Investigation Of Merocyanine Dyes Structure, ESIPT And Spectroscopic Properties By Quantum Chemistry

Fluorescent probe is a prerequisite for optical imaging. An ideal molecular probe has characteristics of targeted, non-toxic, high affinity, high stability and generating aggregates in accordance with the amount of target molecule, treating the same target molecules without difference, emitting the long life, high quantum yield, large strength fluorescence. However, there are few probes which have the above advantages and can be used in clinical, so the synthesis of high-performance fluorescent probe still is a hot research. Merocyanine dyes have been fashionable since were found, which possess high stability, large ability to pass through the cell membrane, large fluorescence intensity, high fluorescence, large Stokes shift, etc., and often been used as fluorescent probes. In the excited state, the dyes may occur excited state intramolecular proton transfer and form new isomers, whose emission spectrum will produce new peak, increase intensity, extend the wavelength, and generate obvious redshift. The main contents of this paper are as follows:(1) Excited state intramolecular proton transfer of merocyanine dyeFirst, optimize three merocyanine dyes' configuration and compute vibration frequency at the ground-state with B3LYP/6-31+G* level. Based on the ground state configuration, computing geometry optimization and vibrational frequency with CIS/6-31+G* level at the excited state. No existence of imaginary frequency indicates energy is a minimum. Then, calculate the spectrum in the ground state and excited states on the basis of optimized structures, absorption and emission spectra will be obtained. Finally, place the N-H bond length in the range of 0.8 to 2.0, flexibly scan all optimized geometries. The results show that each dye molecule in the ground state only has one structure(C=O), while in the excited state A1, A3 have two isomers(O-H, N-H) respectively, A2 exists three isomers(C=O, O-H, N-H). Three dyes only possess one absorption peak, A1, A3 has two emission peaks, respectively, A2 has three emission peaks. In the potential energy curve, there are two energy minima value points for A1, A3, respectively, while A2 has three energy minima. All dyes can occur excited state intramolecular proton transfer proved by infrared spectroscopy, Mulliken charge, dipole moment and energy difference of frontier orbitals.(2) Stability and fluorescence quantum yields of cyanine dyesFirst, optimize seven cyanine dyes configuration and compute vibration frequency at the ground-state with B3LYP/6-31+G* level. Based on the ground state configuration, computing geometry optimization and vibrational frequency with CIS/6-31+G* level at the excited state. No existence of imaginary frequency indicates energy is a minimum. Then, natural bond orbital analysis were done for seven dyes. Finally discuss the relationships between dyes' fluorescent quantum yield and molecular structure, such as the radius of rotation, the molecule relative density, total energy, frontier orbital energy. With the bond level, bond dissociation energy and orbital energy difference between frontier orbitals, we get that the stability of C is the lowest, followed by B, D, G. Fluorescence quantum yield is proportional to the radius of rotation, proportional to the total energy of the molecule, inversely proportional to the energy difference of frontier orbital. At last, we get a relational model of fluorescence quantum yield and molecular radius of rotation, relative density, total energy, frontier orbital energy difference.