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One- and two-photon absorptions in asymmetrically substituted free-base porphyrins: A density functional theory study
KTH, School of Biotechnology (BIO), Theoretical Chemistry.ORCID iD: 0000-0002-1763-9383
2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 7Article in journal (Refereed) Published
Abstract [en]

Electronic spectra and structures of a new family of free-base porphyrin (H2P) derivatives with 4-(diphenylamino)stilbene (DPAS) or 4,4(')-bis-(diphenylamino)stilbene (BDPAS) asymmetric substituents, recently synthesized and studied by Drobizhev [J. Phys. Chem. B 110, 9802 (2006)] are investigated by density functional theory (DFT) using modern density functionals and the 6-31G(*) basis set. The time-dependent DFT technique is applied for calculations of one- and two-photon absorption spectra, electric and magnetic dipole moments, and for prediction of electronic circular dichroism for these chiral molecules. The four-band absorption spectrum of the H2P molecule (Q(x), Q(y), 0-0 and 1-0 bands) is enhanced in single-bond-linked DPAS. This enhancement is explained by hyperconjugation of the almost orthogonal pi systems and by small charge-transfer admixtures. The effect is much stronger for the double-bond- and triple-bond-linked DPAS and BDPAS substituents where absorption in the Q region transforms into a two-band spectrum. These molecules with ethenyl and ethynyl bonding of the porphyrin and donor substituent show very strong two-photon absorption in the near-infrared region. DFT calculations explain this by more efficient conjugation between the H2P and DPAS (BDPAS) chromophores, since they are almost coplanar: "Gerade" states of the H2P molecule occur in the Soret region and transform into charge-transfer states with nonzero transition moments. They are responsible for the strong two-photon absorption effects. Mixing of excitations in both chromophores explains the broadening of the Soret band. Though the calculated two-photon absorption cross sections are overestimated, the qualitative trends are reproduced and help understanding the whole genesis of spectra of these asymmetrically substituted H2P derivatives.

Place, publisher, year, edition, pages
2008. Vol. 128, no 7
Keyword [en]
sac-ci calculations, excited-states, organic-molecules, charge-transfer, zinc porphyrins, energy-transfer, cross-sections, ionized states, pi-systems, basis-sets
URN: urn:nbn:se:kth:diva-17337DOI: 10.1063/1.2838776ISI: 000253336800013ScopusID: 2-s2.0-42949177661OAI: diva2:335381
QC 20100525Available from: 2010-08-05 Created: 2010-08-05Bibliographically approved

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