Manipulating Exciton Dynamics in Glassy Porphyrin Assemblies toward Near-Infrared Luminescence

Despite the high demand of cutting-edge applications for near-infrared (NIR)-luminescent chromophores, their development is hindered by the decrease in the emission efficiency with the narrowing optical bandgap (energy gap law). Herein, we established a correlation between the structural and NIR-luminescent properties of glassy porphyrin assemblies by comparing a systematic series of 11 proquinoidal-arylene-linked porphyrins 1π, each bearing two 3,4,5-tris((S)-dihydrocitronellyloxy)phenyl groups. The elastic and branched alkyl substituents were found to play a pivotal role in defining the distinctive photophysical properties of 1π. Specifically, 1π exhibited a bathochromic shift of the electronic absorption band, which was attributed to the slipped-cofacial ar-rangement of π-stacked porphyrin planes, and a substantial Stokes shift in the solid-state photoluminescence, which was mediated by exciton–phonon coupling within an elastic π-stacking lattice. The double-strand state (1π)2 formed an excimers and exhibited NIR photoluminescence in cyclohexane. The NIR photoluminescence was further enhanced in the abovementioned assemblies, in which case the increased Stokes shift led to solid-state NIR photoluminescence extending into the NIR-II region (> 1000 nm). The results of femtosecond photoluminescence upconversion measurements indicated that the NIR luminescence largely originated from exciton self-trapping.