The emission from transition metal complexes is usually produced from triplet excited states. Owing to strong spin-orbit coupling (SOC), the fast conversion of singlet to triplet excited states via intersystem crossing (ISC) is facilitated. Hence, in transition metal complexes, emission from singlet excited states is not favoured. Nevertheless, a number of examples of transition metal complexes that fluoresce with high intensity have been found and some of them were even comprehensively studied. In general, three common photophysical characteristics are used for the identification of fluorescent emission from a transition metal complex: emission lifetimes on the nanosecond scale; a small Stokes shift; and intense emission under aerated conditions. For most of the complexes reviewed here, singlet emission is the result of ligand-based fluorescence, which is the dominant emission process due to poor metal-ligand interactions leading to a small metal contribution in the excited states, and a competitive fluorescence rate constant when compared to the ISC rate constant. In addition to the pure fluorescence from metal complexes, another two types of fluorescent emissions were also reviewed, namely, delayed fluorescence and fluorescence-phosphorescence dual emissions. Both emissions also have their respective unique characteristics, and thus they are discussed in this perspective.
The photophysics and photochemistry of transition-metal compounds are of great interest, particularly since such materials have been exploited for a wide range of applications including photocatalysis, photosynthesis and photosynthetic model compounds, artificial light-harvesting antenna systems for solar energy conversion, sensing and imaging, supramolecular photochemically driven machines, multiphotonabsorption materials, probes for monitoring biological processes, and the fabrication of high-performance organic lightemitting diodes (OLEDs).[1] A full understanding of the excited-state behavior of organometallic compounds is crucial for the design of new materials for all of these applications. An attractive feature of this class of compounds is that subtle changes in the ligand environment or metal can be used to tune the properties, thereby allowing the control required for a particular application. [1, 3] and both the diimine and C^N cyclometalated complexes can exhibit highly emissive triplet excited states.Mononuclear metal complexes usually show very rapid conversion from singlet into triplet excited states, which is attributed to the "heavy-atom effect". The heavy-atom effect is the promotion of intersystem crossing (ISC) processes by the spin-orbit coupling (SOC) of the metal atom. These effects can begin to be observed with elements as light as sulphur (z = 16).[4] For example, the formation of the 3 MLCT (MLCT = metal-to-ligand charge transfer) excited state of [Ru(bpy) occurs in less than 20 fs, whereas the second-row complexes [Re(X)(CO) 3 (bpy)] + (X = Cl, Br, I) show a much slower interconversion (ca. 100 fs).[6] Furthermore, the order was found to be Cl (85 fs) < Br (128 fs) < I (152 fs), which is contrary to that predicted by the simplistic consideration of the effect of the heavy atom. Tetrahedral [Pt(binap) 2 ] (binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) and [Cu{bis(diimine)}]+ complexes have been shown to have unusually long-lived 1 MLCT states of t = 3 ps and t = 15 ps, respectively, attributed to a distortion towards a squareplanar geometry which reduces the mixing of the 1,3 MLCT states. [7] Our long-standing interest in rhodium-acetylide compounds [8] and luminescent bis(arylethynyl)arenes [9] led us to the development of a high-yielding, one-pot synthesis of a 2,5-bis(phenylethynyl)rhodacyclopentadiene, which we reported to be luminescent.[10] Our subsequent investigations, reported herein, indicate that this new class of luminescent rhodium complexes shows unprecedented excited-state behavior. Our luminescence spectroscopic studies are supported by picosecond time-resolved IR (TRIR) vibrational spectra of the ground and excited states as a means by which to obtain accurate kinetic data on the processes involved. Herein we demonstrate that despite the presence of the second-row transition metal the compounds show remarkable photophysical properties: specifically, long-lived, highly emissive singlet excited states. This new class of material challenges our understandin...
[4+2] cycloaddition reaction and a subsequent -H-shift. We attribute the different photophysical properties of isomers A and B to a higher excited state density and a less stabilized T1 state in the biphenyl complexes B, allowing for more efficient intersystem-crossing S1→Tn and T1→S0. Control of the isomer distribution is achieved by modification of the bis(diyne) linker length, providing a fundamentally new route to access photoactive metal biphenyl compounds.
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