17472
J. Phys. Chem. 1996, 100, 17472-17484
Electron Delocalization in Ruthenium(II) and Osmium(II) 2,2′-Bipyridyl Complexes Formed from Ethynyl-Bridged Ditopic Ligands Vincent Grosshenny,† Anthony Harriman,*,‡ Francisco M. Romero,† and Raymond Ziessel*,† Laboratoire de Chimie, d’Electronique et de Photonique Mole´ cularies and Laboratoire de Photochimie, Ecole Europe´ enne de Chimie, des Polyme` res et des Materiaux, 1 rue Blaise Pascal, B.P. 296, F-67008 Strasbourg, France ReceiVed: June 4, 1996; In Final Form: August 5, 1996X
Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium(II) and osmium(II) 2,2′-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand. In particular, the electrochemical properties are indicative of electron delocalization over an extended π*-orbital in the π-radical anions. The site of attachment of the ethynyl substituent to the 2,2′-bipyridyl ring affects the various properties, especially absorption and emission spectral maxima. In most cases, the rates of nonradiative deactivation of the lowest-energy triplet excited states are slower than expected for a corresponding complex not possessing a conjugated substituent. This effect is rationalized in terms of electron delocalization over part of the ditopic ligand within the triplet state and its significance depends markedly on the triplet energy of the complex in question. The lowest-energy triplet mixes to some extent with an upper-lying triplet that is more strongly coupled to the ground state. According to the nature of the metal complex, this higherenergy triplet might originate from (i) charge transfer from metal center to parent ligand, (ii) a π,π* state localized on the ditopic ligand, or (iii) a metal-centered excited state. For the OsII complexes at 77 K electron delocalization over an extended π*-orbital is accompanied by a reduction in the amount of nuclear displacement between triplet and ground states and by a smaller vibronic coupling matrix element relative to the parent complex. These two factors combine, within the framework of the energy-gap law, to decrease the rate at which electronic energy can be dissipated among medium-frequency vibrational (i.e., -CdC- and -CdN-) modes. This realization permits a quantitative explanation of the measured rate constants for nonradiative decay of the triplet excited states of these ethynyl-substituted metal complexes.
Introduction It has been reported that the triplet lifetimes of certain binuclear ruthenium(II) polypyridine complexes are significantly enhanced with respect to lifetimes calculated solely on the basis of the measured triplet energy.1-9 Such prolonged triplet lifetimes have been recorded both in fluid solution at ambient temperature and in frozen glasses and for complexes formed from either 2,2′-bipyridine1-4 or 2,2′:6′,2′′-terpyridine ligands.5-8 Closely-related effects have also been observed2,9 for the corresponding binuclear osmium(II) and rhenium(I) polypyridine complexes. The two key requisites for attaining an elongated triplet lifetime are associated with the nature of the bridging ligand and it seems essential that this ligand is both highly conjugated and more easily reduced than other ligands coordinated to the metal centers. Thus, upon illumination an electron is promoted from a metal-centered d-orbital into an extended π*-orbital resident on the bridging ligand, giving rise to a metalto-ligand charge-transfer (MLCT) excited triplet state. Ditopic polypyridine ligands coordinated to the two metal centers may be linked directly3 or through spacer moieties comprising ethenyl1,2,4 or ethynyl groups.5-8 The magnitude of this enhancement in triplet lifetime is nontrivial, being sufficient to convert a poor photosensitizer into a viable one, and might exceed a factor of 3000-fold in particular cases. It is also noteworthy that a smaller, but still substantial, enhancement in triplet lifetime is observed for the corresponding mononuclear complexes having the appropriately functionalized ditopic ligand2,5 and for asymmetrical polynuclear complexes bearing †
