A LASER FLASH PHOTOLYSIS STUDY OF PYRENE‐1‐ALDEHYDE. INTERSYSTEM CROSSING EFFICIENCY, PHOTOREACTIVITY AND TRIPLET STATE PROPERTIES IN VARIOUS SOLVENTS

Abstract

Abstract— Triplet‐and singlet‐related photoprocesses of pyrene‐1‐aldehyde (PA) in various solvents have been investigated in detail using 337.1 and 355 nm laser flash photolysis in conjunction with time‐correlated determination of fluorescence lifetimes (τF) and steady‐state photochemical and absorption‐emission spectral measurements. In benzene, the lowest triplet of PA (43 < ET < 46 kcal/mol) has a lifetime of about 50 µs (τT) and displays the absorption maximum at 443 nm with a maximum extinction coefficient (εmax) of 21000 M‐1cm‐1; the corresponding ketyl radical has a sharp absorption maximum at 428 nm (εmax≥ 25000 M‐1cm‐1). The quantum yields (φT) of lowest triplet occupation are high in nonprotic solvents (0.6–0.8), decrease in protic solvents (alcohols) as the polarity of the latter is increased, and maintain a complementary relationship with the quantum yields (φF) of fluorescence. Quantum yields (φPC) of loss of PA due to photoreactions in some solvents have also been determined under conditions of steady irradiation at 366 nm; φPC is in the range 0.1–0.2 in electron‐rich olefinic solvents such as cyclohexene and tetramethylethylene. These results concerning τF, τT, φF. φT and φPC as well as the effects of 1,2,4‐trimethoxybenzene and 2,5‐dimethyl‐2,4‐hexadiene as quenchers for fluorescence, triplet yield, and photochemistry are discussed in the light of possible state orders for PA in polar and nonpolar environments.