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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.1 Concepción mar. 2004

http://dx.doi.org/10.4067/S0717-97072004000100014 

Tuning the Excited States in -fac--[Re(X2dppz)(CO)3(L)]:
Intraligand, Charge Transfer or both?

Rosa López B*, Bárbara Loeb L.*

Facultad de Química y Biología, Universidad de Santiago de Chile,
Av. Bernardo O´Higgins 3363, Santiago, Chile, e-mail: rlopez@lauca.usach.cl
Facultad de Química, Pontificia Universidad Católica de Chile,
Vicuña Mackenna 4860, Santiago, Chile.
Durwin Striplin, Martin Devenney, Kristin Ombergf and Thomas J.Meyefr*
Department of Chemistry, The University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina, 27599-3290, USA.

(Received: July 23, 2003 - Accepted: October 28, 2003)

ABSTRACT

Excited state lifetime measurements - emission, emission lifetime and Resonance Raman - have been conducted on the series -fac--[Re(X2dppz)(CO)3(L)]0,+ (L = Cl-, 4-ethylpyridine (4-Etpy), 4,4´-bipyridine (4,4´-bpy)), with the X2dppz substituted dipyrido[3,2-a:2',3'-c]phenazine ligands (X = CH3 and Cl). The results are consistent with closely lying excited states, with pp* lowest, but MLCT contributing significantly to excited state lifetimes for the neutral complexes. -fac--[Re(Cl2dppz)(CO)3Cl] appears to undergo a crossover from lowest pp* to lowest MLCT through the glass to fluid transition.

INTRODUCTION

In the excited state of polypyridyl complexes of Re(I) there is an interesting interplay between lowest lying metal to ligand charge transfer (MLCT) and ligand localized 3(p-p*) excited states.1 An example has been described with dipyrido[3,2-a:2',3'-c]phenazine, (dppz) as the acceptor ligand, Re(dppz)(CO)3L, in which a dppz-localized 3p-p* state lies lowest, but a low lying MLCT state or states is populated at room temperature and dominates emission and excited state decay.2

Chart 1

Schematic structure of dipyrido[3,2-a:2',3'-c]phenazine, (dppz)

The electronic distribution of dppz as an MLCT acceptor ligand has been investigated by transient resonance Raman measurements on [Ru(bpy)2(dppz)]2+ (bpy is 2,2'-bipyridine), and it has been concluded that the lowest acceptor level in dppz has a high degree of phenazine (phz) character but is delocalized over the whole ligand framework2a. Recently, the temperature-dependent excited state lifetime was measured in protic and aprotic solvents. There are two low lying states, one "bright" and one "dark". The bright state is entropically favored and the dark state energetically favored. They have been associated with the phen and phz fragments acting as the electron acceptors, respectively.3

In this manuscript we explore both themes, excited state ordering and electronic distribution, in complexes of the type -fac--[Re(X2dppz)(CO)3(L)]0,+ (L = Cl-, 4-ethylpyridine (4-Etpy), 4,4'-bipyridine (4,4’-bpy)) with the X2dppz substituted ligands (X = CH3 and Cl) whose structures are shown in Chart 2, which also gives the 1H proton labeling schemes for the ligands.

Chart 2

EXPERIMENTAL

Measurements. UV-visible spectra were recorded on Milton Roy 3000 diode array or Hewlett-Packard 8452A diode array spectrophotometers. 1H-NMR spectra were recorded on a Bruker AC/200, 200MHz spectrometer with TMS as reference. Cyclic voltammetry was performed by using a Wenking HP72 potentiostat, a Wenking VSG 72 signal generator and a Graphtex WX 2300 recorder. A platinum disk was used as working electrode. The counter electrode was a platinum wire and the reference a Ag/AgCl (in aqueous tetramethyl ammonium chloride) electrode, calibrated to SCE. All potentials are reported relative to SCE. Infrared spectra were recorded in KBr mulls in a Bruker Vector 22 FTIR spectrophotometer. Corrected emission spectra, emission quantum yields, excited state lifetimes, transient absorbance difference spectra, and ground and excited state Resonance Raman spectra were obtained as described elsewhere.4

Materials. All chemicals were reagent grade and used as received. The dppz and 11,12-dimethyldipyrido[3,2-a:2',3'-c]phenazine, Me2dppz, ligands were prepared following literature procedures.5.

