Three-ring chromophores

For all compoimds, the nature of the side-chain has a strong effect on the absorption and emission features. The change from alkyl to alkoxy is accompanied by a 0.25 to 0.35 eV red-shift in the absorption and emission maxima. This re- 

The solution photoluminescence quantum yields of the copolymers are liigh (0.4-0.8) and indicate a great polential in light emission applications [60, 64-69], 

---

Remington SJ, Wachter RM, Yarbrough DK, Branchaud B, Anderson DC, Kallio K, Lukyanov KA (2005) zFP538, a yellow-fluorescent protein from Zoanthus, contains a novel three-ring chromophore. Biochemistry 44 202-212... 

The base value for the diene is 214 nm, because the system is heteroannular (if a homoannular diene were present it would take precedence over the heteroannular diene see the following example).There are three ring residues (or alkyl substituents) attached to the chromophore. Through hyperconjugation, the TT system is slightly extended by this type of substitution. The residues are labeled a, b, and c. Each of these substituents is assumed to add 5 nm to the Xmax of the parent heteroannular diene, for a total of 15 nm. The 5,6-double bond in the B ring marked z is exocyclic to the A ring, so empirically we add an additional 5 nm. Finally, for the thiomethyl substituent at the 3 position we add 30 nm. The total is 214 I- 15 + 5 1- 30 = 264 nm. 

FIGURE 46. Retinal chromophore in bR is attached via a protonated Schiff base to Lys-216 on helix G and is tilted toward the extracellular side. To determine its detailed structure, retinal was selectively deuteriated on the three methyl groups on the cyclohexene ring and incorporated into bR from H. Halobium. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

The chromophore of phenylephrine is not extended but its structure includes a phenolic hydroxyl group. The phenolic group functions as an auxochrome under both acidic and alkaline conditions. Under acidic conditions it has two lone pairs of electrons, which can interact with the benzene ring and under basic conditions it has three. Figure 4.11 shows the bathochromic and hyperchromic shift in the spectrum of phenylephrine, which occurs when 0.1 M NaOH is used as a solvent instead of 0.1 M HCl. Under acidic conditions the X max is at 273 and has an A (1 %, 1 cm) value of 110 and under alkaline conditions the X max is a 292 nm and has an A (1%, 1 cm) value of 182. 

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

The ketone group is a useful model for other types of chromophores because it can be selectively excited in the presence of other groups in polymer chains such as the phenyl rings in polystyrene and so the locus of excitation is well defined. Furthermore there is a great deal known about the photochemistry of aromatic and aliphatic ketones and one can draw on this information in interpreting the results. A further advantage of the ketone chromophore is that it exhibits at least three photochemical processes from the same excited state and thus one has a probe of the effects of the polymer matrix on these different processes by determination of the quantum yields for the following photophysical or photochemical steps l) fluorescence,... 

---