Temperature and molecular structure dependences of optical spectra of electrons in liquid diols

Abstract

In 1,2-propanediol (12PD) at 215 K the optical spectrum of e−s shifted unsymmetrically towards higher energy the first 500 ns. Approximating the decay of the low energy side and the growth of the high energy side of the band as first order, the mean decay time at 0.5EAmax (e−s) was τd ≈ 60 ns and the mean growth time at E > EAmax (e−s) was τg ≈ 40 ns. The behavior in 1,3-butanediol (13BD) at 225 K was similar, with τd ≈ 170 ns at 0.5EAmax and τg ≈ 40 ns at E > EAmax. The solvation time constant τs is only slightly greater than τg. The larger value of τd is caused by replenishment of the state at 0.5EAmax by relaxation from lower energy states. In all diols at [Formula: see text] (≥ 298 K for 14BD) the spectra reached their equilibrium shape within 150 ns after the start of the 100 ns radiation pulse. The value of EAmax of e−s in the 1,2-diols is ~0.1 eV greater than those in the other diols and 0.2−0.3 eV greater than that in a primary alcohol at the same temperature. In the secondary–secondary diol 23BD at 296 K, EAmax is 0.6 eV greater than that in 2-propanol. The vicinal diol part of the molecule dominates the intermolecular solvation site of the electron. The temperature coefficient (−dEAmax/dT) increases from 2.6 meV/K in 12 ED to 3.6 meV/K in 23BD. The asymmetry factors Wb/Wr of the e−s bands in the diols are 1.6–2.1, somewhat lower than those in the analogous alcohols, 1.9−2.4. The values of Wr are similar, 0.54 ± 0.04 eV, in all the diols, independent of T, with Wr/EAmax = 0.24 ± 0.02; dWb/dT < 1.0 meV/K. The value of (Gε)max has an upper limit of ~2.5 × 104 e−s/100 eV M cm in the low temperature liquid diols. The value of Gfiεmax is 1.2 × 104 in all diols except 23BD, in which it is 1.3 × 104, independent of T.