Isolated chromophores

Riter R E, Edington M D and Beck W F 1997 Isolated-chromophore and exciton-state photophysics in C-phycocyanin trimers J. Phys. Chem. B 101 2366-71... 

Molecules with two or more isolated chromophores (absorbing groups) absorb light of nearly the same wavelength as does a molecule containing only a single chromophore of a particular type. The intensity of the absorption is proportional to the number of that type of chromophore present in the molecule. Representative chromophores are given in Table 7.9. 

Scheme 1-21. Synthesis of a blue-emitting copolymer with isolated chromophore by Wittig reaction a) base.

Electron-Deficient Polymers - Luminescent Transport Layers 16 Other Electron-Deficient PPV Derivatives 19 Electron-Deficient Aromatic Systems 19 Full Color Displays - The Search for Blue Emitters 21 Isolated Chromophores - Towards Blue Emission 21 Comb Polymers with Chromophores on the Side-Chain 22 Chiral PPV - Polarized Emission 23 Poly(thienylene vinylene)s —... 

When a photoprotein solution (1.3 ml) was shaken with ethanol (0.7 ml) containing one drop of concentrated HC1 and then the mixture was extracted twice with 2 ml each of ethyl acetate, about 75% of the chromophore was extracted into the ethyl acetate extract. The chromophore isolated showed an absorption peak at 398 nm in neutral methanol (Fig. 10.2.5). The isolated chromophore was practically non-fluorescent, like the native photoprotein. However, the acidification of a methanolic solution with HC1 resulted in a sharpening and two-fold increase of the 398 nm absorption peak, accompanied by the appearance of fluorescence. In aqueous 0.1 M HC1, two fluorescence emission peaks (595 nm and 650 nm) were found, together with a corresponding excitation peak (400 nm). Treatment of the 398 nm absorbing chromophore with 0.1 M NaOH resulted in a rapid loss of the 398 nm absorption peak. Dithionite did not affect the 398 peak, suggesting that the chromophore does not contain Fe3+. 

A collection of UV spectra of plasticisers, fluorescent whitening agents (optical brighteners), UV absorbers, as well as of phenolic and aminic antioxidants was published by Hummel and Scholl [21]. UV absorbance data for isolated chromophores are listed elsewhere [22]. A general UV atlas of organic compounds is available [23]. 

Voityuk AA, Michel-Beyerle ME, Rosch N (1998) Structure and rotation barriers for ground and excited states of the isolated chromophore of the green fluorescent protein. Chem Phys Lett 296 269-276... 

Absorption bands due to conjugated chromophores are shifted to longer wavelengths bathochromic or red shift) and intensified relative to an isolated chromophore. The shift can be explained in terms of interaction or delocalization of the 7t and % orbitals of each chromophore to produce new orbitals in which the highest ti orbital and the lowest k orbital are closer in energy. Figure 9.8 shows the conjugation of two ethylene chromophores to form 1,3-butadiene. The n—>n transition in ethylene occurs at 165 nm with an s value of 1500 whereas in 1,3-butadiene the values are 217 nm and 2100 respectively. 

As described above, it is probably adequately clear that the vibrational spectroscopy of water is complicated indeed One can simplify the situation considerably by considering dilute isotopic mixtures. Thus one common system is dilute HOD in D2O. The large frequency mismatch between OH and OD stretches now effectively decouples the OH stretch from all other vibrations in the problem, meaning that the OH stretch functions as an isolated chromophore. Of course the liquid is now primarily D2O instead of H2O, which has slightly different structural and dynamical properties, but that is a small price to pay for the substantial simplification this modification brings to the problem. 

A similar situation occurs for the OD stretch region of dilute HOD in H2O, where now the OD stretch functions as an isolated chromophore. In this case, IR... 

Three-pulse echo experiments on isolated chromophores probe the 1 —> 2 transition in addition to the fundamental Therefore, in order to model nonlinear spectra, in addition to the trajectories for mp(t) and (t), one needs the trajectory for this 1 > 2 transition frequency and in addition to the excited... 

Another very informative nonlinear experiment involves a typical pump-probe technique, but with varying laser polarization. These experiments, again for isolated chromophores, measure the rotational anisotropy TCF [122]... 

The system of dilute HOD in H20 is equally good for probing the structure and dynamics of water with an isolated chromophore (in this case the OD stretch), and it may be even better for two reasons. First, in this case the solvent is water, not heavy water and second, the excited state vibrational lifetime of the OD stretch is somewhat longer (1.45 ps [55]) than that of the OH stretch in HOD/ D20, providing a wider dynamic window before effects of local heating due to energy deposition from population relaxation occur. 

Traditional chemosensor (isolated chromophores) Binding of an analyte molecule quenches only the chromophore to which it binds. 

Table 1). This could suggest that there is a relatively high conformational disorder in the polymer, that the adjacent chromophores cannot be regarded as independent, or that the effective donor strength of the amine is different in the isolated chromophores and in the polymer, where it is shared by two chromophoric units.

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 excited-state kinetics of the chromoprotein were found to differ markedly from the one of the isolated chromophore in solution. A strong and fast biexponential decay is observed and seems to sign a specific deactivation channel, still to be properly identified. This process might well be an electron transfer from the chromophore to the protein, as earlier works had suggested [12]. It is additionally possible to suggest that the nonexponential nature of the fast decay could reveal a structural heterogeneity in the oxyblepharismin-protein complex. 

