J.B. Birks

The photophysics of trans-stilbene

Abstract The fluorescence lifetime of trans -stilbene in dilute methylcyclohexane/ iso -hexane solution has been measured and the mean S 1 radiative ( k F ), radiationless ( k I ) and cis -isomerization ( k C ) rate parameters have been determined from −90 to 60°C. S i consists of a fluorescent trans ( 1 B u * ) state ( k F 0 = 6.0 × 10 8 s −1 ) which undergoes reversible thermal-activated rotational internal conversion (Δ H = 1.75 kcal mole −1 , Δ S = 10.6 cal deg −1 mole −1 ) to a non-fluorescent perp ( 1 A g * ) state. p ( 1 A g * ) lies 610 cm −1 above t ( 1 B u * ) with an intermediate S 1 potential maximum. p ( 1 A g * ) undergoes internal conversion( k I. ∞ = 5.8 × 10 8 s −1 ) to p ( 1 A g ) leading to cis -isomerization. This is the main isomerization channel over the whole temperature range.

The photo-isomerization of stilbene

Abstract The S0 potential of stilbene has minima in the trans (t) and cis (c) configurations and a maximum in the perp (p) configuration. The S1 potential has minima at t, p and c due to crossing of the 1Bu and 1Ag potentials, and it is proposed that the T1 potential has similar minima due to crossing of the 3Bu and 3Ag potentials. Five alternative isomerization mechanisms are considered, the yields of which depend on competition between vertical radiative and radiationless transitions to lower states, and horizontal radiationless transitions between the t, p and c configurations of S1 and T1.

The fluorescence of all-trans diphenyl polyenes

Abstract The temperature dependence of the fluorescence and fluorescence excitation spectra of all- trans diphenyl hexathene (DPH) and octatetraene (DPO) in six solvents confirms the S 1 ( 1 A g *) and S 2 ( 1 B u *) state assignment, and determines their energy difference Δ E . The S 1 fluorescence rate parameter k F depends on Δ E , the solvent refractive index n , the S 2 ( n = 1) fluorescence rate parameter k F20 (2.23 × 10 8 s −1 for DPH, 2.33 × 10 8 s −1 for DPO), and the S 2 -S 1 coupling matrix element V (745 cm −1 for DPH, 500 cm −1 for DPO). The S 1 fluorescence is induced by 1 B u *- 1 A g * potential interaction (PI), via a b u vibrational mode (≈ 900 cm −1 ), and not by vibronic coupling. The main S 1 radiationless transition, rate parameter k R , is thermally-activated internal rotation through an angle θ about the central ethylenic bond(s). The PI distorts the S 1 (θ) potential surface and thus influences k R .

The fluorescence of diphenyl- and retinol-polyenes

The unusual fluorescence properties of the polyenes are listed. The fluorescence lifetimes and/or quantum yields of the first four all-trans diphenylpolyenes have been determined in seven solvents between −50 and 50°C. A model is proposed to account for the unusual fluorescence behaviour in terms of a low-lying 1Ag state, coupled to 1Bu, and which shifts to lower energies on intramolecular rotation.

Fluorescence analysis of trans-2-styrylnaphthalene in solution

Abstract Trans -2-styrylnaphthalene (2-StN) in n -hexane solution at 20°C is a mixture of two species, whose absorption spectra and fluorescence quantum yields ( q ), lifetimes (τ) and spectra ( F ) are determined, and whose relative abundances are estimated. The major species (≈0.67) has τ = 27.6 ns, q = 0.66 and a structured F . The minor species (≈0.33) has τ = 5.2 ns, q = 0.73 and a more diffuse F . These results are obtained by a general method of determining the fluorescence and absorption properties of two species from those of the mixture.

Horizontal radiationless transitions

Abstract Horizontal radiationless transitions (HRT), due to intramolecular rotation, occur in stilbene and polyene derivatives. It is shown that the relative energies and angular dependence of the 1 B * and 1 A * potentials and their interactions can lead to five distinct types of S 1 and S 2 potential. The fluorescence, HRT and other transitions depend on the potential type, and specific examples of each are considered.

The photophysics of azulene

Abstract The derivation of a consistent set of photophysical parameters for azulene involves the revision of several published values.

Fluorescence of the helicenes

Abstract The fluorescence spectra, quantum efficiencies and lifetimes of hexa-, hepta-, octa- and nona-helicene were observed in 1,4-dioxan solution. The fluorescence is assigned to the 1Lb state, which is obscured in the absorption spectra of the higher compounds.

Fluorescence of the higher helicenes

Abstract The fluorescence spectra, quantum yields and lifetimes of [10]-, [11]-, [12]-, [13]-, and [14]-helicene and 3,15-ethanoheptahelicene have been observed in 1,4-dioxan solution at room temperature. The fluorescence occurs from the 1 L b state, which is not observed in absorption. The fluoroscence rate parameter decreases with increase in ring number N , in a manner which depends on whether N is odd or even. The intersystem crossing rate parameter increases with N up to N = 11 and then decreases. The interval between the fluorescence maxima, which is equated to the S o symmetrical vibrational mode, decreases from 1200 cm −1 ( N = 6) to 650 cm −1 ( N = 13).

Influence of environment on the radiative and radiationless transition rates of the pyrene excimer

Abstract From an analysis of the internal rate parameters of the pyrene excimer in solution in propylene glycol, isopropanol and other solvents it is concluded that (i) the radiative transition rate ( k FD ) 0 , normalized to a medium of n =1, is independent of the solvent, (ii) the temperature-independent radiationless transition rate k ID 0 is due to intersystem crossing, and (iii) the temperature-dependent radiationless transition rate k ID T is due to internal conversion, induced by thermally-activated molecular motion which is subject to viscous restraint by the solvent. For the crystal pyrene excimer the radiative transition rate = ( k FD ) 0 , k ID 0 = 0, and k ID T is due to intersystem crossing.