11,12-dichloro[3,2-a:2',3'-c]phenazine, Cl2dppz, was prepared by a condensation reaction as follows: 0.300 g (1.42 mmol) of 1,10-phenanthroline-5,6-dione and 0.252 g (1.42 mmol) of 4,5-dichloro-1,2-diaminobenzene were dissolved in 40 ml ethanol and the mixture heated at reflux for one hour. The solution was evaporated to ~ 10 ml and cooled to room temperature. The product precipitated as a crystalline light brown solid. (Yield: 63.8%). IR (KBr): 1572 (n(CN)) and 1470 cm-1 (n(C=C)). UV-vis (DCE), lmax (e): 390 (~4000), 370 (~4000), 268 nm (~12000 M-1cm-1). 1H NMR (CDCl3): 9.29 (2Ha,dd), 7.81 (2Hb,m), 9.56 (2Hc, dd), 8.48 ppm (2Hd,s). The precursor 1,10-phenanthroline-5,6-dione was prepared by modification of a reported method.6 The starting material Re(CO)5Cl was purchased from Aldrich and used without further purification. The synthesis of -fac--[Re(dppz)(CO)3(L)]+ (L = Cl, 4-Etpy, 4,4'-bpy) has been reported elsewhere.2b

fac-[Re(X2dppz)(CO)3Cl] (X = CH3, Cl). These compounds were prepared by a procedure similar to that given in literature for their bpy analogues.7 To a suspension of 1 mmol of X2-dppz (X = CH3, Cl) in 40 ml of toluene, Re(CO)5Cl (1 mmol) was added. The mixture was heated at reflux for 2 h. After removal of the solvent on a rotary evaporator, the remaining solid was washed with diethyl ether (3 x 5 ml) and dried in high vacuum. For X = CH3, Yield = 93 %. 1H NMR (CDCl3): 9.43 (2Ha,dd), 8.00 (2Hb,m), 9.82 (2Hc, dd), 8.18 (2Hd,s), 2.66 ppm (2 CH3, s). For X = Cl, Yield = 98 %. 1H NMR (CDCl3): 9.48 (2Ha,dd), 8.04 (2Hb,m), 9.79 (2Hc, dd), 8.59 ppm (2Hd,s).

fac-[Re(X2dppz)(CO)3(OTf)] (X = CH3, Cl; OTf = trifluoromethane sulfonate). These compounds were prepared by a procedure similar to the one described for their bpy analogue.8 To a suspension of 0.5 mmol of fac-[Re(X2-dppz)(CO)3Cl] (X = CH3, Cl) in 40 ml anhydrous THF, Ag(OTf) (0.5 mmol) was added. The mixture was heated at reflux for 2 h under inert atmosphere and in the dark. For X = CH3, the AgCl that formed was filtered off, THF removed on a rotary evaporator and the yellow residue re-dissolved in the minimum amount of acetonitrile. Due to the insolubility of the product for X = Cl, the THF was first removed on a rotary evaporator and 40 ml of acetonitrile was added. The AgCl remained as a solid and was separated by filtration, and the solution concentrated on a rotary evaporator. In both syntheses, the final yellow product was precipitated with diethyl ether, separated from the solution by filtration and dried under high vacuum. For X = CH3, Yield = 84.5 %. 1H NMR (CDCl3): 9.36 (2Ha,dd), 8.14 (2Hb,m), 9.94 (2Hc, dd), 8.17 (2Hd,s), 2.63 ppm (2 CH3, s). For X = Cl, Yield = 94 %. 1H NMR (CDCl3): 9.50 (2Ha,dd), 8.22 (2Hb,m), 9.85 (2Hc, dd), 8.69 ppm (2Hd,s).

