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

Organic two-photon absorption (TPA) chromophores have gained much attention among researchers due to their great potential for applications in bioimaging and two-photon microscopy. For high-performance applications, it is better to get a brighter organic fluorophore at a relatively smaller size. The fluorophore brightness is proportional to the TPA cross-section (<i>σ</i>_tpa) and fluorescence quantum yield, and its 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 properties (where donors = BAC and BOC; π-bridges = EY and EN; acceptors = BPZ and HAT; additional substituent groups R = H, F or CN). We have found the following: (a) (BAC-EY)₂-A-R (A = BPZ or HAT, R = H or CN) systems could display mass-averaged TPA cross-sections in the range from 4.8 to 9.3 GM g^-1 mol owing to a cooperative balance between steric hindrance and framework conjugation. (b) Based on three-state model approximation, the large TPA response of the studied BAC-containing systems has been ascribed to pure three-state contribution due to sizable transition dipole moments and small detuning energy. (c) Fully rigid acceptor HAT based systems can only display negligible small emission oscillator strength (<i>f</i>_emi); by contrast, partially flexible acceptor BPZ based systems can retain relatively large <i>f</i>_emi. We guess that the profound geometric changes within the BPZ core might account for the large emission oscillator strength (<i>f</i>_emi) of the latter systems, and the lack of an efficient overlap between transition orbitals might explain the low <i>f</i>_emi of the former systems. (d) Based on the evaluation of radiative and nonradiative rates, BPZ based systems should be better fluorescence emitters compared to HAT based ones, and the (BAC-EY)₂-BPZ-CN system could exhibit not only a not-too-low <i>Φ</i>_fl (0.175) but also a large TPA response (6210 GM). In addition, it is a relatively small and simple chemical structure for experimental synthesis. Therefore, we recommend (BAC-EY)₂-BPZ-CN as a potential high-performance chromophore for possible two-photon fluorescence microscopy applications.