Computational Approach To The Rate Constants Of Excited-state Deactivation Of Organometallic Compounds

Organic light-emitting diodes(OLED)are one of the research hotspots in solid-state lighting and information display.Since organometallic complexes are able to effectively utilize triplet excitons and emit phosphorescence,which may result in an internal quantum yield of 100%,they have become the most popular second generation OLED materials.Due to its outstanding photophysical properties,for instance,short excited-state lifetime and high quantum yield,Ir(Ⅲ)complexes attract much attention and gain great favor.With the development of theoretical methods,quantum chemistry is playing an increasingly important role in molecular design.Therefore,it is of great significance to understand the excited states of organometallic complexes from a theoretical perspective.In this thesis,based on first-principles calculations,we thoroughly investigated the excited-state deactivation processes of several Ir(Ⅲ)complexes via computing rate constants.Focused on the non-radiative pathways,we discussed their main influencing factors,in the hope of providing insights for molecular design and future theoretical work.Based on the characteristics of organometallic complexes,we constructed an excited-state deactivation model,involving vital deactivation processes – photophysical processes,such as radiative decay and intersystem crossing,as well as a non-radiative photochemical process.We calculated the rate constants of the pathways using Einstein spontaneous emission equation,thermal vibrational correlation function(TVCF),and canonical variational transition state theory(CVT),respectively,which achieved in quantitatively describing each possible deactivation process.With the model in place,we investigated several Ir(Ⅲ)complexes,whose emission color varies from blue to yellow,and comput ed the rate constants and photophysical properties of each complex at different temperatures.It is found that all the rate constants will increase as temperature rises,resulting in an ever decreasing excited-state lifetime;and the photochemical process is much more susceptible to temperature changes.Moreover,both as excited-state deactivation pathways,photophysical and photochemical processes compete against each other,which dominate at low temperature and high temperature,respectively.For complexes with a low energy barrier of photochemical process,it may lead to a biexponential decay phenomenon,which is in well accordance with the experimental observation.However,if the energy barrier exceeds 500 meV(ca.11.5 kcal/mol),photochemical process will hardly have any significant effect at room temperature.Finally,we sought for the main influencing factors of individual deactivation pathway at the microscopic level.Here,we proposed a few key points on molecular design,which are specifically targeted at suppressing non-radiative pathways.On the one hand,the non-radiative photochemical process is noticeably influence by temperature;and hence,controlling it is at high priority,which could be achieved by using N-heterocyclic carbine ligands that has stronger Ir-L bonds.On the other hand,intersystem crossing is also a determining factor for complexes that have small energy gap;therefore,limiting the degrees of freedom of ligands is also an option,which could be achieved by using rigid ligands.