Work

Effect of Thionation of the Carbonyl Groups in Naphthalimide‐Phenoxazine Electron Donor–Acceptor Dyads on the Excited‐ State Dynamics: Transient Optical and Electron Paramagnetic Resonance Spectral Studies

Year: 2025

Type: article

Abstract: We performed thionation of the electron acceptor naphthalimide (NI) moiety in donor–acceptor (D‐A) dyads in a series of thio‐substituted D‐A dyads. Thionation significantly enhances the electron‐accepting ability of the chromophore, facilitating the formation of low‐lying charge‐separated (CS) states. Femtosecond transient absorption spectra revealed intersystem crossing (ISC) rate constants of S‐NI‐PXZ ( k ISC = 3.8 × 10 11 s −1 ) and DS‐NI‐PXZ ( k ISC = 2.4 × 10 11 s −1 ), which are interestingly independent of the number of sulfur atoms. Generally, fluorescence quenching was observed due to the rapid ISC process. Native NI‐PXZ exhibits thermally activated delayed fluorescence (TADF). Thionation increased the singlet‐triplet energy gap, making the TADF vanish. In n ‐hexane, nanosecond transient absorption spectroscopy detected the triplet lifetimes shortened from 11.6 µs of NI‐PXZ to 0.66 µs for S‐NI‐PXZ and 0.25 µs for DS‐NI‐PXZ . Time‐resolved electron paramagnetic resonance revealed the large zero‐field splitting D value (4200–9800 MHz) of the triplet state of the thionated derivatives, which is due to the much larger SOC matrix elements of triplet state of DS‐NI‐PXZ (ca. 135 cm −1 ) and DS‐NI (ca. 136 cm −1 ) than the nonthionated NI (3.1 cm −1 ). Thionation of the carbonyl groups in chromophore is a promising method for design of triplet photosensitizers to achieve fast ISC.

Source: Chemistry - A European Journal

DOI: 10.1002/chem.202502885