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TIME RESOLVED PHOTOACOUSTIC
SPECTROSCOPY OF METASTABLE ELECTRONIC STATES OF DYE MOLECULES
W. Keller, W. Schubert, R. Germer, E. Strauss
To cite this version:
W. Keller, W. Schubert, R. Germer, E. Strauss. TIME RESOLVED PHOTOACOUSTIC SPEC-
TROSCOPY OF METASTABLE ELECTRONIC STATES OF DYE MOLECULES. Journal de
Physique Colloques, 1983, 44 (C6), pp.C6-397-C6-400. �10.1051/jphyscol:1983665�. �jpa-00223224�
JOURNAL DE PHYSIQUE
Colloque C6, suppl6ment au nOIO, Tome 44, o c t o b r e 1983 page C6- 397
T I M E RESOLVED PHOTOACOUSTIC SPECTROSCOPY OF METASTABLE ELECTRONIC STATES OF DYE MOLECULES
W.W.A. Keller, W. Schubert, R. Germer and E. Strauss
Universitat Oldenburg, Fachbereich Physik, 0-2900 Oldenburg, F.R. G.
REsumE - En e m p l o y a n t l a s p e c t r o s c o p i e p h o t o a c o u s t i q u e r E s o l u e d a n s l e
-- t e m p s , n o u s a v o n s e x a m i n e l a r e l a x a t i o n non r a d i a t i v e de m o l E c u l e s d e c o l o r a n t p h o t o e x c i t e e s d a n s d e s l i q u i d e s 2 t e m p s r a t u r e a m b i a n t e . La m6thode r e n s e i g n e s u r l a d u r E e d e vie d u t r i p l e t , s u r l e r a p p o r t d ' e m b r a n c h e m e n t s i n g u l e t - t r i p l e t , o u s u r l e s r e n d e m e n t s q u a n t i q u e s r e l a t i f s . L a t e c h n i q u e e s t e m p l o y e e p o u r a n a l y s e r l e t r a n s f e r t d ' g n e r - g i e i n t e r m o l 6 c u l a i r e d ' e r y t h r o s i n e e t s a d g p e n d a n c e a v e c l a c o n c e n t r a - t i o n .
Abstract - The non-radiative decay of o p t i c a l l y excited dye molecules i n l i q u i d s a t room temperature is studied usirq t i m e r e s o l v e d p h o t o a c o u s t i c spectroscopy. The method y i e l d s information about the t r i p l e t l i f e t i m e and the singulet-to-triplet branching r a t i o or the relativ<quantum efficiency. The technique is used t o analyze the intermolecular energy t r a n s f e r of erythrosin and its c o n c e n t r a t i m dependence.
The photoacoustic technique is applied to determine t h e t r i p l e t l i f e t i m e by d i r e c t measurement of the r a d i a t i o n l e s s relaxation r a t e s of xanthen dye solutions. This work r e e v a l u t e s t h e e a r l y p h o t o a c o u s t i c i n v e s t i g a t i o n s o f Hey e t a l . ' . Photoacoustic l i f e t i m e measurements of s o l i d s with cw- and pulsed e x c i t a t i m have been demonstrated successfully 2 r 3 .
E r y t h r o s i n (Merck A r t . 1355) has been choosen on a c c o u n t of its known h i g h S l - f T i branching r a t i o and its t r i p l e t l i f e t i m e i n t h e submillisecond range. The transpa- rency of the sample a t moderate concentrations allows t o check the photoacoustic measurements by monitoring the phosphorescence decay, t h e concurring process t o t h e r a d i a t i o n l e s s relaxation.
For e x c i t a t i o n i n t o the Si s t a t e of erythrosin i n glycerol solution an i n t e n s i t y modulated cw-laser beam has been used. The chopping frequency range was from 10 Hz t o 10kHz. The amplitude and phase of the photoacoustic pressure s i g n a l r e l a t i v e t o t h e incident l i g h t has been detected by a p i e z o e l e c t r i c transducer. Data processing was done o n l i n e by a microcomputer which a l s o does t h e modulation v i a a Pockels cell. The erythrosin/glycerol s o l u t i o n s have been excited by t h e 514 nm l i n e of an argon-ion l a s e r with s o m e mW Fower.
Theory
The e x c i t a t i o n is f o l l o w e d by a " f a s t " and a "slow" h e a t r e l e a s e d u e t o t h e d i f f e r e n t relaxation channels.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983665
JOURNAL DE PHYSIQUE
Thus t h e heat r e l e a s e of t h e sample is a superposition of two components, a "fast"
and a "slow" one r e l a t i v e t o t h e p e r i o d t i m e of t h e m o d u l a t i o n f r e q u e n c y of t h e e x c i t i n g l i g h t .
