Study on charge transfer and photoabsorption characteristics of N-methylpyridinium iodide salt fluorescence probes

To investigate the effect of charge transfer on photoabsorption characteristics of N-methylpyridinium iodide salt-based fluorescence probes, 9B-MVC of asymmetric D-π-A structure together with symmetric A-π-D-π-A structural 9E-BMVC have been investigated using a combination of experimental and theoretical methods. TPA cross-section of symmetric structural 9E-BMVC measured by femtosecond open-aperture Z-scan at 800 nm is 3.3 times larger than that of the asymmetric conjugated structural 9B-MVC, which indicates that the symmetric conjugated structure with two D-π-A subunits obtains great nonlinear enhancement of two-photon absorption (TPA) response, which is closely related to the longer conjugation length. In addition, TPA spectra calculated by Sum-Over-States (SOS) model also indicates obvious redshit, which effective expands the response wavelength region of the N-methylpyridinium iodide salt probes. The excitation processes of the primary states have been visualized in transition density matrix (TDM) and charge density difference (CDD) heatmaps, strong coupling interactions between two D-π-A subunits have been approved in the symmetric A-π-D-π-A structural N-methylphridinium iodide salt fluorescence probe, which is also an important reason for the great nonlinear enhancement of TPA response.