Metal-centered phosphorescent emission

Therefore, this low-energy band is assigned to a metal-centered d->p transition instead of as arising from Au---Au interactions. The solid-state luminescence spectrum (Fig. 25) exhibits a phosphorescence emission band centered at 417 nm. This value compares favorably with those reported for solids K[Au(CN)2]58 and Au2(dmb)(CN)2.63... 

The optical and PL spectroscopies have been undertaken to understand the structure-property correlations of this important family of triplet-emitting polymers. The red shift in the absorption features upon coordination of the metal groups is consistent with there being an increase in conjugation length over the molecule through the metal center. The trade-olf relationship between the phosphorescence parameters (such as emission wavelength, quantum yield, rates of radiative and nonradiative decay) and the optical gap will be formulated. For systems with third-row transition metal chromophores in which the ISC efficiency is close to 100%,76-78 the phosphorescence radiative (kr)y, and nonradiative (/cm)p decay rates are related to the measured lifetime of triplet emission (tp) and the phosphorescence quantum yield ([Pg.300]

The synthesis, optical, and structural properties of another Pt(II) polyyne-containing biphenyl moiety, P12, were reported recently.44 The system has also been extended to the Au(I) and Hg(II) congeners (see Sections IV and V). The influence of the metal center on the spatial extent of S and T excited states was characterized in detail. The ligand-based phosphorescence emissions can be harvested by the heavy-atom effect of these transition metals, which facilitates efficient intersystem crossing from the S] state to the T] state. 

In contrast, emission from [Rh(bipy)2X2] is from a metal-centered, d-d) state. The various low-lying excited states for [Rh(bipy)2Cl2] are represented in Figure 3, with initial excitation into the (ti-te ) ligand state followed by relaxation and eventual phosphorescence from the d-d) state. The rise times for phosphoresence were reported to be 350-630 ns in room temperature solution and at 77 but these values were later found to be artifacts of the detection system. " The emitting dr-d) states of [Rh(bipy)2X2] absorb at 580 (X = Cl) and 550 nm (X = Br), and the lifetimes of their transient absorptions (measured by time-resolved absorption spectroscopy in air-saturated ethanol-methanol (4 1) solution at room temperature) were found to be 84 ns (X = Cl) and 54 ns (X = Br). " (If the solutions are deoxygenated, the lifetimes increase by about a factor of five.) The presumed relaxation path is represented in equation (147), with the rate of internal conversion (IC) a 4 X 10 " s ", followed by intersystem crossing (ISC), localized within the rf-manifold, with the rate constant ca. 8 x 10. 

The intensity of the ligand-localized phosphorescence again suggests that the intersystem crossing efficiency for the tris-chelated complexes is near unity, but the lower intensity for the bis-chelated complexes suggests that population of the metal-centered state is less efficient. Emission from... 

The lowest triplet energy level of the ancillary ligand LX lie well above the energies of LC and MLCT excited states in most of the lr(C N)2(LX)-type complexes, so luminescence of Ir(C N)2(LX) is dominated by LC and MLCT transitions. This leads to similar phosphorescence emission to the/ac-lr(C N)3 complexes with the same cyclometalated ligand [6, 23]. In such cases, density functional theory (DPT) calculations indicate that HOMOs are largely metal-centered, whereas LUMOs are primarily localized on the heterocyclic rings of the cyclometalated ligand. The ancillary is therefore not directly involved in the lowest excited state. 

Perturbation of the photochromic and luminescence properties upon coordination to the metal center (Re, Pt) has been observed. A red shift of the absorption band for the closed isomer is attributed to the perturbation of the metal center in the complex. The emission of both phenl(o) and M-phenl(o) (MLCT) (o open form, c closed form) changes upon conversion to the closed form in the PSS. The strong red-shift of the emission observed for the closed form phenl(c) is attributed to the extension of the 7i-conjugation and has an IL phosphorescence in origin. 

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