Dimers of Quadrupolar Chromophores in Solution: Electrostatic Interactions and Optical Spectra

Abstract

Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized.

Dimers of polar chromophores in solution: role of excitonic interactions in one- and two-photon absorption properties

“…Anyway, the term V plays the role of an effective interaction energy and can implicitly account for additional delocalization not accounted for in the renormalization of 2z. Indeed, imposing a dipole length of B5 A ˚for the molecular arms and a bridge length of B3 A ˚, and renormalizing the interaction by the squared refractive index, 21,57 we obtain V E 0.6 eV. This rough estimate is of the same order of magnitude as the value 0.3 eV in Table 3, obtained from the spectroscopic analysis, suggesting, once again, that the electronic delocalization in the bridge plays a marginal role.…”

“…The bottom-up approach makes direct use of the parameters determined from the analysis of optical spectra of the isolated chromophore in solution to describe optical spectra of multichromophores leading to a large reduction of model parameters and to a better understanding of interchromophore interactions. 18,21,57,58 Following this idea, we quantitatively describe linear and non-linear optical spectra of 1 and 2 obtaining a very good agreement with experimental data. Minor discrepancies are observed in calculated absorption spectra in non-polar solvents that show a more resolved vibronic structure than experimental ones.…”

“…These studies support a steady increase of the TPA cross section going from simple dipolar D-p-A, to quadrupolar D-p-A-p-D (A-p-D-p-A) and octupolar A(-p-D) 3 or D(-p-A) 3 structures. [15][16][17][18][19][20][21] More recently, work has been focused on more sophisticated molecular structures such as macromolecules, [22][23][24][25][26] dendrimers, 27,28 molecular aggregates, 29,30 multichromophores, 18,21,31,32 and organometallic complexes. 10,[33][34][35] The interest in aggregates and multichromophoric systems arises from the possibility to synthesize TPA efficient and comparatively simple molecular units; their organization in supramolecular structures can be exploited to cooperatively enhance the TPA response via specific interactions.…”

Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye

“…Essential-state models( ESMs)p rovide as imple and physically sound description of CT chromophores [54][55][56] and their aggregates, [57][58][59][60][61] and were effective in rationalizing the different propensityt os ymmetry breaking of various families of quadrupolar and octupolar dyes. [28,62,63] ESMsd escribe each dye in terms of am inimal set of basis states, corresponding to the main resonating structures.…”

Homo‐ and Heterodimeric Dyes for Dye‐Sensitized Solar Cells: Panchromatic Light Absorption and Modulated Open Circuit Potential

“…Careful analysis indicates that the absorption peaks for D sq ‐D sq are 614 nm, and 661 nm and broadened peak (Figure a) in the far‐red region, whereas D sq is having transition at 604 nm and 671 nm. It has been seen that λ max of D sq ‐D sq was 6 nm blue shifted and broadened compared to D sq due to the formation of dye aggregates on TiO 2 surface ,. It is very clear from the UV/Vis spectra that dimeric D sq ‐D sq dye showed broadened spectra compared to monomer D sq .…”