INFLUENCE OF TEMPERATURE ON ELECTRONIC SPECTRA OF DYESTUFF SOLUTIONS*

By

J. HEVESI ana L. KOZMA

INSTITUTE OF EXPERIMENTAL PHYSICS, J‰ ATTILA UNIVERSITY, SZEGED

(Presen ted by ~ . Bud£ - - Received 28. IX . 1965)

Analytical formulas based on investigation coneerning the influenee of temperature on electronic spectra of different luminescent systems are given to describe the change of the shape of spectra due to temperature. I t is shown that a simple relation exists between the eonstants in the formulas and the temperature. The experimental values for the systems examin- ed ate in very good agreement with those calculated from the formulas. The changes in the spectra are interpreted by the supposition that the influence of temperature on the distribution of vibrational energy is different for the ground state and the excited state.

l . I n s tudy ing the laws of luminescen t rad ia t ion it p roves useful to examine the t e m p e r a t u r e dependence of the p a r a m e t e r s of luminescence, e. g. of absorp t ion and emission speetra . Ear l ie r invest igat ions , pe r fo rmed ma in ly in the t e m p e r a t u r e range 7~ to 300~ showed changes in the s t ruc ture of spectra as well as in the yield of absorp t ion and emission wi th changing t empe - ra ture . The in t e rp re t a t i on of expe r imen ta l results cannot y e t b e considered as sa t i s fae tory . Seve ra l au thors (e. g. [1 ] - - [3 ] ) der ived formulas on the basis of general physical laws, in an a t t e m p t to describe the shape of speet ra a n d their change with t e m p e r a t u r e , using sŸ models ins tead of luminescent centra . These fo rmulas could not p lov ide Sat isfactory solutions of the p rob lem, ei ther on account of the complex i ty of the relat ions or because of the s implifying supposi t ions used in der iving them.

An empir ical app roach to the solution of the p rob lem was given b y TARASOVA [4], who p roved b y m a n y exper imen t s t h a t the m a x i m a l values of the absorp t ion coefficient k T and of the emission spec t rum fin (more exac t ly t h a t of the spec t ra l yield) f o r a given t e m p e r a t u r e T can be described with the fo rmu la s

kT - - = const [ 1 - - d 1 exp (--AE1/kT)] (1) kTo

and

f~ = c o n s t [ 1 - - d 2 e x p (--AE2/kT)], (2)

f~o

* Delivered at the 8th European Congress on Molecular Spectroscopy in Copenhagen 1965.

Acta Physica Academiae Scientiarum Hungaricae 20, 1966

352 j . HEVESI and L. KOZMA

where kro and fr0 are eorresponding values for tempera ture T o. (d 1, dz, 3 E 1 and 3 E 2 are empirical constants.) TxsAsovx showed tha t the values of AE x and z~E 2, having the character of energies, ate equal to the act ivat ion energy for dielectric relaxation of the solvent in the cases examined. The relations (1) and (2) give ah adequate description of the tempera ture dependenee of kT and f r , as was ascertained by our own investigations. On the other hand, t hey ate not suitable for describing the changes in the shape of speetra due to temperature .

DOMBI et al. [5], tak ing the general equations describing the conneetion between the absorption spectra k(v) and the luminescence spectra fe (v) as a s tar t ing point, obtained the relations

and k(v) = 211 v exp(bv) sech[al(v01--v)]

fq(v) = 21 2 v3 exp (--bv) sech [a,(v02--v)],

(3)

(4)

which give a sat isfactory agreement with experimental resuhs for m a n y cases wi th empirically determined values of a 1 a n d a 2. (211 and 212 in the formulas are normalizing factors, %1 and %2 are values near %, the pure electron t ransi t ion frequeney, and b ~- h/2kT*, where T* is the effective temperature or vibrat ion tempera ture of the molecule. The la t ter can be calculated on the basis of the spectra with the method given in [6]).

According to our experimental resuhs, relations (3) and (4) proved to be adequate for describing the ehanges in the shape of spectra caused by tempera- ture if the dependence of a 1 and a 2 on tempera ture could be determined. One of our most impor tan t resuhs was to give this relation.

2. From relations (1) and (2) giving the in tens i ty of spectra, as well as (3) and (4) describing the changes of their shape with temperature , ir can be concluded tha t the absorption and emission spectra of a luminescent sys tem at t empera ture T can be given in the following forro:

k (v, T) 1 -- d 1 exp (-- AE1/kT ) k(vmax, To) 1 -- d 1 exp ( - - AEIq

v exp (bv) sech [al(v01 -- v)]

max {v exp (bv) sech [al(v01 -- v)]} (5)

and

fq (v, T) f q ( r m a x , To)

_ 1 - - d 2 e x p ( - A E 2 / k T ) v 3 e x p ( - b v ) sech[a 2 ( % 2 - v ) ] , (6)

1 -- d 2 exp ( - - AEzq max {v 3 exp ( -- bv) seeh [a2(Voz-V)]}

where k(vmax, To) and fq(Vmax, To) denote the values belonging to the frequen- cies Vmax of the spectra measured at tempera ture T 0.

