Isolated ultracold porphyrins in supersonic expansions. I. Free-base tetraphenylporphyrin and Zn- tetraphenylporphyrin

Isolated ultracold porphyrins in supersonic expansions. I. Free-base tetraphenylporphyrin and Zntetraphenylporphyrin

Uzi Even, Jacob Magen, and Joshua Jortner

Department of Chemistry, Tel-Aviv University, 69978 Tel Aviv, Israel

Joel Friedman

Bell Laboratories, Murray Hill, New Jersey 06974

Haim Levanon

Department of Physical Chemistry and the Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, 91904 Jerusalem, Israel (Received 3 May 1982; accepted 24 June 1982)

In this paper we report the results of an experimental study of the fluorescence excitation spectra and of the time-resolved emission of Zn-tetraphenylporphyrin (ZnTPP) and offree-base tetraphenylporphyrin (H,TPP) seeded in pulsed supersonic expansions of He. We have studied the S o-+S 1 transition (the Q band) and the SO-+S2 transition (the Soret, B band) of ZnTPP, as weI! as the So-+Si transition (the Qx band), the So--+5'; transition (the Qy band), and the So ...... s, transition (the Bx band) ofH2TPP. Information was obtained on the electronic energy levels, the vibrational level structure, the details oflow-frequency nuciear motion, and some characteristics of electronic relaxation.

I. INTRODUCTION

The techniques of laser spectroscopy of ultracold, isolated, large molecules seeded in supersonic expansions1

were applied recently for studies of excited-state energetics and dynamics of porphyrins and of related compounds. 2

-7 Fitch et al. 2

-4 have studied the Q~ and Qy

bands of phthalocyanine. We subsequently observed the Q and B bands of Mg-tetraphenylporphyrin (MgTPP) in a continuous jet of He, 5 while recent studies explored the Q~ and Qy bands of free-base porphine6 and the Q and B bands of Zn- tetrabenzoporphyrin in pulsed supersonic jets. In this paper we continue our experimental program of spectroscopic studies of ultracold, isolated porphyrins, undertaking an experimental study of lowlying spin-allowed electronic transitions of the freebase tetraphenylporphyrin (H2TPP), and of the Zntetraphenylporphyrin (ZnTPP) molecules in pulsed supersonic expansions of He. The two molecules selected for the present study are synthetiC porphyrins whose ease of preparation and thermal stability makes them attractive model systems for the exploration of excited-state energetics and dynamics of this class of compounds. The use of pulsed supersonic expansions allows for very effective internal cooling of the large molecules, providing spectroscopic data for Hz TPP and of ZnTPP, which are superior to those previously obtained for the related MgTPP molecule. 5 The following information was obtained from energy-resolved spectroscopy of Hz TPP and ZnTPP.

(1) Electronic level structure. The So - 51 and the So

- S2 transition of ZnTPP, whose point symmetry is D4h ,

are expected to correspond to excitations into doubly degenerate electronic states. 8 To the best of our knowledge, unequivocal evidence concerning the degeneracy of the 51 and S2 states of metal porphyrins cannot be in-

ferred from condensed phase spectra, where crystal field splittings are exhibited. 9

,lo Recent evidence for the degeneracy of the Sl and 52 states of the isolated Zn-tetrabenzoporphyrin was reported. 7 The spectroscopy of ZnTPP reported herein provides additional eVidence concerning this point. The D4h symmetry of ZnTPP is reduced in Hz TPP by the presence of the proton-proton axiS. 8

,11 This chemical symmetry breaking effect results in the appearance of the So - Sf and So - 5l transitions, 8,11 whose electronic-vibrational energetics will be reported.

(2) Vibrational level structure. Information on vibronic coupling effects8 in the Q bands of both H2 TPP and of ZnTPP, as well as a Jahn-Teller coupling8- 10

in the Q and B bands of ZnTPP, will be pertinent. In addition, it will be interesting to confront the excited-state vibrational level structure of the TPP compounds with the vibrational structure of other porphyrins, inferred from infrared, Raman, and optical spectroscopy,12 to assert what changes are induced by the presence of the phenyl rings. In the general context such information will be relevant for the vibrational characterization of model systems of biophysical interest.

(3) Low-frequency nuclear motion. In the So state of TPP compounds, large barriers for internal rotation of the phenyl groups with respect to the prophine plane are exhibited, resulting in large phenyl tilt angles. 13-19 An enhancement of the phenyl-porphyrin ring interactions in electronically excited nn* states was inferred from resonance Raman data, which indicate appreciable intensities for the modes of the phenyl rings, whereupon the phenyl rings may rotate towards the porphyrin plane. 20 In the isolated molecule the phenyl rings undergo largeamplitude low-frequency motion involving rotation

4374 J. Chem. Phys. 77(9), 1 Nov. 1982 021·9606/82/214374-10$02.10 © 1982 American Institute of PhYSics

Downloaded 22 May 2011 to 132.66.145.207. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

Even et al.: Ultracold porphyrins. I 4375

around the C-C bond. 17 The equlibrium angle, as well as the characteristic frequency, of this large-amplitude motion may be affected by the IIII* electronic excitation. From the spectroscopic point of View, one eXflects the appearance of a vibrational progression involving this low-energy motion. The changes in the equilibrium configuration and nuclear dynamics upon electronic excitation cannot be interrogated in condensed phases because of several reasons. First, the equilibrium molecular configuration may be modified by interactions with the host, as is the case for biphenyl. 21

Second, this low-frequency motion may be grossly modified, or even clamped, in the condensed phase. Third, inhomogeneous broadening effects may result in complete smearing of such low-frequency progression. Thus, such information regarding low-frequency motion in a "floppy" molecule should be inferred from the spectroscopy of the isolated molecule.

(4) Electronic relaxation from the lowest excited Singlet. The lifetimes of photoselected vibrational excitations in the S1 electronic-vibrational manifold of H2TPP and in the Sl manifold of ZnTPP provide information regarding the mechanisms of interstate electronic relaxation in porphyrins.

