10831
J. Phys. Chem. 1992, 96, 10831-10837
Inhomogeneous Broadening of Organic Dyes In Polymeric Media: Nonlinear Transmisslon Spectra and Photochemical Kinetics Mikhail V. Bondar, Olga V. Przbonska,* and Yevgeniy A. Tikbonov Institute of Physics, Academy of Sciences of the Ukraine, Prospect Nauki, 46, 252650, Kiev-28, Ukraine (Received: January 16, 1992; In Final Form: July 20, 1992)
The paper deals with a new approach to studies of inhomogeneous broadening of vibronic spectra of organic dyes in the polymeric medium at the room temperature. The effects of inhomogeneous broadening are shown to influence such photophysical processes as nonlinear light absorption by dyes solutions and also linear and nonlinear photobleaching. For the first time spectral dependences of the nonlinear light absorption are studied and a 10-fold decrease in the critical saturation intensity of the bleaching in the polymeric matrix as compared to the liquid medium is observed. A model was developed of the spectral inhomogeneous ensemble of molecules in the amorphous polymeric matrix which takes into account the configurational relaxation in the excited state. We also estimated the time of recovery of the primary distribution of molecules at the ground state. The spectral dependence of the bleaching quantum yield of dyes in the polymeric medium is obtained, which extends opportunities for investigations of inhomogeneous broadening at excitations not only in the region of 0 4 transition but also within the whole absorption band.
Introduction Laser photophysics of dye-activated polymeric materials and its application in laser engineering's2 was investigated earlier relative to the following problems: nonlinear optical properties of dyes in solutions and polymer^;^ spectroscopy and photochemistry of dyes in light fluxes of different i n t e n ~ i t i e sreversible ;~~~ and irreversible thermmptical ~henomena;~.' and lasing, energy, and spectral characteristics of polymeric lasers.2.8-'0 Monochromaticity, high intensity, and possibility of frequency tuning of laser excitation source afforded the observation of a number of new regularities in photophysics of complicated molecules. Inhomogeneous broadening of electronic spectra of organic dyes in polymeric media at the rmm temperature is of particular interest. It has been known that complicated organic molecules in solid and liquid solutions may be considered as an ensemble of centers differing in frequencies of 0-0 transitions. This is due to the difference in the configuration of their nearest environment. Until now information on effects of inhomogeneous broadening was gained from spectroscopic studies. Among them the most important are site-selection spectroscopy of complicated molecules in frozen solutions."3l2 The methods permit detecting the fine structured part in the inhomogeneous-broadenedsmooth spectra of absorption, fluorescence, and phosphorescence using monochromatic laser excitation in the 0-0 transition region. The low-temperature 'hole-burning" p r o c e d ~ r e ~ also ~ - ' ~refers to universal methods for revealing the fine structure in spectra. The procedure permits realizing the selective excitation of certain centers from the inhomogeneous ensemble of molecules and affecting them. The systematic investigation of organic molecules in solutions with inhomogeneous configuration of environment is given in a number of publications.i6i8 With the purpose to describe the effects of inhomogeneous broadening of electronic spectra these authors suggest a field scheme of the potential solvate energy (solution cages) and analyze different spectral manifestations of inhomogeneous broadening. In particular, it has been shown that depending on the ratio of fluorescence lifetime ( T ~ ) and intermolecular relaxation time ( T ~ inhomogeneous ) broadening may be of dynamic and static character. However, inhomogeneous broadening of transitions affects not only the spectral parameters but also such important photophysical processes as nonlinear absorption and generation of light by dye solutions and also the processes of linear and nonlinear photochemical bleaching. So,when studying these nonlinear optical phenomena we may gain new information on the model of inhomogeneous ensemble of molecules, estimate times of configu*Towhom correspondence should be addressed.
0022-3654/92/2096-1083 1$03.00/0
rational rearrangement of the environment in the polymeric medium at room temperature, and qualitatively determine the energy function of distribution of centers. Thus, the main effort of the paper was directed to study the spectral dependence of the processes of nonlinear transmission and photochemical bleaching of dyes in the polymeric medium which we consider as new approaches in studying inhomogeneous broadening of their electronic spectra.
