Theoretical Perspective on Two-State "ON-OFF" NLO Switch of Rh(III)-Azobenzene Complex with Considerable First Hyperpolarizability.

In this paper, a light-induced Rh(III)-azobenzene (azo) complex in its two conformations (- and -form) is analyzed via density functional theory methods, focusing on the geometrical and electronic structure, linear and second-order nonlinear optical (NLO) properties, as well as UV-vis absorption spectra. The results show that exhibits more obvious charge separation and more extensive π-conjugation than . The energy gaps () between the highest occupied molecular orbital and lowest unoccupied molecular orbital of and are as small as 0.28 and 0.36 eV, respectively. The unusual small facilitates electronic transition and accompanies large NLO responses. In particular, and possess considerably large second-order NLO responses with the first hyperpolarizability (β) up to 7.0 × 10 a.u. ("OFF" state) and 8.5 × 10 a.u. ("ON" state), respectively. shows good π-conjugation and obvious electric delocalization relative to and thus a considerable β value. The switchable ratio of β ()/β () is equal to 1.2, showing switch characteristics. UV-vis analyses indicate that the ON and OFF states can finely tune the absorption range due to the strongest absorption peaks of (301 nm) and (446 nm) lying in the UV and vis region, respectively. It is expected that the studied Rh(III)-azo complexes may be applied in the field of optoelectronics.