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.

Superfluorescent Squaraine with Efficient Two‐Photon Absorption and High Photostability

“…The largest contribution to the 2PA maximum in the 1 B 3u state comes from NM 12 b 3u (Table 2), which has maximal IR intensity (9696 km mol −1 ≈229.5 D 2 Å −2 u −1 ). The observed relation between IR and 2PA activities has been documented in the literature 53. The shift along this NM, according to the HT mechanism, occurs if the 1 B 3u state acquires a considerable permanent dipole moment (relative to that of the ground state).…”

“…This maximum corresponds to the 2 A g electronically excited state positioned between the two experimental peaks: the first at λ =595 nm of a considerably lower intensity and the second at λ =415 nm with intensity close to that simulated. One cannot exclude the possibility of the existence of a 2PA active electronically excited state, which can be missed or poorly predicted in simulations 53. However, taking into account the excellent agreement between these intensities, we assigned the simulated 2PA maximum at λ =470 nm to the closest experimental peak at λ =415 nm.…”

Using End Groups to Tune the Linear and Nonlinear Optical Properties of Bis(dioxaborine)‐Terminated Polymethine Dyes

“…[28] The calculated transition energies are overestimated (which is typical for this method and can, in part, be attributed to neglect of solvent effects) and the calculated transition dipole moments are also all slightly overestimated. However, the calculations generally reproduce the experimentally observed trends in both quantities well.…”

Molecular design of near infrared polymethine dyes: A review

“…It was established that this transition reveals itself as the first minimum in the fluorescence excitation anisotropy spectra [48] or as the first intense maximum in two-photon absorption spectra. Similarly, twophoton absorption spectra were measured for series of PMDs (see, for example [160][161][162][163][164]). …”