Photophysics of Some Disubstituted Indoles and Their Involvements in Photoinduced Electron Transfer Reactions

Abstract

Steady state electronic absorption, fluorescence emission, polarized spectra of some disubstituted indoles, 2,3-dimethylindole (23DMI), 2,5-dimethylindole (25DMI), and 1,2-dimethylindole (12DMI), were studied in solvents of different polarity coupled with time resolved measurements. The electrochemical measurements demonstrate that DMIs should act as electron donors in photoinduced electron transfer (PET) reactions with 9Cyanoanthracene (9CNA) which serves as the electron acceptor. The photoexcited 9CNA undergoes fluorescence quenching in the presence of DMI both in nonpolar n-heptane (NH) and highly polar acetonitrile (ACN) solvents though no ground state complex formation was apparent between the reactants. In NH, the observation of the reduction of fluorescence emission intensity of 9CNA accompanied by the formation of emissive exciplex was made only for the N-methyl substituted indole 12DMI. Transient absorption spectra recorded by the laser flash photolysis technique show that photoinduced charge separation reaction proceeds both in the excited singlet and triplet states of 9CNA. Moreover, it is also established that production of the monomeric triplet of 9CNA occurs through the ion recombination mechanism. In NH, the possibility of the formation of a contact neutral radical, through the H-atom transfer reaction, as an additional route for rapid nonradiative decay of the exciplex is hinted at in the case of N−H indoles 23DMI and 25DMI. Nevertheless, it is inferred that in the case of 12DMI the observed emissive exciplex in nonpolar medium is due to its inability to form a contact neutral radical. In ACN, the major nonradiative pathway appears to be due to photoinduced ET rather than formation of exciplex.