An approach was developed to determine triplet quantum yields, , which overcomes most of the difficulties usually encountered in flash photolysis studies of triplet states. The combined application of near-IR emission and optoacoustics yields values which are independent of the absorption properties of the triplets. Thus, the triplet energies derived from near-IR phosphorescence spectra together with the values of the heat stored by the triplets determined from optoacoustic experiments afforded = 0.3 ± 0.1 for porphycene (PO) and 0.4 ± 0.1 for its tetra-n-propyl derivative (TPrPO). Several calorimetric references for optoacoustic measurements in the red region were tested. The values, combined with flash photolysis data, yielded accurate triplet-minus ground-state absorption coefficients. The quantum yields for singlet molecular oxygen production determined by time-resolved phosphorescence were = 0.34 ± 0.05 for PO and 0.36 • 0.03 for TPrPO which implies an efficiency near unity for 02(' 8) production upon collision of the triplet states of the porphycenes with ground-state 02.

The quantum yield of triplet formation, phi T, and that of the photosensitized formation of singlet molecular oxygen, phi delta, were determined for a rare nucleoside, 4-thiouridine (4t-Urd), in water and in acetonitrile, using singlet molecular oxygen phosphorescence, laser-induced optoacoustics and time-resolved thermal lensing. These yields, phi T and phi delta, the latter in aerated solutions, were found to be, respectively, in water: 0.67 +/- 0.17 and 0.18 +/- 0.04 and in acetonitrile: 0.61 +/- 0.15 and 0.50 +/- 0.20. The fraction of the 4t-Urd triplet molecules quenched by oxygen leading to singlet molecular oxygen, S delta, was calculated to be between 0.7 and unity in both solvents, this value being indicative of a pi pi*character for the lowest triplet state of 4t-Urd.

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Time‐resolved thermal‐lensing was used to measure the absolute quantum yield (φΔ) of singlet molecular oxygen, O2(1Δg), produced by hematoporphyrin photosensitization in ethanol. Deuteration of the solvent did not affect the value of φΔ. The value of φΔ= 0.53 was then used as reference to evaluate φΔ in O2 (1Δg) phosphorescence experiments with the related porphyrins, monohydroxyethylvinyl deuteroporphyrin and dihematoporphyrin ether. The φΔ values, in conjunction with the respective quantum yields of intersystem crossing (measured using a nanosecond laser flash photolysis technique) served to evaluate efficiencies, SΔ, of O2 (1Δg) production from the porphyrin triplet states. The lifetime TΔ in monodeuterated ethanol was measured as 29 ± 3 μs and 30 ± 1 (xs by time‐resolved thermal lensing and phosphorescence detection, respectively. TΔ in ethanol and fully deuterated ethanol were in good agreement with values reported in the literature.

After excitation at room temperature with a 15 ns dye laser pulse, phytochrome (60 kDalton) from etiolated oat seedlings was studied by optoacoustic spectroscopy, which records the heat emission caused by non‐radiative deactivation processes of the photoexcited molecule. The action spectrum for the heat emitted by Pr deviates from the absorption spectrum around 610 and 695 nm, indicating the build‐up of photoproduct(s) within 15 ns after excitation. It is proposed that the 695 nm product is identical with I700 and lumi‐R known to be the first intermediate on the Pr× Pfr pathway, and that the photochemical back reaction of I700 to Pr occurs on the same time scale. About 90% of the absorbed light energy was lost by radiationless deactivation. Values for the rate constants of excited state deactivation of Pr and for the internal energy content difference of the primary photoproducts are calculated.