Theoretical study of two-photon absorption and fluorescence emission properties of bipyrazine (or hexaazatriphenylene) core based donor-π-acceptor-π-donor framework chromophores

Abstract

Organic two-photon absorption (TPA) chromophores have gained much attention among researchers due to their great potential for applications such as in bioimaging and two-photon microscopy. For high-performance applications, it’s better to get a brighter organic fluorophore at relatively smaller size. The fluorophore brightness is proportional to the TPA cross-section (σ_tpa) and fluorescence quantum yield, and their simultaneous optimization is relatively challenging. In this study, using quadratic and linear response theory within the TDDFT framework, we theoretically explored the effect of structural modifications within the donor-π-acceptor-π-donor framework on the TPA and fluorescence emission property (where, donors= BAC, BOC; π-bridges= EY, EN; acceptors= BPZ, HAT; additional substituent groups R=H, F or CN). We have found: (a.) The (BAC-EY)₂-A-R (A=BPZ or HAT, R=H or CN) systems could display mass-averaged TPA cross-sections in range from 4.8 to 9.3 GM/(g/mol) owing to cooperative balance between steric hindrance and framework conjugation. (b.) Based on three-states model approximation, the large TPA response of the studied BAC-containing systems have been ascribed to pure three-states contribution due to sizable transition dipole moments and small detuning energy. (c.) The fully rigid acceptor HAT based systems can only display negligible small emission oscillator strength ( f_emi), by contrast, the partially flexible acceptor BPZ based systems can retain relatively large f_emi. We guess that the profound geometric changes within the BPZ core might account for the large emission oscillator strengths ( f_emi) of the former systems, and the lack of efficient overlap between transition orbitals might explain the low femi of the later systems. (d.) Based on the evaluation of radiative and nonradiative rates, the BPZ based systems should be better fluorescence emitters compared to HAT based ones, and the (BAC-EY)₂-BPZ-CN system could not only exhibit a not-too-low Φ_fl (0.175) but also a large TPA response (6210 GM). In addition, it’s relatively small and simple chemical structure for experimental synthesis. Therefore, we recommend (BAC-EY)₂-BPZ-CN as potential high-performance chromophore for possible two-photon fluorescence microscopy applications.