fac-[Re(X2dppz)(CO)3(L)]+ (X = CH3, Cl; L = 4-Etpy, 4,4'-bpy). These complexes were prepared from the corresponding OTf precursor by the general procedure described below.7 In a typical preparation 0.20 mmol of -fac--[Re(X2-dppz)(CO)3(OTf)] and 5 to 6 fold excess of 4-Etpy or 4,4'-bpy were dissolved in 30 ml ethanol and heated at reflux under inert atmosphere for 4 h. After cooling the solution to RT, NH4PF6 in excess was added, and the mixture stirred for 12 h. A yellow solid formed, and its precipitation was completed by refrigeration. The solvent was removed by decanting and the solid washed several times with petroleum ether. The product was chromatographed by using an alumina column eluting first with petroleum ether to eliminate traces of 4-Etpy or 4,4'-bpy that might remain, and then with CHCl3 (L = 4-Etpy) or (1/1) acetonitrile/CH2Cl2 (L = 4,4'-bpy). The yellow solution was concentrated and the desired product precipitated by the addition of diethyl ether, collected by filtration and dried under high vacuum. For (L = 4-Etpy, X = CH3), Yield = 70 %. 1H NMR (CD3CN): 9.57 (2Ha,dd), 8.28 (2Hb,m), 9.97 (2Hc, dd), 8.21 (2Hd,s), 2.65 (6H, CH3, s); 4-Etpy: 8.17 (2Hf,d), 7.16 (2Hg,d), 2.5 (CH2, q), 1.0 ppm (CH3, t). For (L = 4-Etpy, X = Cl), Yield = 52 %. 1H NMR (CD3CN): 9.56 (2Ha,dd), 8.23 (2Hb,m), 9.84 (2Hc, dd), 8.52 (2Hd,s); 4-Etpy: 8.00 (2Hf,d), 7.03 (2Hg,d), 2.44 (CH2, q), 1.0 ppm (CH3, t). For (L = 4,4'-bpy, X = CH3), Yield = 87.6 %. 1H NMR (CD3CN): 9.73 (2Ha,dd), 8.31 (2Hb, m), 9.80 (2Hc, dd), 8.19 (2Hd,s), 2.69 (2 CH3, s); 4,4'-bpy: 8.65 (2Hf, dd), 7.57 (2Hg, dd), 7.50 (2Hh, dd), 8.49 ppm (2Hi,dd). For (L = 4,4'-bpy, X = Cl), Yield = 83.5 %. 1H NMR (CD3CN): 9.45 (2Ha,dd), 8.06 (2Hb,m), 9.66 (2Hc, dd), 8.34 (2Hd,s); 4,4'-bpy: 8.31 (2Hf,dd), 7.80 (2Hg,dd), 7.28 (2Hh,dd), 8.21 ppm (2Hi,dd). Elemental analysis (C, H, N) for all compounds and salts were satisfactory.

RESULTS

Representative absorption spectrum for the series -fac--[Re(X2-dppz)(CO)3(L)]0,+ (X = H, CH3, Cl; L = Cl-, 4-Etpy, 4,4'-bpy) is shown in Figure 1 for -fac--[Re(CH3)2-dppz)(CO)3Cl]. For all compounds, the absorptions in the 320-400 nm region are ligand-centered p -> p* bands which also appear in spectra of the corresponding free ligand. Vibronic components are observed, even at RT, with the 0->0 transition at ~ 390 nm. The expected Re -> X2dppz MLCT bands are hidden under the more intense p -> p* bands. Their maxima were determined by difference spectra by using the free ligands as the reference. In Figure 2 the absorption difference spectrum for fac-[Re(CH3)2-dppz)(CO)3(Cl)] is shown. In contrast to -fac--[Re(dppz)(CO)3(PPh3)]+ for which p->p* and MLCT bands are badly overlapped,2b,10 an MLCT band appears at 410 nm (another broad band at ~340 nm can also be seen). A summary of the spectral data is given in Table 1. Data for the free ligands and literature values2b for the dppz complexes are additionally included, for comparison.


Band energies for n(CO) are also listed in Table 1. The general pattern matches well the expected -fac- CO pattern in Re tricarbonyl polipyridinic compounds. The band at highest energy is the totally symmetrical stretching mode. The two bands at lower energy, which are unresolved in some spectra,7b are the symmetrical and antisymmetrical in plane stretching modes. Also in Table 1, are listed E1/2 values in 0.1M [N(n-C4H9)4](PF6) or [N(n-C4H9)4](BF4) in 1,2-dichloroethane (DCE) obtained by cyclic voltammetry. From these data the general trend for the ease of reduction of the dppz derivative is Cl2dppz > dppz > (CH3)2dppz, although in some cases the last two appear interchanged. The potentials of the ReII/I couples are relatively unaffected by changes on the X substituent of the dppz ligand.