(5) Interstate coupling and electronic relaxation in the Soret band. The electronic energy gap separating the S2-S1 states in ZnTPP, as well as the S~-S1 states in H2TPP is - 7000 cm-1

, whereupon the corresponding interstate couplings in these large molecules correspond to the statistical limit for electronic relaxation. 22 The basic spectroscopic implication of interstate coupling in the statistical limit involves homogeneous, Lorentzian line broadening, manifesting the effects of relaxation in a bound level structure. 22 We shall attempt to obtain spectroscopic information on picosecond relaxation lifetime data from line broadening of the vibronic features of the So ret band.

II. EXPERIMENTAL PROCEDURES

The experimental techniques developed by Even et al. 6 for laser spectroscopy in pulsed supersonic expansions were utilized in the present work. A pulsed supersonic nozzle opened by a magnetic valve, which can be operated routinely in the temperature range 20-520°C, was constructed. The gas pulse, which was interrogated by delayed laser-induced fluorescence (LIF) from the seeded jet, was characterized by a width (FWHM) of 200 /lS. The repetition rate of the valve was 10 Hz. The nozzle diameter was 600 /lm. He gas, seeded with the TPP molecule at the stagnation pressure of 1000-2500 Torr, was expanded through the nozzle, which was maintained at a temperature of 380-450°C. The estimated Mach number for these expansions at typical experimental conditions of p = 2000 Torr and T = 420°C is M = 120. The pumping system consisted of a 4 in. diffusion pump backed by a mechanical pump with a pumping speed of 500 I min-1• The high (-103)

duty cycle of the pulsed supersonic source made it possible to employ such a modest pumping system. The pressure in the vacuum chamber was p = 10-3 Torr. The seeded gas was prepared by sending He through the

nozzle chamber containing the solid sample of H2TPP or of ZnTPP (commerically obtained from Porphyrin Products), which was heated to 380-450°C. The vapor pressure of H2 TPP is O. 11 Torr at 395°C, while the vapor pressure of ZnTPP is 0.054 Torr at 393°C. 23 The seeded supersonic expansion was crossed by a nitrogenpumped dye laser (spectral width 0.3 cm-1 and temporal pulse length 5 ns) at distances x = 10-15 mm down the nozzle. We have monitored:

(1) Fluoresence excitation spectra. The fluorescence was detected by a photomultiplier and recorded after normalization to the laser intensity.

(2) Time-resolved decay. The total fluorescence resulting from excitation at a fixed laser wavelength was collected by a photomultiplier, recorded by a Biomation transient recorder with a time resolution of 2 nsl channel and averaged by a signal averager. The laser stray light monitored by this detection system was characterized by a nearly exponential decay with a decay time of 5.5 ns. Molecular decay lifetimes T in the range T= 1-15 ns were determined by deconvolution of the experimental time-resolved fluorescence signal using moment analysis. T was determined from the difference between the normalized first moment of the fluorescence signal and the normalized first moment of the laser pulse. In order to demonstrate that the experimental lifetimes are unaffected by collisional dumping effects within the interrogation region, we have determined T

values by crossing the laser at distances x = 10 mm (x I D = 16) and x = 15 mm (xiD = 25) down the nOZZle, finding that T is independent of x.

Care was exerted to ensure the initial purity of the porphyrin samples and to eliminate any effects of thermal decomposition. The ZnTPP and H2TPP solid samples used in the present work were chlorine free. The spectra of ZnTPP and H2TPP did not change after keeping the nozzle at 420°C for 4 h. We thus conclude that there was no evidence for thermal decomposition of the sample in the nozzle chamber.

III. THE Q BAND OF ZnTPP

Previous work5 on the cooling of MgTPP in supersonic expansions has demonstrated that for this floppy molecule, which is characterized by low-frequency vibrations, effective internal cooling cannot be accomplished in a heavy diluent, such as Ar. This failure was attributed5 to the inefficiency of vibrational predissociation of van der Waals complexes between Ar and the large molecule for vibrational relaxation of these low-frequency modes, as well as to inefficiency of low-temperature collisions for vibrational relaxation of the low-frequency modes in the narrow pressure range, where van der Waals complexing with Ar is not excessive. Consequently, the spectrum of MgTPP seeded in Ar was found to be appreciably broadened by thermal sequence bands and possibly also by van der Waals MgTPP . Ar complexes. Effective internal cooling of MgTPP was accomplished5 by using high pressure He. Analogous results were obtained for ZnTPP and for H2 TPP, which are the subject matter of the present study. Under our experimental conditions, employing

J. Chem. Phys., Vol. 77, No.9, 1 November 1982


4376 Even et al.; Ultracold porphyrins. I

r I(j)

Z w IZ

17750 ENERGY(cm- l )

17650 17550

WAVELENGTH (A)

o I

o

17450

5750

FIG. 1. Fluorescence excitation spectrum in the region 5625-5750 A of ZnTPP seeded in pulsed supersonic expansions of He. ZnTPP was heated in the nozzle chamber to T=450°C, seeded into He at the stagnation pressure p = 1300 Torr and expanded through the D = 600 j1 m nozzle. The laser crossed the expansion at x = 12 mm down the nozzle. The electronic origin is marked 0-0.

a nozzle with D = 600 Mm, effective internal cooling was accomplished by .seedingthese tetraphenylporphyrins in He at the pressure range p = 1000-2300 Torr.

Figure 1 shows the fluorescence excitation spectrum in the range 5625-5725 A of ZnTPP seeded in a supersonic expansion of He at p = 1300 Torr. The spectrum is practically free from vibrational sequence bands, as decreasing the downstream temperature by increasing

ENERGY (em-I)

17540 17520 17500

§LJ~ z W IZ

5700 5705 5710 5715

WAVELENGTH {Al

\

17480

Ne

P=550 TORR

He

P= 1300 TORR

5720 5725

FIG. 2. The electronic origin and low-energy excitations of the So - SI transition of ZnTPP in supersonic expansions of Ne and of He. The stagnation pressures are indicated on the curves. All other conditions as in Fig. 1.

the stagnation pressure in the range p = 1300-2300 Torr did not change the relative intensities of the prominent spectral features. The weak band at 5725 A presumably is due to a hot band. We have demonstrated that the spectrum of Fig. 1 corresponds to the bare molecule rather than to the He-ZnTPP van der Waals complexes, as practically identical spectra were obtained by expansion of ZnTPP in He and in Ne (Fig. 2).

