Mixing of Frenkel and charge-transfer excitons in quasi-one-dimensional one-component molecular crystals

摘要

In molecular crystals like MePTCDI and PTCDA the molecule are regularlyarranged creating quasi-one-dimensional molecular stacks. The intermoleculardistance in a stack (about 3.3 A) is comparable with the electron--holedistance in the excited molecule. The mixing between Frenkel excitons (FEs) andcharge-transfer excitons (CTEs) is very essential for the excitonic andvibronic spectra of both crystals. In this paper, we make simulations of thelinear absorption spectra of the abovementioned crystals. The basic Hamiltoniandescribes the FE--CTEs mixing in the molecular stack (point group $C_i$) causedby two transfer mechanisms, notably of the electron and the hole on theneighbor molecules. The vibronic spectra consist of mixed excitons and onevibrational mode of an intramolecular vibration linearly coupled with FE andCTEs. Using the vibronic approach, we calculate the linear opticalsusceptibility in the excitonic and one-phonon vibronic regions of themolecular stack, as well as of a crystal which contains two types ofnonequivalent stacks. We put the excitonic and vibrational parameters for thecrystals of PTCDA and MePTCDI fitted in the previous studies. We analyze thegeneral structure and some important features of the lineshape of the linearabsorption spectra in the spectral region of 15 000--23000 cm^{-1}. We vary thevalues of the excitonic linewidth and the parameters of the linearexciton--phonon coupling and look for the values which reproduce the absorptionlineshape similar to the absorption spectra of the investigated crystals. Ourstudy exhibits the necessity of introducing the FE--CTEs mixing in theinterpretation of the linear absorption spectra, especially of the MePTCDIcrystal.