Theory 1. spectral Depe" of Nonlinear Absorption (NA) of Light by Liquid and Solid Solutions of Organic Dyes. Nonlinear absorption of nanosecond laser radiation in liquid solution of organic dyes (dynamic inhomogeneous broadening) may be quantitatively described within the threelevel energy diagram with homogeneous broadening of states (Figure la). The consideration of NA of light in the steady state and optically thin-layer approximation may be described by the known expression for the nonlinear absorption c ~ e f f i c i e n t : ~ ~ 1 + bl a = a0 (1) 1 + aI + c12 where a. is the initial absorption coefficient, a = (uoI u ~ ~ ) + 81272, b = u1271+ u2172, c = [(go1 + ~ 1 0 ) ~ 2+1 ~ 0 ~ ~ and~ ~ qjand q are absorption cross sections and fluorescence lifetimes, respectively. Dependences of nonlinear transmission T = f(lgI), where T = exp(-al) (I is the thickness of the absorbing layer), may be of three different types given in Figure 1b. Dependence of type 1 is possible if a > b that means that the probability of So SI transition exceeds the probability of SI S2transition. This corresponds to photoinduced bleaching. The case a = b is characterized by the dependence of type 2. At a < b, i.e., at uol + ul0< uI2the dependence T = f(lg4 is of a more complicated character (curve 3). Up to values of I (corresponding to amax) there is darkening in the system, and then the opposite effectbleaching due to cascade two-step transitions So SIand SI S,-pred~minates.~~ A change in excitation wavelength he within the long wave absorption contour So SIleads to variation in the ratio of the spectroscopic parameters of the molecule and, consequently, of the NA curve shape. Variety of shapes of NA curves may be used to determine photophysical parameters of molecules such as excited state cross section, rate constants, and intermolecular interaction constants. For this purpose the physical-mathematical method package of NA and emission in the UV-vis region were suggested in a previous article.20 We have measured the spectral dependences of NA of light in liquid and solid solutions of dyes at the excitations within the
+
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0 1992 American Chemical Society
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-
T ~ 1
~
~
Bondar et al.
10832 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
the solution of the quadratic equation; for the darkening dependences-by the solution of the cubic equation. Experimentally Is is determined from the NA dependences as the intensity attributing to the transmission of Ts. The spectral dependences of the NA may be used to determine the absorption spectrum from the excited state u12(A),which corresponds to the transition SI S2, see Figure la. If we know the experimental values of nonlinear absorption coefficients a l ( I l ) and a2(12), then with regard to eq 1 we may write 1 + 61, 1 + bI2 a, = a0 a2 = (5) 1 all + cfi aol + a12 cG
1
I
-
I
b
+
+
By dividing al!a2and neglecting quadratic terms in the intensity function, that is valid for the induced bleaching media, we obtain a. - a, a2a - a& 11/12 = (6) a. - a2 ala - a& Expressing a and b via spectroscopical parameters of molecules, we have (a0 - az)Ya, - (a0 - aJa2 u’2(A)= uO1(A)ao[y(ao a2) (a0 ai)]
-
I
5
I I,
7
9
Ig(~,wcm-~l Figure 1. (a) Scheme of energy levels of the dye molecule and (b) calculated dependences T (/SI)at uo, = cm2, ulo = lo-’’ cm2, iI = s, and u12= lo-’’ ( l ) , (2), 2 X cm2 (3).