Acta Physiea ~4caderaiae Scientiarum Hungaricae 20, 1966

I N F L U E N C E O F T E M P E R A T U R E O N E L E C T R O N I C S P E C T R A 353

These relat ions ate suitable for calculat ing the spectra , because the constants can be de te rmined f rom re la t ive ly few exper imenta l data .

3. In the course of our exper iments the speetra of six luminescent systems listed in Table I were measured in t h e t e m p e r a t u r e range 262 to 338~ For measuring the absorpt ion speetra a grat ing spee t ropho tomete r Optica

No S o l v e n t

1

2

3

4

5

Fluorescent m a t e r i a l ( concen t r a t i on )

(molaq

Fluorescein (1 �9 10 -~)

Eosine ( 5 �9 lO -a)

Erythrosine (5 �9 10 -5)

Rose Bengale (5 �9 lO -a)

Aurophosphine ( 1 �9 lO-')

Flavophosphine (5 �9 10 -5)

T (~

NaOH (1%) C2H60H

NaOH (5.10-3 m/l) C2HsOH

NaOH (5.10-a m/l) CzHsOH

NaOH (5" 10-aro/l) C~HsOH

HCI (3 �9 10 -3 m/l) C~HsOH

HC1 (3 �9 10 -3 m/l). (C2HsOH) glycerol (10%)

262

338

262

338

262

338

262

338

262

338 5,000 7,00~

262 7,160 10,000

338 6.5881 9-9~000

12,006 5,9'15

9,625 5,882

10,822 5,660

9,256 5,639

6,6781 10,247 5,680

5.2291 8 ,405 5,661

6,7921 9 ,947 5,360

5.787 I 8 ,934 5,350

6,472 8,750 ] 6,320 l__

7,00~ 6,320

10,000 6,260

[ 9 ,000 6,26O

b.lOt4 (,)

a b s o r p t i o n spectra emiss ion spectra

9,099

6,478

7,336

6,004

6,588

at " lOU w0t "lO-Xt (,) 0- ' )

a• - 10 x~ P0* " 1 0 - 1 ' (,) (,-,)

11,283 5,875

8,059 5,850

10,711 5,625

8,612 5,62O

9,441 5,650

7,599 5,640

10,500 5,390

8,882 5,370

7,944 6,190

6,150 6,190

9,000 6,190

8,400 6,190

Table I

Milano C F - - 4 was used with a supp lemen ta ry deviee for regula t ion of t empera- ture, suitable for measuring layer thicknesses up to 20 cm [7]. In the case of luminescence spect ra the solution was placed in a box with double walls serving to regulate the t empera tu re . The windows of this box were plaeed in f ront of the entrance slit of the spec t rophotometer . The layer thickness of the solution was chosen according to the me thod given in [8] in order to keep the influence of secondary luminescence within the limits of exper imenta l error. Reabsorp- t ion in the sys tem was taken into account in the usual way [9]. The t empera tu re was held cons tan t within • 1 7 6 A high pressure Xenon- lamp (Osram X B O 500) a n d a Hg- lamp (Osram HBO 500) served as l ight source. The near ly monochromat ie exci t ing beam was ob ta ined with a double monochromato r of a meta l in ter ference fi l ter SIF f rom the l ight of the lamps.

4 Acta Physica Academiae Scientiarum Hungarica# 20, 1966

3 5 4 J. EIEVESI and 'L. KOZMA

4. The results show tha t the ehanges in the speetra due to temperature ate of identical eharaeter fo r aH systems. The height of absorption spectra

docreases with inereasing temperature, but the shape of the spectra is also subjeet to changes at the same time. The greatest change in height can be observed in the neighbourhood of the maxima, while the changes in the short wave region are considerably less. In the long wave region the decrease of the absorption eoeffieients is not only less than tha t found near the maxima, but even ah increase can be found in some cases. In the emission spectra a shift towards longer wavelengths can be observed with increasing temperature, evidently due to an increase in the relative transition frequeney of emission belonging to longer wave-lengths.

Figs. 1 and 2 show the measured absorption and emission spectra of fluorescetn and aurophosphine, respectively. (The units are chosen in such a way as to give spectra of the same height for all solutions). I t can be seen from the figures that an increase of temperature resulted in a broadening of the spectra. Aurophosphine exhibits a somewhat different behaviour in so far as no significant broadening in the absorption speetra occurs even in the anti- Stokes region. It should be noted that for this dyestuff the quantum yield function shows no significant temperature dependence, either [10].

