SUBNANOSECOND RELAXATION DYNAMICS OF 2,2'-AZINOBIS(3-ETHYLBENZOTHIAZOLINE-6-SULFONATE) AND CHLORPROMAZINE - ASSESSMENT OF PHOTOSENSITIZATION OF A WIDE-BAND GAP METAL-OXIDE-SEMICONDUCTOR TIO2

Subnanosecond relaxation dynamics of the 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) monoanion, ABTSH(-H+) and the dianion, ABTS(-2H(+)), together with the chlorpromazine cation ClPMZ(+H+) were examined by picosecond laser spectroscopic techniques (excitation wavelength, 355 nm) in the presence and absence of TiO2 colloids to assess their photosensitization of wide band gap metal oxide semiconductors. Fluorescence emissions from ABTSH(-H+) and ABTS(-2H(+)) by excitation with 370-nm light and from ClPMZ(+H+) by excitation at 360 nm are quenched by TiO2 by an interfacial electron transfer process implicating the singlet excited states of the dyes and the conduction band and/or band gap surface states of TiO2 (electron injection). Subnanosecond change-in-absorbance spectra of the substrates showed formation of singlet excited states followed by electron ejection and formation of the corresponding radical cations (zwitterion radicals). Photolysis in the presence of an electron scavenger (e.g., NO3-) at different delay times confirmed these observations. The lifetime of free ABTS(-2H(+))* (S-1) is about 80 ps, and that of ClPMZ(+H+)* (S-1) is pulse-width limited, that is less than 30 ps. The presence of TiO2 colloids in the photolylic system decreased the rate of loss of the radical cation ABTS(.+)(-2H(+)) owing to surface trapping of photoejected electrons from the singlet excited states of the substrates. Formation of the ABTS(2+)(-2H(+)) dication is observed on photolysis of ABTS(.+)(-2H(+)) in the presence of TiO2 colloids and dissolved oxygen; in the absence of either TiO2 and/or O-2, formation of the dication is quenched, consistent with the notion that ABTS(.+)(-2H(+)) is oxidized by surface trapped holes on the TiO2 particle.