entire long wavelength absorption band from the short wavelength edge up to the ChO transition region and then to the anti-Stokes region. These dependences, especially in the anti-Stokes region, where the selectiveexcitation of centers is possible, may be a source of extensive information about the influence of inhomogeneous broadening on nonlinear transmission. For the quantitative characteristic of the change in the NA curve shape depending on I one may use parameter Is, the critical saturation intensity of the So Si transition. For the two-level scheme (uI2= 0) with the homogeneously broadened transition it is twice as low, that is as = a(Is) = a0/2.As shown in a previous articlez1the value of Is is equal to 1 Is = (2)
-
(go1
+ ‘JIO)TF
For transmission-intensity dependence of type 3 (Figure 1b) the introduction of Is and its determination from the experiment according to eq 2 is complicated by the fact that changes in a(l) may be less than 50% and a more general definition of as is required. We consider it is expedient to determine asthe following way: a. + p x , m i n
as =
2
Then it is easy to show that Ts is equal to Ts e ( ‘ ‘ o ~ ~ x J ’ ” ” ) i / 2 (3) amin is attributed to the transmission dependence 1 (Figure lb) and is determined from eq 1 in the neglect of two-step transitions SI S2 with the saturation of state Sz,Le., at a >> c I
-
’
aob/a
is attributed to the extreme of transmission-intensity function 3 (Figure lb). To determine amax it is necessary at first to find intensity I, from the condition of the extremum da/dI = 0 a-
I, =
[e2
+ cb(6 - a ) ] ’ / ’ - c cb
(4)
Then, substituting eq 4 into eq 1 one may determine amax.For induced bleaching dependences the value of Is is determined by
-
-
(7)
where y = Il/12. Thus one can determine uI2(A) knowing the uol(X) and ao(X) from routine measurements; al(A,Il),a2(A,Iz),and y = 11/12from high intensity measurements. 2spectnl~of~BlerchiagQurntum~~q.es in Polymeric Matrix. The measurements of spectral dependences of bleaching quantum yield of dyes in the polymeric matrix afford ample opportunities for studies of inhomogeneously broadened systems at room temperature. The spectral manifestation of the effects of inhomogeneous broadening in luminescence and absorption of dyes at the room temperature is observed only at the selective excitation in the region of 0-0 transition. In contrast, in the photochemical studies the sptctral dependence may be displayed at the excitations within the entire absorption band. The bleaching quantum yield @ (probability of molecular degradation on an absorbed photon) is known to be used for a quantitative description of bleaching of dye molecules. When a dye molecule is simulated by the three homogeneously broadened vibronic singlet states So SI S2(Figure la) in the steady-state approximation and at lower values of the photodecay rates as compared to the photophysical relaxation rates, the value of @ can be presented as f01lows:~
--
+
@2u1271~ @ I
@=
u ~ ~ +T 1~ I
where CPi = k i ~are i the bleaching quantum yields from levels Si (ki are the rate constants of photochemical reactions with Si, i = 1, 2). From eq 8 we see that for weak light irradiation I IS). However at sufficiently high intensities ip becomes also constant: @ = @2 = k2T2, Since for dyed polymeric media the approximation of homogeneous broadening of transition is, as a rule, disturbed, therefore of special interest is a question on spectral dependence of the bleaching constants at the excitations in limit of the long wavelength absorption band. Formula 8 is suitable for determination of photostability of dye molecules in the weak and intensive light fluxes. However to determine a2experimentally is rather difficult, and it is more expedient to make measurements using the notion of dose constant of photobleaching k,-probability of bleaching of one dye molecule per unit of the incident radiation dose.4 The relations between a’, a2,and kD are given by
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The Journal of Physical Chemistry, Vol. 96, No. 26, I992 10833
Organic Dyes in Polymeric Media @I
kD/Qol
@p2
kD-
001
+ QIO
(9)
QOIQ12
ExperimentalSection Mterials. The experiments were made in ethanol-aerated solutions and solid colored polymeric matrices of polyurethane acrylate (PUA). At room temperature PUA-matrix exists in the highclastic state characterized by higher mobility of the polymeric chain segments rather than in glassy polymers. Organic dyes are laser dyes from polymethine (PD) and styryl (SD) classes: indodicarbocyanine dye PD 643 (the position of the absorption band maximum in ethanol is at 643 nm), imidadicarbocyanine dye PD 637,and styryl dye SD 530. The dyes were obtained from the Institute of Organic Chemistry (Kiev, Ukraine); see structure formulas.
0.8
-
E:
2
c
44
-
I
I
I- Me
Me
PD643 Ph
Ph I
7
I
I
I- Et
Et
PD637
The dye molecules (with the concentration of 10-4-10-5mol/L) were dissolved into a liquid photosensitive composition (a mixture of oligomer-oligourethaneacrylate and benzophenone initiator with concentration of 0.4% wt) and polymerized between two glass plates. Photopolymerization was carried out by exposing the sample to the ultraviolet light centered at the absorption band of the i n i t i a t ~ r .The ~ glassy plates are necessary to provide for a high optic quality of the polymeric layer as well as the photopolymerization reaction to occur since the atmospheric oxygen is an inhibitor of the reaction. NA Measurement. The specimens were excited by the frequency-tunable polymeric dye laser2 having the following characteristics: the tuning band Ahe = 560-690 nm, the energy per pulse E = 3 mJ, the half-height pump pulse duration re = 15 ns, the spectral half width
