Photodecarbonylation of α-Diketones:  A Mechanistic Study of Reactions Leading to Acenes

Poly(acene)s are significant compounds for various electronic applications. A clean, one-step synthesis involves α-diketones (2−4), which undergo facile Strating−Zwanenburg photodecarbonylation producing the corresponding poly(acene)s (i.e., anthracene, hexacene, and heptacene, respectively). Compounds 2−4 show weak fluorescence (λ_F = ∼525−530 nm and Φ_F = ∼0.1−0.4%) and phosphorescence (λ_Ph = ∼565−570 nm) and have a small singlet−triplet energy gap (S₁ − T₁ gap, ∼4 kcal/mol) that facilitates rapid intersystem crossing from the singlet to the triplet state. Both the singlet states (τ_S = ∼20−218 ps) and the triplet states (τ_T = ∼370 ps to <7 ns) of 2−4 are short-lived, while the decarbonylation of 2−4 is a rapid process occurring within 7 ns from both the singlet and the triplet manifolds. The nanosecond laser flash photolysis of 4 also reveals the T−T absorption of heptacene (580 nm, τ = ∼11 μs).