Theoretical Study of Energy Gaps for Naphthalimide-based Charge Transfer Compounds

Thermally activated delayed fluorescence (TADF) materials are promising for the next generation of organic light-emitting diodes. To realize reverse intersystem crossing (RISC) effectively for TADF molecules requires basically small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states. In this work, 1,8-naphthalimide was selected as a fixed acceptor (A) because of its large rigid conjugate structure with a high radiative decay rate. Twelve naph- thalimide-based intramolecular charge transfer (ICT) compounds were constructed by the conjunction of naphthalimide moi- ety with each of donors (D) including 9,9-dimethyl-9,10-dihydroacridine, phenoxazine, etc. Their energy gaps were calculat- ed with the Gaussian 09 package at TD-DFT level using the optimal Hartree-Fock exchange method reported by Huang Shuping recently. The results show that D-A structure of naphthalimide-based ICT compounds has smaller ΔEST and longer starting fluorescence emission wavelengths (λ) than those for corresponding D-phenyl-A type. However, strong electron do- nor like 9-(9H-carbazol-3-yl)-9H-carbazole (DCZ) seems to have no significant effect on ΔEST and λ at high CT amount. To achieve small ΔEST of these naphthalimide-based ICT compounds, increasing the twisting angle between D and A, together with enhancing electron donating ability of D is found to be a practical strategy to stabilize the energy of the lowest local- ly-excited triplet state ( 3 LE) and substantially lower the energy of its lowest CT triplet excited state ( 3 CT). As a result, 4-(9,9-dimethyl-9,10-dihydroacridine)-N-phenyl-1,8-naphthalimide (4b) and 4-(phenoxazine)-N-phenyl-1,8-naphthalimide (5b) show small ΔEST of 0.01 eV and 0.02 eV, respectively. Their λ and oscillator strengths of vertical absorption from ground state (S0) to S1 are 575 nm and 0.0002 for 4b, while 621 nm and 0.0025 for 5b, respectively. In addition, the λ of compound 5b obtained from its photoluminescence curve is 600 nm, which consists with the computed one with an error of 0.07 eV. Therefore, compounds 4b and 5b can be expected to be potential reddish-orange and red TADF emitters. Keywords naphthalimide; thermally activated delayed fluorescence; energy gaps; quantum chemical calculation