The UV Absorption And Emission Properties Of Carbon Dot Models: High-level Theoretical Benchmark Investigations Of Paradigmatic Polycyclic Aromatic Hydrocarbons

Carbon dots(CDs)including carbon nanodots(CNDs),graphene quantum dots(GQDs)etc.,are important new carbon-based nanomaterials with outstanding optical properties(absorption and photoluminescence)and photostability allowing fascinating applications in many fields,such as nano-photocatalysis,optoelectronic and energy devices and bio-applications.Therefore,a large number of experimental studies have been reported to synthesize CDs with various properties.Because CNDs and GQDs are supposed to consist of a predominantly graphitic sp² hybridized carbon core,polycyclic aromatic hydrocarbons(PAHs),conjugated aromatic molecules composed of fused bezenoid rings,can be regarded as model for the theoretical calculation of CDs.Many theoretical investigations for ground states and UV absorption and fluorescence transitions of CNDs and GQDs by means of PAHs models have been carried out.Most of them are based on the TDDFT method.However,it has been reported that TDDFT with several functionals can give incorrect order and even including large deviation of excitation energies for the first two low-lying excited states in some PAH cases.But these difficulties may be avoided by another DFT-based multireference method,the density functional theory/multireference configuration interaction(DFT/MRCI)method as reported.Other than DFT-based methods,many wave function-based ab initio methods,for instance second-order approximate coupled-cluster(CC2),have also been reported with good performance for PAHs.Moreover,some MR methods,including multi-reference singles and doubles-configuration interaction(MRSD-CI),?-complete active space internally contracted second order perturbation treatment(?-CASPT2C)and multiconfigurational quasi-degenerate second-order perturbation theory(MCQDPT2)also describe correctly the first two excited states of pyrene,but only small PAHs are studied in these MR methods with unsatisfying accuracy and thus more MR calculations are still needed.Besides,it is controversial in current literatures that whether double excitation make contribution to excited state calculation of different?-conjugated systems.Overall,the absorption and PL processes of CDs based on paradigmatic PAHs models(pyrene,circum-1-pyrene,coronene,circum-1-coronene,and circum-2-coronene)have been simulated by means of state-of-the-art quantum chemical methods to explain and characterize the electronic transitions.The performance of three commonly used single-reference(SR)methods,TDDFT-B3LYP,TD-Coulomb-attenuating method-B3LYP(TD-CAM-B3LYP)and algebraic diagrammatic construction up to second-order(ADC(2)),and also three MR methods,DFT/MRCI,spectroscopy oriented configuration interaction(SORCI)and strongly contracted-n-electron valence state perturbation theory to second order(SC-NEVPT2)were studied.To the best of our knowledge,it is in the present study the first time that SC-NEVPT2 and SORCI have been applied to excited state calculations of larger PAHs.Unlike most of current studies,higher-lying states were also investigated.Furthermore,the S₁ excited state geometry optimization is performed by means of SR methods and the emission energies are also calculated.In our study,DFT/MRCI was found to perform best for pyrene and coronene in terms of excitation energies by comparing with experimental values.Therefore,DFT/MRCI was used as a benchmark for other approaches where experimental values are unavailable.NEVPT2 and TD-B3LYP methods were found to agree well with experimental or DFT/MRCI results,except for TD-B3LYP which gave incorrect order of the lowest two states in pyrene-based cases.The good performance of TD-B3LYP may be attributed to an error cancellation owing to the increasing importance of a multi-reference description with increasing molecular size both for excited and ground states.SOS-ADC(2)performs reasonably well in terms of excitation energies for smaller systems,but the error increases as the size of the system gets bigger.TD-CAM-B3LYP was found to give the biggest error and also gave incorrect order of the lowest two states in circum-1-pyrene.The natural transition orbital(NTO)analysis for SOS-ADC(2)results indicated that at least two configurations were essential to characterize most of the lower-case electronic states.A strong density increase of dark states was observed in the UV spectrum with size increasing except for the lowest few states which remained well separated.For higher states,the photodynamics based on excitations will be strongly influenced by the density of dark states.In terms of the emission process,geometrical relaxation effects in the S₁ state could be well correlated with the nodal structure of the NTOs involved in the excitation.The emission energies obtained by the SOS-ADC(2)method were close to experimental values in most cases.TD-B3LYP approach gave almost the same emission energies as SOS-ADC(2)except for the state-ordering problem in pyrene-based compounds.Obviously,overestimation of emission energies was found in TD-CAM-B3LYP.