Laboratoire de Chimie, d’Electronique et de Photonique Moleculaires. Laboratoire de Photochimie. X Abstract published in AdVance ACS Abstracts, September 15, 1996. ‡
S0022-3654(96)01626-7 CCC: $12.00
a spectator metal center.7 We may surmise, therefore, that the effect is general and might apply to many MLCT triplet states provided an extended π*-orbital is available and of the appropriate energy.2 The origin of the enhanced triplet lifetimes, as realized in this way, lies in a combination of two important effects. First, the presence of an electron-withdrawing substituent pushes the redox potential for one-electron reduction of the bridging ligand to a more positive value relative to that of the parent ligand. This serves to ensure that the bridging ligand is reduced preferentially, thereby lowering the energy of the MLCT triplet excited state. The latter effect, which is apparent from a redshifted emission spectrum, reduces the extent of mixing between the MLCT triplet and any metal-centered (MC) excited states situated at higher energy.10 Since such MC excited states usually undergo rapid nonradiative deactivation to the ground state, it is clear that decoupling the MLCT and MC states is likely to increase the triplet lifetime. Second, the rates of nonradiative deactivation of MLCT triplet excited states of metal complexes are controlled by vibrational relaxation.11 It is wellknown that these rates tend to increase with decreasing triplet energy12 but other factors, especially the size of the nuclear displacement between ground and triplet states, contribute to the overall process. As first described by Meyer and coworkers,1,2 electron delocalization over an extended π*-orbital reduces the degree of structural distortion within the triplet state. This effect, in turn, decreases the electron-vibrational coupling constant and thereby slows the rate at which electronic energy can be dissipated as heat.13 We have further explored the concept of electron delocalization over the bridging ligand by studying the photophysical © 1996 American Chemical Society
Bipyridyl Complexes with Ethynyl-Bridged Ditopic Ligands
(a)
J. Phys. Chem., Vol. 100, No. 44, 1996 17473
(b)
Figure 1. (a) Structures of the ditopic ligands used in this study. (b) Structures and abbreviations of the various metal complexes synthesized during this work.
properties of a series of ethynyl-bridged multinuclear ruthenium(II) and osmium(II) polypyridine complexes (polypyridine ) 2,2′-bipyridine). Particular attention has been given to symmetrical binuclear complexes because of the possibility of photon migration occurring between the terminal metal centers.14 We consider that such events are likely because of the long triplet lifetimes, the short metal-to-metal separations, and the selective charge injection into the bridging ligand that takes place upon illumination with visible light.8 It is shown that the site at which the ethynyl group is attached to the bipyridyl ligand is of importance, as is the nature of the metal cation. Because of their stereochemical integrity, chemical stability, and lack of isomerization, our interest has focused on the use of alkynes as spacer moieties but we have included a critical comparison of the relative efficacies for electron delocalization as exhibited by alkenes and alkynes.
