Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy

Abstract

We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)(NPh), NH, OCH, t-butyl, CH, H, F, CF, CN, benzothiazole, and NO, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S → S transitions to higher energy states, above the lowest-energy S → S transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ ∼ 300 GM (1 GM = 10 cm s), for R = NPh. At transition energies overlapping with the lowest-energy S → S transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ < 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S → S transition gradually increases with increasing EW strength, reaching values σ = 700 GM for R = NO, while in the S → S transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S → S transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S → S transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S → S transition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.

Observation of Distinct Two-Photon Transition Channels in CdTe Quantum Dots in a Regime of Very Strong Confinement

“…It is interesting to note that the FOM for the 2nd and 3rd maxima increase with larger QDs, i.e., FOM 2nd = 245 GM/nm 3 and FOM 3rd = 322 GM/nm 3 for CdTe-507 and FOM 2nd = 300 GM/nm 3 and FOM 3rd = 454 GM/nm 3 for CdTe-554. This behavior corroborates our previous results because a reduction of surface defects leads to a decrease in the 2PA forbidden transition strength (as pointed by the FOM for the 1S 3/2 (h) → 1S(e)), while it increases for 2PA-allowed transition, analogous to what occurs in organic chromophores [20,36]. …”

“…Some mechanisms, however, can break the inversion symmetry of the wave functions that describe the electronic states. For example, interactions with a solvent, the Stark transient effect, a magnetic field, surface defects, and structural irregularities, among others, may prevent the wave function from presenting a well-defined parity [17,18,19,20,21]. In this context, one-photon-allowed states may also be accessed via 2PA.…”

Symmetry Breaking in Pyrrolo[3,2‐b]pyrroles: Synthesis, Solvatofluorochromism and Two‐photon Absorption

“…Note that for 8 and 20 , where the NLT data is directly compared to the 2PEF measurement, the spectral shapes are closely matched, but the absolute cross‐section value obtained by NLT appears as a factor 1.5–2 higher, even though both methods used fluorescein in pH 11 aqueous buffer as 2PA reference standard . This discrepancy may be related to the absorption from the excited state, which effectively increases the NLT response but does not affect directly the 2PEF signal . In both cases, the experimental error is on the order of 20–30 %.…”

“…[32] This discrepancym ay be relatedt ot he absorption from the excited state, which effectively increases the NLTr esponse but does not affect directly the 2PEF signal. [33] In both cases, the experimental error is on the order of 20-30 %.…”

New Styryl Phenanthroline Derivatives as Model D−π−A−π−D Materials for Non‐Linear Optics

“…One of the most successful approaches to achieve high coefficients in NLO properties rely on the use of dipolar‐type molecules with a donor−bridge−acceptor (D−π−A) motif and quadrupolar‐type molecules with a D−π−A−π−D structure . The optical properties of these class of compounds depend not only on the optimal choice of the electron donor/acceptor pair but also on the nature of the π‐conjugated spacer; moreover, heterocyclic fragments have been considered to be promising for organic chromophores both as D/A moieties and as elements for the π conjugation ,…”