TABLE 1. Energetics of the electronic origin of the SO-Sl transition of ZnTPP and of the So-S1 transition of H2TPP.

Molecule

ZnTPP

ZnTPP

ZnTPP

ZnTPP

Medium

Pulsed supersonic expansion of He

Gas phase 718 K

Benzene solution 300 K

Ethanol glass 103 K

Pulsed supersonic expansion of He

Gas phase 421 K

Peak Spectral energy bandwidth (cm-1) (cm-1)

17490 1.2

16835 ~ 800

16949 ~400

16670 -250

15617 2

15060 ~ 700

Assignment Reference

0-0 Present work Rotational broadening

Unresolved vibrational 23 sequence congestion and rotational broadening

Unresolved vibrational sequence congestion and inhomogeneous broadening

Unresolved vibrational sequence congestion and inhomogeneous broadening

0-0 Rotational broadening

Unresolved vibrational sequence congestion and rotational broadening

24

25

Present work

23

J. Chern. Phys., Vol. 77, No.9, 1 November 1982



The narrow spectral features of Z nTPP, as portrayed in Figs. 1 and 2, are attributed to the low-lying vibrational excitations of the So - SI transition, which originates from the vibrational electronic origin So( 0) of the ground state. The lowest-energy, intense, spectral feature of ZnTPP is located at 5717.4 A (17490 cm-1

).

The absolute accuracy of the energy scale is ± 4 cm-1,

being determined by the calibration of the laser wavelength. This lowest-energy intense excitation is attributed to the electronic origin 0-0 of the So - SI transition. The linewidth of the electronic origin (FWHM) is 1. 2 cm- l , being due essentially to unresolved rotational structure. This width of 1. 2 cm-1 for the rotational envelope of the 0-0 transition of ZnTPP in pulsed supersonic expansions of He, accomplished at p = 1600 Torr and D = 600 j.Lm, is considerably lower than the value of 3.0 cm-1 previously observed by us5 for the 0-0 transition of MgTPP in continuous supersonic jets of He at p = 10 atm and D = 150 j.Lm, demonstrating the efficiency of rotational cooling of the large ZnTPP molecule under the present experimental conditions.

It is instructive to compare the characteristics of the electronic origin of the internally cold isolated ZnTPP molecule with solution spectra24 ,Z5 and with the vapor spectrum. 23 The data of Table I demonstrate the dramatic effect of internal cooling on the electronic origin of this huge molecule.

In the energy range Ell = 0-100 cm-1 above the electronic origin, two additional intense spectral features are exhibited at 23 ± 2 and 45 ± 2 cm-1

• These two vibrational features can be attributed either to vibronically induced Herzberg-TeUer false origins, or to ordinary vibrational excitations in the SI manifold. We reject the first alternative on the basis of three arguments. First, in isolated rigid porphines and related compounds, the Q band is dominated by an intense 0-0 tranSition, while the relative intensities of high-frequency vibronicaUy induced transitions are weak. The assignment of the two low-frequency features to "false origins" is incompatible with their high relative intensities. Second, the low frequency involved originates from the motion of the phenyl groups, which is expected to be ineffective for vibronic coupling with higher electronic states. The low-frequency vibrational excitations are attributed by us to low-frequency excitations in the SI manifold, which involve the torsional motion of the phenyl groups. The intense 23 ± 2 and 45 ± 2 cm-1 vibrational features of ZnTPP are assigned to a single vibrational mode, being due to the 0-1 and 0-2 excitations of the torsional mode. The 72 ± 2 cm -1 vibrational feature (Table II) is probably due to the 0-3 excitation ofthis torsional mode. A detailed discussion of these torsional excitations of ZnTPP, together with the corresponding motion in HzTPP, will be presented in Sec. V. Some of the low-energy features in the energy range 39-100 cm-1 (Table II) may be due to some other type of motion of the phenyl groups, e. g. , out-of-plane bending. At present, we cannot provide a definite identification or an assignment of these features.

Table II lists the vibrational excitations of ZnTPP in the low energy range up to Ell = 280 cm-1• Following the

TABLE II. Vibrational &tructure and experimental radiative IUetimes for the 51 state of ZnTPP electronic origin at 17490 em-I. a

Relative intensity AV (cm-I)b Assignment r(ns)C

(6) -25 Hot band 100 0 0-0 3.3 105 23 w; 4.0

8 39 70 45 2w; 3.6 20 48 15 72 3w~ 3.2 25 80 3.2

9 81 5 93 5 98 6 118

10 170 15 178 2.0 18 186 F 20 203 F 2.3 12 207 18 211 186+w; 22 222 203+~ 2.2

7 228 8 233 186 +2w~ 5 241

17 245 203+2~ 2.2 5 263 8 271

aAbsolute accuracy of wavelength scale ± 2 'A (± 6 em-I). bAccuracy of energies AV relative to the electronic origin is ± 2 em-I.

cAccuracy of lifetimes ± 30%.

prominent torsional excitations, several additional moderately weak vibrational features are exhibited in the range E II =100-280 cm-1

• The assignment of fundamental vibrations in this energy range rests on the observation of combination bands with the prominent 0-1 and 0-2 torsional excitations of the phenyl groups. Two fundamental frequencies of 186 and 203 cm-1 could then be identified. Additional weak spectral features in the energy range Ell = 100-280 cm-1 do not reveal the lowfrequency torsional progreSSions (Table II). Accordingly, we were reluctant to assign these weak features to vibrational excitations of the So(O) - SI transition. Further work is required to establish whether these weak spectral features are intrinSiC, being due to Fermi resonances or originate from impurities.

