Photonically-Activated Molecular Excitations for Thermal Energy Conversion in Porphyrinic Compounds

Heterocyclic, macrocycle organic compounds, structurally characterized with porphyrins, are not only abundant in nature but also environmentally friendly. These porphyrinic compounds have recently been extensively studied for their fascinating structures, physical properties, and high potentials in engineering applications. We report experimental results on the photonically activated thermal energy conversion via irradiations of white light (simulated solar light). The photothermal effects have been well studied for metallic conductors with a large number of charge carriers based on the so-called localized surface plasmon resonance (LSPR). However, the LSPR model may not apply to the porphyrinic materials with a very limited number of charge carriers. In this study, we have found several porphyrinic compounds to exhibit pronounced photothermal effects including chlorophyll, chlorophyllin, hemoglobin, and phthalocyanine, which all share similar structural characteristics. Raman data show characteristic molecular vibrations from these compounds that are responsible for photon-to-thermal energy conversions near the optical absorption frequencies. We attribute the porphyrin molecular vibrations to the photothermal effects observed from these compounds and predict that all porphyrinic materials can be optically activated for pronounced photothermal effects. Also established is a newly defined specific photothermal coefficient (SPC), a unique photothermal property of the thin films investigated in this study.