Synthesis and linear and nonlinear optical properties of low-melting π-extended porphyrins

A large and diverse library of trans-A2B2 and A2BC-porphyrins possessing two arylethynyl substituents at the meso positions has been efficiently synthesized and tested for their two-photon absorption (2PA) behavior. All compounds fall into three general types A–π–A, D–π–D or D–π–A, where A is an electron-acceptor and D is an electron-donor moiety. These porphyrins contain two polyalkoxyaryl substituents, resulting in very low melting points (typically 110–125 °C) and superb solubility in non-polar solvents. Some of these porphyrins exhibit two different crystal phases in addition to an isotropic liquid state. Their linear and nonlinear optical properties were thoroughly elucidated and analyzed. π-Extended porphyrins emit light in the NIR and have moderate triplet state lifetimes. The increase of 2PA cross-section in the Soret region for porphyrins bearing strong electron-donating groups has been attributed to resonance enhancement (caused by intensification, redshift and broadening of the lowest Q-band) of gerade–gerade transition. The combination of high two-photon absorption cross-sections (>500 GM) and low melting points makes them perfect candidates for nonlinear optical materials in the 600–900 nm range.