Transport of Quasilocalized Excitons in Molecular Crystals

Abstract

Articles you may be interested inEffects of impurity scattering and transport topology on exciton migration and trapping: An experimental study of quasionedimensional molecular crystals A model is presented for electron-phonon coupling in molecular crystals ~nd its effect on transport?f excitons and charge carriers in aromatic hydrocarbon crystals. The model differs from the usual ones In that the coupling is taken quadratic instead of linear in the vibrational coordinates. It is shown that this coupling dominates in aromatic hydrocarbons, leading to large frequency shifts in out-of-plane molecular vibrations on electronic excitation or ionization. The present study is limited to weak intermolecular electronic coupling, implying quasilocalized excitons (or electrons). There are then two limiting mod~s of diffusion, governed by the rate either of exciton transfer between molecules or ~f phonon transfer. In el:her limit, at high temperatures, transport occurs by incoherent jumps between adjacent molecules (hOppll~g) and increases or decreases gently with temperature, while at low temperatures, coherent transport decreaSIng rapidly with increasing temperature occurs. The diffusion coefficient is larger for normal than for deuterated aromatic hydrocarbons except for hopping transport in the slow-exciton limit. The observed temperature dependence and deuterium effect of triplet exciton diffusion in the c' direction of anthracene are reproduced by the model in this case. The transition from hopping to coherent diffusion is not readily observable for anthracene, but may be observable for naphthalene or benzene. Preliminary results indicat~ that t~e. ~od.el also accounts satisfactorily for the magnitude and temperature dependence of charge-carner moblhtles In anthracene.

A model calculation on the transport of excitation energy in a molecular crystal

“…Munn and Siebrand [30] have shown that a one-dimensional crystal with a single impurity has three types of normal modes: (1) symmetric, (2) antisymmetric, and (3) localized (around the impurity site). If the number of sites is n, then there are (n -1)/2 symmetric modes, (n -1)/2 antisymmetric modes, and one localized mode ( Table I).…”

“…Three parameters, namely J (electronic coupling between adjacent molecules), wl (vibrational coupling between adjacent molecules), and w2 (exciton-phonon coupling) are important in the consideration of excition diffusion [30]. For quasilocalized or slow excitons in a molecular crystal J is very much smaller than w , (the slow exciton limit).…”

“…In (12), &,k = Eek -E,, where E, is the energy of the ground state. Interactions among different modes and different bands are usually neglected while describing excitonic motion in a molecular crystal [19,30].…”

Two-level system coupled to phonons: Full analytical solution

“…While very simple, this model is not trivial and already allows studying a variety of phenomena that include spectral lineshapes [9], electron transfer [10,11], electron-phonon interaction in quantum dots [12,13], and quantum control [1,[14][15][16][17][18][19][20][21][22]. The independent boson model fundamentally deals with the consequences of electronic excitations that induce a spatial shift in the vibrational modes [23][24][25], and in some materials like aromatic hydrocarbons, a frequency change [26][27][28][29][30][31][32]. Instead of using the physically relevant phonons that depend on the occupied level of the electronic system, i.e.…”

Phosphorescence microwave double resonance in exciton states of molecular crystals and coherent states of triplet spin ensembles

“…r( k). tim es the group velocity of the wavepacket : (3) l(k) is thus equivalent to a mean-free path and r(k) corresponds to a correlation time for the wave vector state kor linear combination of k states at an energy E;(k) associated with the zeroth order state. From a dynamical point of view the important feature of coherent migration is that excitons can propagate in the crystal at a variety of velocities and a variety of distances depending upon the k states populated.…”