Electronic and Vibronic Contributions to Two‐Photon Absorption in Donor–Acceptor–Donor Squaraine Chromophores

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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 (1Bu) 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 1Bu state with bu 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.

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