Porphyrins exciton effects

J > D. There is rapid excitation energy transfer between the two porphyrins (exciton model). The values of the zfs and kinetic parameters for the dimer are determined by the monomer values and relative orientation of the porphyrin planes (5-8,24). (Assuming that the porphyrin structure is not modified by dimerization). For instance, with parallel porphyrin planes, the value of D is not affected by dimerization, whereas the effect on E depends on the relative orientation of the in-plane axes of the two rings ( ). 

The exciton effect in aromatic chromophores, e.g., porphyrins, is more complex since excitation leads to two dipoles of the same energy in perpendicular orientations. In stacks one still finds only shortwavelength shifts, but... 

Instead of quinones one may use other porphyrins as electron acceptors. Prominent covalent porphyrin dimers are connected by mei-o-acetylene units. Strong lateral exciton effects are observed in electronic spectra (Fig. 6.5.2) (Officer et al., 1996 Ruhlmann and Giraudeau, 1996). 

The exciton effects observed in absorption spectra of porphyrin monolayers are often different from those in stacked or lateral fibrous assemblies. One usually finds exclusively long-wavelength shifts of the Soret bands or no effect at all. Porphyrins either lie parallel to each other on the subphase (no shift) or are oriented in a slipped stack-of-cards configuration tilted by 10-20° angles against the subphase surfaces, which produce file observed 20-100 nm red shifts (Fig. 6.8.2). 

Dimerization causes shifts of optical absorption and emission bands of the order of several hundred cm (cf. Table I). This suggests that the triplet EPR data must be interpreted in terms of exciton and charge transfer effects. In the case of ZnTCP the effect of dim.erization on zfs values can be accounted for on the basis of rapid triplet excitation transfer between essentially unperturbed porphyrin moieties. If the exciton model applies the principal components of the zfs tensor in the dimer (X, , Z ) can be related... 

The secondary structure of poly(iV-alkynylamides) is influenced by the position of the chiral center and amide group.The position of the chiral center mainly affects the helical pitch, which becomes short when the chiral center is positioned away from the main chain. The stability of the helical structure is also influenced by the position of the amide group. Based on molecular orbital study, it is concluded that poly(iV-propargylamides) with right-handed helical structure display a plus Cotton effect around 390 nm. This is also confirmed by the exciton chirality method using porphyrin as a chromophore. ... 

Presented experimental data reveal that for CdSe/ZnS quantum dots with two ZnS monolayers values follow a monotonous function drastically decaying with the QD core diameter. From the physico-chemical point of view, we conjecture that upon interaction of P with QD surface, the electron wave function may be locally modified (via inductive and/or mesomeric effects [9]) forming a surface local state capable to trap the electron of the photogenerated exciton (Fig. 2A). Thus, we will consider the behaviour of the electron wave function at the interface to the functional pyridyl group of the attached porphyrin. The single-carrier envelope wave functions y/a in a spherical core/shell QD are determined by the Schrodinger equation... 

A number of reports on phthalocyanines and porphyrins have been published. Spectral diffusion and thermal recovery of spectral holes burnt into phthalocyanine doped Shpol skii systems has been examined . An absorption, emission, and thermal lensing research on carboxylated zinc phthalocyanine shows the influence of dimerization on these properties. Fourier transformation of fluorescence and phosphorescence spectra of porphine in rare gas matrices has yielded much structural and electronic state data on this compound . Exciton splitting is an effect which is seen in the spectra of covalently linked porphyrins . A ps fluorescence study of the semirigid zinc porphyrin-viologen dyad has provided evidence for two dyad conformers . Spectral diffusion in organic glasses has been measured by observing the hole recovery kinetics over the time scale of 1 to 500 ms for zinc tetrabenzoporphyrin in PMMA . 

The luminescence quenching for CdSe/ZnS nanocrystals passivated by organic ligands (pyridyl-substituted porphyrins, pyridine, 2,2 -bipyridine, 1,10-phenathroline) has been studied on the basis of steady-state and time resolved measurements in toluene at 295 K. The porphyrin Jt-conjugated macrocycle plays the principal role in non-radiative exciton relaxation in NC-organic ligand composites (via mesomeric effects and possible partial HOMO and LUMO overlap of porphyrin and meso-pyridyl rings). 

