Excited state relaxation

Weiner A M and Ippen E P 1985 Femtosecond excited state relaxation of dye molecules in solution Chem. Phys. Lett. 114 456-60... 

The lifetime of an analyte in the excited state. A, is short typically 10 -10 s for electronic excited states and 10 s for vibrational excited states. Relaxation occurs through collisions between A and other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational deactivation, or nonradiative relaxation, the excess energy is released as heat thus... 

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 requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Siemiarczuk A, Ware WR (1987) Complex excited-state relaxation in p-(9-Anthryl)-N, N-dimethylaniline derivatives evidenced by fluorescence lifetime distributions. J Phys Chem 91 3677-3682... 

Excited-state relaxation, in photochemical technology, 19 109-111 Excitons. See also Frenkel exciton in double heterostructure OLEDs, 22 217... 

Picosecond Excited-State Relaxation of Some Iron Porphyrins and Hemoproteins... 

STRAUB AND RENTZEPIS Picosecond Excited-State Relaxation... 

The metaiioporphyrins form a diverse class of molecules exhibiting complex and varied photochemistries. Until recently time-resolved absorption and fluorescence spectroscopies were the only methods used to study metailoporphyrln excited state relaxation in a submicrosecond regime. In this paper we present the first picosecond time-resolved resonance Raman spectra of excited state metaiioporphyrins outside of a protein matrix. The inherent molecular specificity of resonance Raman scattering provides for a direct probe of bond strengths, geometries, and ligation states of photoexcited metaiioporphyrins. 

See also Iron hemoproteins excited-state relaxation, 173,175f Fe(II) protoporphyrin dimethyl ester... 

Iron porphyrins—Continued picosecond excited-state relaxation, 168-181 picosecond... 

Fig. 4. Cr(CO)s excited state relaxation dynamics comparison of semi-classical trajectory surface hopping (left), and MCTDH wave packet dynamics (right). Trajectory shows molecule passing through TBP Jahn-Teller geometry within 130 fs, then oscillating in SP potential well afterward. Wave packet dynamics plotted for the Si and S0 adiabatic states in the space the symmetric and asymmetric CCrC bending coordinates.

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

Chemical structure effect on the excited-state relaxation dynamics of the PYP chromophore... 

In order to better understand the early photophysics of PYP, we have carried out a comparative study of three model chromophores, the deprotonated frans-p-coumaric acid (pCA2 ) and its amide (pCM ) and phenyl thioester (pCT) analogues, in aqueous solution (see structures in Fig. 1). The excited-state relaxation dynamics was followed by subpicosecond transient absorption and gain spectroscopy. 

It is worth noting that the fluorescence decays and quantum yields are the same for 2 -deoxynucleosides and 2 -deoxynucleotides in the case of purines (dA/dAMP and dG/dGMP) while for the pyrimidines (dC/dCMP and dT/TMP), the fluorescence quantum yields of nucleotides are higher and the fluorescence decays slower as compared to those of the corresponding nucleosides. This shows that the phosphate moiety does affect the excited state relaxation to a certain extent. 

Auger emission to neutralize incoming ions leaves the solid surface in an excited state relaxation of the surface results in secondary electron generation (23, 24). Secondary electrons are ejected when high-energy ions, electrons, or neutral species strike the solid surface. These electrons enhance the electron density in the plasma and can alter the plasma chemistry near a solid surface. Radiation impingement on a surface can induce a number of phenomena that depend upon the bombardment flux and energy. 

---