Spin-dependent electron transfer dynamics in a platinum-complex–donor–acceptor triad studied by transient-absorption detected magnetic field effect

For realization of efficient organic light-energy conversion systems, controlling the lifetime of photogenerated charge separated states in donor (D)–acceptor (A) molecules is of much importance; the spin dynamics is one of the important controlling factors. We previously reported that the covalently-linked 1,3-bis(2-pyridylimino)-isoindolate platinum (BPIPt)–dimethoxytriphenylamine (D)–naphthaldiimide (A) triad molecule (BPIPt–DA) exhibits a triplet-born long-lived charge separated state (BPIPt–D^•+A^•−), the lifetime of which is significantly increased from 4 µs to 10 µs by an applied magnetic field of 270 mT in room temperature tetrahydrofuran (THF). The purpose of the present study is to clarify detailed dynamics of spin-dependent generation and the decay of BPIPt–D⁺A⁻. For this purpose, we measured transient optical absorption (TA) and the TA-detected magnetic field effect (MFE) as functions of temperature and dispersion media. In THF at 183 K, MFE-detected transient spectra of the intermediate BPIPt^•−–D^•+A state are observed. We have successfully quantified the recombination loss at this state by a kinetic simulation of MFE without using any reference molecules. The lifetime of the final BPIPt–D^•+A^•− state in a cellulose acetate polymer matrix at room temperature is significantly prolonged to 20 µs at 0 mT and 96 µs at 250 mT compared to those in THF. From the comparison of temperature dependences of the two media, effects of molecular motions on the electronic coupling and the spin relaxation are discussed.