Photoluminescence from Charge-Transfer States

The recent discovery of emission from interfacial charge-transfer states in blends of conjugated polymers has challenged the assumption that exeiton ionization at the heterojunction leads to pairs of charges without any wavefunction overlap between electrons and 

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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 photoluminescence of lattice oxide ions of transition-metal oxides mixed or supported on conventional carriers has also been reported (160b). The luminescence is shown to occur from oxo complexes (M04)" (M = V, Mo, W, Cr) in which the transition-metal ion exists in a high oxidation state with a d° electronic configuration. Since the d orbitals of the transition-metal ion are not occupied and therefore the d-d transitions impossible, S0 -)-S charge-transfer electronic transitions occur in the oxo complexes upon absorption of light. The result is that an electron is transferred from a filled molecular orbital localized mainly on the O2 anions to a d orbital of the transition-metal ion. This leads to the formation of an excited singlet electronic state S, with two unpaired electrons, in which the total electron spin,... 

The photoluminescence of dipyridophenazine complexes of ruthenium ) in the presence and absence of DNA has been well-characterized (38-40, 46-52). Excitation of the dppz complexes with visible light (440 nm) leads to localized charge transfer from the metal center (39, 40). In aqueous solution, the emission resulting from the metal-to-ligand charge-transfer excited state is deactivated via nonradiative energy transfer... 

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

From the Stern-Volmer plots shown in Fig. 13, the values of were determined to be 57,71, and 85 Torr when the MgO catalyst was degassed at 1273, 1173, and 873 K, respectively. Although the exact values of the photoluminescence lifetime of MgO and of the absolute quenching rate constant are unknown, the assumption that the quenching rate constant of O2 toward the excited emitting sites on the catalyst is approximately equal to the value toward the similar charge-transfer excited triplet state of the... 

The results obtained from such dynamic photoluminescence studies shown in Fig. 41, together with the results obtained for O2 and CO. permit calculation of the absolute rate constants of quenching for the various molecules, as follows 9.34 x 10 for O2, 3.52 x 10 for C2H4, 2.24 x 10 for rram -2-butene, and 1.51 x 10 for N2O, all in units of (g/mol s), respectively (33,34,56,69,115-117). Consequently, the reactivities of these molecules toward the charge-transfer excited state of the vanadyl species decrease in the order O2 > CO > C2H4 > CsHg > trans-l-C Wg > N2O (120, 208-210). 

The Franck-Condon analysis of the vibrational fine structure of the photoluminescence spectrum of the anchored vanadium oxide observed at 77 K indicates that the equilibrium V-0 bond distance of the vanadyl group is elongated in the charge-transfer excited state by 0.013 nm compared with the ground state value (725). UV irradiation of the anchored vanadium oxides at 280 K in the presence of CO led to the photoformation of CO2. Since the photoformation of CO2 from CO is accompanied by the removal of oxygen from the oxide (i.e.. the photoreduction of the oxide), such an elongation of the equilibrium nuclear distance of the V-0 bond in the excited state is closely associated with the facile photoformation of CO2 on the anchored vanadium oxides. In other words, the O hole trapped centers in the electron-hole pair state of the (V -0 ) complex exhibit a high reactivity similar to 0 anion radicals 66). 

Fluorescence and phosphorescence are emission processes which originate directly or indirectly (see 5 section ll.B) from the electronically excited singlet state and triplet state, respectively, produced by charge-transfer processes (Eqs. 1 and 2). Many publications deal with such charge-transfer transitions by diffuse reflectance spectroscopy (DRS) (2-6) showing the link between the latter technique and photoluminescence. It is worthwhile to recall that the emergence of the coordination chemistry of solid-state anions, namely, of surface lattice oxide ions, has almost entirely been based on the results of both photoluminescence and DRS analyses (7, 66). For some catalytic systems, vibrational structures can be detected (see Section IV.B) with an associated vibrational constant, which may be determined directly and independently by IR or Raman spectroscopy, evidencing the relation between these spectroscopies and photoluminescence (33, 34). 

Figure 3 shows that the ex-Ti-oxide/Y-zeolite catalyst exhibits a photoluminescence spectrum at around 490 nm by excitation at around 290 nm at 77 K. The observed photoluminescence and absorption bands are in good agreement with those previously observed with the highly dispersed tetrahedrally coordinated Ti-oxides prepared in silica matrices [3,9]. We can therefore conclude that the observed photoluminescence spectrum is attributed to the radiative decay process from the charge transfer excited state to the ground state of the highly dispersed Ti-oxide species in tetrahedral coordination as shown in the hv... 

Fig. 48 Energy levels in the pristine polymers and blend system of PCNEPV and MDMO-PPV. Since the triplet state of MDMO-PPV is the lowest excited state of the system, excitations relax to it and diminish the possibility of charge carrier separation. Notation singlet (S), triplet (T), exciplex (ex), and charge-separated states (CSS) and transitions (ET = energy transfer CS = charge separation PL = photoluminescence ISC = intersystem crossing) between these states. Crosses indicate processes that do occur in the pure materials, but that are quenched in the blend. (Reprinted with permission from [229], 2005, American Physical Society)...

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