Photophysical and photochemical properties of 1,4-tetracenequinone in solution

Abstract

Photophysical and photochemical properties of 1,4-tetracenequinone (1,4-TQ), whose lowest singlet (S₁) and triplet states (T₁) are both of π,π^* type, have been investigated in solution by means of emission measurements, a time-resolved thermal lensing technique and nanosecond laser flash photolysis at 295 K. The fluorescence spectrum of 1,4-TQ is shown to shift to the red with increasing solvent polarity. Based on the Lippert–Mataga equation, the change in the dipole moment between S₁ and the ground state (S₀) is estimated to be as large as 10.4 D, which is considered to originate from the aromatic subsystem of 1,4-TQ. The fluorescence quantum yield and lifetime are obtained to be 0.03 and 1.4 ns, respectively, and the quantum yields for the S₁→T₁ intersystem crossing (ISC) and the S₁→S₀ internal conversion are determined to be 0.78 and 0.19, respectively, in CCl₄ at 295 K. The relatively slow ISC rate indicates that the energy level of the T₂(n,π^*) state lies slightly higher than that of the S₁(π,π^*) state. The transient absorption spectrum of triplet 1,4-TQ is measured in CCl₄ at 295 K and its molar absorption coefficient is determined to be 25 300 dm³ mol⁻¹ cm⁻¹ at 440 nm. In CCl₄ , triplet 1,4-TQ is quenched by the ground state 1,4-TQ with a self-quenching rate constant of 5.3 × 10⁷ dm³ mol⁻¹ s⁻¹. Absence of H-atom abstraction from phenol by triplet 1,4-TQ confirms that the T₁ state is π,π^* in nature.