Photophysics and nonlinear absorption of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings

The photophysical properties of a series of 4,4′-diethynylazobenzene derivatives terminally capped with substituted aromatic rings (1a: R = phenyl; 1b: R = 4-(diphenylamino)phenyl; 1c: R = 4-(9H-carbazol-9-yl)phenyl; 1d: R = 9H-fluoren-2-yl; 1e: R = biphenyl-4-yl; 1f: R = naphthalen-2-yl) were systematically investigated. All compounds exhibit strong 1π,π* absorption bands in the UV region; and a broad, structureless charge-transfer band/shoulder in the visible region (except for 1a), which systematically red-shifts when electron-donating substituents are introduced to the terminal phenyl rings, but blue-shifts when π-conjugation of the terminal aromatic ring increases. All compounds are emissive in solution at room temperature and at 77 K. When excited at the low-energy absorption band, the compounds emit fluorescence between 369 and 419 nm, which can be attributed to 1π,π*/1ICT (intramolecular charge transfer) state. Density functional theory (DFT) calculations on 1a–1f in gas phase were also performed to gain insight into the nature of the ground electronic state and the low-lying excited electronic states. 1d–1f exhibit strong triplet transient absorption band(s) in the visible spectral region, which are mainly attributed to the 3π,π* state. Reverse saturable absorption (RSA) of these compounds was demonstrated at 532 nm using ns laser pulses. The degree of RSA follows this trend: 1b > 1c ≈ 1a > 1e >1f > 1d, which is mainly determined by the ratio of the triplet excited-state absorption cross-section to that of the ground-state and the triplet excited-state quantum yield.