Fluorescent coumarin derivatives with large stokes shift, dual emission and solid state luminescent properties: An experimental and theoretical study

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Coumarin derivatives containing 8-benzothiazole ( C-2) and its difluoroboron bound derivative ( C-3) were prepared. Both derivatives show dual emission at 322 nm and 513 nm and large Stokes shift (188 nm), compared to the unsubstituted coumarin ( C-1), which shows emission at 356 nm with small Stokes shift of 46 nm. C-2 and C-3 show fluorescence in solid state, in contrast the C-1 is non-fluorescent in the solid state. The excited state intramolecular proton transfer (ESIPT) process of C-2 was fully rationalized by DFT/TDDFT calculations with optimization of the ground state (S 0) and excited state (S 1) geometries. TDDFT calculations propose that the large Stokes shift of C-2 and C-3 are due to the re-distribution of the frontier molecular orbitals at excited states. Study of the potential energy curve of C-2 indicated that the dual emission of the C-2 is due to the simultaneous S 1 and S 3 emission, not the rotamer of the enol form.