Photoinduced intermolecular Charge-Transfer Systems for Optical Limiting
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Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
t
-Ns
•
EtNs -%.'
S
SS,7
El
DTTCP
E+ -!.•/ Et
0O2Et
'
-03S
El Et
M= Zn or H2
(ZnOEP or H2 OEP)
(N) --
~-N+~
Et
N- -N I
C1O 4-
HITMl
El Et.
N
R-N-
so3 -
R= methyl- or n-heptyl
Et3NH IR144 Figure1. Structure of the molecular systems used in these studies.
Viologen
change for the charge-transfer reaction is a critical factor governing the rate of the reaction and the selection of the donor/acceptor systems. Second, a diffusion controlled step, wherein the sensitizer encounters the acceptor, leads to formation of a complex ([S* ---A]) in which charge transfer occurs ([S+---A-]). Finally, ion separation, which is in competition with charge recombination, leads to the formation of two radical ions S+" and A-*. One or both of these radical ions (S+" and A-) may absorb strongly at the pumping wavelength, leading to optically-induced absorption. For efficient PICT-OL, a number of conditions must be fulfilled: (a) the ratio of the radical-ion absorption cross-section (oi) to the photosensitizer ground-state value (eg) must be much greater than unity, (b) the rate of charge transfer (kcT) should be fast compared to the laser pulse duration, and (c) the rate of radical-ion separation must exceed or be comparable to the rate of charge recombination (kcR), as needed in order to obtain a high yield of the radical ions [6]. RESULTS AND DISCUSSION Absorption Spectra and Spectroelectrochemistry In the absence of methyl viologen (MV), HITCI (in methanol) exhibits an intense absorption band at 740 nm with a small shoulder to the blue (at - 685 nm). Upon adding MV, up to a concentration of 0.3 M, no change in the band intensity of HITCI was observed (Fig. 3), suggesting that the formation of ground-state charge-transfer complexes between MV and HITCI is not significant. The absorption near 350 nm, as seen in Fig. 3, is due to MV itself. Porphyrin/benzyl alcohol solutions show a strong absorption band at - 400 nm (H2 OEP) and 410 nm (ZnOEP) with weak features to the red (H2 OEP: 499, 534, 564 and 617 nm; ZnOEP: 538 and 573 nm). In the presence of MV, a broad absorption tail extending out to - 600 nm is observed indicating formation of ground-state charge-transfer complexes. However, at the wavelengths of interest here, the absorbance due to the free porphyrin was dominant. The spectroelectrochemistry of the tricarbocyanines shows a new band due to C2+' at 560 nm that builds up with time, while under an oxidizing potential, and the intensity of 740 nm absorption band decreases (see Fig. 4 where the spectra of IR144 are shown as a representative case). The estimated oxidation potential (EI/ 2 (D+/D) vs Ag/AgCI)) for HITCI is - 0.25 V compared with a reduction potential, E1 /2 (A/A-), for MV of - - 0.45 V. MV+" has a well known absorption band around - 609 nm [5]. The free energy change for the photoinduced electrontransfer reaction between the cyanines and MV is highly exoergic (- - 0.84 eV). FLUORESCENCE QUENCHING RATES The cyanines and porph
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