Our spectroscopic data for ZnTPP revealed vibrational excitations in the SI manifold at low energies, while no evidence was obtained for "symmetry breaking" of the doubly degenerate SI(IEy) state of the "isolated" molecule.

IV. THE Ox AND Oy BANDS OF H1TPP

Figures 3 and 4 show the fluorescence excitation spectra in the range 5700-6430 A of HzTPP seeded in supersonic expansions of He. On the basis of the invariance of the spectra with changing the stagnation



4378 Even et al.: Ultracold porphyrins. I

ENERGY (em-I)

15980 15900 15820 15740 15660 15580

o WAVELENGTH (A)

pressure in the range 1300-2000 Torr, we have demonstrated again that the prominent features correspond to excitations from So(O) of the bare ultracold molecule. No spectral features are exhibited in the range 6415-6550 A. The first moderately weak band observed at 6412 A is presumably due to a hot band. The intense spectral feature at 6403.3 A (15617 cm-1

) is assigned to the electronic origin of the first spin-allowed So - S~ electronic excitation, i. e., the Q" band. The widths of the spectral features of the 0-0 and of the lowest vibrational excitations (FWHM) are 1-2 cm-1

, reflecting again unresolved rotational structure. In Table I the characteristics of the 0-0 of the S1 state of Hz TPP are confronted with solution and gas phase spectra. The energy range Ell = 0-100 cm-1 is dominated (Fig. 3 and Table III) by two additional intense low-energy spectral features at 18 ± 2 and 40 ± 2 cm -1 above the electronic origin. The qualitative features of the spectrum of HzTPP for Ell <f 100 cm-1 are very similar to those of ZnTPP. Following the analysis of Sec. III, we assign the two intense equidistance low-frequency vibrational excitations of HzTPP to torsional excitations of the phenyl groups with respect to the porphine plane. The striking qualitative similarity of the energetics and of relative intensities of the low-energy vibrational excitations in the lowest spin-allowed electronically excited

o I o

r---r---l FIG. 3. Fluorescence excitation spectrum in the range 6250-6430 A of H2 TPP in supersonic expansions of He nozzle temperature p=450·C, stagnation pressure p = 1200 Torr. All other experimental conditions as in Fig. 1. The electronic origin is marked 0-0. Each triplet, consisting of a fundamental vibrational frequency together with its 0-1 and 0-2 torsional excitations, is marked"-'-'. The vibrational frequency (in cm -1) is marked in parenthesis.

state of HzTPP and of ZnTPP indicates that the intense vibrational "triplet," consisting of the electronic origin and of the two lowest torsional excitations provides a spectroscopic characterization of the electronic-vibrational excitations of "isolated" TPP molecules. In Sec. V we shall return to a more detailed analysis of the torsional motion. Some additional low-energy spectral features in the range Ell = 0-100 cm-1 may correspond to out-of-plane bending of the phenyl groups. However, we are uncertain whether these moderately weak features are indeed intrinsic.

The spectral range 5700-6370 A, which corresponds to Ell = 100-1900 cm-t, (Fig. 4) reveals a rich, wellresolved, vibrational structure. The sub-range 6125-6250 A exhibits only a few, very weak vibrational features (Table III) and was not reproduced in Fig. 4. As is evident from Fig. 4, the vibrational structure in the range E II = 100-1900 cm-1 is characterized by the appearance of triplets of equidistant spectral features separated by 20 ± 2 cm -1. Each of these triplets corresponds to a fundamental vibration followed by combination bands of this fundamental with the 0-1 and 0-2 torsional excitations of the phenyl groups. The appearance of the combination bands of the torSional excitations provided us with a diagnostic tool for the indenti-

17600 17200 ENERGY (em-I)

16800 16500 16300 !l5151

rn 115111

no

5850 595

(9501 r--o--l

110001 ",

WAVEI F NGTH (A)

~ 18181 ,-,--,17391

,---,-----, 17001 ,--,-,

6125

FIG. 4. Fluorescence excitation spectrum in the range 5650-6125 A of H2 TPP in supersonic expansions of He. Experimental conditions and notations as in Fig. 3.




TABLE m. Vibrational structure and experimental radiative lifetimes for the Sf state of H2TPP electronic origin at 15617 cm-I • a

.c.v .c.v (cm-I)b,c Assignment T(ns)d (cm-I)b,c Assignment T(ns)d

-22 o

18 22 (w)

40 43 (w) 58 61 69 (w) 94 (w)

146 151 160 166 184 188 191 207 (w) 208 214 232 236 256 331 351 380 388 (w) 398 400 487

Hot band 0-0

""

F

146+"" 146+2wf

F

F

191 +"" 208+w1 191+2"" 208 +2"" F (?)

331 +w1 F (?)

380+"" F

507 487+wt 522 (w) 530 487+ 2wt 539 (w) 543 (w) 547 (w) 654 (w) 672 (w) 685 (w) 691 (w) 700 F 718 700+w1 739 F

12.0 12.0

11.5

12.0

741 758 778 785 818 824 840 848 862 866 (w) 872 (w) 950 971 986 994

1000 1019 1022 1041 1211 1241 1258 1279 1301 1316 1351 1365 1384 1404 1511 1515 1533 1536 1550 1559 1589 1607 1710 1731 1752 1874 1896 1921

700+ 2w1 739+wt 739+2wt

F F(?)

818+"" 824+"" 818+ 2w1 824+ 2w1

F 950+wt

950+2w~ F

1000 +"" 1000+2wt

F 1258+wt 1258 +2""

F 1365+wt

F F

1511 +"" 1515+wt 1511 +2wt 1515+2wt

F(?)

1710+w1 1710+2wt F(?) 1874+w1 1874+2wt

aAbsolute accuracy of wavelength scale ±2 A (±6 cm-I). bAccuracy of energies .c.v relative to 0-0 is ±2 cm-I. C(w) denotes a weak spectral feature. dAccuracy of lifetimes ± 10%.

