Inter- and Intramolecular Interactions in Some Supramolecular Photochemical Systems (original) (raw)
Related papers
Coordination Chemistry Reviews, 2015
The role of ligand-field states for the photophysical properties of d 6 systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the 3 MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the 3 T 1 (t 2g 5 e g 1) state, at similar energies. If this state lies above the 3 MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy) 3 ] 2+ , m-bpy = 6-methyl-2,2-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the 3 MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the 3 dd state drops to below the 3 MLCT state, and that furthermore the population of this state from the 3 MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the 3 MLCT state.
Inorganic Chemistry, 1988
1,4,5,&Tetraazaphenanthrene (TAP) is a chelating ligand similar to 2,2'-bipyrazine (bpz), which differs from the latter by being much more rigid and having a more extended delocalized r-system. It was thus deemed interesting to compare the properties of three complexes, Ru2+(bpy),(TAP),, (n = 0, 1, 2), with their corresponding Ru2+(bpy)n(bpz)h counterparts. The pK, values of the singly protonated forms of the three TAP complexes in their ground and first excited MLCT states have been determined, the basicities increase with the increasing number of bpy ligands. In the pH range of the measurements, the protonation of the excited state is irreversible; the so-called "apparent pK,*'s" thus reflect only the sequence of the luminescence lifetimes, whereas the sequence of the basicities of the three complexes in their excited state follows that of the pK**'s estimated from a F6rster cycle. The complexes are more basic in the excited state than in the ground state. Interestingly, the luminescences are also quenched by organic buffers in pH domains where protonation cannot occur; this is attributed to the formation of hydrogen bonds between the carboxylic acid of the buffer with one of the free nitrogen atoms of the TAP ligand(s). It was also found that the quenching rate constant by the carboxylic acids decreases when the corresponding carboxylate ion is present; a mechanism is proposed where ion pairing is responsible for this loss of efficiency.
Density-functional study of luminescence in polypyridine ruthenium complexes
Journal of Photochemistry and Photobiology A: Chemistry, 2013
A density-functional theory (DFT) study of five ruthenium complexes has been carried out with the goal of gaining deeper insight into factors governing luminescence lifetimes. The five compounds are [Ru(bpy) 3 ] 2+ (1), [Ru(L1) 2 ] 2+ (2), [Ru(tpy) 2 ] 2+ (3), [Ru(L1)(tpy)] 2+ (4), and [Ru(L2) 2 ] 2+ (5), where bpy = 2,2 -bipyridine, tpy = 2,2 ;6 ,2 -terpyridine, L1 = 1,1 -[2,6-pyridinediylbis (methylene)]bis [3-methylimidazolium] hexafluorophosphate and L2 = 1,1 -[2,6-pyridinediylbis (methylene)]bis [3-methylbenziimidazolium]. Experimental work, including the synthesis and photophysical properties of 5 is also reported in the context of this study. Gas phase geometries optimized using X-ray crystallography geometries as start geometries were found to be close to the start geometries. Gas phase absorption spectra calculated using time-dependent DFT were found to be in good agreement with spectra measured in solution. A partial density of states (PDOS) analysis of the molecular orbitals shows that it is possible to recover a ligand field theory (LFT)-like picture. On the basis of this PDOS-derived LFT-like picture we propose two orbital-based luminescence indices, both motivated by the idea that luminescence quenching results from a low 3 MLCT → 3 MC barrier. The first luminescence index is E, the difference between the e * g and lowest energy * PDOS bands. The second luminescence index is d × , the product of the amount of character in the t 2g band with the amount of ruthenium d character in the 1 * band. These luminescence measures are intended as qualitative rather than quantitative predictors. Low values of E and high values of d × are shown to correlate with lack of luminescence for the five compounds studied in this paper, while high values of E and low values of d × correlate well with luminescence.
