Excited states characteristics

In photoirradiated solid cyclohexane (freezing point 6.5°C), much higher fluorescence quantum yields and longer fluorescence lifetimes were observed than in the liquid phase [89]. In solid Ar matrices, the fluorescence characteristics, energy dependence of the lifetime and intensity, were found to be very similar to these characteristics in the gas phase. This points to the importance of cyclohexane-cyclohexane interactions to determine the excited-state characteristics in the liquid phase [76]. 

The proposed mechanism may explain such excited-state characteristics as the temperature dependence of lifetime or Aex dependence of the fluorescence intensity at low excitation energies. However, in order to explain the energy dependence of the photodecomposition at high energies at least one more dissociative state should be included in the mechanism, which decompose to radicals. 

In dichloromethane solution, the [Ru(bpy)2(l)]2+ complex (Scheme 1) exhibits an absorption band at 455 nm (emax = 10400 M em Figure 5) and an emission band at 619 nm (x = 733 ns, cf> = 0.05, Figure 5, Table 1). These bands can straightforwardly be assigned to spin-allowed and, respectively, spin-forbidden metal-to-ligand-charge-transfer (MLCT) excited states, characteristic of Ru(II) polypyridine complexes[6a,c,e]. 

The lack of significant electronic interactions between the electroactive constituents in the ground state prompts to the excited state characteristics. This has been accomplished by various photophysical methods. A short summary of the results will be presented in this chapter to set the tone for the following energy transfer discussions. 

Thus, it has been shown that the porphyrin chromophores act as an antenna system for transmitting its excited energy to the noncovalently associated fullerene moieties. Furthermore, the examination of excited-state characteristics revealed significant energy transfer properties of these complexes upon photoexcitation. 

These complexes also usually exhibit substantial photostability under visible light irradiation and, due to their relatively long-lived triplet excited-state characteristics, the emission lifetimes are easily quenched by bimolecular electron- and/or energy-transfer processes in solution [6, 76], The electronic structures of MLCT excited molecules of diimine rhenium(I) tricarbonyl complexes can be viewed as a charge-separated species, [LRen(CO)3(diimine ")], with an essentially oxidized... 

Recently, the excited state characteristics of HPTS have been probed with mid-infrared pulses providing insight into state-specific vibrational modes [76]. In Fig. 14.3 the absorbance changes in the fingerprint region of HPTS are shown to be solvent dependent. The fact that these vibrational band patterns appear within the time resolution of 150 fs, without any additional changes up to several tens of picoseconds, indicates that previous observations of a 2.5 ps time component in UV/vis pump-probe experiments [59, 60] previously assigned to a iL], —> level... 

The triplet excited state characteristics of the crown-ether squaraines, determined by energy transfer sensitization, are also summarized in Table 1. The triplet states of the crown ether squaraine derivatives underwent a selfquenching process with bimolecular quenching rate constants varying from 0.5 X 10 to 1.9 X 10 M s with the ground state of the dyes to yield the radical cation and anion (Reactions 1 and 2). 

This case study deals with different photophysical properties of a variety of diimine rhenium(I) tricarbonyl complexes. The exceptionally diverse photophysical behavior of these complexes is largely dependent on the nature of their lowest excited states. Varying the substituents on either the diimine ligands or ancillary ligands can easily change the relative order of these excited states and provides a way to tune the excited-state characteristics. A range of important applications is now becoming apparent, based on the richness of the photophysical and photochemical properties of diimine rhenium(I) tricarbonyl complexes. 

Mosshauer effect The resonance fluorescence by y-radiation of an atomic nucleus, returning from an excited state to the ground state. The resonance energy is characteristic of the chemical environment of the nucleus and Mossbauer spectroscopy may be used to yield information about this chemical environment. Used particularly in the study of Fe. Sn and Sb compounds. 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

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). 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

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