Twisted chromophores

Figure 11. Hemicyanine dye with a sterically hindered bridge which gives it a twisted chromophore.

While the planar geometries are the ground state minima this is not necessarily so for the excited state, in fact in some cases the excited state has an energy minimum with a perpendicularly twisted chromophore. According to the calculations the ground and excited states for the t-OBF and HT in the neutral form (A) and the cp-OBF in the zwitterionic form come very close to each other. It has been proposed that this can lead to fluorescence quenching nonadiabatic crossing (NAC). 

The two lowest energy minimaareatr = 27°and

headed arrows show the low-energy hula-twist pathway from the planar chromophore to the perpendicularly twisted chromophore. 

Twisted chromophores [50] are even more complex (see Figure 6.3 and Figure 6.4) and various theoretical methods do not agree. This divergence in predictions can he partially attributed to singularities and to parameterization inherent in semiempirical methods hut clearly theory does not provide... 

FIGURE 6.3 Molecular first hyperpolarizability, p, values calculated by semi-empirical INDO methods are shown as a function of twist angle for a twisted chromophore (see upper part of the figure). The maximum predicted... 

Figure 10. Twisted chromophores with outstandingNW responses.

Similarly, this macrocycle adopts a loop conformation in which the whole chromophoric system is twisted. 

Raman intensity in the K spectrum suggests the chromophore has relaxed about its single and/or double bonds. The HOOP Raman bands begin to increase in intensity again after 100 ps when twisting in the chromophore is regained owing to formation of the KL intermediate. ... 

Olsen S, Lamothe K, MartiAAnez TJ (2010) Protonic gating of excited-state twisting and charge localization in GFP chromophores a mechanistic hypothesis for reversible photoswitching. J Am Chem Soc 132 1192-1193... 

In molecules like 2, or 3, the 1,3-diene chromophore is planar. Formally, the presence of R differentiates the two vertical halves of the molecule, which becomes chiral10. The physical meaning of this differentiation is that it induces dissymmetric vibrations, which determine, in turn, a dynamic twist of the chromophore. 

This point of view finds its justification in the following observations. Compounds 8 (pyrocalciferol) and 9 (isopyrocalciferol), having opposite absolute configurations of the stereogenic centres near the dienes, show lA —> 1B Cotton effects at about 275 nm of the same sign and intensity. The reason for this is that only the twist of the chromophore determines the optical activity in fact the diene moieties are distorted in the same sense in 8 and 9, as found by X-ray diffraction16. 

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

Furthermore, for both 28 and for 26, MM calculations indicate that even the individual double bonds are highly twisted (15-20°). This could be (vide infra) a further mechanism giving rise to optical activity of the tt-tt transition of the diene chromophore. 

The planar conformations s-cis and s-trans cannot be appreciably populated owing to the repulsive steric interaction between the Cg—H and C5-methyl (in the s-cis) or between the same Cg—II and the methyl group on (Q in the s-trans). This repulsion is minimized by introducing a twist around the Q,—( 7 bond. Limiting conformations, with skew angles of about 40° and 140°, can be assumed, as shown in Scheme 8. In this way, an intrinsically dissymmetric chromophore is created. 

Direct linkage of aromatic halves by a biphenyl bond is also possible, which leads to a large family of molecular glasses. The twist angle of this linkage is 30°, which couples the halves electronically. However, the extent of this coupling depends on the nature of the chromophore halves. Most compounds of this family are based on the common diaminobiphenyl core (Fig. 3.7). The classical... 

In several examples including nitroanilines, the effect of twisting the chromophore from planarity decreases the absorption intensities. The reasons for the bathochromic effect as the angles of twist in the 4-aniline series increase is subject to discussion. When considering this (as well as in all attempts to obtain definitions of polarity of solvents by quantitative parameters) it is important to exclude or minimize the presence of hydrogen bonding overlapping158 other interactions. 

R. K. Guo and S. Tazuke, Microheterogeneity oflocal segment mobility in solid polymethacrylates as studied by a polymer-bound twisted intramolecular charge transfer chromophore, Macromolecules 22, 3286 (1989). 

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