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.