The Study Of The Rotational Potential Energy In The Electronic Excited State And Different Environmental Sensitivity In Cyanine Dyes Derivatives

Recently, with the significant application of fluorescence detection technologies in life sciences and medical diagnosis, cyanines as fluorescent labels for biomolecules such as proteins and nucleic acids, and as environmentally sensitive probes for reporting on local properties like viscosity, polarity, pH, temperature and others are becoming the the focus of attention. Therefore, the mechanism of the relationship between structure and properties of the dye becomes more important. Therefore, the mechanisms of different sensitivity of cyanine dyes derivatives in rotation-restricted environments (DNA or viscosity) have been investigated systemically by density functional theory and tuning the potential energy in the electronic excited state.The fluorescence quantum yields of unsymmetrical dyes (TO-1and TO-3) and symmetrical dyes (Cy3and Cy5) in H2O, DNA and90%glycerol were tested. In DNA micro environment, unsymmetrical dyes (TO) typically exhibit a larger fluorescence quantum yields enhancement than the corresponding symmetrical counterparts (Cy). Cyanine dyes with different methine dyes chain length have different environmental sensitivity. Computational studies on these dyes reveal that the potential energy in the electronic excited state is controlled by C-C bond rotational motions, which causes mainly nonradiative deactivation, according to the activation energies for the rotation. The rotations of different C-C bonds in molecules have much different rotating energy barrier. Symmetrical dyes (Cy) possess obviously higher rotating energy barrier as well as larger energy gap compared to the unsymmetrical dyes (TO). The C-C bond rotation close to quinoline moiety of unsymmetrical thiazole orange (TO) lets the dye possess the most low energy barrier and also the most low energy gap. This rotation plays a major role in reducing fluorescence quantum yields and providing low fluorescent background in the free states of the unsymmerical cyanine dyes.Meso-substituted and unsubstituted cyanine dyes are very different in environmental sensitivities in rotation-restricted environments (DNA or viscosity). The potential energy curves of Cyan46and Cyan2dyes were compared. The C-C bond rotations of the two dyes exhibit that substitution on the bridge is expected to decrease the energy barriers for rotations around different chemical bonds, which play a major role in reducing fluorescence quantum yield in a free state and providing a high environmental sensitivity for the meso-substituted cyanine dyes. Following the stepwise rational approach, we compared the photophysical properties of Cy5, Cy5-CH3and Cy5-CHO through experimental analysis and DFT computations, which further support the proposed mechanism. Basing on this mechanism, we also have tried to design and synthesize meso-substituted Cy3models on (Cy3-CH3and Cy3-Ph). Revealing photophysical properties for Cy3and its analogues will inevitably promote the research on structure and application.A systematically spectroscopic analysis is presented of Cy7, Cy5and Cy3aminocyanine dyes. The important structural modication exhibited significant large Stokes shifts and different viscosity sensitivities in viscous environment. Computational studies on these dyes reveal that the twist ability of amino group plays an important role toward dual fluorescence phenomenon and viscosity response. In low-viscous solvents, the amino group was described as a rotor and was decoupled with the polymethine chain by photo-excited, displaying a long emission peak. In high-viscous solution, the amino group is almost parallel to polymethine chain and the lone-pair of the amino group is in the right orientation for efficient overlap with the polymethine chain, displaying a short emission peak. The overall balance of the rotating ability for amino group and polymethine chain affects the competition between radiative and non-radiative processes in viscous environment.Three carbazole-based hemicyanine dyes with different heterocycles have different background quantum yields, and thereby have different sensitivities on the viscosity detection, which display large Stokes shift and high sensitivity. Computational studies on these dyes reveal the C-C bond rotation close to the carbazole moiety of the three dyes drives the molecule toward low energy gap between ground state and the first excited state, which causes mainly nonradiative deactivation. The oscillator strengths in lowest singlet excitation drop rapidly between0°and95°, which lead to a dark state when fully twisted at95°for these dyes. We have drawn a mechanistic picture at molecular level to illustrate how these dyes work as viscosity-sensitive fluorescent probes. The activation barriers and energy gaps of C-C bond rotation strongly depend on the choice of heterocycle, which play a major role in reducing fluorescence quantum yield in a free state and providing a high sensitivity on the viscosity detection in viscous environment for the carbazole-based hemicyanine dyes.