title: Electronic and vibronic contributions to two-photon absorption in donor-acceptor-donor squaraine chromophores.
abstract: Many squaraines have been observed to exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions. Here, the electronic and vibronic contributions to these low-energy two-photon absorptions are elucidated by performing correlated quantum-chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two-photon active states approach those of the lowest energy one-photon active (1B(u)) states. This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these types of chromophores. On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low-lying electronic states of appropriate symmetry for TPA. Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1B(u) state with b(u) vibrational modes. Through implementation of Herzberg-Teller theory, we are able to identify the vibrational modes responsible for the low-energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.
authors.affiliation: School of Chemistry and Biochemistry and Center of Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
authors.forename: Shino
authors.initials: S
authors.lastname: Ohira
authors.forename: Indranil
authors.initials: I
authors.lastname: Rudra
authors.forename: Karin
authors.initials: K
authors.lastname: Schmidt
authors.forename: Stephen
authors.initials: S
authors.lastname: Barlow
authors.forename: Sung-Jae
authors.initials: SJ
authors.lastname: Chung
authors.forename: Qing
authors.initials: Q
authors.lastname: Zhang
authors.forename: Jon
authors.initials: J
authors.lastname: Matichak
authors.forename: Seth R
authors.initials: SR
authors.lastname: Marder
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Brédas
mesh_terms: D003503:Cyclobutanes / Q000737:chemistry*; D004583:Electrons; D010636:Phenols / Q000737:chemistry*; D017785:Photons; D011789:Quantum Theory; D013057:Spectrum Analysis / Q000379:methods*; D014732:Vibration
chemical_list: D003503:Cyclobutanes; D010636:Phenols; C480596:squaraine