Experimental Section Materials. The various ligands used in this study were synthesized15 as described previously and their structures are drawn in Figure 1a. The metal-containing precursors, [Ru(bpy)2Cl2]‚2H2O16 and [Os(bpy)2Cl2],17 where bpy refers to 2,2′bipyridine, used in the assembly of the various complexes were prepared according to literature methods. A general procedure was adapted for preparation of all the complexes described herein; their structures and abbreviated nomenclature are shown in Figure 1b. Thus, a stirred solution of [Ru(bpy)2Cl2]‚2H2O or [Os(bpy)2Cl2] (1 or 2 equiv) and the appropriate ditopic ligand (50 mg scale, 1 equiv) in ethanol (20 mL) was heated at 80 °C for 10-18 h. The warm solution was filtered over celite before addition of solid NH4PF6 (6 equiv). Slow evaporation of the solvent resulted in precipitation of the crude complex as the
17474 J. Phys. Chem., Vol. 100, No. 44, 1996 hexafluorophosphate salt. Pure compounds were obtained by chromatography (alumina, eluting with 3-10% methanol in dichloromethane, or silica, eluting with 10-15% water in acetonitrile) and recrystallization from acetone/diethyl ether or acetonitrile/toluene mixtures. The complexes were characterized by thin-layer chromatography (TLC), FAB mass spectrometry (positive mode, m-nitrobenzyl alcohol matrix), electrospray mass spectrometry (ESMS, acetonitrile solution, low voltage), UVvis spectroscopy, and elemental analysis. The quoted Rf values were measured by TLC on alumina with CH2Cl2/CH3OH (9/1) as eluent. Ru1(1): yield 82%; Rf ) 0.63; FAB+ 949 [M - PF6], 804 [M - 2PF6], 648 [M - 2PF6 - bpy], 492 [M - 2PF6 - 2bpy]; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 448 (12 000), 289 (75 700). Anal. Calc. for C46H38F12N8P2Ru (Mr ) 1093.868): C ) 50.51, H ) 3.50, N ) 10.24; Found C ) 50.36, H ) 3.33, N ) 10.11. Ru2(2): yield 16%; Rf ) 0.37; FAB+ 1653 [M - PF6], 1508 [M - 2PF6], 1363 [M - 3PF6], 949 [M - 3PF6 - Ru - 2bpy], 803 [M - 4PF6 - Ru - 3bpy - H]; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 447 (23 000), 325 (sh, 50 500), 310 (sh, 56 500), 288 (115 900). Anal. Calc. for C66H54F24N12P4Ru2 (Mr ) 1797.243): C ) 44.11, H ) 3.03, N ) 9.35; Found C ) 43.93, H ) 2.91, N ) 9.19. Ru2(3): yield 50%; Rf ) 0.59; FAB+ 1677 [M - PF6], 1532 [M - 2PF6], 973 [M - 3PF6 - Ru - 2bpy]; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 446 (18 500), 366 (sh, 20 100), 347 (27 200), 289 (102 100). Anal. Calc. for C68H54F24N12P4Ru2 (Mr ) 1821.266): C ) 44.85, H ) 2.99, N ) 9.23; Found C ) 44.63, H ) 2.71, N ) 9.02. Ru1(4): yield 66%; Rf ) 0.74; ESMS m/z 892.8 [M - PF6]+, 373.6 [M - 2PF6]2+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 465 (16 600), 288 (81 300). Anal. Calc. for C42H30F12N8P2Ru (Mr ) 1037.759): C ) 48.61, H ) 2.91, N ) 10.80; Found C ) 48.56, H ) 2.84, N ) 10.69. Ru2(5): yield 76%; Rf ) 0.63; FAB+ 1597 [M - PF6], 1452 [M - 2PF6], 1307 [M - 3PF6], 1162 [M - 4PF6], 748 [M 4PF6 - Ru - 2bpy], 591 [M - 4PF6 - Ru - 3bpy - H], 435 [M - 4PF6 - Ru - 4bpy - H]; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 488 (25 100), 443 (sh, 21 300), 288 (107 800). Anal. Calc. for C62H46F24N12P4Ru2 (Mr ) 1741.133): C ) 42.77, H ) 2.66, N ) 9.65; Found C ) 42.58, H ) 2.47, N ) 9.46. Ru2(6): yield 76%; Rf ) 0.62; FAB+ 1621 [M - PF6], 1476 [M - 2PF6], 1331 [M - 3PF6], 917 [M - 3PF6 - Ru - 2bpy], 772 [M - 4PF6 - Ru - 2bpy], 615 [M - 4PF6 - Ru - 3bpy - H]; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 487 (29 200), 442 (sh, 21 700), 288 (98 600). Anal. Calc. for C64H46F24N12P4Ru2 (Mr ) 1765.16): C ) 43.55, H ) 2.63, N ) 9.52; Found C ) 43.25, H ) 2.33, N ) 9.31. Ru1(7): yield 29%; Rf ) 0.64; ESMS m/z 892.9 [M - PF6]+, 374.2 [M - 2PF6]2+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 465 (16 600), 339 (49 900), 288 (81 250). Anal. Calc. for C42H30F12N8P2Ru (Mr ) 1037.759): C ) 48.61, H ) 2.91, N ) 10.80; Found C ) 48.28, H ) 2.56, N ) 10.59. Ru2(8): yield 92%; RF ) 0.47; ESMS m/z 748.9 [M - 2PF6 + C2H5OH]2+, 451.0 [M - 3PF6 + C2H5OH]3+, 302.3 [M 4PF6 + C2H5OH]4+, 436.0 [M - 3PF6]3+, 291.1 [M - 4PF6]4+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 453 (24 100), 370 (40 300), 341 (34 600), 319 (38 700), 288 (129 900). Anal. Calc. for C62H46F24N12P4Ru2 (Mr ) 1741.139): C ) 42.77, H ) 2.66, N ) 9.65; Found C ) 42.56, H ) 2.49, N ) 9.39. Ru2(9): yield 40%; Rf ) 0.52; ESMS m/z 737.6 [M 2PF6]2+, 443.4 [M - 3PF6]3+, 296.3 [M - 4PF6]4+; UV-vis
