Alternative selection rules for one- and two-photon transitions in tribenzotetraazachlorin: Quasi-centrosymmetrical π-conjugation pathway of formally non-centrosymmetrical molecule

Abstract

We compare the two-photon absorption (2PA) spectra of non-centrosymmetrical metal-free tribenzotetraazachlorin (H 2 TBTAC) and analogous symmetrical tetra-tert-butyl-phthalocyanine (H 2 TtBuPc). Surprisingly, despite formal lack of center of inversion, the 2PA spectrum of H 2 TBTAC displays a two-photon allowed transition at 935 nm, similar to gerade-gerade (g-g) transitions observed in H 2 TtBuPc and in other symmetrical phthalocyanines. This transition is even better resolved in the singlet-singlet excited-state absorption spectrum. We tentatively explain the survival of the g-g transition in H 2 TBTAC by assuming that the main π -electron conjugation pathway in the tetraazasubstituted tetrapyrrole macrocycle bypasses the outer parts of the two oppositely located isoindole rings and thus renders the optically responsive core of the chromophore quasi-centrosymmetrical. By using the independently measured ground-and excited-state absorption extinction coefficients, we also show that the two-photon absorptivity can be quantitatively explained by a simple threelevel model with the lowest energy Q 1 state serving as an intermediate level.

Bright Long-Lived Luminescence of Silicon Nanocrystals Sensitized by Two-Photon Absorbing Antenna

“…The four lowest energy transitions of mTBT exhibit alternating 1PA/2PA pattern,[53] indicating that the selection rules for centrosymmetrical molecules still prevails in spite of the lack of an inversion center, as observed for other non-centrosymmetrical chromophores. [56] The enhancement of the 2PA of S 2 in TBT vs mTBT may be attributed to the contribution of the static dipole moment terms in eq. (1) considering that the transition dipole moment to S 2 is negligible for mTBT and not for TBT .…”

Linear Photophysics and Femtosecond Nonlinear Spectroscopy of a Star-Shaped Squaraine Derivative with Efficient Two-Photon Absorption

“…This is evidence of vibronic coupling with asymmetrical vibration modes, 11,21,71 which violates the selection rules. 72,73 The most intense short wavelength 2PA maximum with δ 2PA ≈ 8000 GM (at ≈820 nm) corresponded to the spectral range of the lowest 1PA (see insert in Figure 3a) and two-photon allowed S 0 →S n transitions (S 0 and S n are the ground and high excited electronic states of 1, respectively). The nature of the short wavelength 2PA band can be analyzed using the simplest three-level energy model 74 based on the sum-over-states (SOS) approach, 53 when δ 2PA is determined by the expression: 75 with an extremely large maximum extinction coefficient ε(λ ab max ) ∼ 10 6 M −1 cm −1 .…”

Stabilizing g-States in Centrosymmetric Tetrapyrroles: Two-Photon-Absorbing Porphyrins with Bright Phosphorescence

“…Various methods of enhancement of 2PA in tetrapyrroles have been explored over the years, including construction of expanded porphyrins, porphyrin oligomers and arrays, ,,,− adding polarizing substituents to create asymmetric dipolar push–pull-type ,− or symmetric quadrupolar systems, ,,, and extending porphyrin π-system by way of asymmetric aromatic fusion, , as well as using indirect approaches based on 2PA antenna chromophores and subsequent energy transfer onto porphyrins. ,,− In some cases, remarkably high 2PA cross sections have been achieved; however, triplet states in complex dye systems are frequently affected by undesirable quenching pathways (e.g., intramolecular charge transfer), which lead to a decrease or even complete loss of phosphorescence . Furthermore, 2P-active porphyrin-based systems are usually large hydrophobic molecules, lacking intrinsic aqueous solubility and often exhibiting strong tendency to aggregate even in organic solutions.…”