SOLVENT AND TEMPERATURE EFFECTS ON DUAL FLUORESCENCE IN A SERIES OF CAROTENES - ENERGY-GAP DEPENDENCE OF THE INTERNAL-CONVERSION RATE

Solvent and temperature effects on the dipole-allowed S-2 --> S-0 and the symmetry-forbidden Si --> S-0 transitions of all-trans-carotenes, with 5 (m-5), 7 (m-7), 8 (m-8), 9 (m-9), 11 (all-trans-beta-carotene>, 15 (decapreno-beta-carotene), and 19 (dodecapreno-beta-carotene) conjugated double bonds (N), have been investigated by steady-state and time-correlated single-photon counting (SPC) experiments. The measured fluorescence quantum yields of the S-1 --> S-0 emission (Phi(f1)) decrease from 7 x 10(-3) to about 4 x 10(-6) at room temperature when going from m-5 to beta-carotene. For the longest compounds N = 15 and 19 only the S-2 emission was observed, with fluorescence yields (Phi(f2)) of about 5 x 10(-5). The measured S-1 fluorescence lifetime of m-5, m-7, m-8, and m-9 was found to decrease with decreasing energy gap between S-1 and S-0 (Delta E(S-1-S-0)), in accordance with the energy gap law (EGL). Phi(f2) indicates that the S-2 lifetime is on the order of 100 fs for all compounds. Fluorescence emission from the S-1 state of beta-carotene in room temperature liquids was observed with the 0-0 transition located at 14 200 +/- 500 cm(-1). The intensity ratio I-2/I-1, where I-2 represents the integrated S-2 --> S-0 emission and I-1 the S-1 --> Sd emission spectrum determined by time-resolved methods, depends on the Delta E(S-1-S-0) in a similar way as Phi(f2)/Phi(f1) (=k(r2)k(1)/(k(r1)k(21))). When N increases from 5 to 11, I-2/I-1 (approximate to Phi(f2)/<Phi(>f1)) increases about 2000 times, while the rate of internal conversion between S-1 and S-0 (k(1)) increases by a factor of 300. Thus, the term k(r2)/(k(r1)k(21)) is also affected by N, where k(r2) and k(r1) are the radiative rate constants of the S-2 and S-1 state, respectively, and k(21) is the rate of internal conversion between S-2 and S-1. The solvent polarizability (alpha) affects the dual emission pattern (Phi(f2)/Phi(f1)), as was clearly observed for m-8 and m-9. This is mainly due to an enhancement of k(21) at larger alpha, since the larger the alpha, the smaller is the S-2-S-1 energy gap (Delta E(S-2-S-1)). tau(f1) is about 2-3 times longer for m-7, m-8, and m-9 in 77 K glasses than in room-temperature liquids. The weak temperature dependence indicates that no large-amplitude vibronic motion couples the S-1 and S-0 states. The steady-state anisotropy of the S-1 --> S-0 transition of m-7 and m-8 in 77 K glasses is about 0.38 and 0.37, respectively. At room temperature the anisotropy is lower, as a result of rotational diffusion motion. Because of the short S-2 lifetime, the fluorescence anisotropy of the S-2 --> S-0 transition is always close to 0.4, irrespective of the temperature.