Photoinduced intramolecular charge-transfer state of p-dimethylaminobenzoic acid in CdS and TiO2 colloid solutions

A photoinduced intramolecular charge transfer (ICT) of p-N,N-dimethylaminobenzoic acid (DMABA) has been investigated in TiO2 and CdS colloidal solutions by steady-state and time-resolved fluorescence spectroscopy as well as ns time-resolved transient absorption spectroscopy. The intensity ratio of the ICT emission to the normal emission of DMABA was largely enhanced by the addition of CdS colloids with the decay times of the ICT emission increasing from 1.7 to 2.3 ns. However, the ICT emission of DMABA adsorbed on TiO2 colloids was greatly quenched with constant fluorescence decay times. Concomitantly, the dual-emission bands of CdS (530 and 630 nm) and the single-emission band of TiO2 (340 nm) were significantly changed upon adsorption of DMABA. These illustrate that electron transfer takes place efficiently between CdS or TiO2 colloids and DMABA. For an efficient electron transfer, CdS interacts with DMABA in the ICT excited state, while TiO2 interacts with DMABA in the LE state. In addition, we observed that the formation of the ICT state of DMABA became more favorable for the electron transfer from CdS to DMABA at the DMABA-CdS interface, while the electron transfer at the DMABA-TiO2 interface inhibited the formation of the ICT state of DMABA. The ns time-resolved transient absorption experiments demonstrated further that the locally excited singlet state of DMABA was more affected by the interfacial electron transfer than the triplet state. (C) 2000 Elsevier Science S.A. All rights reserved.