According to the present results and our earlier investigati0ns [10] con- cerning the dependence of quantum yield on the wave-length of exciting light, the similar temperature dependence of quantum yield and of absoprtion spectrum can be ascribed to the fact that the population in higher vibrational levels of the ground state is altered by changes of temperature. The relatively increasing frequency of transitions with low energy in the emission spectra indicates that the changes in the vibrational configurations of molecules produeed by the changing temperature ate different f0r ground state and exeited state. This possibility as well as the influence of vibrational energy distribution on the shape of spectra has been referred to by several authors [11]--[131 in earlier papers. The constants in relations (5) and (6) were deter- mined from experimental absorption and emission spectra with the methods given in [4] and [5].

Values of the activation energy A E 1 in relation (5) were found to be nearly equal for all systems examined and were ealeulated to A E 1 ~ 2 �9 10 -13 erg/partiele, in good accordance with the dielectric relaxation time of ethanol which was used a s a solvent. The divergence between the calculated values is less than ! 3%. I t is to be noted that these values of A E 1 are based on tempe- rature dependence of the maxima of spectra .

The value of the "mirror frequency" v 0 in the formulas was determined in the usual way from the intersection of the spectra. Calculation from the extreme values of function T(v) given in [5] leads practieally to the same results. Our calculations show that relation (5) is fulfilled in a very good

Acta Physica Acaderniae Scientiarum Hungaricae 20, 1966

INFLUENCE OF TEMPERATURE ON ELECTRONIC SPECTRA 355

"�91

0,5

7,o

O~

8

Flu oresce/n x |

x T=262~ ~ 0 0 T=298~

[] T = 318~ x | A T = 338~ ~ I:3 0 /',

x DZ~

[2 o

E]�91

k 0 | • [] ~fa) kmmo~) e [] • o A tq f~)r.o~

/,, ~) Z& Z~ x

|

rn & A ~ ~ x cJ D |

$

8 ~ Q8o, , , I I

450 500 550 J(m H

Fig. 1

0 I I 400 450 500 550 600

A ~x ~ ~ ~ ~ ~ ~ Auropho~phine [ ] ~ x ~ x T= 262 ~ 0 • 0 T=298~

,., T= 338OK

o • x ~

k(~~ P fqr~)

o x A

" ~ 8 8 [ ] x x A x �91

A 0

(;2 9~ >' 9 " e ~@ BO

Fig. 2

approximation using these values of %. However, the emission spectra fq(~,,T)

calculated with the same values on the basis of formula (6) show significant divergences from the measured values. For most of the systems examined (except Bengali Red) the values of frequency had to be taken with %2 < %1

4* Acta Physica Academiae Scientiarum Hungaricae 20, J966

35• J. HEVESI av.d L. KOZMA

( : v0) (see eolumns 7. and 9. of Table I) to obtain a good aer with the experimental results.

In the Table the luminescent dyestuffs, their concentrations, and the composition of the solvent as weU as the values of constants playing a tole in

~. lO-"ys-')

5~0 5,5 6~0 6,5 v . l O - ~ ( s - ~ ) - - - - - - , . - -

O: 5,0 &O F,O ~.10 -~4 (s-U

T= 26 ?~

~ F I l,O- uocescein l Eo#ine

I ' ~ 0 0 ~ 0 O I ~ .l~"-,rl i 6,0 6,5 5,0 5Ÿ 6,0 G, 5

~- u. lO - '

~ L o~ : Ro,~e Bengale O~ k(~)

o �91 5,o ~.i0_,%_,)5,5_~._ 6,o

i ~ [ v ) ~ , h~176176

6,0 v . I 0 - ( 8 - I ) - - - - - , . - -

Fig . 3

relations (5) and (6) are given onlyfor two temperatures, though measurements were also made at T = 298~ and T = 318~

One of the possible explanations of the fact that in caleulating the emission spectra the frequeney v02 had to be taken less than v01 is to be found in the supposition that the energy system of complex luminescent molecules can be described with the three level term-scheme suggested by NrPOm~r~T [14]. It can

Acla Physica Academiae Scienliarum Hungaricae 20, 1966

LNFLUENCE OF TEMPERATURE ON ELECTRONIC SPECTRA 3 5 ~

be easily understood on the basis of this scheme that greater pure eleetronie transition frequeneies %will belong to absorption processes than those belonging to emission aets.