The dependence on t h e modulation frequency w of t h e "slow" heat can be described by
w h i l e t h e " f a s t " one r e m a i n s i n phase w i t h t h e i n c i d e n t l i g h t a t a l l modulation frequencies in t h e experiment considered
T is t h e l i f e t i m e of the t r i p l e t s t a t e . Es is the energy d i f f e r e n c e Ti-SO because t h e phosphorescence is n e g l i g i b l e . Ef is t h e sum of a l l o t h e r e n e r g y d i f f e r e n c e s leading to the "fast" heat production.
The spectrum of t h e amplitude M(w) and phase $(w) is given by
Due to t h e heat f r a c t i o n released delayed from t h e metastable t r i p l e t state t h e amplitude shows a c h a r a c t e r i s t i c s t e p l i k e kehaviour while a phase minimum occurs a t
The maximum phase difference a t wm is given by
On the b a s i s of t h e measured modulation frequency of the amplitude and phase of t h e p h o t o a c o u s t i c s i g n a l one is a b l e t o c a l c u l a t e from eq. ( 6 ) and ( 5 ) t h e r a t i o Es/Ef and the l i f e t i m e T.
Experimental results
PA s p e c t r a were measured a t various dye concentrations i n a mcdulation frequency range from 10 Hz t o more than 10 kHz. Cue t o c e l l resonancies a t higher frequencies r e l i a b l e a d reproducable measurements w e r e - l i m i t e d t o t h a t frequency interval.
The l i f e t i m e of e r y t h r o s i n i n glycerol s o l u t i o n s a t low concentrations (c~10-%) h a s been d e t e r m i n e d t o be 380 k19 us. The measurements show a s h i f t of t h e p h a s e minimum t o h i g h e r modulation f r e q u e n c i e s w i t h i n c r e a s i n g d y e c o n c e n t r a t i o n s a c d o r d i n g t o a d e c r e a s e of t h e t r i p l e t l i f e t i m e . At c = 2 ' 1 0 - ~ m t h e l i f e t i m e was found to 68 vs.
The s o l i d l i n e s i n f i g . 1 a r e calculated with t h e values Es/Ef and T cbtained from eq. (5) and (6) showing an e x c e l l e n t agreement 'of theory and experiment.
A l l the photoacoustic r e s u l t s were checked o p t i c a l l y by measuring t h e decay of t h e
weak phosphorescence of t h e T1 s t a t e a f t e r p u l s e d e x c i t a t i o n . The d a t a a r e summarized in Fig. 2. O p t i c a l l y ard photoacoustically determined l i f e t i m e s a r e i n e x c e l l e n t agreement.
From t h e photoacoustic d a t a w e found t h e amplitude of t h e "slow" heat t o be 70% of the t o t a l heat released. This f r a c t i o n is independent of t h e dye c o n c e n t r a t i m up t o 2 ' 1 0 - ~ mol/l. T h i s c o n f i r m s t h a t t h e observed s h i f t o f t h e phase minimum i s s o l e l y due t o a decrease of the t r i p l e t l i f e t i m e .
Fig.1 Transfer functions of erythrosin/glycerol s o l u t i o n s s o l i d l i n e s a r e t h e t h e o r e t i c a l l y expected curves
The c o n c e n t r a t i o n d e p e n d e n c e of t h e i n c r e a s e of t h e decay r a t e from t h e T i t o So i n d i c a t e s an i n t e r m o l e c u l a r p r o c e s s . Three d i f f e r e n t mechanisms have t o be considered: Forster-type energy t r a n s f e r to a quencher m l e c u l e , TI-TI quenching and d i f f u s i o n l i m i t e d concentration quenching.
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,RI.LE, 0"EIIC"lnG DUE TO SOeSIEIl-TYPE E n F I G V TRIVEFERERYTHROSIN CONCENTRRTION s Inol/Li
Fig. 2 T r i p l e t l i f e t i m e versus dye concentration
s o l i d l i n e : Forster-type energy t r a n s f e r
C6-400 JOURNAL DE PHYSIQUE
The observed dependence of t h e T i l i f e t i m e on t h e c o n c e n t r a t i o n is i n good agreement with Forster-type energy t r a n s f e r (Fig. 2). However t h e c h a r a c t e r i s t i c nonexponential decay of the phosphorescence i n t e n s i t y was not &served and there- f o r e t h i s mechanism was not confirmed.
T r i p l e t - t r i p l e t quenching has to be excluded below dye concentrations c ~ 1 0 - ~ mol/l because t h e expected dependence of the l i f e t i m e on the square of concentration and the i n t e n s i t y of t h e e x c i t a t i o n was not observed.
So f a r d i f f u s i o n l i m i t e d conentration quenching agrees b e s t with the experimental d a t a even s o its c o n c e n t r a t i o n dependence is t o weak f o r cs10-~ mol/l. I n p a r t i c u l a r it is i n agreement with the observed exponential phosphorescence decay.
The experimental d a t a do not exclude d i f f u s i o n l i m i t e d formation of an eximer a s a c o l l i s i o n p r d u c t of an e x c i t e d dye molecule and an unexcited one. The eximer is supposed t o have a s h o r t e r Ti l i f e t i m e than t h e monomer.
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