Theoretical Investigation On The Spectra And Rate Constants Of The Molecular Aggregates Based On The First-principles Calculation

With the advancements of organic optoelectronic materials and devices,a number of novel organic molecular aggregates luminescence phenomena have been discovered,for instance,aggregation-induced emission,ultralong glow room temperature organic phosphorescence,and nano-particle size-dependent optical property.Basically,these novel luminescence phenomena stem from intermolecular interaction and intramolecular structure modifications,through influencing the electronic state,vibronic coupling,as well as excited state decay processes in the aggregates.In order to get deeper understanding of these novel phenomena,we studied the aggregation effect on the photophysical properties of organic materials by first-principles calculations.Firstly,for the crystalline aggregates,we use vibrationic coupled Frenkel-exciton model and quantum mechanics and molecular mechanics(QM/MM)calculations to consider intermolecular excited state interactions,and investigated the vibrationally resolved absorption and emission spectra for a series of organic molecular aggregates.It is found that the competition between the intramolecular vibronic coupling(?)and the intermolecular excitonic coupling(J)dictates the spectral features of the aggregate.We find that at room temperature,the optical spectra lineshape is mainly influenced by the ratio of the exciton coupling with respect to the excited state reorganization energy(?).Namely,only when J/? is larger than a critical value(~0.17),the lineshape of the aggregate starts to deviate from that of the single molecule.Secondly,we investigated the optical spectra for the sexithiophene nano-aggregate by combining a number of computational techniques ranging from molecular dynamics and QM/MM approach to the vibronic coupled exciton model.Stucture wise,we can identify two types of cluster,A and B,formed in the aggregates.Large intramolecular structure distortion was found in cluster A,which leads to blue shifted absorption and emission through breaking conjugation.In addition,irregular intermolecular packing of cluster A leads to weak intermolecular excitonic couplings.While for cluster B,planar monomer and ordered packing structure induces a stronger excitonic coupling and a more H-type like aggregation.Both the intramolecular structure distortion and intermolecular packing irregularity determine the optical spectra for molecular nano-aggregates.Comparing to those in solution,the spectra of aggregates in nanoparticle clusters with distorted monomer and disordered packing structure are shifted to the blue region,stemming from the higher excitation energy and minor excitonic coupling.While for the cluster with planar monomer and ordered packing structure,the blue-shifted absorption is induced by the synergism of vibronic coupling and excitonic coupling.Finally,we present an analytical thermal vibration correlation function formalism to calculate the nonradiative decay rate constant considering excitonic coupling for molecular aggregates basing on split-operator approximation.We find that the nonradiative decay rate constant is always enhanced by excitonic coupling effect.Combining with first-principles calculations,we find the excitonic coupling can increase the nonradiative decay rate constant about 12%-33% for the AIEgens(aggregationinduced emission luminogens).