In Figure 3 the emission spectrum for -fac--[Re(CH3)2-dppz)(CO)3Cl] in DCE at room temperature (RT) is shown. The spectrum of the same compound in 2-Methyltetrahydrofuran (Me-THF) at 77 includes a vibronic progression (544, 588, and 636(sh) nm) characteristic of a p-p*, ligand-centered emission. The presence of a pyridine type ligand instead of chloride has a more meaningful effect. For the cases with neutral ligands (Et-py, 4,4´-bpy) emission spectra even at room temperature exhibit the characteristic vibronic structure for ligand-based emission. As a general rule, excitation spectra match well with the corresponding absorption spectra, as can be seen in Figure 4 for -fac--[Re(CH3)2-dppz)(CO)3Cl] in DCE at room temperature. Emission maxima, quantum yields (fem) and lifetimes (t) data for the series of complexes are presented in Table 2.

Excited state Resonance Raman band energies by transient Resonance Raman measurements (354.7 nm pulse and probe) are listed in Table 3 for [(X2dppz)Re(CO)3Cl] X = Me, Cl, H. Representative spectra are shown in Figure 5.

DISCUSSION

It is evident from the photophysical data in Table 3 that there is a wide variation in excited properties among the various ReI complexes containing X2dppz (X = H, CH3, Cl) as the acceptor ligands. The absorption data are not especially revealing. In an earlier study based on [Ru(bpy)2(dppz)]2+ and [Ru(dmb)2(dppz)]2+ (dmb is 4,4´-dimethyl-2,2´-bipyridine) it was noted that in the low energy absorption spectrum there are overlapping Ru -> dppz, Ru ->bpy, dmb bands. There are two Ru ->dppz transitions: one is a Ru to a p* level largely localized on the phen portion of the ligand. The second is at lower energy and lower intensity5b. In this case the p* acceptor level is largely localized on the phenazine portion of the ligand and is the acceptor level for the emitting MLCT state which is lowest lying3. In the case of the Re complexes, their low energy spectra are dominated by p -> p* (X2dppz) bands. There is evidence for Re -> X2dppz absorption in the tailing to the low energy side, Figure 1, and in the difference spectra, Figure 2.

Emission spectra and lifetimes are more revealing. For the cases with neutral ligands, (L = Et-py, 4,4`-bpy) room temperature lifetimes are long, 10´s to 100´s of ns and it can be concluded that these emitting states are dppz localized 3p -> p* states with lifetimes shortened due to the heavy atom effect of the chemically bound Re. This conclusion is reinforced by the transient infrared (TRIR) results reported earlier.10 In TRIR spectra of MLCT excited states such as [ReII(bpy-.)(CO)3(4-Etpy)]2+11, large positive shifts occur in the n(CO) bands (Dn = 50 - 100 cm-1) due to the change in electronic configuration from dp6 to dp5p*1 which decreases electron density at the metal and, with it, dp(Re)-p*(CO) backbonding.12 In the TRIR spectrum of [Re(dppz)(CO)3(PPh3)]+* n(CO) shifts are to lower energy by 8 cm-1. The lowest excited state is a dppz-localized 3pp* state and 3dppz is a slightly electrodondonating ligand.10

Replacement of the backbonding permiting pyridyl or phosphine ligands by Cl- leads to dramatic changes in photophysical properties. Qualitatively, replacement of these ligands by Cl- is expected to stabilize the MLCT states relative to p->p*; Cl- is a s and p- donating ligand in a relative sense, and donation to partially oxidized Re in the MLCT excited state stabilizes the MLCT state electronically. The lowest lying excited state in the binuclear complex [(4-Etpy)(CO)3Re(m-bbpe)Re(CO)3(4-Etpy)](PF6)2-4, (bppe is 1,2--trans---bis-(4´-methyl-2,2´-bipyrid-4-yl)ethene) was found to be p->p* (bbpe) with a close-lying MLCT. Even so, excited states properties near room temperature are dominated by MLCT decay nd emission. Temperature-dependent transient absorption (TA) measurements showed that the two states coexist at room temperature and undergo relatively slow interconversion.4a

The appearance of broad structureless emissions at RT and greatly shortened lifetimes in the series of -fac--[Re(X2dppz)(CO)3Cl], (X = CH3, Cl) points to lowest lying MLCT states for these complexes. That conclusion must be drawn with care, given the appearance of vibronic structure in the room temperature spectra of -fac--[Re(X2dppz)(CO)3(L)]+, (X = CH3, Cl, L = 4-Etpy, 4,4'-bpy) which points to lowest p ->p* states. This point is further illustrated by the variations in lifetimes in Table 2.

