Determination of Fluorescence Polarization and Absorption Anisotropy in Molecular Complexes Having Threefold Rotational Symmetry

Abstract

The current work concerns investigation of the polarization properties of complex molecular ensembles exhibiting threefold (C,) rotational symmetry, particularly with regard to the interplay between their structure and dynamics of internal energy transfer. We assume that the molecules or chromophores in such complexes possess strongly overlapped spectra both for absorption and fluorescence. Such trimeric structures are widely found in biological preparations, as for example the trimer of Cphycocyanin (C-PC). Higher order aggregates, e.g. hexamers and three-hexamer rods, are also investigated and compared with the trimer case. The theory addresses both steady-state and 6-pulse excitation and establishes some links between them. Monochromophoric, bichromophoric and trichromophoric molecular complexes are individually examined.For steady-state excitation, analytical formulas are reported for the degree of fluorescence polarization and absorption anisotropy. It is shown that the polarization is dependent on the chromophore inclination relative to the symmetry axis, the relative efficiencies of absorption and fluorescence by chromophores of different spectral types, and the rates of energy equilibration. To assess the validity of the theory, it has been applied to C-PC aggregates. Here it was found that different C-PC aggregates provide practically identical polarization response. For S-pulse excitation we give analytical formulas for determination of the fluorescence depolarization, and also the depolarization associated with absorption recovery, both for a monochromophoric trimer and some particular cases of bichromophoric trimer. More complicated systems are analyzed by computer modeling. Thus it transpires that the initial polarization anisotropy r(t = 0) takes the value 0.4 for all considered aggregates; the long-time limit r(t -+ m) has about the same value as is associated with steady-state excitation. We also show that with steady-state excitation the degree of fluorescence polarization is practically equal for various C, aggregates of C-PC, and that the major factor determining the polarization is the chromophore orientation relative to the symmetry axis.

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

“…The reason for this difference could lay in a deviation of the directions of DSB transition dipoles from the directions of DSBnitrogen bonds in the plane of the principal axis of the molecule. Such deviations lead to a different value of residual anisotropy 60 ͑an inclination angle relative to the N-C plane of about 15.8°could lead to the residual anisotropy of 0.06 observed in our experiments͒. This result suggests the transition dipole arrangement in this molecule in solution to slightly deviate from planar geometry.…”

Charge‐Transfer Interactions in Tris‐Donor–Tris‐Acceptor Hexaarylbenzene Redox Chromophores

“…[32] This effect can be observed in symmetry-broken or asymmetric chromophores, in which energy-transfer between chromophoric branches occurs. This value is somewhat lower than the expected r = 0.4 for parallel transition moments, which may be due to residual molecular motion in the sucrose octaacetate matrix at room temperature.…”

Electrochemistry and Photophysics of Donor‐Substituted Triarylboranes: Symmetry Breaking in Ground and Excited State

“…In case of the D 3 -symmetric TAB 3 the excitation anisotropy is low (< 0.1) over a wide spectral range, but increases significantly for the low-energy CT transition. This red-edge excitation effect has been observed before [57,58] and is due to an intramolecular excitation transfer between the three chromophore subunits of 3 in combination with symmetry breaking of the molecule. For molecules with a threefold symmetry (e.g., triphenylene) in which the transition moments are arranged within one plane, the additivity law of anisotropy yields a value of r 0 = 0.1.…”