none (3). After that, kDincreases (when I grows) for polymethine and rhodamine dyes up to fcgax, but remains the same for the phenalenone dye. That is why the ...
M. V. B ondar et al.: Photobleaching o f Laser Dyes in Polym eric M edia
103
Photobleaching of Laser Dyes in Polymeric Media*) Photoausbleichen von Laserfarbstoffen in polymeren Medien By M.V. B o n d a r , O. V. P r z h o n s k a y a , and E. A . T i k h o n o v , Kiew, USSR (Received Sept. 27, 1989)
Abstract The initial part o f the bleaching kinetics in the case o f weak light irradiation follows a second-order kinetics. In intensive light irradiation the photodecay yield increases with the increase of the intensity which is explained by the existence o f an effective channel o f irreversible transitions from the second ex cited singlet state. Zusammenfassung Der Anfangsteil der Ausbleichkinetik im Falle schwacher Lichtanregung folgt einer Kinetik zweiter Ordnung. Im Falle einer intensiven Lichtanregung wächst die Ausbeute des Photozerfalls m it dem A n steigen der Intensität, was mit der Existenz eines effektiven Kanals irreversibler Übergänge vom zweiten angeregten Singulett-Zustand erklärt wird.
1. Introduction The photostability of dye-activated polymeric materials determines the possibility of their application in the capacity of scintillators, sun energy collectors, laser elements etc. The phenomena of photochemical degradation of dye molecules in polymeric materials are diverse and strongly depend on the parameters of light irradiation (wavelength, intensity) and on the properties of the matrix itself. Photochemical properties of dye-activated poly meric materials are studied rather seldom [1...3] compared to those of liquid solutions. In vestigations on the photochemistry of dye-activated elastomers were presented apparently in our articles only [4... 6]. The peculiarity of photochemical reactions of dyes in polymeric m e dia is determined by the segmental mobility of the polymer molecules. Therefore, for elastic polymers characterized by a higher mobility of the polymeric chain segments than glass poly mers, the kinetics of photobleaching of dyes can show some special features. This contribu tion is devoted to research the irreversible photochemical degradation of several dyes in an elastic polymer—polyurethanacrylate—under monochromatic irradiation by a light flux of different intensities ( / ^ Is; Is being the saturation intensity of the singlet transition S0^ SJ.
2. Experimental The samples to be studied were prepared as “triplexes”, i.e. the dyed layer of a polymer 0.2...0.8 mm thick was placed between two glass plates. The dye molecules (rhodamine, phenalenone, and polymethine) were dissolved into a liquid photosensitive composition (a mixture of oligomer and initiator with a concentration of 10_4...10“5 mol/1). Photopolymeri zation was carried out by exposing it to ultraviolet light centered to the absorption band of the initiator [4].
*) This paper is based on a poster contribution at the 5th Symposium on Optical Spectroscopy Sep tember 2 5 -2 7 , 1988, Eisenach (GDR).
104
E xperim entelle T echnik der Physik 38 (1990), 2, 103-107
3. Results 3 .1 . W e a k lig h t i r r a d i a t i o n The measurement of photobleaching kinetics of dyes for the case I < Is was carried out with an argon laser line of 514.5 nm, excited S0-* Si states for all used dyes. The spatial beam profile was formed approximately rectangular, the intensity was attenuated to S0.5 W /cmz in order the beam thermal blooming and heating effect were not important within measurement accuracy. The variation of the transmission (7) in time due to the photobleaching is shown in Fig. 1.
t/m in — —
t/m in — —
Fig. 1. The experimental dependences T from t for PD 560 (1,5 - without 0 2), Rh6G (2,6 without 0 2), PD 555 (3,7 - without 0 2) and 6-aminophenalenone (4)
Let us analyze the dependence received. In the given field approximation, the molecule decomposition rate equals dN(t) dt
■aNV(t),
where N(t) is the concentration, a and (3 are the rate constant and the order of reaction, resp. For the first-order reaction (P = 1) holds N(t) = exp(- tk the diffuse flow of oxygen provided the first order of photochemical reaction. The direct proof of the singlet oxygen generation by the polymethine dyes was presented early in [7, 8]. An important photochemical characteristics of a dye molecule is the bleaching quantum yield, i.e. the probability of molecular degradation on an absorbed photon. W hen a dye mole cule is simulated by the 3-vibronic singlet level S0'^ S 1- ^ S 2 the bleaching quantum yield can be presented as follows: ЛФ