Faetors a x and a z were determined from experimental data with the method given in [5]. One of the main results of our invest igat ions was to

T=838*K

1 rescein I , U - ~ ~ o / [ n : l v )

I f q ( v )

o

,,o_ /y\, , L r qr

f~m ,,c/ 9 ~ ~ krw

o,JJ L~, 5,5 60

1~0 k R o s e Bengote

o,, o ~

6,5, 5,0 ~.10_74(s_7) 5,5 6,0 , "~

o ~Io ~.'~c~-'9 7 , 0 , o / J 5,0 ~.10-~(s-7) 6,0

m e

~ 0

Fig. 4

give the temperature dependenee of the faetors a x and a z, whieh was not known earlier. Our results show that a 1 and a 2 ate inversely proportional to the temperature, i. e. a i = e l ~ T * and a 2 -~ c J T * , where e 1 and e z ate eonstants charaeteristie o f the luminescent system.

Emission and absorption speetra for temperatures T = 262~ and T = 338~ are shown in Figs. 3 and 4. The spectra calculated with the values

Acta Physica Academiae Scientiarum Hungarieae 20, 1966

3• 3. HEVESI and L. K'OZMA

20

10

10

10

l ~~Q Fluot'esce[n 1 \ o r = 2 6 2 o K

~r,,) f ~ o ~ x T='~176

y % X I ,

5,5 ~.lO_~~t~) =-- 6,5

f % \ o r=2s2oK

xl I "

A I I

s~.1o_,4( _ v so= s5

I ~ Aurophosph/ne | / ~ k~ or-282 ~

&O 65 70 %5 v.lO-~4(s-7) =

O F I ~ ~,\ 0 T:262~K

sg.lo_~(~_,) so= s,s

o ~'£ ~ . , o - @ - , ) " _~ 6,

5,0~- I ~ FIo~op~o~ph/n~

k('v) ~ K

I i e,O.lo_~(s_,) s 5 = 7,o

Fig. 5

a, b and % given in Table I are indicated by solid lines, while measured spectra are indicated by smaU circles. The spectra ate plotted in arbitrary units. As can be seen from the figures the values calculated with the formulas ate in very good accordance with the experimental data.

The values of k(v), both measured and caleulated with formula (5) using the aetivation energy ,dEt, ate plotted in Fig. 5 in order to show the tempera-

A c t a P h y s i c a A c a d e m i a s Sr H u n g a r i c a e 20, 1966

INFLUENCE OF TEMPERATURE ON ELECTRONIC SPECTRA 359

t u r e d e p e a d e n c e o f t h e rea l c o n d i t i o n s o f i n t e n s i t y . I t c an b e seen t h a t t h e

e h a n g e s b o t h in t h e s h a p e of t h e s p e e t r a a n d in t h e i r i n t e n s i t y , due to t h e in .

f l uenee o f t e m p e r a t u r e a re v e r y wel l d e s e r i b e d b y r e l a t i o n (5).

T h e a u t h o r s a t e i n d e b t e d to P r o f . D r . �93 B u r £ M e m b e r o f t h e H u n g a r i a n

A c a d e m y of Se i enees , t o w h o m t h e y of fe r t h e i r s inee re t h a n k s fo r m o s t v a l u a b l e

d i s e u s s i o n s a n d a d v i e e d u r i n g th i s w o r k .

REFERENCES

1. B. I. STEPANOV, Lumineszeencia szlozsnª molekul (Minszk, Izd. A. Nauk BSSR 1956.) 2. Sz. I. KUBAREV, Optika i Spektr., Toro. I. (Izd. A. Nauk USSR, Moszkva--Leningr•

1963.) 3. J. HORVkTH, Acta Phys. et Chem. Szeged, 11, 3, 1965. 4. T. M. TARASOVA, ZSETF., 21, 189, 1951. 5. J. DOMBI, I. KETSKEM• and L. KOZMA, Acta Phys. et Chcm. Szcged, 1O, 15, 1964;

Optika i Spektr., 18, 710, 1965. 6. M. N. ALENCEV, Optika i Spcktr., 4, 690, 1958; L KETSEM• J . DOMBI and R.

I~IORVAt, Ann. Phys., 8, 342, 1961. 7. J . HEvEsI, Dissertation, Szeged, 1965. 8. ~. BuD£ and I. KETSKEM• Acta Phys. Hung., 7, 207, 1957. 9. TH. F6IISTER, Fluoreszenz organischer Verbindungen (G6ttingen 1951.)

10. J. HEVESI and L. KOZMA, Optika i Spcktr., 19, 434, 1965. 11. A. N. NIKITINA, G. S. TER-SARKISJAN, B. M. MIKHAILOV a n d L. E. MINCHENKOVA, Ac ta

Phys. Polon. Sci., 24, 483, 1964; Optika i Spcktr., 14, 655, 1963. 12. J. B. BIRKS and D. J. DYSON, Proc. Roy Soc., A275, 135, 1963. 13. J. HORV�93 Magyar Fizikai Foly£ 13, 195, 1965. 14. B. Sz. NEPORENT, Izv. A. Nauk. USSR, 22, 1372, 1958.

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.Acta Physica Acaderniae Scientiarurn Hungarieae 20, 1966