Electronic energy transport in aromatic vinyl-polymers: Nonexponential picosecond trapping in poly-(N-vinylcarbazole)

Abstract

The theoretical concept of nonexponential electronic S 1 energy relaxation in nonrandom, polychromophoric polymers has been tested experimentally by means of picosecond time‐resolved fluorescence spectroscopy. For the low‐energy, sandwich‐type excimer E 2 of poly‐(N‐vinylcarbazole), p‐N‐VCz, in dilute liquid solution the fluorescence rise‐profile F E 2 (t), collected at λem =460 nm, has been analyzed in terms of nonconventional relaxation kinetics. A time‐dependent trapping function, k(t)=b+c t − 1 / 2, which reflects both the ‘‘effective’’ diagonal disorder and the pronounced low dimensionality of carbazole hopping sites in the fluid regime has been used in a first attempt to model migrational sampling in a sequence of excited‐state relaxation processes. The kinetic scheme consists of a distribution of transport states {X 1}, a small ensemble of energy‐relaxed monomeric chromophores X 2, and a discrete state of the mobile excimer X 3 (E 2) coupled to X 2. Exact solutions to the δ‐pulse response behavior {X 1}, X 2, and X 3, respectively, can be found which contain typically nonexponential terms of the form of time‐dependent pre‐exponentials A i j (t). The functional forms of A i j (t) as well as their relevancy to picosecond and nanosecond time scales have been demonstrated by synthetic data simulation. The excimer δ‐pulse trial function based upon this scheme has been shown to recover satisfactorily the experimental data. The limitations of the model, the uncertainties of rise curve analysis, in general, and the main problems encountered in rationalizing excited statetransport and trapping parameters in the presence of rotational sampling have been discussed.