Vibrationally Resolved Electronic Spectra Of Seveal Benzene Ring Aromatic Compounds And Their Application In DSSCs

In this work, equilibrium geometrical parameters of ground and the two low-lying states（1La and 1Lb）, vibrational frequencies, oscillator strengths, vertical and adiabatic excitation energies of indole-like organic molecules were calculated using density functional theory（DFT） and the time-dependent extension（TD-DFT） methods（B3LYP, PBE0, m PW1PW91, BHand HLYP, M06 L, M062 X, CAM-B3 LYP, and LC-BLYP） with 6-311+G（2d, p） basis set. Based on these parameters, vibrational resolved ¹L_b（?）S₀ electronic spectra were simulated using Franck-Condon approximation including Herzberg-Teller（HT） and Duschinsky effects. Based on the experimental data and other theoretical results, most of the vibrational normal modes which emerged in the experimental spectra of the four molecules were assigned tentatively. The HT and Duschinsky effects played important roles in weakly dipole-allowed electronic transitions, leading to the spectral profiles full and rich. In order to explore the correlation of vibronic structure associated with the spectrum and efficiency of dye sensitized solar cells（DSSCs）, a series of metal-free molecules with a donor-Ï€-acceptor（D-Ï€-A） configuration were designed based on the heteroatoms with different atomic sizes and electronegativity on the five-member ring of NKX-2587 or using oligothiophene as Ï€-conjugated linker of NKX-2587. To provide reliable molecule design guideline, we investigated the performance of light harvesting efficiency, charge collection efficiency, charge injection efficiency and dye regeneration efficiency for newly designed dyes relative to their model dyes. Results showed that the low-frequency modes play an important role in enhancing the intensities of the electronic spectra and structures can affect light harvesting efficiency（LHE） of the the heteroatoms with different atomic sizes and electronegativity on the five-member ring of NKX-2587. The maximum absorption wavelengths（λmax） are slightly red-shifted and the spectra broadened with the increasing thiophene units. 1. Vibronic analysis of ¹L_b（?）S₀ transition of indole and 3-methylindole: the influence of Anharmonic, Duschinsky and Herzberg-Teller contributionsThe spectral similarity and difference between indole and 3-methylindole were investigated with density functional theory（DFT） and the time-dependent extension（TD-DFT）. The geometries, energies, dipole moments, oscillator strengths and normal-mode vibrational frequencies of ground and the two low-lying excited states（1Lb and 1La） were calculated and compared with the available experimental and previous theoretical values. The vibrational resolved ¹L_b（?）S₀ electronic spectra were simulated using Franck-Condon approximation including Herzberg-Teller（HT） and Duschinsky effects. Furthermore, the first-order anharmonic correction was also taken into account in harmonic approximation. The HT and Duschinsky effects played important roles in weakly dipole-allowed electronic transitions, leading to the spectral profiles full and rich. The simulated spectra of ¹L_b（?）S₀ of indole and 3-methylindole were in good agreement with the experimental findings, which indicated that the present theoretical method is a useful tool to simulate the optical spectra of the indole-like organic molecules. 2. Vibrationally resolved ¹L_b（¹A′） （?）S₀（¹A′） electronic spectra of benzimidazole and indene: influence of Duschinsky and Herzberg-Teller effects on weak dipole-allowed transitionsGeometrical optimizations of the ground and first excited states of benzimidazole and indene were performed using the density functional theory（DFT） and its time-dependent extension methods（TD-DFT）, respectively. Their vibrationally resolved 1Lb（1A′） ? S0（1A′） absorption and fluorescence spectra were simulated within the Franck-Condon approximation including the Herzberg-Teller（HT） and Duschinsky effects. Calculated results revealed that, with the HT and Duschinsky effects getting involved, the simulated weak 1Lb（1A′） ? S0（1A′） electronic spectra of the two molecules excellently reproduce the experimental findings. Based on the experimental data and other theoretical results, we tentatively assigned most of the vibrational normal modes which emerged in the experimental spectra of the two molecules. The present theoretical insights are expected to help us understand the nature of electronic transitions in the vibrationally resolved absorption and fluorescence spectra of benzimidazole and its analogues. 3. Exploring vibronic contributions affect the light harvesting efficiency through the vibrationally resolved electronic spectra of NKX-2587 derivativesThe vibrationally resolved electronic spectra of five metal-free NKX-2587 derivatives containing heteroatom with different atomic size and electronegativity, were simulated within the Franck-Condon approximation including the Herzberg-Teller and Duschinsky effects, aimed at exploring the correlation of vibronic structure associated with the spectrum and efficiency of dye sensitized solar cells（DSSCs）. Ground and excited state potential energy surfaces were simulated by the density functional theory（DFT） and its time-dependent extension（TD-DFT）. The parameters of short-circuit current density（Jsc） and open circuit voltage（Voc） involving efficiency of DSSCs, such as total dipole moments（μnormal）, the light harvesting efficiency（LHE）, injection driving force（?Ginject）, and the number of electrons in the conduction band（nc）, were calculated and discussed in detail. Through the assignment of the vibrational modes and potential energy distribution（PED） analysis, the vibronic contributions on LHE of NKX-2587 derivatives were investigated. Infrared（IR） and Raman spectra were also simulated to reveal the structural properties of the five dye molecules. Results showed that the low-frequency modes play important roles in enhancing the intensities of the electronic spectra and structures can affect light harvesting efficiency（LHE）. In this sense, our results provided a guidance for understanding the sources of spectral intensities of dye molecules, and a valuable help for rational design of new molecules to improve the energy conversion efficiency（η） of DSSCs. 4. Exploring the effect of vibronic effect on light harvesting efficiency of NKX-2587 derivatives with oligothiophene as Ï€-conjugated linker through vibrationally resolved electronic spectraThe vibrationally resolved electronic spectra of five metal-free NKX-2587 derivatives with oligothiophene as Ï€-conjugated linker, were simulated within the Franck-Condon approximation including the Herzberg-Teller and Duschinsky effects, aimed at exploring the correlation of vibronic structure associated with the spectrum and efficiency of dye sensitized solar cells（DSSCs）. The parameters of short-circuit current density（Jsc） and open circuit voltage（Voc） involving efficiency of DSSCs, such as total dipole moments（μnormal）, the light harvesting efficiency（LHE）, injection driving force（?Ginject）, and the number of electrons in the conduction band（nc）, were calculated and discussed in detail. The effects of both the length of the Ï€-conjugated unit upon the photophysical, electrochemical, and photovoltaic properties of the dyes were also investigated. Results indicated that the maximum absorption wavelengths（λmax） are slightly red-shifted and the spectra broadened with the increasing thiophene units. The intrinsic vibronic contribution plays a relevant role in differentiating the absorption width of the five dyes. In addition, compared with the spectra in vacuum and ethanol, the solvent effect which can broaden the vibronic spectra was discussed. In this sense, our results provided a guidance for understanding the sources of spectral intensities of dye molecules, and a valuable help for rational design of new molecules to improve the energy conversion efficiency（η） of DSSCs.