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The fluorescence and laser properties of ten aromatic compounds, specially chosen from the p-oligophenylenes (D(2) symmetry) or m-oligophenylenes (C(2) or D(3) symmetry) are studied experimentally (at 293 K) and quantum chemically. The quantum yields, gamma and the decay times, tau(f) of fluorescence are measured for deaerated and non-deaerated cyclohexane solutions. The oscillator strengths, f(e) of the S(0)--S(p) (1A--(1)L(a)) and S(0)--S(alpha) (1A--(1)L(b)) transitions, fluorescence, k(f) and intersystem crossing, k(ST), rate constants, and natural lifetimes, tau(0)(T) are calculated. The lowest 1L(b), 1L(a) and 3L(b) (77 K) levels are determined. It is found that all p-oligophenylenes from p-terphenyl onwards are excellent, photochemically stable laser dyes although the solubility in this series decreases dramatically. On the basis of trends observed in p-oligophenylenes (D(2)-series) and on the properties of the experimentally studied m-oligophenylenes of the C(2)- and D(3)-series, the fluorescence and laser properties of other compounds from these series are estimated/predicted. It is shown, for the first time, that m-oligophenylenes of the C(2)-series, from 1,3-di(p-terphenyl)benzene will acquire fluorescence of 1L(a)--(1)A nature and could be extremely effective laser dyes. It is also shown that m-oligophenylenes of the D(3)-series, from 1,3,5-tri(p-quaterphenyl)benzene will also acquire 1L(a)--(1)A nature fluorescence and laser ability, although this would not be as good as that of compounds in the C(2)-series. It is concluded that m-oligophenylenes can be used not only for passive mode locking but some may also be used as laser dyes and scintillators. The results obtained are important for various practical purposes and theoretical considerations.

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, fe, quantum yield, gamma, decay time, tauf, fluorescence rate constant, kf (Einstein coefficient, A) and intersystem crossing rate constant, kST, 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 kST 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 Sp(1La) and Sinfinity(1Lb) 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.

Fluorescence properties of 36 specially chosen aromatic compounds are experimentally studied at a temperature of 293 K. The compounds are arranged in pairs so that even numbered compounds, because of symmetrical substitutions, are designated a higher symmetry group than those of odd numbered compounds. The pairs are, however, family-related in π-structure. The quantum yield of fluorescence γ, and decay times τf, of deaerated and nondeaerated cyclohexane solutions are measured. The oscillator strength fe, the fluorescence rate constants Kf, natural lifetimes τ0T and intersystem crossing rate constants Kst, are calculated. Investigations showed that transition from a lower to higher symmetry, but family-related in π-structure molecule depending on the nature of the S1 excited state, has different effects on parameters such as γ, τf, fe, Kf and Kst. If the S1 excited state has the nature of the Sp(1La) state, then the value of ν00 decreases. This is accompanied by an increase in the values of γ and τf. Parameters fe and Kf may change unpredictably. However, if the development of π-structure occurs in the direction of polarization of the S0 → S1 transition, the values of fe and consequently Kf are also found to increase. If the S1 excited state has the nature of the Sα(1Lb) state then generally, γ, fe and Kf are found to decrease, with τf increasing. Furthermore, the Kst value in most cases of symmetrical substitution, decreases, sometimes very significantly, though the energy gap between S1 and T1 states becomes less. For example, the transition from 9-phenylanthracene (C2 symmetry group) to 9,10-diphenylanthracene (D2 symmetry group) is accompanied by a decrease in the value of Kst, by a factor of 18. This phenomenon is qualitatively treated using the first-order spin-orbit coupling theory and simulation of singlet and triplet levels.

Absorption, fluorescence and laser properties of ten selected aromatic compounds from the oligophenylene family are studied experimentally at room temperature (293 K). The first eight compounds are arranged in such way that odd numbered compounds reveal 1Lb --1A fluorescence, while even numbered compounds show 1La --1A fluorescence. All compounds are family related in pi-structure and are of the same degree of planarity and rigidity. The quantum yield of fluorescence, gamma, and the decay times, tau(f), of non-deaerated and deaerated cyclohexane solutions are measured. The oscillator strength, f(e), the fluorescence rate constant, Kf, natural lifetimes, tau(o)T, and intersystem crossing rate constant, K(ST), are calculated. The lowest 1Lb and 1La singlet and 3La, triplet (77 K) levels are determined. Investigations showed that transition from a polyphenyl molecule which shows 1La --1A fluorescence to a family related in the pi-structure molecule which reveals the 1Lb --1A fluorescence is accompanied by certain changes in all the fluorescence parameters. This indicates that gamma decreases, tau(f) increases, Kf and the FWRE of the fluorescence spectrum decrease. Moreover, K(ST) also decreases, sometimes very significantly. The decrease in the K(ST) value is explained by the fact thatpsiS alpha/H(SO)/psiTimatrix elements of the spin-orbit coupling of the S alpha and Ti states are much lower in value than analogous elements of the spin-orbit coupling of Sp and Ti states. It is shown that all p-polyphenyles exhibit excellent laser action, while m-polyphenyles do not produce laser oscillation under any conditions. The values of K(ST) and other fluorescence parameters measured can be used for various practical purposes and theoretical considerations.