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LASER DYES PHOTOCHEMISTRY : PICOSECOND AND NANOSECOND LASER STUDY
M. Meyer, J. Mialocq
To cite this version:
M. Meyer, J. Mialocq. LASER DYES PHOTOCHEMISTRY : PICOSECOND AND NANOSEC- OND LASER STUDY. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-541-C7-544.
�10.1051/jphyscol:19877129�. �jpa-00226948�
JOURNAL DE PHYSIQUE
Colloque C7, supplbment au no12, Tome 48, dbcembre 1987
LASER DYES PHOTOCHEMISTRY : PICOSECOND AND NANOSECOND LASER STUDY
M . MEYER* and J . C . MIALOCQ
CEA, CEN/Saclay, IRDI/DESICP/DPC/SCM CNRS-UA 331, F-91192 Gif-sur-Yvette Cedex, France
and
"
~ u a n t e l SA, F-91941 Les Dlis Cedex, FranceResume
On e t u d i e e n s p e c t r o s c o p i e l a s e r nanoseconde e t picoseconde l e s p r o p r l e t e s photophysiques e t photochimiques d e s c o l o r a n t s l a s e r s : rhodamines e t merocyanines.
La p o l a r i t & d e l a molecule de c o l o r a n t a un r 6 l e e s s e n t i e l . Dans l e c a s d e s merocyanines, l a s o l u b i l i t e , l e deplacement r e s p e c t i v e m e n t bathochrome ou hypsochrome du s p e c t r e d ' a b s o r p t i o n d e l ' e t a t fondamental 5, d e s m6rocyanlnes f a l b l e m e n t ou f o r t e m e n t p o l a i r e s , l e deplacement I m p o r t a n t v e r s l e rouge du s p e c t r e d e f l u o r e s c e n c e dans l e s s o l v a n t s p o l a l r e s dependent f o r t e m e n t d e l a p o l a r i t e du c o l o r a n t dans l ' e t a t fondamental So, mais a u s s i de s a p o l a r i t e dans l ' e t a t s i n g u l e t e x c i t e f l u o r e s c e n t S1. On montre q u ' a p r e s excitation lumlneuse du DCY ( % -
d~cyanomethylene-2-methyl-6-p-dimethylam~nostyr~l-4H-pyran). l e moment d i p o l a l r e e s t augmente de 5.6 D a 26.3 D . c e g u l e s t dO a un t r a n s f e r t d ' e l e c t r o n l n t r a m o l e c u l a l r e du groupe a-lno-, d o m e u r d ' e l e c t r o n aux groupes cyano-, a c c e p t e u r s d ' e l e c t r o n .
On determrne egalement l e s s p e c t r e s d ' a h s o r p t i o n d e s e t a t s t r a n s i t o i r e s s i n g u l e t s e x c i t e s ( d e d u r e e d e v i e nanoseconde) e t t r l p l e t s e x c i t e s ( d e d u r e e d e v i e microseconde) a i n s i que l e rendement q u a n t l q u e d e p a s s a g e l n t e r s y s t e m e S1 + T I . La c o n n a i s s a n c e de c e s p r o p r l e t e s photophysiques e s t e s s e n t i e l l e pour l ' o p t i m l s a t i o n du f o n c t i o m e m e n t d e s l a s e r s a c o l o r a n t .
A b s t r a c t
P h o t o p h y s i c a l and photochemical p r o p e r t i e s o f l a s e r dyes (rhodamines and merocyanines) a r e s t u d i e d through nanosecond and picosecond l a s e r s p e c t r o s c o p y .
The p o l a r i t y of t h e dye molecule i s one of t n e most imporcar.!
p a r a m e t e r s . I n t h e c a s e of merocyanines, t h e dye s o l u b i l i t y . t h e r e d o r b l u e s h i f t o f t h e ground s t a t e So a b s o r p t i o n spectrum r e s p e c t i v e l y fcr
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877129
C7-542 JOURNAL DE PHYSIQUE
the less or more pclar dyes. the large red shift of the fluorescence spectrum in poiar solvents depend strongly on the dye poiarity in its So ground state but also in its fluorescent flrst singlet excited state S1.
Upon light excitation of DCM (4-dicyanomethylene-2-methyl-6-=- dimethylaminostyril-4H-pyran) a large change of the dipole moment from 5.6 D to 26.3 D is evidenced due to an intramolecular electron transfer from the electron donor amino-group to the electron acceptor cyano- groups.