The solution to this apparent dilemma was suggested earlier by the results of ground state Resonance Raman (RR) on -fac--[Re(NO2dppz)(CO)3Cl]13a in CH2Cl2. These results demonstrated that the lowest lying excited state was p ->p* even though emission and excited state decay are dominated by the lowest MLCT state or states. Raman enhancement was observed for the characteristic bands of the entire framework of the dppz acceptor, considered to be formed by two parts, a phen moiety bound to Re and a remote phz (phenazine) moiety. In the RR spectrum a band near 1400 cm-1 was strongly enhanced. The band energy responded to substitution on the dppz ligand consistent with assignment to a dominant ring stretching involving the entire framework.13a Similarly, for the complexes in this study, a strongly enhanced band appears at ~ 1400 cm-1 for -fac--[ Re(Cl2dppz)(CO)3Cl] , consistent with resonance enhancement from an extended ring mode. A second band at 1306 cm-1 is also strongly enhanced. From Table 3 and Figure 4 the bands obtained in the transient resonance Raman spectra (1000-1800 cm-1) for the -fac--[ Re(X2dppz)(CO)3Cl] series (X = Cl, H, Me), provide additional information about the excited state, with a band near 1404 cm-1 (assigned as previously to a ring vibration of phenazine) strongly enhanced.

The results of the photophysical measurements on -fac--[Re(dppz)(CO)3Cl] lead to the suggested energy level diagram in Scheme 1. For this compound, emission decay kinetics are simple exponential, consistent with rapid interconversion between MLCT and p ->p* states (k2, k-2 >> k1, k3). This is consistent with the overlap between excitation and absorption spectra and formation of the initial state or states in an equilibrium distribution. In this kinetic limit, the MLCT and p ->p* states are in facile equilibrium. The overall rate constant for excited state decay k (= t-1) is given by:

k = t-1 = (k3 + k1K2) / (1 + K2) (Eq. 1)

For -fac--[Re(dppz)(CO)3Cl], the MLCT is thermally accessible, k1K2 >> k3 and MLCT decay dominates, qualitatively consistent with Scheme 1. For -fac--[Re((CH3)2dppz)(CO)3Cl], t = 0.91 ms at room temperature and t = 10.2 ms at 77 K. In either case, the transient RR (TR3) results point to small, if any, MLCT contribution. Thermal population of MLCT is small and the p ->p* state is observed in the TR3 experiment at 354 nm. Supporting this conclusion is the fact that at 354 nm excitation wavelength, the ReII(dppz-) moiety of the MLCT excited state is strongly absorbing but dppz p->p* is relative transparent.2a. This interpretation neglects the temperature dependences of k1 and k3 which are expected to be small.14


Scheme 1

The cationic complexes -fac--[Re(X2dppz)(CO)3(L)]+, (L = 4-Etpy, 4,4'-bpy) are pp* emitters at room temperature with long lifetimes. The pp*-MLCT energy difference is high, and the MLCT states do not contribute significantly to excited state decay.

For -fac--[Re(Cl2dppz)(CO)3Cl], an emission with t < 5.3 ms was observed at 77 K but there is no detectable emission at RT, giving evidence for a short lived excited state. This is presumably a case where there is a change in excited state ordering between the rigid medium at 77 K and fluids solution, with MLCT the lowest state in the fluid. Stabilization of MLCT relative to pp* is an expected result in the fluid, because of relaxation of the surrounding solvent dipoles.16

ACKNOWLEDGMENTS.

Financial support from Fondecyt-Chile (Grant 1940577, and líneas complementarias 898007), Conicyt-Chile (Grant 2950063) and DICYT-Usach (Grant 020141LB) is gratefully acknowledged. Travel between the laboratories at PUC and UNC (R.L., B.L.) has been supported by the US and Chilean Science Foundations (NSF and Conicyt), through Grant INT-9123215.

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