Synthesis of Schiff Base–BF2 Complexes and Characterization of Their Excited State Dynamics

The photophysics of a series of Schiff base–BF2 complexes was investigated using steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as theoretical computations. The native chromophore has a weak fluorescence and a short fluorescence lifetime (<30 ps, determined using the femtosecond fluorescence upconversion method). No triplet state formation was observed for the compounds. Moreover, attachment of a heavy atom (iodine) to the phenyl ring did not enhance intersystem crossing (ISC), which was different from that observed with Bodipy analogs. We attribute the lack of ISC to the short lifetime of the S1 state, which decays rapidly through an efficient non-radiative decay channel, possibly geometry torsion. We also studied a Schiff base–BF2 complex with a twisted molecular structure, which showed a similarly short S1 state lifetime (<100 ps) and weak fluorescence. Using nanosecond transient absorption spectroscopy and intermolecular triplet–triplet energy transfer, we determined the triplet state lifetime of the Schiff base–BF2 complexes to be ca. 20 μs, which is much shorter than that of the Bodipy chromophore (100–800 μs). Based on femtosecond transient absorption spectra, we inferred that the decay of the emissive S1 state takes about 17–20 ps, leading to a non-emissive (dark) state, followed by the formation of a long-lived non-emissive singlet excited state. Theoretical computations demonstrated large spin–orbit coupling matrix elements (SOCMEs, up to 29 cm−1), but the fast non-radiative relaxation of the S1 state inhibits ISC. Thus, we propose that the fast internal conversion inhibits ISC of the iodo-containing molecules. A theoretical study of the zero-field splitting (ZFS) parameters of the triplet state indicates that the ZFS D parameter (45 cm−1) was overestimated for the iodo-containing compounds, whereas a reasonable value was obtained for the iodo-free compounds (ca. 0.03–0.06 cm−1).