Photochemical reactions of bifluorenylidene. Part 2.—Flash photolytic investigations

Abstract

A short lived species which absorbs between 360 and 530 nm is produced when a dilute solution of bifluorenylidene in a viscous paraffin solvent is flashed. The transient is assigned to the triplet phosphorescent state since the phosphorescence lifetime and relaxation time of the species at 77 K are identical within experimental uncertainties. The decay of the transient is strictly first order and the data at room temperature can be rationalized in terms of the equation τ^–1_transient=k₃+k₄[A(S₀)] with k₃(paraffin)=(440 ± 25) s^–1 and k₄(paraffin)=(3.7 ± 0.2)× 10⁷ dm³ mol^–1 s^–1. The absence of a second order term in the kinetic equation is apparently due to a compensation of two opposing effects; a decrease of a triplet–triplet annihilation term and an increase of the self quenching term with the disappearance of A(T_R).

On flashing bifluorenylidene in isopropanol, a transient with a lifetime independent of the ground state concentration is monitored. It is assigned to the bifluorenyl radical AH since the amount of transient formed is always proportional to the amount of bifluorenylidene phototransformed by the flash. The rate constant for hydrogen abstraction from isopropanol by AH, k₆, is determined to be 61 ± 7 dm³ mol^–1 s^–1.

In viscous paraffin + isopropanol mixtures, A(T_R) and AH are monitored in succession by kinetic spectroscopy. The rate constant for hydrogen abstraction from isopropanol by A(T_R) in a long chain paraffin solvent is found to be k₅= 550 ± 100 dm³ mol^–1 s^–1.

Various attempts to find direct spectroscopic evidence for the elusive triplet state(s) of lower energy than T_R were unsuccessful. Nevertheless, kinetic spectroscopy showed that the T₁ state of anthracene is efficiently quenched by bifluorenylidene even though the T₁ state of the donor is 60 kJ mol^–1 below the phosphorescent level of bifluorenylidene.

The photoreduction in isopropanol can be sensitized by triplet energy donors such as naphthalene and triphenylene. The quantum yield of the sensitized reaction is smaller than that of the reaction under direct irradiation. This is tentatively discussed in terms of a competition between energy transfer to the T_R state and to the triplet state(s) of lower energy than T_R.