Photoinduced formation of a long-wavelength Soret absorption state of TPPS aggregates in polymer films under 405 nm laser irradiation
Description
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 indicate a photoinduced structural reorganization of aggregates that is kinetically stabilized under polymer confinement. 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 kinetically trapped, unconventional state in porphyrin aggregates, providing new insight into photoinduced processes in molecular assemblies and offering a pathway toward controlling their optical properties. Absorption spectra were measured using a xenon lamp (Hamamatsu Photonics, L2273) as the probe source. A continuous-wave 405 nm laser (MLL-III-405-500 mW) was used for irradiation. The laser spot diameter was measured as 610 µm (FWHM) using the knife-edge method. The beam was used as-emitted, without focusing. A Thorlabs PM100D was used for power measurements. In PVP films, clear spectral changes were induced at intensities below ~15 mW, whereas higher intensities (>100 mW) were typically required for PVA and PAA films. To ensure precise spatial overlap between probe and irradiation beams, a 128-channel multichannel lock-in detection system (Signal Recovery 7210 × 4) was employed, based on previously reported methods [28]. The time constant was set to 1 s, and each spectrum was acquired within 5 s.