12.0 11.6

11.0

12.0

11. 7

11.3

11.8

11.8

10.4 11.4 12.3

12.3

fication of fundamental vibrational excitations in the S~ manifold of HzTPP (Figs. 3 and 4 and Table m). We were able to identify 14 fundamental vibrational frequencies of Hz TPP, which are compared in Table IV with the low-frequency modes of the SI state of ZnTPP (Sec. III) , as well as with the vibrational modes of Sf state of free-base porphine (H2P) recently investi-· gated by laser spectroscopy in supersonic expansions. 6

A cursory comparison between the vibrational fundamentals and HzTPP and of HzP is of interest for the establishment of the role of vibrational excitations of the benzene groups in the S~ state. At the risk of triviality

we point out that the low frequency motion (20±2 cm-1

for HzTPP and 23±2 cm-l for ZnTPP) is absent in the rigid HzP molecule, providing conclusive evidence for its assignment to the motion of the phenyl groups in the TPP compounds. We have searched for the appearance of phenyl-ring modes in the S~ manifold of HzTPP. The So - S~ spectrum of Hz TPP reveals several weak vibrational frequencies in the range 700-1000 cm-I

, which do not have a clear counterpart in the HzP spectrum, i. e., 739 or 824 cm-I

• The 739 cm- l feature is close in energy to the 1~ vibrational excitation of alkyl benzenes. 20.26

In view of the low intensity of the frequencies, which may involve the phenyl group, we conclude that the So - Si electronic excitation of HzTPP is accompanied by small configurational changes of the normal modes of the phenyl groups.

We were able to observe and analyze well-resolved vibrational structure in the Si manifold up to excess energy of Eu= 1800 cm-l

• With increasing Ell the vibrational level structure becomes congested, whereupon no vibrational structure can be resolved within the quasicontinuous, weak absorption in the spectral range below 5350 A. The drastic drop in the intenSity ofthe fluorescence excitation spectra in the range Ell> 1800 cm -1 does not originate from the shortening of the excited-state lifetimes which exhibit (see Sec. VI) only a weak dependence on Eu' Thus, in the range Ell> 1800 cm-l the weak absorption is blamed on two effects: (i) Less favorable Franck-Condon factors for individual vibrational excitations, and (ii) intrastate vibrational anharmonic scrambling effects, which lead to redistribution of the intensity of individual vibrational excitations over a wide energy range. The appearance of the "vibrational

TAB LE IV. Fundamental vibrational frequencies of isolated ZnTPP, (present work), H2TPP (present work) and free-base porphine (H2P from Even et al. Ref. 6). a,b

ZnTPP H2TPP H2P

23±2 18 ±2 146 148

186 191 203 208

331 304 (380) (487) 700 712 739 818 783 824 950 961

1000 984 1049 1258 1209 1365 1327 1345 1354 1368 1511 1556 1515 1584

(1710) (1874)

aEnergies in cm-! units. bAccuracy of frequencies ±2 cm-!.




ENERGY(cm- l )

19150 18950 18750 18550 18350

WAVELENGTH (A)

FIG. 5. Fluorescence excitation spectra in the range 5200-5450 A of H2TPP in supersonic expansions of He. Experimental conditions as in Fig. 3.

quasicontinuum" in this energy range manifests the effects of many-level Fermi resonances. Z7 In the case of HzTPP the low-frequency torsional vibrational modes presumably participate in the anharmonic coupling, resulting in effective smearing of the intensity for high vibrational excitations of the 5f manifold.

The spectral range 5250-5330 A reveals the appearance of a new intense spectral band (Fig. 5). This spectral feature is ill-defined, showing some moderately sharp "spikes" superimposed on a broad background. The overall width (FWHM) of this spectral band is huge, being -100 cm- l

• This broad spectral band is attributed to the 50 - 5 f electronic transition of Hz TPP. The large width of this spectral band originates from the superposition of two effects.

(a) Effective electronic relaxation. The electronic origin and the low-lying vibrational excitations are broadened due to effective interstate 5f- 5f electronic relaxation. Previous work6 on the HzP molecule has established that the relaxation homogeneous widths of low electronic-vibrational excitations of the 5 f state in that molecule are 10-20 cm-1

•

(b) The appearance of low-frequency torsional motion of the phenyl groups in the 5 f manifold of Hz TPP . In analogy to the 5f state, we expect the appearance of high intensity 0-1 and 0-2 excitations of the torsional excitation, which are located at - 20 and at - 40 cm-1

above the electronic origin.

In the spectral range Ev = 0-100 cm-1 above the electronic origin of the 50 - 5 f excitation of Hz TPP we expect the appearance of several intense electronic-vibrational transitions, whose homogeneous widths are comparable to their spacings. The broad band in the range 5250-5300 A is attributed to superposition of the homogeneously broadened electronic origin and of the lowlying Vibrational excitations of the 50 - 5 f transition. In view of this congestion, the homogeneous linewidths of

individual electronic-vibrational excitations cannot be determined. From the energy of the onset of the broad absorption at 5300 A, we can roughly estimate the splitting of the electronic origins of the 5 f and 5f states of the HzTPP to be AII=3250 cm-1

, within an accuracy of 50 cm-1

• This value of All for Hz TPP is close to but slightly lower than the 5 {-5f splitting All = 3564 cm-1

for free-base porphine. 6

v. TORSIONAL MOTION OF THE PHENYL GROUPS

In the energy range Ev = 0-100 cm-1 above the electronic origin of the 50 - 51 transition (Q band) of ZnTPP and of the 50 - 5f transition (Qx band) of Hz TPP, we have observed the low-energy excitations which are characterized by the following features:

(1) Two intense vibrational excitations are observed above the electronic origin at energies of 23 ± 2 and 45 ± 2 cm-1 for ZnTPP and at energies of 18 ± 2 and 40 ± 2 cm-l for HzTPP.

