Photoinduced formation of a long-wavelength Soret absorption state of TPPS aggregates in polymer films under 405 nm laser irradiation

Understanding how molecular aggregates respond to photoexcitation under confinement is essential for designing functional optical materials, yet photoinduced structural modification in polymer environments remains poorly understood. In particular, the emergence of new absorption features beyond conventional excitonic frameworks under photoexcitation has not been clarified. In this study, we investigate the spectral evolution of Tetra-Phenyl Porphyrin tetra-Sulfonic acid (TPPS) aggregates embedded in polymer films (PVP, PVA, and PAA) under continuous-wave 405 nm laser irradiation. Absorption spectroscopy was used to monitor time-dependent changes during and after irradiation. We find that laser irradiation induces the formation of a distinct long-wavelength Soret-side absorption band at ∼510 nm, significantly red-shifted from the conventional J-aggregate band (∼490 nm). Notably, this band appears without a corresponding feature in the Q-band region. The formation pathway depends strongly on the polymer matrix: the band develops mainly after irradiation in PVP and PVA films, whereas it forms during irradiation in PAA films and persists over extended periods. These observations are difficult to reconcile with conventional excitonic descriptions, including current CT-coupled exciton frameworks, and are inconsistent with photobleaching, thermal effects, or excimer formation. Instead, the observations demonstrate the formation of a distinct photoinduced long-wavelength spectral state under polymer confinement. A structural reorganization of aggregates provides one plausible interpretation of the observed behavior, although direct structural evidence is not yet available. The strong polymer dependence suggests that local mobility and hydration-mediated effects govern the formation pathway. This work demonstrates that polymer-confined environments enable the formation of a long-lived, unconventional photoinduced spectral state in porphyrin aggregates, providing new insight into photoinduced processes in molecular assemblies and offering a pathway toward controlling their optical properties.