Photo-induced proton transfer in fluorene- and carbazole-based compounds as red- and orange-light-emitting molecules: A TD-DFT study

• Two fascinate molecules were studied.
• PBE0/TD method was used to study the ESIPT process.
• Potential energy curves in ground and excited states were calculated.
• Unusually high Stokes shifts were expected.
• The photophysical properties of the compounds were calculated.

In this explore, intramolecular photo-induced proton transfers in fluorene- and carbazole-based compounds of 5H-indeno[1,2-b]pyridin-9-ol (IPO) and 5H-pyrido[3,2-b]indol-9-ol (PIO) are investigated using TD-DFT method at PBE0/6-311++G(2d,2p) level of theory. The potential energy surfaces were calculated for ground and excited states. In contrast to the ground state, photoexcitation from S0 state to S1 one induces the operation of the excited-state intramolecular proton transfer process. The calculated fluorescence emission wavelength for both molecules is >600 nm. Both compounds exhibit unusually high Stokes shifts fluorescence. The structural parameters, H-bonding energy, absorption and emission bands, vertical excitation and emission energies, oscillator strength, fluorescence rate constant, dipole moment, atomic charges and electron density at critical points were calculated. Molecular orbital analysis shows that vertical S0 → S1 transition in the studied molecules corresponds effectively to the excitation from HOMO (π) to LUMO (π∗). The potential of these molecules as emissive materials in designing and generation of new displays and light sources are predicted based on their high Stokes shifts (374 and 291 nm).

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