(2) A third weaker member of the progression is observed at 72 ± 2 cm- l for ZnTPP and at 61 ± 2 cm- l for HzTPP.

(3) The relative intensities of the three vibrational excitations, listed in (1) and (2) with respect to the electronic origin, are 1. 0: O. 55: O. 21 for HzTPP and 1. 05: O. 70: 0.15 for ZnTPP.

We have assigned these low-energy vibrational excitations in the two compounds to the torsional motion of the phenyl groups with respect to the plane of the porphyrin ring. As the three Vibrational excitations, listed in (2) and (3), are equidistantly spaced in energy, they are attributed to a single torsional vibrational mode within the 51 (or 51) manifold. The torsional motion of the phenyl groups in TPP compounds in their ground electronic state is characterized by a double minimum potential,17 with the phenyl groups being nonpolar with respect to the plane of the porphyrin ring. Crystallographic data indicate that the equilibrium angle cp of the phenyl groups with respect to the porphyrin ring exceeds 600. 13,a This angle may be somewhat lower in the isolated molecule, where model calculations17 yield cp = 44° in So. The barrier height is quite large, being E B "" 8000-10000 cm-l in 50' as inferred from kineticZ8

(a)

and magnetic resonanceZ8 (b) data. Nothing is known concerning the torsional motion in the first electronically excited Singlet state; however, one may expect that the torsional motion is also characterized by a double minimum potential in the 51 (or 5f) state. Figure 6 portrays a sketch of the relevant sections of the potential surfaces for torsional motion in the 50 and 51 states of TPP compounds. The 50 potential was adopted from the model calculations of Wolberg. 17 The 51 potential was characterized by the same curvature in the vicinity of its minima as for the 50 state, while the position of the equilibrium angle in 51 was taken to be slightly lower than for 50. To provide a semiquantitative description of the energetics and spectroscopy of the torsional motion (Fig. 5), one has to specify the vibrational frequencies and the potential barriers in the two electronic states. The characteristic vibrational frequency




5,

16000 ., E u

~

> 8000

So

0 1T

FIG. 6. Artist's view of potential surfaces for the torsional motion of the phenyl groups relative to the porphyrin ring in TPP compounds in their ground and first electronic singlet states.

w~ for the electro nically excited state is 23± 2 cm- I for ZnTPP and 18±2 cm-I for H2TPP, these frequencies being close within the large experimental uncertainty. An "intelligent guess" for the frequency"" of torsional motion in So can be provided from the prominent hot bands appearing for both compounds (Tables II and III), which yield w1 = 22 ± 2 cm-1 for H2TPP and w~ = 25 ± 2 cm-1 for ZnTPP, the ground state frequencies being close for both compounds. Accordingly, we infer that w1 is close to w~ for both compounds. Finally, it is reasonable to assume that the planarity barrier in the excited state is comparable to that in the ground state. Accordingly, the planarity barrier ~E B"" 8000 cm-t , both in the ground state and in the excited state, is huge with respect to the level spacing (w~ or w~). The following features of the energetics and spectral intensities of the system portrayed in Fig. 6 will be exhibited:

(i) The relevant quantum levels are located well below the barrier.

(ii) The level spacing is nearly harmonic, or rather weakly anharmonic, in the vicinity of the minima of the potential surfaces in both electronic states.

(iii) The level spacings wf and w~ in both electronic states are nearly equal.

(iv) The allowed electronic-vibrational excitations from the low vibrational levels of So correspond essentially to tram itions between the states of two displaced harmonic oscillators.

(v) The symmetry-allowed spectral transition from the electronic origin of So involve both odd-parity and even-parity O-v (v = 0,1,2,3, ... ) transitions.

The relative intensity lo_v for a O-v transition normalized to the intensity 10_ 0 of the 0-0 transition can be well approximated by the simple relation

(V. 1)

where S=A2/2, with A=AI/>/{Acf}) representing the reduced displacement Acf> between the minima of the two potential surfaces (Fig. 6), is expressed in units of the rms zero-point displacement (AI/>2) for the torsional motion in its ground Vibrational state. Equation (V. 1) is valid provided that anharmonicity corrections are minor and that frequency changes between So and SI are small . Both conditions seem to be well satisfied for the torsional motion in TPP compounds. The relative intensities for the torsional progression can be reasonably well reproduced by the simple intensity relation [Eq. (V. 1)] with S = 1. We then obtain the relative intensities: 1. 0 (for 0-0), 1. 0 (for 0-1), 0.5 (for 0.2), 0.17 (for 0-3), and 0.04 (for 0-4), which are in good agreement with the experimental data [see point (3)J, e. g., for H2TPP we found 1. 0 (for 0-0), 0.55 (for 0-1), and 0.21 (for 0-3). The weak intensity for the 0-4 transition precludes its unambiguous identification. The value of S = 1 for the electronic-vibrational coupling parameter, which characterizes the low-frequency motion, implies that the reduced displacement is AI/> = 1. 4 {AI/>2)1/2. A cursory examination of Wolberg's potential surfaces (Fig. 6) indicates that the equilibrium displacement upon electronic excitation is moderately small, being AI/> "" 10. This modest value of AI/> is sufficient to exhibit a pronounced Franck-Condon progression for the torsional motion.

Our approximate treatment did not focus attention on chemically specific effects on the torsional motion. The characteristic excited-state frequency ~ = 18 ± 2 cm-I for H2 TPP seems to be slightly lower than the value w~ = 22 ± 2 cm-1 for ZnTPP, although the difference between these two values is barely outside the (large) experimental uncertainty. Also, the intensity pattern [point (3)] for the two compounds is slightly different. These interesting second-order chemical effects deserve further study.

