Theoretical Investigation On The Photophysical Properties Of Photosensitizers Based On Isoalloxazine

Flavin is the main photoactive substance of biochrome in nature,and isoalloxazine ring is its catalytic core.Flavin and their derivatives（FLs）exist in many redox forms.The excited states can not only participate in the redox reaction through electron transfer,but also have the ability to transfer energy,and participate in the photochemical process with its mild and effective characteristics.Alloxazine（Az）is tautomeric structure of isoalloxazine.Alloxazine and its derivatives（Azs）are complementary in the catalytic process,moreover they can be quickly regenerated during the catalytic process.As photosensitizers,FLs and Azs can catalytic many chemical reactions.In this work,the two compounds are the main objects,and the theoretical calculations based on Density Functional Theory and Time-Dependent Density Functional Theory are used to study and predict photophysical properties of several flavin and alloxazine derivatives.Mainly explore the effect of modification methods such as extendingπ-conjugation,introducing carbonyl functional groups and building electron donor-acceptor dyes on the photophysical properties of organic photosensitizers.The specific findings are as follows:Firstly,1,8-naphthalimide,a chromophore with excellent optical properties,shares a benzene ring with isoalloxazine.According to different synthesis methods,threeπ-conjugated extension compounds,Np FL1,Np FL2,and Np FL3,are obtained.The absorption peaks of the three compounds have different degrees of red shift compared to FL,which is the expected result of extendedπ-conjugation.Moreover,the absorption spectra are not a simple linear addition of the two parts,indicating that theπ-conjugation of isoalloxazine is effectively extended.The theoretical fluorescence emission spectra and the rates of S₁→S₀ process show that the extendedπ-conjugated compounds maintain good fluorescence luminescence characteristics.The non-radiation transition processes corresponding to the fluorescence and phosphorescence luminescence process,the energy consumption(E_reorg)through structural reorganization by intrinsic normal modes of isoalloxazine framework.The introduction of 1,8-naphthalimide does not increase E_reorg.In addition,the carbonyl functional groups reduce the energy difference between S₁ and T₁ states,enhance the spin-orbit coupling,and increase the intersystem crossing rates of the three compounds by 3 orders of magnitude.And the triplet quantum yields of the three compounds are higher than that of isoalloxazine.Secondly,the electronic structure and photophysical properties of a series of N-Methyl and N-Acetyl substituted Az were investigated.The absorption spectra show that amidation will not have a great influence on the distribution of peaks,but AAz13 and AAz3 will greatly reduce the intensities.AAz3Me with methyl substitution at N1 position can greatly increase the intensity of AAz3.Non-radiative decay from singlet and triplet excited states of amidated Az derivatives are dominant over their radiative counterparts.The fast non-radiative decay of excited states can be attributed to the E_reorg through structural reorganization facilitated by intrinsic normal modes of Az framework,as well as their coupling with those of the functional groups.Some functional groups like carbonyl or acetyl can lead to electronic states that have large spin-orbit coupling matrix elements withπ,π^*states,which may enhance ISC.Due to different bonding of N1 and N3 within the Az framework,substitution may result in different photophysical properties of amidated Az compounds.Specifically,functionalization at N1 may help to maintain or even reduce E_reorg and would promote the absorption and radiative decay from excited states.However,the strong coupling of vibrational modes of acetyl at N3 with the intrinsic normal modes of Az framework would contribute significantly to E_reorg,and benefit the non-radiative decay of excited states.Finally,establish electron donor-acceptor（D-A）photosensitizers with FL or Az Me and BODIPY with excellent optical properties.Among them,BODFL and BODAz Me,which are tightly connected by carbon-carbon single bonds,have the structural and photophysical characteristics of typical D-A photosensitizers.The two units of the ground state structures are orthogonal,in line with the typical characteristics of the Spin Orbital Charge Transfer Intersystem Crossing mechanism.In the frontal orbit,Highest Occupied Molecular Orbital are localized on BODIPY,and Lowest Unoccupied Molecular Orbital are localized on FL or Az Me,which has the potential of Photoinduced Electron Transfer.Natural Transition Orbital analysis shows that the S₀→S₂ transitions are charge transfer states.The absorption spectra show that BODFL has a shoulder peak near the strongest peak in the low-polar solvent,which gradually weakens to disappear with polarity increase of solvent,which is accorded with the characteristics of the charge transfer peak of D-A photosensitizers.The fluorescence spectra of BODFL and BODAz Me keep the fluorescence characteristics of BODIPY while showing charge transfer peaks.However,the phosphorescence spectra are more similar to BODIPY because the T₁ states of two are localized on the BODIPY unit.