting to point out that there are some

carbazoles whose Ep=2 are not known or aredifficult to be measured by cyclic voltammetryowing to their high irreversibility. As when thecalculated adiabatic ionization potentials (Ia;method: PM3) of carbazoles are related to theone-electron oxidation potential (Ep=2) values agood linear correlation is obtained, this linearcorrelation may be used for estimating the Ep=2

values of other carbazoles just knowing athermodynamic parameter such as Ia; easilycalculated by computational chemistry. Also,the Ia2Ep=2 relationship allows to calculatethe free solvation energy (DGðsÞ) value of carba-zoles radical cations formed as intermediatesduring electrochemical and photochemicalprocesses.Finally, on the basis of the comparison of

experimental and calculated absorption spectra(Tables 1 and 2; f ; Dm; lmax; method: PM3) onecan conclude that, at least for carbazoles Sn’S0(n ¼ 1; 2) transitions are quite accurately predictedwhile those involving the T1 state are still hard topredict by the semiempirical methods employed inthe present study.

Acknowledgements

The authors thank Universidad de Buenos Airesand CONICET for partial financial support;

Dr. H. Nonami (Ehime University, Japan) forHyperChem Suite and Dr. F. Quina (FederalUniversity of San Pablo, Brasil) for the use ofEdinburgh OB900 fluorometer and generoushospitality. R. Erra-Balsells and S.M. Bonesi areresearch members of CONICET.

References

[1] B.F. Plummer, L.K. Steffen, T.L. Braley, W.G. Reese,

K. Zych, G. van Dyke, B. Tulkey, J. Am. Chem. Soc. 115

(1993) 11543.

[2] F. Lewitzka, H.G. Lohmannsroben, J. Photochem.

Photobiol. Chem.: A 61 (1991) 191.

[3] D.O. Cowan, R.L. Drisko, Elements of Organic Photo-

chemistry, Plenum Press, New York, 1976, pp. 250–259.

[4] S.P. Glynn, T. Azumi, M. Kinishita, Molecular Spectro-

scopy of the Triplet State, Prentice Hall, Englewood Cliffs,

NJ, 1969.

[5] A.K. Chandra, N.J. Turro, A.L. Lyooms, P. Stone, J. Am.

Chem. Soc. 100 (1978) 4964.

[6] G.W. Robinson, R.P. Frosch, J. Chem. Phys. 38 (1962)

1187.

[7] P.S. McClure, J. Chem. Phys. 17 (1949) 905.

[8] M.J. Kasha, J. Chem. Phys. 20 (1952) 72.

[9] N.J. Turro, Modern Molecular Photochemistry, Benjamin,

Menlo Park, CA, 1978.

[10] S.M. Bonesi, Ph.D. Thesis, Universidad de Buenos Aires,

1995.

[11] S.M. Bonesi, R. Erra-Balsells, An. Assoc. Quim. Argent.

87 (1999) 127.

[12] S.M. Bonesi, R. Erra-Balsells, J. Lumin. 93 (2001) 51.

[13] S.M. Bonesi, R. Erra-Balsells, J. Chem. Soc. Perkin Trans.

2 (2000) 1952.

[14] S.M. Bonesi, R. Erra-Balsells, J. Photochem. Photobiol.,

A: Chem. 110 (1997) 271.

[15] S.M. Bonesi, R. Erra-Balsells, J. Heterocycl. Chem. 34

(1997) 887.

[16] S.M. Bonesi, R. Erra-Balsells, J. Heterocycl. Chem. 34

(1997) 891.

[17] HyperChem. Suite, Autodesk Inc., Ontario, 1998.

[18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria,

M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski,

J.A. Montgomery, Jr., R.E. Stratmann, J.C. Burant,

S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin,

M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,

R. Cammi, B. Mennucci, C. Pomelli, C. Adamo,

S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,

Q. Cui, K. Morokuma, D.K. Malick, D. Rabuck,

K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz,

B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,

I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox,

T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara,

C. Gonzalez, M. Challacombe, P.M. Gill, B. Johnson,


W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez,

M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian

98, Revision A.3, Gaussian, Inc., Pittsburgh PA, 1998.

[19] A. Bree, R. Zwarich, J. Chem. Phys. 49 (1968) 3355.

[20] J.F. Adams, W.W. Mantulin, J.R. Huber, J. Am. Chem.

Soc. 94 (1973) 5477.

[21] A. Sakar, S. Chakravorti, J. Lumin. 78 (1998) 205.

[22] S. Fukuzumi, M. Fujita, J. Otera, Y. Fujita, J. Am. Chem.

Soc. 114 (1992) 10271.

[23] J.F. Ambrose, R.F. Nelson, J. Electrochem. Soc. 112

(1975) 876.

[24] J.F. Ambrose, R.F. Nelson, J. Electrochem. Soc. 115

(1968) 1159.

[25] N. Mataga, Y. Toriashi, K. Ezumi, Theor. Chim. Acta 2

(1964) 158.

[26] J.R. Huber, J.E. Adams, Ber. Bunsenges. Phys. Chem. 78

(1974) 217.

[27] S.C. Chakravorty, S.C. Ganguly, J. Chem. Phys. 52 (1970)

2760.

[28] J.L. Huber, W.W. Mantulin, J. Am. Chem. Soc. 94 (1972)

3755.

[29] W.W. Mantulin, J.R. Huber, Photochem. Photobiol. 17

(1973) 139.

[30] W. Siebrand, J. Chem. Phys. 46 (1967) 440.

[31] W. Siebrand, J. Chem. Phys. 47 (1967) 2411.

[32] H.J. Hasink, J.R. Huber, J. Mol. Spectrosc. 60 (1976) 31.


Documents

Electronic spectroscopy of N- and C-substituted chlorocarbazoles in homogeneous media and in solid matrix