Grosshenny et al. (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 439 (21 600), 380 (49 300), 355 (56 700), 328 (54 800), 288 (134 200). Anal. Calc. for C64H46F24N12P4Ru2 (Mr ) 1765.16): C ) 43.55, H ) 2.63, N ) 9.52; Found C) 43.29, H ) 2.45, N ) 9.36. Os1(1): yield 84%; Rf ) 0.27; ESMS m/z 1184.1 [M + H]+, 1038.1 [M - PF6]+, 519.3 [M + H - PF6]2+, 446.4 [M 2PF6]2+, 297.9 [M + H - 2PF6]3+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 590 (3430), 473 (12 800), 434 (11 400), 348 (21 300), 292 (81 400). Anal. Calc. for C46H38F12N8P2Os (Mr ) 1182.998): C ) 46.70, H ) 3.24, N ) 9.47; Found C ) 46.58, H ) 3.01, N ) 9.18. Os2(5): yield 24%; Rf ) 0.37; ESMS m/z 837.7 [M - 2PF6 + C2H5OH]2+, 814.7 [M - 2PF6]2+, 510.0 [M - 3PF6 + C2H5OH]3+, 494.8 [M - 3PF6]3+, 346.2 [M - 4PF6 + C2H5OH]4+, 334.9 [M - 4PF6]4+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 620 (6450), 487 (20 000), 443 (17 700), 384 (14 200), 367 (14 500), 291 (86 500). Anal. Calcd for C62H46F24N12P4Os2 (Mr ) 1919.345): C ) 38.80, H ) 2.42, N ) 8.76; Found C ) 38.68, H ) 2.24, N ) 8.49. Os2(8): yield 32%; Rf ) 0.21; ESMS m/z 832.8 [M - 2PF6 + 2H2O]2+, 814.4 [M - 2PF6]2+, 506.8 [M - 3PF6 + 2H2O]3+, 494.9 [M - 3PF6]3+, 344.3 [M - 4PF6 + 2H2O]4+, 335.1 [M - 4PF6]4+; UV-vis (CH3CN) λmax (nm) ( (dm3 mol-1 cm-1)) 600 (4000), 461 (15 200), 378 (28 400), 348 (41 600), 331 (41 400), 290 (88 700). Anal. Calc. for C62H46F24N12P4Os2 (Mr ) 1919.345): C ) 38.80, H ) 2.42, N ) 8.76; Found C ) 38.69, H ) 2.28, N ) 8.59. Methods. Absorption spectra were recorded in dilute acetonitrile solution at ambient temperature with a Kontron Instruments Uvikon 930 spectrophotometer. Luminescence spectra were recorded in deoxygenated acetonitrile solution at ambient temperature with a Perkin-Elmer LS50 spectrofluorimeter equipped with a red-sensitive photomultiplier tube. The emission spectra were corrected for imperfections of the instrument by reference to a standard lamp and a total of five individual spectra were averaged for each sample. Only a small correction factor was required for wavelengths between 600 and 830 nm, but this factor became progressively more important between 830 and 850 nm. Above 850 nm the correction factor was too high to be reliable. Emission maxima were reproducible to within (5 nm. Luminescence quantum yields were calculated relative to tris(2,2′-bipyridyl)ruthenium(II) in acetonitrile,18 using dilute solutions after deoxygenation by purging with argon. Low-temperature luminescence spectra were recorded for ethanol glasses at 77 K using a fully-corrected Perkin-Elmer LS5 spectrofluorimeter and were also referenced to tris(2,2′-bipyridyl)ruthenium(II).19 In all cases, nonfluorescent glass cutoff filters were used to separate emission from scattered excitation light. Luminescence quantum yields were taken as the average of three separate determinations and were reproducible to within (8%. Luminescence lifetimes were measured following excitation of the sample with a 30-ps laser pulse at 532 nm as delivered from a frequency-doubled, mode-locked Quantel YG402 Nd-YAG laser. The laser intensity was attenuated to 5 mJ per pulse and incident pulses were defocused onto an adjustable pinhole positioned in front of the sample cuvette. Luminescence was collected with a microscope objective lens at 90° to excitation and isolated from any scattered laser light with nonemissive glass cutoff filters. The emergent luminescence was focused onto the entrance slit of a Spex high-radiance monochromator and thereby passed to a fast-response photomultiplier tube operated at 900 V. The output signal was transferred to a Tektronix SCD1000 transient recorder and subsequently to a microcomputer for storage and analysis.
