Excited states chromophores

Chen KY, Cheng YM, Lai CH et al (2007) Ortho green fluorescence protein synthetic chromophore Excited-state intramolecular proton transfer via a seven-membered-ring hydrogen-bonding system. J Am Chem Soc 129 4534 -535... 

Fig. 7 EET in the CC P4 including solvent induced modulations. Shown are the chromophore excited state populations, blue curve rn = 1, red curve m = 2, black curve m = 3, green curve rn = 4. Upper panel averaged populations (across a time slice of 10 ps), lower panel non-averaged populations in a 5 ps time window.

Fig. 11 Normalized time and frequency resolved emission spectrum of the CC P4. A 6 ps time averaging has been carried out to mimic the apparatus function of the single photon detector. Radiative and non-radiative decay has been accounted for by a common chromophore excited-state life time of 5 ns.

It describes single chromophore excited state decay where the statistical operator Rme defines intra chromophore vibrational equilibrium in the excited electronic state. The whole mefl has to be taken at time argument t — t and, then, to be multiplied to A (f,f k) in Eq. (68). 

Triads containing two Ru(II)(terpy)2 end groups connected by one or two ethynyl spacers to a central Co(terpy)2 moiety have been described by Ziessel, Harriman, and co-workers [77]. Upon excitation of the ruthenium chromophore, excited-state electron transfer from the central cobalt site occurs, as shown by spectral identification of the transient species. The electron transfer thus occurs at a distance of 15 A. 

Scheme 34. Mechanism for pyrimidine dimer cleavage by blue photolyase and fully reduced enzyme (163). FIH-, FAD neutral blue radical FIH2, FADH2 SC, second chromophore , excited state due to absorption of light +, radical cation radical anion fT, pyrimidine dimer 2T, repaired pyrimidine monomers.

Organic photochemistry deals with exploring and generalizing the excited state behavior of various chromophores. Excited state behavior of most other chromophores is understood on the basis of carbonyls, olefins, enones, and aromatics, which have played leading roles in this process. In Chapter 3, Nau and Pischel highlight the excited state behavior of azoakanes by comparing them directly with alkanones. 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

Excited-State Relaxation. A further photophysical topic of intense interest is pathways for thermal relaxation of excited states in condensed phases. According to the Franck-Condon principle, photoexcitation occurs with no concurrent relaxation of atomic positions in space, either of the photoexcited chromophore or of the solvating medium. Subsequent to excitation, but typically on the picosecond time scale, atomic positions change to a new equihbrium position, sometimes termed the (28)- Relaxation of the solvating medium is often more dramatic than that of the chromophore... 

The UV spectra of thiirane 1-oxide and (15,25)-(+)-2-methylthiirane 1-oxide show a broad maximum at about 205 nm (e —23 000). The latter shows a positive Cotton effect at low energy followed by a negative effect at high energy. The lowest excited states of thiirane 1-oxide involve excitations from the two lone pairs of the oxygen atom (79G19). 2,3-Diphenylthiirene 1-oxide and 1,1-dioxide show absorption due to the 1,2-diphenyl-ethylene chromophore. 

It is generally believed that the absorption (and fluorescence excitation) peak at about 400 nm is caused by the neutral form of the chro-mophore, 5-(p-hydroxybenzylidene)imidazolin-4-one, and the one in the 450-500 nm region by the phenol anion of the chromophore that can resonate with the quinoid form, as shown below (R1 and R2 represent peptide chains). However, the emission of light takes place always from the excited anionic form, even if the excitation is done with the neutral form chromophore. This must be due to the protein environment that facilitates the ionization of the phenol group of the chromophore. This is also consistent with the fact that the pACa values of phenols in excited state are in an acidic range, between 3 and 5 (Becker, 1969), thus favoring anionic forms at neutral pH. 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

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