Theoretical Investigations on the Roles of Intramolecular Structure Distortion versus Irregular Intermolecular Packing in Optical Spectra of 6T Nanoparticles

It is of vital importance to theoretically understand unique nanoparticle size-tunable and excitation wavelength-dependent multiple optical properties in organic nanoparticles. In this work, we proposed a theoretical protocol to calculate the optical spectrum of the organic nanoparticles, which combines molecular dynamics (MD) simulation, the quantum mechanics/molecular mechanics (QM/MM) approach, and vibronic-coupled Frenkel exciton spectrum theory. By using the protocol, we explored the relationship between intramolecular structure distortion, irregular intermolecular packing, and optical spectra in α-sexithiophene nanoparticles. Two representative clusters cutting from the simulated amorphous nanoparticle were investigated and found to exhibit a blue shift for absorption and emission spectra compared to the solution, which is totally different from the blue-shifted absorption and red-shifted emission in crystal. For the cluster with distorted monomer and disordered packing, the blue shift results from the higher excitation energy and larger vibronic coupling of low-frequency vibration modes, while for the cluster with planar monomer and ordered packing, the blue shift is induced by the synergism of vibronic coupling and excitonic coupling. Strikingly, the superposition of the spectra of two clusters reproduces the experimental spectra and well explains the unusual blue-shifted emission observed for α-sexithiophene nanoparticles. Our theoretical protocol is general and applicable to other organic nanoparticles, thus aiding the rational design of high-quality organic nanoparticles.