The present work provides spectroscopic information on low-frequency intramolecular torsional motion in large TPP molecules. A qualitatively similar set of low-frequency excitations was reported previously by us for the Q band of MgTPP molecule in supersonic jets. 5 We believe that the present phYSical picture assigning the torsional excitations of the phenyl groups to a O-v vibrational progression supersedes the assignment presented in previous work,s which attributed the low-energy vibrational structure ofthe Q band of MgTPP to two types of excitations involving even-parity torsional excitations and even-parity transitions for outof-plane bending of the phenyl groups. The prominent low-frequency features of the Q band of MgTPP correspond to O-v torsional excitations. Some weak lowfrequency features in the range Ev = 0-100 cm-I in the spectra of H2TPP, ZnTPP, and MgTPP may correspond to out-of-plane bending. However, a definite assignment of these excitations cannot be provided at present. From the point of view of general methodology, the appearance of the low-energy vibrational progression for the torsional motion of the phenyl groups is expected to be universal for this class of molecule, providing a general "fingerprint pattern" for the characterization of electronic excited states of tetraphenylporphyrins.




VI. ELECTRONIC RELAXATION IN THE FIRST SINGLET STATE

We have studied the time-resolved fluorescence decay of ZnTPP and H2TPP following selective excitation. The experimental radiative decay lifetimes T of photoselected states provide direct information on electronic relaxation of individual vibrational-electronic excitations in the SI

manifold of ZnTPP and in the Sr manifold of H2TPP. From the lifetime data assembled in Tables II and III, the following information emerges:

(1) The lifetimes TO of the electronic origin of H2TPP is TO = 12 ± 1 ns, while for the electronic origin of ZnTPP To = 3. 3 ± 0.7 ns. To obtain a rough estimate of the emission quantum yields from the electronic origins of these bare molecules, we utilized the pure radiative lifetimes Tr inferred from the integrated oscillator strengths whict are Tr"'" 60 ns for ZnTppB and Tr"'" 120 ns for H2TPP. B

Thus, TO/Tr"'" O. 05 for ZnTPP and TO/Tr"'" 0.1 for H2TPP. The shortening of the decay lifetime and the reduction of the emission quantum yield of ZnTPP relative to the free-base compound reflects the heavy atom enhancement of intersystem crossing from the SI state of ZnTPP. B Apart from this indirect information, no direct evidence could be inferred concerning the relative contributions of SI - Tl and of SI - So radiationless channels in these molecules.

(2) The lifetimes of the electronic origin and of the torsional excitations of the phenyl ring in the energy range Ev = 0-100 cm- l are practically independent of the initial vibrational state, both for H2TPP and for Z nTPP. This result is not surprising as, in view of the close proximity between the equilibrium configuration and the Similarity between the vibrational frequencies w~ and w~, these torsional modes do not act as active accepting modes in the electronic relaxation process.

>f-(/)

Z W fZ

25300

3950

ENERGY (em-I)

25200 25100 25000

3970 3990 WAVELENGTH (AI

FIG. 7. Fluorescence excitation spectrum in the range 3950-3990 A of H2TPP in supersonic expansions of He. Experimental conditions as in Fig. 3.

>f-(/)

Z W fZ

3940 WAVELENGTH (Al

FIG. 8. Fluorescence excitation spectrum in the range 3940-3980 A of ZnTPP in supersonic expansions of He. Experimental conditions as in Fig. 1.

(3) The lifetimes of photoselected vibrational states in the S~ manifold of H2TPP over the broad range Ev = 0-1700 cm- l are practically constant, assuming the value T = 11 ± 1 ns. The practical independence of T

on the excess vibrational energy was observed recently for two classes of large molecules, which exhibit the following characteristics:

(i) Threshold behavior. For moderately high vibrational excitations in the SI state of tetracene and pentacene, which are located above some threshold energy Ev, i. e., Ev> 1500 cm- l for tetracene and Ev> 700 cm-l for pentacene, T is independent of E v , assuming a value which is considerably lower than the lifetime To of the electronic origin.

(ii) Total independence. For several porphyrins. i. e., free-base porphine6 and Zn-tetrabenzoporphyrin,7 E v = 0 and T in the SI manifold is independent of Ev with T= TO'

The Si state of H2TPP studied herein corresponds to class (ii). The weak dependence of T on Ev in the first electronically excited singlet manifold of porphyrins and related molecules seems universal, reflecting the manifestation of effective intrastate scrambling between vibrational levels ("Vibrational energy redistribution") on intrastate electronic relaxation. 27

VII. THE SORET BAND

In the spectral range 3950-3990 A the fluorescence excitation spectrum of H2TPP reveals an additional electronic transition (Fig. 7), which is attributed to the So

- S~ transition (the B" band). We were unable to detect the By band of H2 TPP, which is expected to be located at higher energies. No evidence was obtained for the So-SI transition down to 3700 A, while spectroscopic studies at higher energies were precluded because of

J. Chem. Phys .• Vol. 77, No.9, 1 November 1982



experimental difficulties. The So - S~ transition of the "isolated" H2TPP molecule reveals a well-resolved vibrational structure (Fig. 7). The intense spectral feature at 3981. 9 A is tentatively assigned to the electronic origin of the first component of the Soret band of Hz TPP. The mean spacing between the low-lying adjacent vibrationallevels is 22 ± 2 cm-!. This vibrational progression is tentatively assigned to the O-v (v=O, 1,2, and 3) vibrational excitations of the torsional motion of the phenyl groups. This large amplitude motion in the S~ Soret band is qualitatively very similar to the torsional motion in the Sf state. The vibrational frequency 22 ± 2 cm-l is close to the value w~ = 18 ± 2 cm- l

, while the intensity pattern in the Bx band is also quite close to that observed for the Qx band.

The fluorescence excitation spectrum of ZnTPP in the spectral range 3940-3980 A (Fig. 8) reveals a second electronic transition, which is tentatively attributed to the So - Sa transition, 1. e., the Soret band. The spacing between the lowest lying vibrational excitations in the Sz state of ZnTPP is 25 ± 3 cm- l , which is quite close to the torsional frequency w~ = 23 ± 2 cm- l in the SI manifold. As for Hz TPP, the low frequency torsional motion for the ZnTPP molecule is similar in the Q band and in the Soret band, so that both the Sa and SI states can be described qualitatively by the potential surfaces of Fig. 6.

