The Preparation And Study Of Broadband Light-Harvesting Organic Triplet Photosensitizers

The triplet photosensitizers are compounds which can efficiently produce triplet state upon photoexcitation, via vertical excitation and intersystem crossing (ISC). These compounds are able to initiate various photochemical or photophysical processes by intermolecular triplet energy transfer or electron transfer. Triplet photosensitizers have been widely used in various areas, e.g. photodynamic therapy, triplet-triplet annihilation upconversion, and photoredox catalytic organic reactions, etc. However, most of the known triplet photosensitizers are based on single chromophore molecular structural protocol, thus there is only one major absorption band in visible spectral region. As a result, these conventional triplet photosensitizers are unable to efficiently harvest the excitation energy of broadband light source, such as solar light. Generally, multi-chromophore can overcome above drawbacks, while the photophysical processes involved in the photophysics of these multi-chromophore triplet photosensitizers are not fully understood. To this end, our group prepared multi-chromophore triplet photosensitizers showing broadband visible light absorption, basing on intramolecular resonance energy transfer. New triad triplet photosensitizer based on naphthalenediimide (NDI) was prepared. The bromo-NDI moiety was used as triplet energy donor and the heavy atom-free diaminoNDI was used as singlet/triplet energy acceptor. This novel triplet photosensitizer was designed to study the intramolecular electron and energy transfer interaction, between the different components of the triad triplet photosensitizer, as well as the competing between ISC and singlet energy transfer processes. Meanwhile, this molecule extends the absorption range of NDI derivatives, realizing broadband absorption.This unique structural protocol and thus alignment of the energy levels ensures the competing ISC and FRET in the triad. The photophysical properties of the triad and the reference compounds were studied with steady-state UV-vis absorption spectra, fluorescence spectra, nanosecond transient absorption spectra, cyclic voltammetry, and DFT/TDDFT calculations. FRET was confirmed with steady-state UV-vis absorption and fluorescence spectroscopy. Intramolecular electron transfer was observed in polar solvents, demonstrated by the quenching of both the fluorescence and triplet state of the energy acceptor. In non-polar solvents, the fluoresence is not quenched, indicating weak electron transfer. Nanosecond transient absorption spectroscopy shows that the T1 state of the triad is exclusively localized on the 2,6-dialkylaminoNDI moiety in the triad upon selective photoexcitation into the energy donor, which indicates the intramolecular triplet state energy transfer. The intermolecular triplet state energy transfer between the two reference compounds was investigated with nanosecond transient absorption spectroscopy. The photophysical properties were rationalized by TDDFT calculations.