Formulas of the two molecules studied in this work (the structures shown are the tautomers stable in the ground state in solution at room temperature).
Absorption (1), fluorescence [excitation at (curve 2)], and fluorescence excitation [observation at (curve 3)] spectra of HNAN in MCHX at room temperature. Curve 3 is normalized to the maximum of curve 1. Curve 4 represents the absorption spectrum of HNAQ normalized to the “red tail” of curve 1. Inset: comparison of the absorption spectrum of HNAQ at room temperature (4) and the fluorescence excitation spectrum of HNAN at (5) according to Ref. 13.
Comparison of calculated energies of and transitions of and orbital character (bars) with the absorption spectrum at room temperature (curve). transitions are marked with bars pointing upwards and transitions with bars pointing downwards. Solid line is used for and dashed for transitions. The height of the bar for transitions having zero oscillator strength [all and ] is arbitrary, while for transitions the height of the bar denotes the oscillator strength on a logarithmic scale.
Main molecular orbitals involved in the lowest and excited states of HNAQ [(a) and (b)] and HNAN (c).
Transient absorption spectra of HNAN for selected delays between pump and probe pulses.
Transient absorption spectra of HNAQ for selected delays between pump and probe pulses.
Kinetics of transient absorption of HNAN measured at wavelengths corresponding to each of the characteristic spectral bands. Experimental data are plotted with points, for the clarity of the graph only each 20th point is plotted for delays greater than . Fits of four-exponential decays convoluted with the instrumental function are drawn with solid lines.
Width of the Gaussian instrumental response function (IRF) determined from coherent signals originating from the solvent (dots) together with a polynomial fit of power of 4 (line).
Decay associated difference spectra (DADS) for HNAN after deconvolution.
Diagram of the proposed relaxation routes of an excited HNAN molecule. Straight and wavy arrows denote the observed radiative and nonradiative transitions, respectively.
Comparison of the relative cross section for the stimulated emission of HNAN and the DADS corresponding to the decay time of . The curve plotted for the cross section is scaled and vertically shifted to match the valley in the DADS.
Numerical values of decay times obtained by simultaneous fitting of the convoluted four-exponential functions to the kinetics of HNAN measured at five different wavelengths.
Numerical values of the time constants obtained by global analysis for HNAN and HNAQ.
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