Ru(CO) and RhCl porphyrins with a meso-(p-pyridyl) substituent give rise to the formation of macrocyclic tetramers 40 and 41, respectively. The Ru tetramer structures were converted by an excess amount (103-fold) of pyridine into monomers. The Soret band sharpened during the change, showing the effect of excitonic interactions between the cofacially arranged porphyrins [71,72]. 

The photophysics of these six metallacycles has been studied in chloroform [71]. As expected for weakly interacting systems, the absorption spectra of the homonuclear species 9,10 and 9Zn, lOZn are very similar to those of the parent free-base and zinc-porphyrin chromophores in the Q-band region (Fig. 22), except for minor spectral shifts. A prominent difference between the planar and the slipped cofacial macrocycles is found in the Soret band region, in which a clear exciton splitting (of ca. 500 cm ) is present only for the latter compoimds (10 and lOZn). This result is as expected on the basis of the relative center-to-center distance in the two types of metallacycles (10.1 A in the slipped cofacial geometry as compared to 14.1 A in the planar one). The photophysics of the homo-dimers is very similar to that of the corresponding monomeric species. In particular, 9 and 10 exhibit the typical fluorescence of the free-base or zinc-porphyrin units (9 A. ,ax = 655, 716 nm, T = 5.7 ns 10 Amax = 656, 716 nm) and 9Zn and lOZn that of Zn-porphyrins (9Zn Amax = 608, 651 nm, t = 1.1 ns lOZn Amax = 600, 651 nm). The fife-times (9 and 10, 5.5 ns 9Zn and lOZn, 1.04 ns) are somewhat shortened (by 30-40%) with respect to the porphyrin components, as a consequence of the heavy-atom effect of the external ruthenium centers (see above for a detailed account of this phenomenon). 

For example, Kobuke and Miyagi made porphyrins bearing methylimidazole substituents at the 5- and 15-meso positions on the macrocycle.- Both the cis- and trans-atropisomers formed dimers in which Zn- + was pentacoor-dinated. In particular, the rrans-isomer (17) formed an extremely stable dimer (18), even at concentrations used for fluorescence measurements, that is, 10 molL (Figure 14). Dimerization had pronounced effects on the electronic absorption and fluorescence spectra the Soret band was split by 18 nm, and the Q bands were shifted to the red by more than 20 nm. The same was true for the fluorescence maxima. The splitting of the Soret band is usually explained by excitonic interactions. In this case, the interactions between the porphyrins are important, since the amount of splitting observed corresponds to an interaction energy of 1035 cm . ... 

Homogeneous films of [M-TRP](CF3 SOJ) can be easily deposited on glassy carbon or ITO substrates, by dip-coating from a methanolic solution. The transmission UV-Vis spectra generally exhibit a broadened and less intense Soret band, probably due to exciton coupling with neighboring porphyrin rings. This hypothesis is supported by the fact that such effect is virtually absent in the less intense Q bands. 

The porphyrin addition largely decreased the performance of BHJ SCs with limited enhancement in the absorbance spectra broadening (Table 28). The AFM imaging of the films showed that the porphyrin addition to the blend affected morphology of the resulting films. Moreover, electronic properties of the active materials were changed. The porphyrins acted as recombination sites for the excitons within the blend. However, the bulkier the molecule of the mc o-peripheral substituent, the less pronounced was this effect [128]. 

Recently, an example of an efficient BHJ SC was assembled of commercially available components by careful D-A interface optimization (Table 30) [108, 109]. Here, self-assembled porphyrins stacks were used as the charge mediators to assist effective exciton dissociation at the photocathode. This approach allowed con-stmcting BHJ SC with the PCE as high as 7.13 %. 

A novel face-to-face dimeric porphyrin system was reported recently by Lehn et al. [5,6] in which the two porphyrin rings are held apart by [18]-N204-aza-oxamacrocycles (1). Spectroscopic studies showed that the two porphyrin rings interact together via exciton coupling [7]. This effect serves to shorten the excited singlet state lifetime (Xg = 1.22 ns) and to reduce the fluorescence quantum yield with respect to the corresponding monomeric porphyrin. For 1 in acetonitrile, the quantum yield for formation of the porphyrin excited triplet state was only 0.18, compared to a value of 0.67... 

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