Multichromophoric systems

M. J. Shephard, M. N. Paddon-Row, Large Predicted Changes in Geometry Accompanying Charge Separation in Various Rigid Multichromophoric Systems in the Gas Phase An ab Initio MO Study , J. Phys. Chem. A 1999,103, 3347-3350. 

Multichromophoric systems consisting of two benzoate chromophores and another chro-mophore are subject to additivity relation in the amplitudes of exciton Cotton effects145, similar to that found for polybenzoate systems (Section 4.4.2.4.). 

Our objective is to understand how the noncovalent interactions responsible for nucleic acid secondary structure (i.e. base stacking and base pairing) affect the photophysics of these multichromophoric systems. Here we describe initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides... 

The dye-loaded dendrimer 110 can be detected by means of single-molecule fluorescence.48,57,281,282 This multichromophoric system was embedded into a thin polymeric film at very low concentrations so that individual dendrimer molecules could be studied by far-field fluorescence microscopy at room temperature. Temporal fluorescence intensity fluctuations, fluorescence spectra, and decay times were measured separately or simultaneously. Collective on/off jumps of the fluorescence intensity as well as different emissive levels were observed for single dendrimer molecules with eight perylenemonoimide chromo-... 

Fig. 33 A schematic example of the formation of a giant photoinduced CS state, +DA1A2A3 , in a multichromophoric system using a redox cascade. Note that each ET event in the cascade is exergonic.

Nevertheless, other chromophores have been investigated and they have provided interesting insights, particularly porphyrin and Cso groups, since these serve as useful mimics of the cofactors present in the photosynthetic reaction center (Figure 37). Electron transfer involving porphyrins and fullerenes will be presented in more detail elsewhere in this Handbook Volume III, Part 2, Chapter 2 and Volume II, Part 1, Chapter 5 respectively), and so only a brief discussion is presented here. An excellent overview of photoinduced ET processes in Cso-based multichromophoric systems has been produced previously [116]. 

Figure 43. Some multichromophoric systems for investigating superexchange through aromatic groups.

One of the basic mechanisms in multichromophoric systems, electronic excitation transfer has been in the past and still is in many studies largely described using Forster theory. As stated by Forster [20], this model is developed for the weak coupling limit as it is based on an equilibrium Fermi Golden Rule... 

Information about the rate constant of hopping ( hopp) through excitation energy transfer can be derived from the fast anisotropy decay time (02)- In order to take into account the possibility of multiple energy transfer channels in the case of a multichromophoric system containing identical chromophores, among which efficient dipole-dipole interactions occur, the measured anisotropy decay time 02 can be related to fchopp by Eq. (5), where the value of i represents the number... 

Two other Forster allowed processes can occur in multichromophoric systems under condition of multiple excitations, namely, singlet-triplet quenching and singlet ion/radical quenching if either the triplet or ion/radical absorption spectra do overlap with the fluorescence spectrum of the donor. These processes were not observed for these systems at the ensemble level because of the low probability of formation of these species resulting in a small relative abundance at the ensemble level. However, at the single molecule level they could be visualized [25, 44],... 

Scholes G D 1999 Theory of coupling in multichromophoric systems Resonance Energy Transfer ed D L Andrews and A A Demidov (New York Wiley) pp 212-43... 

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