| Photochromic material refers to a type of material that can undergo color change after being excited by a light source.Over the years,they have been the focus of research by inorganic,organic and material chemists,and they are recognized as the most promising functional materials.Azobenzenes are currently the most widely used type of photochromic materials in the world.The photochromic properties of these compounds are excellent,and they have ultra-high storage density and non-destructive information readout characteristics.Under different wavelengths of light irradiation,the molecules which possess two configurations have distinctly different ultraviolet-visible absorption spectra,and they have wide application prospects in many fields,such as light-triggered switches,holographic data erasure,image storage,light control materials and biology.As a result,the photoisomerization reaction mechanism of azobenzene has aroused the intense interest of researchers and has become a hot topic in the theoretical chemistry community in recent decades.In this work,quantum chemical theory was used to study the structure modification and optimization and optical properties of a series of azobenzene derivatives.The geometric structure,absorption spectrum,frontier molecular orbital and single triplet excitation state were studied systematically,as well as the photoisomerization and electro-isomerization mechanisms of different molecules were analyzed to investigate them in essence.It is hoped that our theoretical calculation may pave the way towards the rational design of novel photoswitches that fulfill a set of desired characteristics.The main research content is summarized as follows:1.Theoretical Study of the cis-trans Isomerization Mechanismof a Pendant Metal-bound AzobenzeneIn this work,we have theoretically studied the influence brought by the additional metal complex on AB isomerization and analyzed the thermal cis?trans isomerization mechanism of Re?CO?3-AB.Calculations reveal that the energy transfer process of metal complex occurs due to partial diazo antibonding character of the LUMO orbital,allowing the MLCT band to induce isomerization.We have found that binding metal diimine group to benzene rings for AB drastically lowers the energy barrier of the cis?trans isomerization,making it easier to isomerize.It is concluded that the thermal isomerization of Re?CO?3-AB mainly follows the NNC rotation-assisted inversion mechanism in the S0 state.We also searched the potential energy profiles of the vertical excitation for the excited states along the inversion and rotation pathways.By detailed analysis,we presented a hypothesis that can be accomplished in the following steps:?i?the isomerization can easily travel through the conical intersection?CI?of S0/S1 along the rotation path and then descend to the S0state to reach the trans isomer;?ii?a relaxation from the S1 state to the T1 state may occur,followed by an isomerization reaction through the intersection area of S0-T1-S0.2.Theoretical Study on the Thermal cis-trans Isomerization of Azoheteroarene PhotoswitchesIn this work,a novel class of five-membered azo photoswitches arylazopyrazole derivatives 2,5,8 and 11 has been described based on DFT and TD-DFT methods.For all of them,cis and trans isomers have been calculated and transition states have been searched for.We have demonstrated that the five-membered azoheteroarenes hold significant potential in comparison to their more common AB counterparts.The conformation of the arylazopyrroles is highly dependent on the substitution pattern on the heteroarene,which in turn has a significant impact on the electronic properties of the compounds,such as the more stabilization of the frontier molecular orbitals,decreased energy barriers of cis?trans thermal reversions,effective separation of n??*bands of the trans and cis isomers,and fast isomerization rates,furthermore,azopyrazole 8 has a very long thermal half-life?6.9×104 h?,which is comparable to some of the slowest azo photoswitches reported.These outstanding features for five-membered azoheteroarenes are readily tuned,using steric interactions alone,by choice of the substituents on the heteroaromatic ring.Ultimately,our results reveal that the isomerization reaction is not reached by pure inversion along the angle N1=N2-C3 or C5-N1=N2 but rather by simultaneous rotation around the C5-N1=N2-C3dihedral angle,that is a rotation-assisted inversion mechanism.Azopyrroles 2 and 5also exist the C5-N1=N2-C3 rotation mechanism along the pathway making this pathway the most likely way of isomerization.3.Theoretical Study on the Thermal cis-trans Isomerization of Novel Acylhydrazone PhotoswitchesIn this work,In this work,a novel class of acylhydrazone photoswitches 13,16,11,2,14 and 12 with a broad range of unique properties,depending on their substituents has been analyzed and showcased.For all of them,cis and trans isomers have been calculated and transition states have been searched for.We herein detailed a comprehensive investigation of the acylhydrazone family,and demonstrated that they hold significant potential and uniqueness.The impact of conformational modifications and solvent effects are all very significant for the properties of acylhydrazone family.The conformation of the acylhydrazones are highly dependent on the substitution pattern on the crucial positions of the backbone fragment,which in turn has a significant impact on the electronic properties of the compounds,such as the more stabilization of the frontier molecular orbitals,decreased energy barriers of cis?trans thermal reversions and faster isomerization rates.Especially noteworthy feature is the very long thermal half-life?1.20×105 h and 1.09×105 h?,the optimal structures modeled in this work were acylhydrazone 14 and 12 with EWGs substituents.These outstanding features for acylhydrazones are readily tuned,using steric interactions alone,by choice of the substituents on the R1 and R2 position.Actually,all calculated properties appear corresponding regularity with the increase of the polarity of the solvents.The inclusion of solvents has shown similar trends and we observed that the greater the solvent polarity,the stronger the property,except the absorption spectra,which is not appreciably affected by solvent.Last but not least,our results reveal that the isomerization reaction proceeds through a linear transition state indicative of an inversion mechanism.The transition state,however,is not reached by pure inversion along the N1-N2=C3 angle but rather by simultaneous rotation around the C5-N1-N2=C3 dihedral angle,that is a rotation-assisted inversion mechanism.In view of our theoretical calculation results,it may provide useful guidelines to modify the photophysics of the compounds of interest.4.Theoretical Study of Substituent and Charge Effects on the Thermal cis?trans Isomerization of ortho-Fluoroazobenzenes PhotoswitchesIn a nutshell,the thermal cis?trans isomerization reaction mechanism of neutral,cationic and anionic photoswitch ortho-fluoroazobenzenes with a series of electron-donating and electron-withdrawing substituents have been full-scale analyzed and showcased.Our results can be summarized as following.First,the conformation of the F4-azos is highly dependent on the substitution patterns on the crucial positions of the backbone fragment,which in turn has a distinct impact on the electronic properties.Cationic F4-azos can dramatically decrease reaction energy barrier,and electron withdrawing substituents are especially more preferable ones.In addition,different substituent patterns have significant influence on reaction mechanism,electron donating groups have rotation mechanism,while electron withdrawing groups have inversion mechanism for the cationic F4-azos.Second,extending the?system in the trans-F4-azos caused remarkably red-shifts with electron withdrawing groups in the para-substitution.Moreover,when we introduce a positive or a negative charge,there will be significantly red-shifts and the HOMO-LUMO gap show obvious decline compared to the neutral ones.And then,our work deciphered the mechanism and universality of anterior observations regarding the electrochemical switching of cis-F4-azos.Anionic cis?trans is 1012 times faster than neutral cis?trans,and driven by the buildup of the thermodynamically a more stabilization trans isomer.Finally,through our computational methods,we elucidate that the potential energy surface of the radical anion/cation allows for a dramatically decrease in the reaction energy barrier,it decreases 18.8 and 24.2 kcal mol-1 for anion and cation of thermal cis?trans isomerization compared to the neutral one owing to the change of electron density distribution.Our theoretical calculation may pave the way towards the rational design of novel photoswitches that fulfill a set of desired characteristics. |
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