Bipyridyl Complexes with Ethynyl-Bridged Ditopic Ligands
J. Phys. Chem., Vol. 100, No. 44, 1996 17475
TABLE 1: Photophysical Properties Recorded for the Various Ruthenium(II) and Osmium(II) Complexes in Deoxygenated Acetonitrile Solution at Ambient Temperature and in an Ethanol Glass at 77 Ka acetonitrile solution compd
λabs/nm
[Ru(bpy)3]2+ Ru1(1) Ru2(2) Ru2(3) Ru1(4) Ru2(5) Ru2(6) Ru1(7) Ru2(8) Ru2(9) [Os(bpy)3]2+ Os1(1) Os2(5) Os2(8)
451 448 447 446 465 488 487 452 453 439 478 473 487(443) 461(500)
a
/dm3
mol-1
14 600 12 100 23 000 18 500 16 600 25 100 29 200 13 600 24 100 21 600 11 100 12 800 20 000 15 200
cm-1
ethanol glass
ΦL
λem/nm
τT/ns
ΦL
λem/nm
τT/µs
0.0620 0.0002 0.0002 0.0004 0.0470 0.0520 0.0660 0.0390 0.0430 0.0350 0.0046 0.0050 0.0040 0.0028
620 612 615 625 668 690 705 670 680 710 745 755 805 835
980 2.2 4.1 8.6 1,430 1,850 2,430 1,170 1,270 1,330 60 58 65 52
0.38 0.19 0.20 0.24 0.19 0.21 0.24 0.19 0.23 0.28 0.065 0.118 0.057 0.030
600 589 591 596 650 670 685 650 655 685 710 725 770 805
5.2 3.5 3.8 4.6 4.9 5.5 6.8 4.7 5.9 8.1 0.83 1.24 0.81 0.51
See Experimental Section for error limits.
Approximately 100 individual laser shots, collected at 10 Hz, were averaged for kinetic measurements. The temporal resolution of this instrument, determined by the rise time of the photomultiplier tube, was ca. 2 ns. Emission lifetimes measured with this setup were reproducible to within (5%. Improved time resolution was achieved by replacing the photomultiplier tube with a Hamamatsu single-shot streak camera. Again, approximately 100 individual records were averaged before data analysis. The available dynamic range for the streak camera was 0.1-10 ns and, in this case, the laser pulse was deconvoluted from the sample luminescence. Emission lifetimes measured with the streak camera were reproducible to within (0.5 ns. All kinetic measurements were made with samples previously deoxygenated by purging with argon and the absorbance of each solution was adjusted to be ca. 0.15. Data analysis was made by a nonlinear, least-squares iterative fitting routine that utilized a modified Levenberg-Marquardt global minimization procedure. Transient adsorption studies were made with a frequencydoubled, Q-switched Quantel 580 Nd-YAG laser (pulse duration 10 ns). The laser intensity was attenuated to ca. 2 mJ and defocused onto the sample cuvette. Solutions were adjusted to possess an absorbance of ca. 0.25 at 532 nm and were purged continuously with argon. The monitoring beam was provided by a pulsed, high-intensity xenon arc lamp and, after passing through the sample at 90° to the laser pulse, was directed onto the entrance slits of a high-radiance monochromator. Output from the photomultiplier tube was sent to a Tektronix SCD1000 transient recorder and then to a microcomputer for storage and analysis. Transient spectra were compiled point by point with five individual laser shots being averaged at each wavelength. Kinetic measurements were made at fixed wavelength with 100 individual laser shots, collected at 1 Hz, being averaged and analyzed by nonlinear, least-squares computer iteration. The time resolution of this setup was ca. 15 ns (fwhm ∼ 10 ns). Where necessary, improved temporal resolution was achieved using a frequency-doubled, mode-locked