Enhancing the Triplet yield in compact dibenzofuran-napthalimide donor/acceptor dyad based on Charge Recombination Induced Intersystem Crossing via substitution of one atom

Abstract

Donor-acceptor dyads with efficient charge recombination (CR) induced intersystem crossing (ISC) capability, always remain of particular interest due to their key role in fascinating photochemical application. Two compact donor-acceptor dyads (DBF-NI and DBT-NI) were devised by linking the naphthalimide (NI) chromophore with donor dibenzofuran (DBF) and dibenzothiophene (DBT) moieties, with the aim to investigate CR induced ISC. The photophysical properties of the dyads were explored using steady-state and time resolved transient absorption spectroscopy as well as density functional theory. DBT-NI showed higher triplet yield (ΦΔ = 75%) compared to another dyad DBF-NI (ΦΔ = 58%), although both are with difference of only one atom. Electrochemical analysis and transient absorption studies revealed efficient charge separation (CS) in polar solvents, while no CS was observed in non-polar solvents. Interestingly, ISC channel in current dyads switch from spin orbit induced ISC (nπ*→ππ* transition) to spin orbit charge transfer intersystem crossing (SOCT-ISC) with increase in solvent polarity. In non-polar solvent, nπ*→ππ* transition is dominant ISC pathway while in polar solvent CR leads to triplet population. Through femtosecond transient absorption studies, we found the ultrafast CS (2.6 ps) and CR (78 ps) in DBT-NI, while in DBF-NI comparatively slow CS (3.9 ps) was observed. We found efficient and strong CS process in DBT-NI results in the lowering of CS state while triplet energy level remains nearly unaffected, thus the energy gap between the CS state and triple energy level reduces which is beneficial for spin orbit charge transfer intersystem crossing (SOCT-ISC), thus higher triplet yield was observed. This work not only introduce tactic for future designing of new efficient triplet photosensitizers, but also of particular importance in the field photochemistry, photovoltaics and photodynamic therapy.