PHOTOPHYSICAL PARAMETERS OF CHLOROPHYLLS a AND b·FLUORESCENCE AND LASER-INDUCED OPTOACOUSTIC MEASUREMENTS

Abstract— The quantum yields of internal conversion (energy loss) of the photoexcited chlorophylls a and b in ethanol at 24°C have been determined by measuring the quantum yields of the competing radiative (fluorescence) and non-radiative (intersystem crossing) deactivation processes (which lead to energy storing species) by means of emission and laser-induced optoacoustic spectroscopy, respectively. The values for internal conversion of chlorophyll a are 10% and 14% in the presence and absence of molecular oxygen, respectively. The corresponding yields for chlorophyll b are negligibly small. The internal conversion in chlorophyll a is attributed to vibronic coupling with the substituents of the porphin system, since internal conversion was found to be substantially reduced for unsubstituted Mg-porphin in ethanol and for chlorophyll a when dissolved in a viscous medium.