The appearance of low-resolved vibrational structure of the S~ state of HzTPP and the Sa state of ZnTPP provides significant qualitative information concerning the time scale for intramolecular electronic relaxation from the vibrational excitations of the Soret band of these molecules. The linewidths ~ of the low-energy vibrational features (FWHM) are ~(S~) "" 3 cm-! for Hz TPP and ~(Sz) "" 6 cm- l for ZnTPP, being higher than the corresponding widths ~(Sf) = 2 cm- l for HzTPP and ~(SI) = 1. 2 cm-l for ZnTPP in their first electronically excited Singlet state. A rough estimate for the contribution 1i to the homogeneous linewidth of the Soret band can be obtained by invoking the reasonable assumption that the rotational broadening in the B band and in the Q band is identical. The homogeneous relaxation widths of the Sa (or S~) states are 1i = ~(Sn - ~(Sn = 1 cm-l for H2TPP and 1i =~(52) - ~(S!) =4 cm-! for ZnTPP. The upper limit for the lifetime T(52) of the electronic origin and of low-lying vibrational excitations in the Soret band is T(S2) <1i/~. Thus, T(52) > 5x 1O-!2 s for H2TPP and T(52)

> 10-12 s for ZnTPP. The upper limit for the lifetime of the S2 state of the isolated ZnTPP molecule is consistent with the lifetime T(Sz) = 3. 6 X 10-1Z s estimatedZ9 from solution quantum yield data. The picosecond time scale for electronic relaxation from the Soret band of H2TPP and ZnTPP obtained herein is similar to the value T(S2) > 4 x 10-12 s recently determined for the Soret band of the isolated Zn-tetrabenzoporphyrin molecule' and for this molecule in solution. 3o These results are of considerable interest for the elucidation of the features of intramolecular electronic relaxation of highly excited electronic states of isolated porphyrins.

ACKNOWLEDGMENT This research conducted at Tel-Aviv University was

supported by the United States Army through its European Research Office.

ID. H. Levy, Annu. Rev. Phys. Chern. 31, 197 (1980). 2p. S. H. Fitch, L. Wharton, and D. H. Levy, J. Chern. Phys.

69, 3424 (1978). 3p • s. H. Fitch, L. Wharton, and D. H. Levy, J. Chern. Phys.

70, 2019 (1979). 4S. H. Fitch, C. A. Hayman, and D. H. Levy, J. Chern. Phys.

73, 1064 (1980). 5(a) U. Even, J. Magen, J. Jortner, and H. Levanon, J. Am.

Chern. Soc. 103, 4583 (1981); (b) U. Even, J. Magen, J. Jortner, and H. Levanon, J. Chern. Phys. 76, 5684 (1982).

Su. Even, J. Magen, and J. Jortner, Chern. Phys. Lett. 88, 131 (1982).

7U. Even, J. Jortner, and J. Friedman, J. Phys. Chern. 86, 2273 (1982).

8M• Gouterman, The Porphyrins, edited by D. Dolphin (AcademiC, New York, 1978), Vol. m, p. 1.

9G. W. Canters, J. van Egmond, T. J. Schaafsma, and J. H. van der Waals, Mol. Phys. 24, 1203 (1972).

lOG. W. Canters and J. H. van der Waals, The Porphyrins, edited by D. Dolphin (Academic, New York, 1978), Vol. III.

liS. Voelker and J. H.van der Waals, Mol. Phys. 22, 1703 (1976).

12R. H. Felton and N. T. Yen, The Porphyrins, edited by D. Dolphin, (Academic, New York, 1978), Vol. m, p. 347.

13E. B. Fleischer, Acc. Chern. Res. 3, 105 (1970). 14S. J. Silvers and A. Tulinsky, J. Am. Chern. Soc. 89, 3331

(1967). 15M. Neot-Ner and A. D. Adler, J. Am. Chern. Soc. 94, 4763

(1972). I~. O'Rourke and C. Curran, J. Am. Chern. Soc. 81, 5111

(1969). 17A. Wolberg, J. Mol. Struct. 21, 61 (1974). 18S. S. Eaton and G. R. Eaton, J. Am. Chern. Soc. 97, 3660

(1975). 19G• M. La Mar, G. R. Eaton, R. H. Holm, and F. A. Walker,

J. Am. Chern. Soc. 95, 63 (1973). 20J. M. Burke, J. R. Kincaid, and T. G. Spiro, J. Am. Chern.

Soc. 100, 6077 (1978). 21A. Baca, R. ROsse Hi, and L. E. Brus, J. Chern. Phys. 70,

5575 (1979). 22J. Jortner and R. D. Levine, Adv. Chern. Phys. 47, 1 (1981). 2:Jr,. Edwards, D. H. Dolphin, M. Gouterman, and A. D. Adler,

J. Mol. Spectrosc. 38, 16 (1971). 24G. D. Dorough, J. J. Miller, and F. M. Heunnekens, J. Am.

Chern. Soc. 73, 4315 (1951). 25A. Scherz and H. Levanon, J. Phys. Chern. 84, 324 (1980). 26J. B. Hopkins, D. E. Powers, and R. E. Smalley, J. Chern.

Phys. 72, 5039 (1980). 21A. Amirav, U. Even, and J. Jortner, J. Chern. Phys. 75,

3770 (1981). 28(a) L. K. Gottwald and E. F. Ullman, Tetrahedron 36, 3071

(1969); (b) F. A. Walker and G. L. Avery, ibid. 52. 4949 (1971).

29M. P. Tsvirko, G. F. Stelmakh, V. E. Pyatosin, K. N. Solovyvov, and T. F. Kachura, Chern. Phys. Lett. 73. 80 (1980).

30L. Bajema, M. Gouterman, and C. B. Rose, J. Mol. Spectrosc. 39, 421 (1971).



Documents

Isolated ultracold porphyrins in supersonic expansions. I. Free-base tetraphenylporphyrin and Zn- tetraphenylporphyrin