Evolution of absorption, fluorescence, laser and chemical properties in the series of compounds perylene, benzo(ghi)perylene and coronene

Absorption, fluorescence and laser properties of perylene, benzo(ghi)perylene and coronene are studied experimentally (under the same conditions) and quantum chemically at room (293 K) and at low (77 and 4 K) temperatures and direct comparison is made between the results for each molecule. All the main absorption and fluorescence parameters such as oscillator strength, f_e, quantum yield, γ, decay time, τ_f, fluorescence rate constant, k_f (Einstein coefficient, A) and intersystem crossing rate constant, k_ST, are measured or calculated. The systems of singlet and triplet levels for these compounds are simulated and analyzed. Triplet states mixing with the lowest singlet S¹ state are determined. The low values of k_ST found are explained. The possible vibronic coupling in the molecule coronene is discussed. The nature of the three fluorescence bands of coronene observed is interpreted. The change in the arrangement of the singlet and triplet levels of the studied compounds is interpreted quantum-chemically. It is found that at room temperature (293 K), only perylene shows laser action, while all three compounds show good laser oscillation at low temperature (<100 K). The differences in the laser properties of these compounds are explained by the inversion of the S_p(¹L_a) and S_α(¹L_b) levels which occurs in the transition from perylene to benzo(ghi)perylene. Chemical properties of the compounds studied are outlined. Linear and quasi-linear fluorescence spectra of perylene and benzo(ghi)perylene, obtained at 77 and 4 K, can be used in the identification of these compounds.