Coumarin-Based Hybrids: From Linear Photophysical Properties to Multiphoton Excitation Behavior.

Coumarin-based derivatives are recognized as tunable photonic building blocks due to their strong light-matter interaction and relevance for both linear and nonlinear optical applications. This work presents an investigation of the linear optical properties and multiphoton excitation response of four derivatives coupled with benzothiazole and benzimidazole moieties. The compounds 7-diethylamino-coumarin-benzothiazole, 6-bromo-coumarin-benzothiazole, and 7-diethylamino-coumarin-benzimidazole exhibit nearly identical absorption maxima (∼423-428 nm) and emission peaks (∼485 nm), while displaying pronounced differences in molar absorptivity and ground to first excited-state transition dipole moment (μ01). The 7-diethylamino-substituted derivatives show enhanced molar absorptivity (∼5.3 × 104 L mol-1 cm-1) and larger transition dipole moment (μ01 ∼ 8.1 D) compared to the bromo-substituted (μ01 ∼ 5.5 D). In contrast, 7-hydroxyl-coumarin-methyl-benzimidazole exhibits a distinct spectral signature characterized by a larger Stokes shift and lower molar absorptivity, with an intermediate μ01 ∼ 7.0 D reflecting a different electronic balance within the conjugated framework. Multiphoton excitation experiments using femtosecond laser pulses demonstrate efficient two-photon (800 nm) and three-photon (1200 nm) excited fluorescence for all derivatives. Remarkably, the hydroxylated derivative combines an exceptionally high fluorescence quantum yield (∼48%) with a measurable excited-state lifetime (∼3 ns), identifying it as the most promising candidate for bright photoluminescent probing under one-, two-, and three-photon excitation.