Porphyrin–azoheteroarenes: synthesis, photophysical, and computational studies

Azobenzenes (Ph-N[double bond, length as m-dash]N-Ph) are well-known photochromic compounds with widespread applications. Replacing one or both phenyl rings of azobenzenes with heteroarenes leads to a new class of compounds known as azoheteroarenes (Het-N[double bond, length as m-dash]N-Ph/Het). Azoheteroarenes have gained attention as promising alternatives to traditional azobenzenes in the field of photopharmacology due to their ability to undergo photoswitching under visible light. Interestingly, the chemistry of porphyrin-containing azoheteroarenes has been underexplored. In this study, we present the synthesis of hitherto unknown hybrid molecules: porphyrin-azopyrroles (porphyrin-N[double bond, length as m-dash]N-pyrrole) and porphyrin-azoindoles (porphyrin-N[double bond, length as m-dash]N-indole), which also feature porphyrins with five-membered <i>meso</i>-substituents, such as 2-furyl and 2-thienyl groups. The porphyrin-azoheteroarenes with <i>meso</i>-tris(2-furyl/2-thienyl) substitutions demonstrate red-shifted absorption and emission bands, more significant Stokes shifts, and smaller optical bandgaps compared to hybrids containing only <i>meso</i>-aryl groups. Additionally, these porphyrin-azoheteroarenes exhibit higher fluorescence emission intensities than their corresponding precursor porphyrins.