Similar curves are obtained with other synthetic polypeptides, and in most cases they are reasonably independent of the nature of the amino acid side chains. In synthetic polypeptides and proteins the observed Cotton effects do not arise from isolated chromophores but are composite curves resulting from several transitions assigned to the amide bonds in the 200-m/x region. The a-helical curve, for example, results from three optically active absorption bands. One around 222 m/ arises from an n — 7T transition of nonbonding electrons, and the other two at 208 and 191 m/ji are attributed to w — tt transitions parallel and perpendicular to the axis of the helix. These transitions of the a-helix and the resulting Cotton effects characteristic of the a-helix are at present of great interest in interpreting ORD curves of membranes. 

For organic materials, ultraviolet absorption spectra are substantially determined by the presence of functional groups. Identical functional groups in different molecules may not absorb at precisely the same wavelength due to different structural environments which modify the local electric field. The magnitude of the molar extinction coefficient ( e ) for a particular absorption is directly proportional to the probability of occurrence of the particular electronic transition. Spectral features of some isolated chromophoric groups are presented in Table 2... 

A — 1.064 /xm using the instrumentation and data analysis procedures described above. Typical d33 values at zero time were found to be in the range 0.1 -1.0 x 10 esu. These magnitudes agree well with those expected for the chromophore number densities employed (N = 0.4-1.9 x 10 /cm ), assuming literature Mz zzz values for the chromophore and the applicability of an isolated chromophore, molecular gas description of the field-induced chromophore orientation process (7,8). 

Meyer and coworkers investigated the photophysical behavior of vinyl containing Ru(II) and Os(II) complexes electropolymerized into the channels of silica sol-gel modified ITO electrodes. The monomeric complexes, [Ru(vbpy)3]2+ and [Os(vbpy)3]2+ (vbpy = 4-methyl-4/-vinyl-2,2/-bipyridine), have excited state lifetimes of approximately 900 and 60 ns, respectively. Incorporation into the sol-gel pores and polymerization (reductive polymerization initiated at the ITO electrode) results in chromophores that exhibit a remarkably small amount of self-quenching and have domains that reflect relatively isolated chromophores with excited state lifetimes longer than the solution values [125]. 

When one metal ion is used as a donor for sensitizing the emission of a second accepting metal ion, the characteristic lifetimes r of their excited states, which are related to their deactivation rates by r = k l, are affected by the metal-to-metal communication process. This situation can be simply modeled for the special case of an isolated d-f pair, in which the d-block chromophore (M) sensitizes the neighboring lanthanide ion (Ln) thanks to an energy transfer process described by the rate constant k 1 ". In absence of energy transfer, excited states of the two isolated chromophores decay with their intrinsic deactivation rates kxl and kLn, respectively, which provides eqs. (32) and (33) yielding eqs. (34) and (35) after integration ... 

It should be emphasized that solvation of excited electronic states is fundamentally different from the solvation of closed-shell solutes in the electronic ground state. In the latter case, the solute is nonreactive, and solvation does not significantly perturb the electronic structure of the solute. Even in the case of deprotonation of the solute or zwitterion formation, the electronic structure remains closed shell. Electronically excited solutes, on the other hand, are open-shell systems and therefore highly perceptible to perturbation by the solvent environment. Empirical force field models of solute-solvent interactions, which are successfully employed to describe ground-state solvation, cannot reliably account for the effect of solvation on excited states. In the past, the proven concepts of ground-state solvation often have been transferred too uncritically to the description of solvation effects in the excited state. In addition, the spectroscopically detectable excited states are not necessarily the photochemically reactive states, either in the isolated chromophore or in solution. Solvation may bring additional dark and photoreactive states into play. This possibility has hardly been considered hitherto in the interpretation of the experimental data. 

The UV photochemistry of phenol and related systems (such as indole, pyrrole, imidazole) is dominated by a hydrogen detachment reaction which is driven by repulsive 1ira states [33,35 10], For the isolated chromophores, the 1 mr -driven photodissociation has been explored in unprecedented detail by high-resolution photofragment translational spectroscopy [40], The OH (or NH) bond is broken homolytically, resulting in the formation of two radical species, the hydrogen atom and the phenoxy (or indolyl, etc.) radical. Ion pair formation (abstraction of protons) is energetically not feasible for isolated photoacids. 

Figure 3.41 Free energy surfaces S0 and 5, for a frozen solvent situation with the solvent equilibrated to the S0 charge distribution at the Franck-Condon geometry, indicating the lack of a Cl This should be contrasted with Figure 3.40 for the isolated chromophore.

Creed et al. [125]. The 2,6-di-substitution pattern was chosen so that the anthracene moiety would be more likely to be elongated and to act as a mesogen. This polymer has an LC mesophase that is most probably N. Evidence was reported for chromophore association in this polymer even when it is highly diluted in a good solvent such as dichloromethane, but spectral perturbations due to aggregation are especially noticeable in films. Aggregation, as evidenced by a strongly blue-shifted band at 235 nm (the isolated chromophore absorbs at... 

Fig. 19. Potential energy curves and energy relationships in rhodopsin. Curve I Excited state of rhodopsin and bathorhodopsin. Curve II Ground state of rhodopsin and bathorhodopsin. Curve 111 Ground stale of isolated chromophore. Symbols , and 2 are quantum yields for reaching the single potential minimum along the 11,12 torsional coordinate. From Rosenfeld et al. [201].

The major excited-state structural dynamics observed in UMP [173] were ascribed to modes at 1231, 783, 1680, 1396, and 1630 cm-1, in that order, while the major excited-state structural dynamics in the isolated chromophore occur along the 1231, 1680, 1396, and 1630 cm-1 modes in that order, with very minor contributions... 

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