Unveiling the effect of amino substitution on intramolecular electron/energy transfer and intersystem crossing in the naphthalimide-pyrene donor-acceptor dyad

We synthesized a molecular dyad (NINH-Py) featuring pyrene (Py) as an electron donor and naphthalimide (NI) as an electron acceptor. The molecular design incorporates an orthogonal geometry facilitated by steric influence of the 4-amino group on the NI unit to study the structure-property relationship (electron transfer and intersystem crossing) for tuning the excited state dynamics. The photophysical characteristics of the dyad were comprehensively examined in solution and polymer films (in comparison with a previous dyad without an amino group, NI-Py) using a suite of spectroscopic techniques, complemented by quantum chemical calculations. The interaction between NINH and Py units in NINH-Py is limited at ground and excited states, which is evident by the unquenched fluorescence even in polar solvents. Femtosecond transient absorption spectra exhibited environment-dependent photophysics: in solution, the Förster resonance energy transfer (FRET) from Py unit to NINH moiety (¹Py∗ → ¹NINH∗ process) occurring within ca. 0.65 ps, conceivably in competition with charge separation. However, in polymer film triplet formation is observed within ca. 1.0 ns. Contrarily, NI-Py dyad (without amino group) showed efficient charge separation at ca. 0.20 ps. Interestingly, in contrast to the previously reported NI-Py, upon introducing amino substitution on NI unit, the localization of triplet state is shifted from pyrene chromophore to NI moiety in NINH-Py. Notably, the triplet lifetime is significantly longer than that achieved for the dyad without amino group (281.4 μs vs 84 μs). The solvent polarity-independent low singlet oxygen quantum yield of only ∼15 % shows lack of charge transfer mediated ISC. Moreover, P-type delayed fluorescence was observed for the dyad (τ_DF = 56.4 μs). Theoretical calculations support the experimental observation.