Chromophores separation

While this pessimistic view indeed holds true for chromophores separated in a vacuum, such need not be the case if a medium is placed in between the chromophores. Indeed, rapid ET is expected, and is found, to take place if the chromophores are connected by a conjugated network of Jt bonds (vide infra). 

Short-range intramolecular excimers (chromophores separated by 13 carbon atoms or less) are formed exclusively between nearest-neighbor rings (3 atom separation) in PS, P2VN, P1VN, and PVK. 

Chromophores (or oxochromes) are small groups of atoms responsible for characteristic absorptions. By extension, the chromophore in a molecule corresponds to the site responsible for the electronic transition. A chromogene is a species formed by a skeleton on which many chromophores can be found. For a series of molecules containing the same chromophore, the position and intensity of the absorption bands are constant (Table 11.1). When a molecule contains several isolated chromophores separated by at least two single bonds, the overlapping of individual effects is observed. If the chromophores are adjacent to one another, a different situation results. 

The hb-PAEs of hb-P13 and hb-P15 contain NLO-active azo-functionalities, which are soluble, film-forming, and morphologically stable (Tg > 180 °C). Their poled films exhibited high SHG coefficients ( 33 up to 177pm/V), thanks to the chromophore-separation and site-isolation effects of the hyperbranched structures of the polymers in the three-dimensional space (Table 5) [28]. The optical nonlinearities of the poled films of the polymers are thermally stable with no drop in d33 observable when heated to 152 °C (Fig. 8), due to the facile cross-linking of the multiple acetylenic triple bonds in the hb-PAEs at moderate temperatures (e.g., 88 °C). 

Thus, the semi-classical Marcus theory of non-adiabatic ET expresses the ET rate constant in terms of three important quantities, namely Vel, A, and AG°. It therefore follows that an understanding of ET reactions entails an understanding of how these three variables are dependent on factors such as the electronic properties of the donor and acceptor chromophores, the nature of the intervening medium and the inter-chromophore separation and orientation. 

Given the complexity of excimer behaviour believed to be present in all the decays measured it is perhaps not surprising that there are shortcomings in the different kinetic interpretations offered by Equations 3 and 4. Indeed, particularly for BuPBD which is reluctant to form an intermolecular excimer, it is possible to envisage that there exists in Poly(VBuPBD) a distribution of preformed excimer conformations each with differing overlap, molecular constraint, chromophore separation and decay rate. What is interesting is that these features also look to be apparent in Poly (VPPO) even though PPO itself readily forms an intermolecular excimer. Even if just two excimer conformations are present in these polymers the apparent anomalies of Tables I, II and III now start to appear more rational. 

In a recent report, Holden et al (9) postulated that kj2 vras low compared to for polymers with naphthyl chromophores separated by greater than three carbon atoms. The naphthyl group in the backbone of NDI-650 are certainly separated by more than three carbon atoms and thus our results are quite consistent with those of Holden et al (9). 

Compounds with two insulated chromophores, i.e. chromophores separated by one or more methylene groups or meta-oriented about a benzene ring, exhibit absorption in the same region as an isolated chromophore. From Table 4.10 it can be seen that meta-polyphenyls containing 14 benzene rings absorb at nearly the same wavelength as biphenyl. 

The primary ET in RC presents many aspects which are very challenging to understand. Despite the many efforts, as yet unresolved by either experiment or theory are the following features (a) the remarkably fast rate 3 ps) of this electron exchange between two chromophores separated by 17 A (b) the fact that the M branch acts as spectator in the reaction, despite structural similarities and quasi-C2 symmetry between the L and M subunits (c) the role of the accessory bacteriochlorophyll in Van der Waals contact with P and Hi, in the mechanism of ET. 

The first mechanism, called exciton delocalisation, occurs when the inter-chromophore separation is about 1-2 nm, and is dependent on the inverse cube (i.e. r ) of that separation. What happens is that when the chromophore molecules are held rigidly together (as they would be, more or less, in the chloroplast membrane) at about 1-2 nm, their excited states become perturbed enough to form a new set of excited states that are delocalised over the whole array of chromophores. Thus, as soon as an electron is excited, it is delocalised over all the pigment molecules (Figure 3.39). 

Spectroscopy of the Polymers in Solution. The emissions of the pyrene and naphthalene groups attached to the polymers are sensitive to small changes in the chromophore separation distances. A short separation distance (ca 4 to 5 A) can be monitored with pyrene labeled polymers via changes in the features of the pyrene emission, and a longer scale (ca 15 to 50 A) by measuring the extent of non-radiative energy transfer between the two chromophores, either in solutions of the doubly-labeled copolymer or in mixed solutions of pyrene- and naphthalene-labeled materials. 

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