Linear and Two‐Photon Absorption Properties of Interacting Polar Chromophores: Standard and Unconventional Effects

Abstract

Investigations into the effects of electrostatic interactions on linear and two-photon absorption (TPA) properties are probed using a reference polar chromophore and different related compounds. Compounds in which a passive charge is present near the active chromophore, as well as dimers of the reference molecule, were studied. The combined experimental and theoretical investigation demonstrates that strong modulation of the properties can be attained. In particular, the TPA response of dimers is heavily affected by unconventional excitonic effects, that is, beyond the standard Heitler-London approximation. These effects, which stem from purely electrostatic interactions, lower the nonlinear optical response in the investigated case by an additional amount (up to 20%), and are expected to enhance the TPA cross section in other supramolecular alignments. Electrostatic interactions cannot be overlooked when modelling or investigating highly concentrated and/or confined samples, as are usually needed in many applications. The correct knowledge of their effects can be exploited to guide engineering at the molecular and supramolecular level.

Enhanced Two‐Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments

“…For multichromophoric systems, the increased complexity can be addressed through either the well-known excitonic model (subsection 3.2) or semi-empirical models for interacting chromophores, [127,219] depending on the nature and the strength of interchromophore interactions.…”

“…The resulting two-level model has proved to be adequate to describe the linear and nonlinear absorption of push-pull chromophores. [127,153,221] Molecules with quadrupolar symmetry described by donor-π-acceptor-π-donor or acceptor-π-donor-π-acceptor structures, provide another simple case allowing for a reduced description. For this class of chromophores, a three-state model can be adopted, which is based on the neutral and two zwitterionic (degenerate) states.…”

“…This has already been demonstrated for branched chromophores built from the gathering of either dipolar [108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123] or quadrupolar [97,102,[124][125][126] sub-chromophores via common conjugated core moieties and multichromophore structures in which subchromophores interact only via electrostatic interactions. [127] Alternative routes such as those based on porphyrins, [128][129][130][131][132][133][134][135][136] oligomers and polymers [103,[137][138][139] have been explored as well. The level of complexity has been increased even further by studying dendritic species such as conjugated dendrimers, [115,118,[140][141][142] multichromophoric dendrimers [143] and nanodots.…”

“…[146][147][148][149] Since only a few electronic transitions often predominate in the nonlinear resonant spectra of organic molecules, effective few-state models have become very popular for rational molecular design of NLO-phores. [90,103,127,[150][151][152][153][154][155][156][157][158][159][160] For branched structures, the Frenkel exciton model has been shown to provide a valuable qualitative tool to connect the photophysical properties of branched chromophores to those of their corresponding monomeric counterpart. [113,120,[161][162][163] Theoretical limits for TPA activities have also been explored as well.…”

Cooperative Two‐Photon Absorption Enhancement by Through‐Space Interactions in Multichromophoric Compounds

“…[15] Dipole-dipole interactions were found to be responsible for a reduction of the TPA response per chromophoric unit, while modeling suggested that changing the relative orientation/ distance of the chromophores would allow cooperative TPA enhancement to be achieved. [15] This prompted us to design and investigate multichromophoric molecular structures in which through-space interactions-instead of through-bond interactions [16,17] -would be exploited to increase the TPA response. In contrast to supramolecular chemistry, our aim was to use covalent bonding to confine chromophores and control their number and relative distance/positioning by grafting on suitable platforms.…”