Quantel YG402 NdYAG laser (pulse duration 30 ps, maximum energy 25 mJ per pulse). Residual 1064 nm output from the laser was used to generate a white light continuum for use as the analyzing pulse, which was delayed with respect to the excitation pulse with a computer-controlled optical delay stage. Excitation and monitoring pulses were split into two equal beams and directed almost collinearly through the sample cell with short fiber optical cords. Unabsorbed 532 nm excitation light was removed with a notch filter and the residual pulses were directed to the entrance slits of a Spex spectrograph. After attenuation, the dispersed signals
were transferred to a Princeton image-intensified, dual-diode array and subsequently to a microprocessor for storage. Approximately 300 individual laser shots were averaged at each delay time and about 40 separate delay times were used to generate a kinetic profile. Data analysis, after background subtraction, was made as above but a global procedure was used in which decay curves collected at 15 different wavelengths were averaged. The temporal resolution of this instrument was ca. 60 ps while the maximum delay time attainable was restricted to ca. 10 ns. Electrochemical measurements were made by cyclic voltammetry using a Pine Instruments potentiostat equipped with a Pt microdisc working electrode and a Pt wire counter electrode. Ferrocene was used as an internal reference and was calibrated against a saturated calomel reference electrode (SCE) separated from the electrolysis cell by a glass frit presoaked with electrolyte solution. Solutions contained the electrode-active material (ca. 2 × 10-4 mol dm-3) in deoxygenated acetonitrile solution containing tetra-N-butylammonium perchlorate (0.2 mol dm-3). The quoted half-wave potentials were reproducible to (20 mV. Results Photophysical and Electrochemical Properties of Monoand Binuclear Ruthenium(II) Complexes. Absorption spectra recorded for the RuII complexes bearing a 6,6-ethynylsubstituted ditopic ligand (i.e., Ru1(1), Ru2(2), and Ru2(3)) in dilute acetonitrile solution exhibit the expected absorption transitions in the UV and visible regions. Thus, an intense ligand-centred π,π* transition is clearly apparent around 288 nm while the MLCT transition has a definite maximum (λabs) around 447 nm with molar absorption coefficients at the peak () of ca. 104 dm3 mol-1 cm-1 per RuII center (Table 1). These MLCT transitions extend into the red region of the spectrum in the form of a low-intensity absorption tail but there is no indication of the bands being split into two components. Splitting of the MLCT absorption bands might be expected in view of the fact that there are two types of ligand (i.e., parent and ditopic) coordinated to the metal center but this situation is not observed experimentally. The spectra, when compared to that recorded for the parent complex [Ru(bpy)3]2+ under identical conditions,20 are slightly blue-shifted and contain additional LC absorption bands in the UV region that can be assigned to the ditopic ligand. These extra bands appear as doublets located at 320 and 340 nm for Ru1(1) and Ru2(2) and at 350 and 360 nm for Ru2(3). The small red shift noted for
17476 J. Phys. Chem., Vol. 100, No. 44, 1996
Grosshenny et al.
