Photochemistry of far red responsive tetrahydroquinoxaline-based squaraine dyes

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 (lambda(max) approximately 715 nm, epsilon(max) approximately 1.66 x 10(5) M(-1) cm(-1) in acetonitrile). Their excited singlet states ((1)SQ*) 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 (Phi(ISC) < 0.001). Their excited triplet states ((3)SQ*), 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 (epsilon(triplet) approximately 4.2 x 10(4) 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.