Expanding π-Conjugation in Chlorins Using Ethenyl Linker.

A series of chlorin monomers and dyads has been prepared to probe the effect of ethenyl vs ethynyl linkers on the electronic conjugation and optical properties in resulting derivatives. Styryl-substituted chlorins have been prepared either by a Heck reaction or by microwave-assisted olefin metathesis, while beta-beta ethenyl-linked dyads have been synthesized from the corresponding vinyl-substituted chlorin monomer using microwave-assisted olefin metathesis. It has been found that when an ethenyl linker is connected at the beta-position of chlorin it provides stronger electronic conjugation than an ethynyl one, which is manifested by a greater bathochromic shift of the longest wavelength absorption (Q(y)) and emission bands. Stronger electronic coupling is particularly evident for dyads, where ethenyl-linked dyad exhibits a strong near-IR absorption band emission (lambda(abs) = 707 nm, lambda(em),= 712 nm Phi(f) = 0.45), compared to the deep-red absorption and emission of a corresponding ethynyl-linked dyad (lambda(abs) = 689 nm, lambda(em) = 691 nm, Phi(f) = 0.48). The reactivity of ethenyl-linked dyads with singlet oxygen is discussed as well. The results reported here provide further guidelines for molecular design of deep-red and near-IR absorbing and intensely emitting chlorin derivatives and chlorins with extended pi-electronic conjugation for a variety of photonic applications.