Inorganic Chemistry, 1988
(1, L= bpy, T= trz; 2, L= bpy, T= trz-Q; 3, L= biq, T= trz; 4, L = biq, T = trz-Q; bpy = 2,2'-bipyridine, biq = 2,2'-biquinohne; trz = 4-amino-3,5-di-2-pyridyl-4H-1,2,4-triazole; trz-Q = 4(4'-N, N-dimethylamino-phenyl)imino-3,5-di-2-pyridyl-4H-l,2,4-tri~ole) have been synthesized, and their absorption spectra, luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K), and electrochemical behaviour have been investigated. The absorption spectra of the complexes show intense absorption bands in the UV region (E in the range 10"-105 M-r cm-') that are assigned to ligandcentred transitions and moderately intense absorption bands in the visible (E in the range 103-lo4 M-' cm-') that are attributed to metal-to-ligand charge transfer (MLCT) transitions. The absorption bands in the visible of the biq-containing complexes are at lower energies than those of the bpy-containing ones. The four complexes emit from a MLCT excited state both at 77 K and at room temperature, with lifetimes in the range lo-'-10m6 s and lo-'-lo-* s, respectively. The luminescence lifetimes and quantum yields are practically the same for 1 and 3 and for 2 and 4, respectively, indicating that the presence of the N,N-dimethylamino unit on the triazole ligand does not affect the radiative and radiationless rate constants of the chromophores and does not cause an electron-transfer quenching process. On electrochemical oxidation, 1 and 3 exhibit a reversible oneelectron wave at + 1.22 and + 1.37 V versus SCE, respectively, that are assigned to metal-centred oxidations, while 2 and 4 undergo two successive one-electron oxidations at + 1.30 and + 1.56 V (2) and + 1.30 and + 1.71 V (4). By comparison with the redox behaviour of the free ligands, in both 2 and 4 the first process is attributed to oxidation of the N,N-dimethylamino moiety, and the second one to metal-centred oxidation. Two reversible reduction processes occur in all the complexes at about -1.15 and -1.40 V (1 and 2) and at about -0.60 and -0.85 V (3 and 4). Such processes are assigned as bpy-and biq-centred reductions, respectively. The positive shift of the metal-centred oxidation on passing from 1 and 3 to 2 and 4 is attributed to electronic 'communication'
Journal of Photochemistry and Photobiology A-chemistry, 2002
We report three new systems containing tris(bipyrazine)ruthenium II attached to bis(bipyridine)chlororuthenium II/III and/or pentacyanoferrate II/III complexes via the bipyrazine (bpz) bridging ligand, i.e. [Ru(bpz) 3 Ru(bipy) 2 Cl] 3+/4+ , [Ru(bpz) 3 Fe(CN) 5 ] −/0 and [(CN) 5 Fe III (bpz)Ru II (bpz) 2 Ru II (bipy) 2 Cl] + , (bipy: 2,2 -bipyridine). The excitation at 440 nm of the [Ru II (bpz) 3 Ru II (bipy) 2 Cl] 3+ complex leads to a 1 MLCT state which undergoes efficient intersystem crossing to the corresponding 3 MLCT state, [Ru III (bpz − ) 3 Ru II (bipy) 2 Cl] 3+ . The conversion to the inverted mixed valence state, [Ru II (bpz − ) 3 Ru III (bipy) 2 Cl] 3+ , proceeds via electron transfer mechanisms, competing with the radiative and nonradiative decay of the 3 MLCT state. A similar behavior was observed for the [Ru II (bpz) 3 Fe II (CN) 5 ] − complex. In the case of the [(CN) 5 Fe III (bpz)Ru II (bpz) 2 Ru II (bipy) 2 Cl] − triad, the excited state centered on the [Ru(bpz) 3 ] 2+ moiety can undergo intramolecular electron transfer with the peripheral Ru II and Fe III groups, promoting effective charge-separation in the inverted mixed valence complex, [(CN) 5 Fe II (bpz)Ru II (bpz) 2 Ru III (bipy) 2 Cl] + .
Coordination Chemistry Reviews, 2015
The photophysical behavior of Ru(II) and Os(II) diimine complexes having complex aromatic hydrocarbon diimine ligands has received considerable attention as systems exhibiting intramolecular energy transfer to yield excited states with lifetimes much longer than the parent diimine complexes. Here we present a focused discussion of the photophysical behavior of transition metal complexes with modified terpyridyl ligands. The overview includes, as an example of approaches used to evaluate such systems, spectroscopic studies of a pair of Ru(II) mono-and bis-terpyridyl complexes modified with vinylpyrene (Pyr-v-tpy) to have ligand localized excited states that are equal to or lower than the energy of the known MLCT state of the parent complexes, [Ru(Mpt) 2 ] 2+ and [Ru(Mpt)(dien)] 2+ (Mpt = 4'-tolyl-2,2',6',2"-trpyridine, dien = diethylenetriamine). The common observation is that the presence of Pyr-v-tpy serves to lengthen the excited state lifetime of the complex through interaction of MLCT and ligand localized (IL) states. For [Ru(Pyr-v-tpy) 2 ] 2+ the excited state lifetime increases by a factor of more than 10 4 relative to [Ru(Mpt) 2 ] 2+. For [Ru(Pyr-v-tpy)(dien)] 2+ , the 3 IL state is close in energy to the MLCT state of the parent [Ru(Mpt)(dien)] 2+ and, while the transient absorption spectrum is significantly perturbed relative to [Ru(Mpt)(dien)] 2+ , the excited state decay rate changes by only a factor of four. The long-lived excited state is formed in less than a ps, indicating strong coupling of the MLCT and ligand localized manifolds.