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

Abstract

The photophysical properties, nonlinear absorption characteristics and the influence of the substituted aromatic rings on the nature of excited states of a series of 4,4′-diethynylazobenzene derivatives ( 1a – 1f ) were systematically investigated. All compounds exhibit reverse saturable absorption at 532 nm and could potentially be used as nonlinear absorbing materials. ► Photophysics of 4,4′-diethynylazobenzene derivatives was systematically investigated. ► Excited state properties can be tuned by alternation of the terminal substituents. ► Compounds exhibit varied reverse saturable absorption at 532 nm for ns laser pulses. 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-(9 H -carbazol-9-yl)phenyl; 1d : R = 9 H -fluoren-2-yl; 1e : R = biphenyl-4-yl; 1f : R = naphthalen-2-yl) were systematically investigated. All compounds exhibit strong 1 π , 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 1 π , π */ 1 ICT (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 3 π , 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.