Theoretical Study On The Molecular Structure And Electronic Spectra Of Several Organic Systems

With the improvement of the computing performance of computers,quantum chemical calculation has been used in many fields of research and has become an effective method to study the photophysical properties and photochemical reactions of materials.In this paper,the molecular structure and luminescence properties of several organic systems have been analyzed by theoretical methods,and the excited state proton transfer process and fluorescence detection mechanism have been also discussesed.Specific research contents are displayed as follows:（1）A detailed theoretical investigation on the excited state intramolecular proton transfer（ESIPT）directionality and dynamics behavior of 3-（benzo[d]thiazol-2-yl）-2-hydroxy-5-methoxybenzaldehyde（BTHMB）with two asymmetric proton acceptors has been performed.The structure parameters and infrared vibration frequency analysis confirm that the hydrogen bond O₁-H··N or O₁-H··O₂ formed by hydroxyl O₁-H and the proton acceptor N atom or O₂ atom is strengthened in the excited state.The CVB indexes indicate that the strength of hydrogen bond O₁-H··N is stronger,which will drive the proton to the N atom.The constructed potential energy curves confirm that the energy barrier of ESIPT occurring at the N atom is smaller（0.39 kcal/mol）.The dynamics simulation shows that the proton is transferred to the N atom at 48 fs.While the proton is transferred to the O₂ atom at 151 fs and the reverse transfer occurs at 294fs.This result further confirms that the proton turns to the N atom instead of the O₂ atom.From the analyses of charge distribution and surface electrostatic potential,we conclude that the direction of proton transfer is related to the basicity of N atom.This work will provide some help for the research on regulating the direction of proton transfer.（2）The spectral properties of a salicylideneaniline-based fluorescent chemosensor（E）-2-（4-（（4-（diethylamino）-2-hydroxybenzylidene）amino）benzylidene）malononitrile（SB1）and its sensing mechanism for cyanide anion（CN^-）have been studied.Structural parameters show that the intramolecular hydrogen bonds in SB1 and the product SB1-CN are enhanced in the excited state,which induce the intramolecular proton transfer in 114 fs and 167 fs by the results of nonadiabatic dynamics,respectively.The calculation of the electronic spectra indicates that the fluorescence of SB1 at 626 nm is attributed to the proton-transfer tautomer rather than the excited state intramolecular charge transfer state.The emission peak at 523 nm appearing after adding of CN^-is attributed to the proton-transfer tautomer of SB1-CN,which is blue-shifted by 103 nm compared with SB1.Through charge transfer analysis,it is known that the blue shift of fluorescence is caused by the weakening of the charge transfer degree duo to the destruction ofπconjugated structure of SB1 by the addition reaction of CN^-.（3）The detection mechanisms of a bifunctional ratio fluorescent probe2-（4-（benzo[d]thiazol-2-yl）-3-（（tertbutyldimethylsilyl）oxy）benzylidene）malononitrile（HBT-FS）for fluoride（F^-）and sulphite(SO₃^2-)have been investigated.F^-and SO₃^2-make the fluorescence of HBT-FS red-shift and blue-shift,respectively.The reaction sites of F^-and SO₃^2-are determined through the Laplacian bond order and orbital-weighted dual descriptor,respectively.F^-cleaves the Si-O bond in HBT-FS to form the product compound 2 with a hydrogen-bonded six-membered ring and SO₃^2-is added to the C₅site of HBT-FS to form the product compound 3.Potential energy curve and dynamic simulation confirm that the excited state proton transfer（ESIPT）of compound 2 can occur at 155 fs to form a keto structure with an emission at 623 nm,which is consistent with the measured value of 634 nm after adding F^-.Therefore,the fluorescence red-shift of HBT-FS after adding F^-is caused by ESIPT.The calculated fluorescence of compound 3 is at 404 nm,which is in accord with the measured value of 371 nm after adding SO₃^2-.Charge transfer analysis confirms that the intramolecular charge transfer extent is weakened because of the destruction of the conjugated structure of HBT-FS by the addition reaction of SO₃^2-,which induces the fluorescence red-shift of HBT-FS.The research will provide some theoretical guidance for the design of multifunctional fluorescent probes.（4）The excited state intramolecular proton transfer（ESIPT）process of a fluorescent probe2-（3-（3-formyl-4-hydroxystyryl）-5,5-dimethylcyclohex-2enylidene）malononitrile（EP1）and its detection mechanism for cyanide anion（CN^-）have been studied theoretically.Orbital-weighted dual descriptor isosurface and condensed local nucleophilicity indices confirm that the carbon atom on the aldehyde group of EP1 is the addition site of CN^-.Gibbs free energy calculation shows that the reaction of CN^-with EP1needs to overcome the energy barrier of 14.29 kcal/mol and then get the product EP1-CN with a lower energy of 6.61 kcal/mol than the reactant.The structural parameters indicate that the hydrogen bond O₁-H₂···O₃ in EP1 is enhanced in the excited state,and the proton on the O₁-H₂ group in the product EP1-CN is spontaneously transferred to the O₃ atom.The potential energy curves confirm that the proton transfer cannot occur in EP1 because the energies of the ground state and excited state increase with the extension of the O₁-H₂ bond length.While proton transfer in EP1-CN is unobstructed because energies of the ground state decrease with the O₁-H₂ bond length.This confirmes that EP1 detecting CN^-is through the fluorescence variation by enhancing the intensity of charge transfer rather than by hampering ESIPT.（5）The sensing mechanism of the ratiometric fluorescence chemosensor2-（（6-（diethylamino）quinolin-2-yl）methylene）malononitrile（3A）for the cyanide anion（CN^-）has been investigated theoretically.The C₂ site of 3A is the addition site of CN^-determing by the condensed dual descriptor calculation and the energy barrier of the reaction is 8.58 kcal/mol.The large interaction energy of25.75 kcal/mol between 3A and CN^-indicates a high selectivity to CN^-.Compared to 3A,the fluorescence of the addition product is blue-shifted,which is explained by frontier molecular orbital（MOS）and Hirshfeld analysis.