Phosphorescence excitation

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

Barakat, K.A., Cundari, T.R. and Omary M.A. (2003) Jahn-Teher distortion in the phosphorescent excited state of three-coordinate Au(I) phosphine complexes. Journal of the American Chemical Society, 125, 14228-14229. 

Bojan, V.R., Fernandez, E.J., Laguna, A., L6pez-de-Luzuriaga, J.M., Monge, M., Olmos, M.E. and Silvestru, C. (2005) Phosphorescent excited state of [Au2 (Ph2Sb)20 3] + Jahn-Teller distortion at only one gold(I) center. Journal of the American Chemical Society, 127, 11564-11565. 

A number of experimental techniques are available for the determination of triplet energy levels. Those most commonly employed are phosphorescence spectroscopy, phosphorescence excitation spectroscopy, singlet to triplet... 

This fluorescence and phosphorescence (curve IVa and IVb) originate only from those excited molecules whose intramolecular hydrogen bond is broken. This is proven by the phosphorescence excitation spectrum where the long wavelength band (intramolecular hydrogen bond) is lacking, Fig. 5, curve II. 

Merrill and Roberts (Z) have examined both PET films and fibers and have attributed the fluorescence (excitation 342 nm, fmision 388 nm) to a 1(n,n ) transition. They have proposed a (n,ir ) transition, since the observed fluorescence is at lower energy than the observed phosphorescence (excitation 313 nm, emission 452 nm, 1.2 sec), which they have proposed from a (tt,tt ) state. 

Figure 1. Uncorrected phosphorescence excitation and emission spectra of dimethyl terephthalate (5 X 10 3M) in 95% ethanol at 77 K, excitation scan Em A 418 nm emission scan Ex A 250 nm lifetime (t) 2.2 sec (9)...

The uncorrected phosphorescence excitation and emission spectra of PET yarn at 77°K show an excitation maximum at 310 nm and emission at 452 nm with a lifetime, t, equal to 1.2 seconds... 

When photoluminescence spectra were recorded for a Ti(OSi(CH3)3)4 model compound, upon excitation at 250 nm only one emission band was detected (at 500 nm), which was assigned to a perfect closed Ti(OSi)4 site. The excitation of these species is considered to be a LMCT transition, 02 Ti4+ —<> (0-Ti3+), and the emission is described as a radiative decay process from the charge transfer state to the ground state, O Ti3+ — 02 Ti4+. Soult et al. (94) also observed an emission band at 499 nm, which they attributed to the presence of a long-lived phosphorescent excited state. The emission band at 430 nm of TS-1 was tentatively assigned to a defective open Ti(OSi)3(OH) site (49). 

The very weak Tm - So transitions are hard to observe directly by absorption spectroscopy. Even with long cells, the high concentrations required present solubility — and what is more important — purity problems. An impurity of 1 10 may give rise to absorption bands which have the same intensity as the expected Ti So absorption. The experimental conditions, therefore, have to be chosen to allow an increase of the Ti- - So oscillator strength to be achieved through perturbation by paramagnetic species (O2 or NO) or heavy atoms. Alternatively, an indirect method, phosphorescence excitation spectroscopy, which has high sensitivity and selectivity, may be applied. 

Let us consider in detail a recent paper by Jones, Kearns, and Wing which represents one of the most thorough investigations of singlet-triplet transitions and reveals much of the power of the phosphorescence excitation method. 

Let us note that there is a small gap of about 30 cm-i between the O —0" band observed in phosphorescence excitation and phosphorescence emission (Fig. 20). Such an effect is typical for crystals where emission occurs generally from traps that lie 20—50 cm i below the host level. The phosphorescence spectrum is dominated by the strong 0 - 0" band, an indication that there is little geometry change between So and T. ... 

Phosphorescence excitation spectroscopy also allows us to observe the transitions starting at 389 nm to the second triplet state, which is of (n,n ) nature. Direct spin-orbit coupling (mechanism I) to a Sn n,n ) state introduces strong in-plane, long-axis polarization. Indeed, in-plane polarization is preferred over out-of-plane polarization by 3 1, and long-axis polarization is about four times stronger than the short-axis contribution. 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

Rothman, W., Case, A., Kearns, D. R. Determination of singlet-triplet absorption spectra from phosphorescence excitation spectra a-bromonaphthalene. J. Chem. Phys. 43, 1067 (1965). 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

In practice, metal complexes of bpy and phen, such as [Ru(bpy)3]2+ and [Ru(phen)3]2+, exhibit long-lived phosphorescent excited states, arising from ligand-centred triplet charge transfer states (3MLCT). Lifetimes are of the order of 102—103 ns in fluid solution at room temperature. 

Figures. 3 and 4 show highly resolved phosphorescence and phosphorescence excitation spectra of Pd(2-thpy)2 for the energy range of the lowest triplet state [56]. The compound is dissolved in an n-octane matrix (Shpol skii matrix). Due to the specific chromophore-cage interaction, one obtains only one strongly dominating site. Interestingly, the situation is different for deuterated n-octane matrices. (Compare Sect. 3.1.6 and see also Table 4.)...

The properties of the low-lying excited singlet and triplet states of 19 different steroidal enones have been investigated by phosphorescence excitation spectroscopy, at 77 and 4.2 Information was obtained on the ordering of excited... 

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