TABLE 2: Reorganization Energies and Electrochemical Properties Measured for the Various Metal Complexes in Deoxygenated Acetonitrile Solution at Ambient Temperaturea EM/cm-1 b
compd ]2+
[Ru(bpy)3 Ru1(1) Ru2(2) Ru2(3) Ru1(4) Ru2(5) Ru2(6) Ru1(7) Ru2(8) Ru2(9) [Os(bpy)3]2+ Os1(1) Os2(5) Os2(8)
1450 2050 1710 1370 1880 1960 2010 1540 1760 1790 950 1370 1620 1650
E1/2ox/V vs SCEc 1.27 (65, 1e) 1.42 (70, 1e) 1.35 (90, 2e) 1.31 (110, 2e) 1.38 (70, 1e) 1.31 (100, 2e) 1.29 (90, 2e) 1.40 (70, 1e) 1.38 (90, 2e) 1.35 (90, 2e) 0.83 (65, 1e) 0.85 (70, 1e) 0.82 (100, 2e) 0.81 (110, 2e)
E1/2red/V vs SCEd -1.35 (1e), -1.54 (1e), -1.79 (1e) -1.30 (1e), -1.52 (1e), -1.65 (1e) -1.02 (1e), -1.26 (1e), -1.51 (2e) -1.00 (1e), -1.18 (1e), -1.51 (2e) -1.24 (1e), -1.45 (1e), -1.55 (1e) -0.94 (1e), -1.19 (1e), -1.40 (2e) -0.86 (1e), -1.04 (1e), -1.35 (2e) -1.22 (1e), -1.49 (1e), -1.60 (1e) -0.92 (1e), -1.16 (1e), -1.35 (2e) -0.90 (1e), -1.12 (1e), -1.33 (2e) -1.25 (1e), -1.54 (1e), -1.80 (1e) -1.11 (1e), -1.21 (1e), -1.48 (1e) -1.04 (1e), -1.14 (1e), -1.38 (2e) -0.97 (1e), -1.11 (1e), -1.38 (2e)
KD/meV
11410 1100 16850 1100 11500 5250 50 230
a Values given in parentheses after the oxidation potentials refer to the different in potential (in mV) between anodic and cathodic peaks measured at 100 mV/s scan rates. The number of electrons accompanying each process is also given in parentheses. b Reorganization energy, (150 cm-1. c Oxidation potential, (20 mV. d Reduction potential, (20 mV.
Ru2(3) is consistent with the increased level of π-conjugation possessed by this compound. Unlike the situation found with [Ru(bpy)3]2+, luminescence from these ethynyl-substituted compounds was difficult to isolate from scattered excitation light and the derived luminescence quantum yields (ΦL) were very low (Table 1). The emission maxima (λem) are similar to that of the parent complex, appearing around 620 nm (Table 1) while the corrected excitation spectra were found to correspond closely to the absorption spectra recorded across the visible region. Relative to the parent complex, the luminescence lifetimes (τT) recorded in deoxygenated acetonitrile at ambient temperature were extremely short, being on the order of a few nanoseconds (Table 1). Increasing the number of ethynyl groups in the bridging ligand and/or adding the second RuII cation causes a slight increase in both ΦL and τT and is accompanied by small red shifts in the maxima of the emission spectra. In an ethanol glass at 77 K, however, the emission lifetimes and quantum yields more closely resembled those of the parent complex (Table 1). For each compound the emission maximum is blueshifted by ca. 20 nm upon moving from fluid solution to the [less polar] low-temperature glass.19 Complexes Ru1(4), Ru2(5), and Ru2(6) differ from those described above only in respect of the site of attachment of the ethynyl spacer moiety in the bridging ligand (Figure 1b). Even so, the absorption spectra recorded in dilute acetonitrile solution indicate that the maxima of the MLCT transitions are significantly red-shifted (Table 1) but the additional LC absorption transitions in the UV region are less apparent. Close examination of the absorption spectra indicates that the MLCT transitions might be split since there are obvious shoulders appearing around 445 nm. This being the case, it is tempting to assign the lower energy MLCT transitions to charge injection into the bridging ligand, with the higher-energy MLCT bands being associated with the parent ligands. There are concomitant red shifts in the luminescence spectra, which display maxima around 700 nm in acetonitrile solution at ambient temperature (Table 1). The emission lifetimes and quantum yields measured in acetonitrile solution are greatly enhanced with respect to those determined for the 6,6-substituted compounds (Table 1), although the various values are comparable when measured in an ethanol glass at 77 K. In particular, it should be noted that the emission lifetimes recorded at room temperature, being on the order of a few microseconds, exceed that measured for the parent complex despite their lower triplet energies. The decay profiles collected at both temperatures gave satisfactory fits to single-exponential processes.
