Yukio INOUE¹, Hirokazu HORI¹, Takeki SAKURAI¹, Yukiko TOKITOMO², Junko SAITO³ and Taku MISONOU^3,*

¹Department of Electronics, Faculty of Engineering, Yamanashi University ²Department of Food Science, Education and Human Science, Yamanashi University ³Department of Biology, Faculty of Education, Yamanashi University, Takeda, Kofu 400-8511, Japan

(Received December 4, 2001; Accepted January 18, 2002)

Optical properties of an ultraviolet-absorbing substance (UVAS) extracted from the marine red alga, Porphyra yezoensis, have been investigated. The substance is excited by UV light, and the emitted fluorescence is detected using an intelligent fluorescence detector. The fluorescence of UVAS is weaker by four orders of magnitude than the fluorescence intensity emitted by anthracene in the same optical system. The absorbed energy is apparently not transferred to the photosynthesis process and is believed to be consumed as heat. Using photothermal spectroscopy, a signal is observed indicating that the absorbed photon energy has transferred to the heat. The waveform of the photothermal signal of UVAS is similar to that of quinoxaline, whose fluorescence quantum yield is known to be zero. It is determined that the fluorescence quantum yield and the energy of the triplet state of UVAS are 1.7 ± 0.7 × 10^-4 and 21000 ± 1000 cm^-1, respectively. The conclusion is that the excited molecules of UVAS decay by passing through the triplet state and dissipate all absorbed energy as heat.

Key words: ultraviolet, absorption, red alga, fluorescence, quantum yield, photothermal spectroscopy, heat, energy