Photochemistry of Far Red Responsive Tetrahydroquinoxaline-Based Squaraine Dyes

Abstract

The photochemical and redox properties of two newly synthesized tetrahydroquinoxaline-based squaraine dyes (SQ) are investigated using femto- and nanosecond laser flash photolysis, pulse radiolysis, and cyclic voltammetry. In acetonitrile and dichloromethane, these squaraines exist as monomers in the zwitterionic form (λmax ≈ 715 nm, εmax ≈ 1.66 × 105 M−1 cm−1 in acetonitrile). Their excited singlet states (1SQ*) exhibit a broad absorption band at 480 nm, with singlet lifetimes of 44 and 123 ps for the two dyes. Both squaraines exhibit poor intersystem crossing efficiency (ΦISC < 0.001). Their excited triplet states (3SQ*), however, are efficiently generated by triplet−triplet energy transfer using triplet excited 9,10-dibromoanthracene. The excited triplet states of the squaraine dyes exhibit a broad absorption band at ca. 560 nm (εtriplet ≈ 4.2 × 104 M−1 cm−1) and undergo deactivation via triplet−triplet annihilation and ground-state quenching processes. The oxidized forms of the investigated squaraines (SQ•+) exhibit absorption maxima at 510 and 610 nm.