The absorption spectra of che transient excited singlets (nanosecond lifetime) and excited triplets {microsecond lifetime) and the quantum yield of intersystem crossing are also determined. 'he knowledge of these photophysical properties is essential to the dye lasers optimization.
The wide use of dye lasers in various fieldsis due to their large tunability.
Nevertheless in order to achieve a perfect stability and to optimize the functioning of the laser, a good knowledge of the photophysical properties of the dye is funda- mental. One of the commonly used dyes : 4-dicyanomethylene-2-methyl-6-p-dimethylami- no-styril-4H-pyran (DCM, Figure 1) which shows a high conversion efficiency under different pumping conditions ( 1 - 9 ) has been investigated.
H
H
N
Figure 1 : DCM molecular structure.
The absorption and emission spectrum overlap of DCM is very small but strongly solvent dependent as shown in Figure 2.
wavelength (nm)
Figure 2 : absorption and fluorescence spectra of DCM in various solvents : isooctane ; --- methanol ;
. ... . ...
chlorobenzene ;-
- -- pyridine.DCM which is a merocyanine type of dye has a moderately polar ground state. By means of vector addition using the dipole moments of the various groups involved in the molecule (p(CH ) = 0-4 D ; p(CN) =
-
3.8 D ; p((CH ) N) = 2.6 D) the DCM ground3 3 2
state dipole moment has been calculated : p = 5.6 D. According to Brooker and all.
g
(101, weakly polar merocyanines are soluble in solvents of low polarity and show a red shift of the ground state absorption spectrum for increasing solvent polarity.
Using the transition energy of merocyanine VII whose polarity is similar to DCM, Brooker established a solvent polarity parameter :
xR.
In fact, the DCM absorption transition energy (EA in kcal.mo1-l) correlates perfectly with thex
values of 25R
solvents ranging from isooctane to propargyl alcohol (AEA = 4.1 kcal.mo1-l) : , E = 0.386
xR
+ 43.72A
According to the Lippert and Mataga theories (11-12) describing the dipole interactions between a polar solute and its solvent cage, a linear $elation ship can be established between the Stokes shifts and Af (Af =
a -
- E : dielec-2&+ 1 2nL+ 1'
tric constant, n : refraction index). The slope m of this straight line is related to the dipole moment of the excited state via :
pe - p = 0 . 0 1 0 ~ g
a being the radius of the Onsager cavity. By applying this theory to the spectral properties of DCM in 32 solvents, we obtained an estimate for the first excited singlet state dipole moment of p = 26.3 D. This rather high value emphasizes the charge transfer characterof the excited state of DCM which has two amino electron donor sites and two cyano electron acceptor sites.
By means of the nanosecond absorption spectroscopy set up depicted in figure 3, the differentiel absorption spectrum of the first singlet excited state of DCM has been determined in methanolic solution.
figure 3 : Nanosecond absorption spectroscopy set up.
1 : Nd-YAG laser cavity (Quantel NHG24) 2 : Nd-YAG preamplifier
3 : Nd-YAG amplifier
4 . 5 . 6 : W P cristals used to generate the second ( 5 3 2 nm), third ( 3 5 5 nm) or fourth ( 2 6 6 nm) harmonic
7 : sample cell 8 : filter
9 : pulsed Xenon arc lamp (XBO 450 V/1 Oaram) 10 : monachrometor (M25 Huet)
11 : photomultiplier (Hamamatsu R928T) 12 : oscilloscope (Tektronix 7 8 3 4 ) .
JOURNAL DE PHYSIQUE
With a 5.4 x mol.dm -3 DCM solution complete photobleaching could be achie- ved by exciting the sample with Nd-YAG frequency doubled 6 ns fwhm laser pulses. The S absorption spectrum obtained shows a maximum of absorbance at 450 nm with a molar
1 4 3
extinction coefficient of E~~~ = (6.5
+
0.6) x 10 dm .mol-l.cm-'. A second experi--5 -3
ment with a 2.0 x 10 mol.dm solution performed under low laser intensity condi- tions in order to avoid stimulated emission, confirmed the previous results.
In conclusion, it has been shown that DCM exhibitsa considerable Stokes Shift which has been measured in a large variety of solvents, in order to calculate the difference between the ground state and the first singlet excited state dipole moments. The absorption spectrum of the first singlet excited state has been obtai- ned for the first time. Triplet state absorption measurements by means of energy transfer via naphtalene (QISC < could be achieved and picosecond time scale fluorescence investigations are gnder way in order to investigate the intramolecular charge transfer process.
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