Effect Of Substitution Position On Intramolecular Charge Transfer Property Of Carbazole-pyridine Derivatives

Intramolecular Charge Transfer(ICT)is the process of electron transfer between donor and acceptor groups within a molecule.Donor(D)and acceptor(A)are linked by ?-conjugated bridges to form a D-A type organic small molecule.In recent years,D-A type organic small molecules have been widely used in organic light-emitting diodes,organic solar cells and organic field-effect transistors.The types,numbers and relative positions of electron-donating groups,electron-accepting groups,and ?-conjugated bridges can all affect the properties of intramolecular charge transfer.In our group's preliminary study,we found that the same carbazole-pyridine derivatives can change the intramolecular charge transfer intensity greatly by simply changing the substitution position of carbazole on the pyridine ring.This interesting phenomenon has aroused great interest in our research.Figuring out this problem is beneficial to the deeper understanding of the effects of atomic charges,distances and other factors on intramolecular charge transfer from the atomic level,and provides new ideas for molecular design.We have studied a series of carbazole-pyridine derivatives with a symmetric D-A-D structure centered on the pyridine ring and the carbazole distributed on both sides of the pyridine ring,and have investigated how the relative positions of the electron donor and acceptor in organic small molecules with symmetric structures affect the intramolecular charge transfer strength of small molecules,laying a theoretical foundation for the future development of high-performance D-A organic small molecules.The main contents of the paper are as follows:1.The effect of different substituent positions on the intramolecular charge transfer properties of organic small molecules was investigated using 26-carbazolepyridine derivative(26-CzPy),35-carbazole-pyridine derivative(35-CzPy)and 24-carbazole-pyridine derivative(24-CzPy)molecules.From cyclohexane to acetonitrile solvent,the UV maximum absorption peaks of all three molecules were around 290 nm,and the changes in the maximum absorption peaks of all three molecules were small with increasing solvent polarity,which shows that the electronic structures and properties of their Franck-Condon(FC)excited and ground states did not change much compared to each other.The maximum emission peaks of 26-CzPy,35-CzPy and 24-CzPy were red-shifted by 30 nm,53 nm and 61 nm,respectively,from cyclohexane to acetonitrile solvent,and this obvious solvatochromism phenomenon indicates that intramolecular charge transfer occurs in excited molecules.It is surprising that three molecules,with identical functional groups and only the substitution positions of the two carbazole groups on the central pyridine ring are different,can cause such a big difference in the property of intramolecular charge transfer.2.In order to further investigate the influence of intramolecular charge transfer properties in detail,we have investigated the molecular structure,molecular frontier orbitals and intramolecular charge transfer by using density functional theory(DFT)and at time dependent density functional theory(TD-DFT)methods to explore in depth the influence of molecular structure on intramolecular charge transfer property.The calculation of the difference between the excited and ground state electron densities of three compounds allowed to determine that Electron transfer from carbazole group to pyridine group.We further quantified the intramolecular charge transfer properties of 26-CzPy,35-CzPy and 24-CzPy using the atomic dipole moment correction Hirshfeld(ADCH)method model,and their overall dipole moments are in the following order from largest to smallest: 24-CzPy ? 35-CzPy ?26-CzPy.We obtained experimentally that the excited states of intramolecular charge transfer in three molecules vary by 14 D,19.6 D and 19.6 D,respectively,with respect to the ground state dipole moment.The order of the intramolecular charge transfer intensity of the series of molecules from largest to smallest is: 24-CzPy ? 35-CzPy ? 26-CzPy.This is because the different positions of the substituents lead to a significant difference in the amount of charge transfer from the donor to the acceptor in the molecule,and the charge transfer distances are almost identical,making the overall dipole moment of the molecule different and affecting the intramolecular charge transfer intensity.3.Mechanofluorochromic materials exhibit reversible and dynamic fluorescence transition behavior when stimulated by mechanical forces,and have the advantages of being clean and pollution-free and easy to modulate during the transition process,which has attracted a lot of attention from scientists.Torsional intramolecular chargetransfer molecules are usually torsional between the donor(D)and the acceptor(A),and there exists a tunable torsion angle that is expected to be ideal for mechanofluorochromic materials.To explore this issue,we have investigated the mechanofluorochromic properties of DPAOXDBEN,a 1,3,4-oxadiazole derivative molecule containing a diphenylamine moiety.After grinding DPAOXDBEN changed from cyan to green,and the color returned to cyan after annealing,with the maximum emission peak red-shifted by 32 nm.It may be due to the fact that the molecular conformation tends to be planar after grinding from crystalline to amorphous state,the intermolecular hydrogen bonding is weakened,leading to an increase in the degree of?-conjugation and there is a planar intramolecular charge transfer generated.