Photophysical properties of quinoxaline-fused [7]carbohelicene derivatives

Helicene and its derivatives have received considerable attention as candidates for organic photoelectronic materials. Recently, novel quinoxaline-fused [7]carbohelicene derivatives have exhibited unique structural and photophysical properties, especially in the crystal state. However, their structure-property relationships have not been fully understood from their micromechanisms, which is also important to further improve their performance. Herein, the electronic transitions, electronic circular dichroism (CD), second-order nonlinear optical (NLO) responses and charge transport properties of five quinoxaline-fused [7]carbohelicene derivatives have been investigated based on density functional theory calculations. The experimental UV-Vis/CD spectra of the studied compounds were reproduced well by our calculations. Thus, we can assign their electron transition properties and absolution configurations (ACs) with high confidence. It is found that the CD bands of quinoxaline-fused [7]carbohelicene derivatives mainly originate from exciton coupling between quinoxaline, phenyl or 4-methoxyphenyl groups and [7]carbohelicene, which is in sharp contrast to [7]carbohelicene. More interestingly, these derivatives possess large first hyperpolarizability values. For example, the β value of compound 6 is 32.96 × 10 esu, which is about 190 times larger than that of the organic urea molecule. The bandwidth of the valence band of compound 2 is comparable to that of the conduction band and slightly larger than that of tris(8-hydroxyquinolinato)aluminium. This means that compound 2 is a potential candidate as an ambipolar charge transport material.