Multichromophore molecules

Another aspect of supramolecular photochemistry concerns the use of covalently linked bichromophoric and multichromophoric molecules. In the example of Figure 8.19 a two-step sequential, photoinduced electron transfer is observed in the trichromophoric species D-D-A. Local excitation of the cyanonaphthalene acceptor results in charge separation first to the amine and then to the methoxyaniline donor in times of a few tens of ns... 

Gudipati, M.S., Exciton, exchange and through-bond interactions in multichromophoric molecules an analysis of the electronic excited states, ]. Phys. Chem., 98, 9750,1994. 

Figure 10 An intensity time trace of a multichromophoric molecule, showing both progressive and digital bleachings and blinking of individual chromophores.

I., Beljonne, D., Champagne, B., Mullen, K., Sauer, M., Hofkens, J. andDe Schryver, F. C. (2004) Multichromophoric dendrimers as single-photon sources a single-molecule study. J. Phys. Chem. B., 108, 16686-16696. 

Figure 14. Confocal fluorescence images of single molecules of (a) the model compound 69 and (c) the multichromophoric dendri-mer 70 in a polyvinylbutyral (PVB) film. The fluorescence intensity transients (fluorescence intensity versus time) exhibit typical on-off behavior for 69 (b) and jumps between different emissive levels for 70 (d). Pictures taken from ref. [48],...

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-... 

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],... 

PMl-substituted hexa-peri-hexabenzocoronene (HBC) 35 was designed as model compound for intermolecular energy and electron transfer studies because of its Dgh symmetry, electronic and self-assembling properties [33-35]. The multichromophore (HBC-6PM1) 35 has six PMl chromophores attached to HBC via a 3 -dodecyl-4, 5, 6 -triphenyl-1,l 2, l"-terphenyl spacer unit (Scheme 8). HBC-6PM1 35 was obtained by Diels-Alder reaction of hexa-[4-(tetradec-l-yn-l-yl)phenyl]-HBC 34 [36] and well-established building block PMI-decorated Cp 29 (as discussed above) in diphenyl ether at 250°C in 58% yield [33-35]. Indeed, electronic excitation of the HBC core of this molecule resulted in efficient energy transfer to the PMl shell [37, 38]. 

The calculated spectra agreed very well with the experimental results (see Fig. 23) and allowed an assignment to specific molecular vibrations. Two main systems of vibronic bands were found. One involved molecular vibrations between 200 and 600 cmT. These were mainly breathing modes of the perylene backbone. The second system extended from 1,300 to 1,650 cm Here, predominantly C-C stretch modes were involved (mixed with CH deformation modes see Fig. 23). The second series of transitions was mainly responsible for the vibronic progression seen in ensemble spectra. They did not involve the imide groups, which explained the similarity of the monomer spectra when the substitution pattern at N is changed, or if the molecules are part of a multichromophoric species. 

Hofkens J, Mans M, Gensch T, Vosch T, Cotlet M, Kohn F, Herrmatm A, Mullen K, De Schryver F (2000) Probing photophysical processes in individual multichromophoric dendrimers by single-molecule spectroscopy. J Am Chem Soc 122 9278... 

Hernando J, Hoogenboom JP, van Dijk EMHP, Garci a-Lopez JJ, Crego-Calama M, Reinhoudt DN, van Hulst NF, Garcia-Parajd MF (2004) Single molecule photobleaching probes the exciton wave function in a multichromophoric system. Phys Rev Lett 93 236404... 

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