Influence of Molecular Geometry, Exchange-Correlation Functional, and Solvent Effects in the Modeling of Vertical Excitation Energies in Phthalocyanines Using Time-Dependent Density Functional Theory

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• 作者：Victor N. Nemykin ; Ryan G. Hadt ; Rodion V. Belosludov ; Hiroshi Mizuseki ; Yoshiyuki Kawazoe
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：December 20, 2007
• 年：2007
• 卷：111
• 期：50
• 页码：12901 - 12913
• 全文大小：626K
• 年卷期：v.111,no.50(December 20, 2007)
• ISSN：1520-5215

文摘

A time-dependent density functional theory (TDDFT) approach coupled with 14 different exchange-correlationfunctionals was used for the prediction of vertical excitation energies in zinc phthalocyanine (PcZn). In general,the TDDFT approach provides a more accurate description of both visible and ultraviolet regions of theUV-vis and magnetic circular dichroism (MCD) spectra of PcZn in comparison to the more popularsemiempirical ZINDO/S and PM3 methods. It was found that the calculated vertical excitation energies ofPcZn correlate with the amount of Hartree-Fock exchange involved in the exchange-correlation functional.The correlation was explained on the basis of the calculated difference in energy between occupied andunoccupied molecular orbitals. The influence of PcZn geometry, optimized using different exchange-correlationfunctionals, on the calculated vertical excitation energies in PcZn was found to be relatively small. The influenceof solvents on the calculated vertical excitation energies in PcZn was considered for the first time using apolarized continuum model TDDFT (PCM-TDDFT) method and was found to be relatively small in excellentagreement with the experimental data. For all tested TDDFT and PCM-TDDFT cases, an assignment of theQ-band as an almost pure a_1u (HOMO) e_g (LUMO) transition, initially suggested by Gouterman, wasconfirmed. Pure exchange-correlation functionals indicate the presence of six ¹E_u states in the B-band regionof the UV-vis spectrum of PcZn, while hybrid exchange-correlation functionals predict only five ¹E_u statesfor the same energy envelope. The first two symmetry-forbidden n * transitions were predicted in theQ_0-2 region and in the low-energy tail of the B-band, while the first two symmetry-allowed n * transitionswere found within the B-band energy envelope when pure exchange-correlation functionals were used forTDDFT calculations. The presence of a symmetry-forbidden but vibronically allowed n * transition inthe Q_0-2 spectral envelope explains the long-time controversy between the experimentally observed low-intensity transition in the Q_0-2 region and previous semiempirical and TDDFT calculations, which were unableto predict any electronic transitions in this area. To prove the conceptual possibility of the presence of severaldegenerate ¹E_u states in the B-band region of PcZn, room-temperature UV-vis and MCD spectra of zinctetra-tert-butylphthalocyanine (Pc^tZn) in non-coordinating solvents were recorded and analyzed using banddeconvolution analysis. It was found that the B-band region of the UV-vis and MCD spectra of Pc^tZn canbe easily deconvoluted using six MCD Faraday A-terms and two MCD Faraday B-terms with energies closeto those predicted by TDDFT calculations for ¹E_u and ¹A_2u excited states, respectively. Such a good agreementbetween theory and experiment clearly indicates the possibility of employing a TDDFT approach for theaccurate prediction of vertical excitation energies in phthalocyanines within a large energy range.