Similarly, absorption spectra recorded for Ru1(7), Ru2(8), and Ru2(9), having adjacent 2,2′-bipyridyl units linked at the 5,5positions in the ditopic ligand, exhibit the expected LC transition centered around 290 nm that is assigned to the parent ligand. Intense MLCT transitions are also apparent around 450 nm (Table 1) while additional LC absorption transitions are clearly evident as intense multiplets appearing in the UV region. Presumably, these additional LC absorption bands are associated with the ditopic ligands. Luminescence from these same compounds is readily observed in deoxygenated acetonitrile solution at ambient temperature and in an ethanol glass at 77 K. Luminescence quantum yields and lifetimes recorded in deoxygenated acetonitrile solution at room temperature and in ethanol at 77 K remain comparable to those of the parent complex (Table 1). In each case, corrected excitation spectra were found to agree well with the corresponding absorption spectra recorded over the visible region and the luminescence decay profiles were well described in terms of a singleexponential component. In order to estimate the magnitude of the total reorganization energy (EM) that accompanies decay of the lowest-energy MLCT triplet state the absorption spectrum was examined for a high concentration of compound in acetonitrile solution at room temperature. The MLCT spectral region, covering the range 380-600 nm, was assumed to consist of overlapping bands for [nominally] spin-allowed and spin-forbidden transitions.20 The onset of the spin-allowed MLCT transition was identified by spectral curve-fitting routines as being around 550 nm and any significant absorbance occurring at lower energy was assigned to the spin-forbidden MLCT transition.21 The modified luminescence spectrum, compiled according to22
L(ν)M dν ) [L(ν)/ν3] dν
(1)
where L(ν) refers to the luminescence intensity at wavenumber ν (in cm-1), was normalized to the peak of the modified absorption spectrum, compiled as follows23
(ν)M dν ) [(ν)/ν] dν
(2)
where (ν) is the molar absorption coefficient (in dm3 mol-1 cm-1) at wavenumber ν. The difference in energy between the crossover point of modified absorption and emission spectra and the peak of the modified emission spectrum was attributed to the reorganization energy.13 The derived values are collected in Table 2 and the procedure is illustrated in Figure 2. Apparently, there are minor variations in EM among the various
Bipyridyl Complexes with Ethynyl-Bridged Ditopic Ligands
Figure 2. Illustration of the method used to derive total reorganization energies for the triplet excited states of the various metal complexes. The peak of the modified emission spectrum (which corresponds to νmax) is normalized to the highest point of the modified absorption spectral profile recorded for the spin-forbidden MLCT transition. The crossover point, designated as χ, corresponds to the triplet energy (∆ETS) while the reorganization energy (EM ) ∆ETS - νMAX) is taken as the difference between the triplet energy and the emission maximum. The spectra are for Ru2(5).
complexes, the average value being ca. 1750 cm-1 in acetonitrile at room temperature. These EM values refer to a combination of structural changes and differences in solvation between ground and triplet states. It should be noted that the derived EM values do not depend on the magnitude of the absorption coefficient but they are somewhat sensitive to the absorption wavelength taken as the normalization point (Figure 2). For the OsII compounds the near-infrared absorption region is a plateau such that normalization was made at the maximum extinction coefficient around 600 nm. For the RuII complexes, where there is no plateau, we have assumed that the lowintensity absorption band present at wavelengths longer than 550 nm is due entirely to a [nominally] spin-forbidden, singletto-triplet MLCT transition.20 Normalization was fixed at 550 nm. The electrochemical properties of these ethynyl-substituted RuII complexes were studied by cyclic voltammetry in deoxygenated acetonitrile solution at ambient temperature. For each compound, including the parent complex, one-electron oxidation of the RuII center occurred by way of a quasi-reversible process with a half-wave potential (E1/2ox) in the region of 1.2-1.4 V vs SCE (Table 2). The oxidation peak was not split for the binuclear complexes, indicating that the metal centers are not in strong electronic communication with each other. However, the separation between anodic and cathodic peaks (∆Ea,p) measured at slow scan rates for the binuclear complexes, being on the order of 90-110 mV, exceeded that for the parent (∆Ea,p ∼ 65 mV) and those for the corresponding mononuclear complexes (∆Ea,p ∼ 70 mV). This finding is consistent with the half-wave potentials for oxidation of the two metal centers in the binuclear complexes being inequivalent, with removal of the second electron occurring at slightly higher potential. The slightly larger ∆Ea,p found for the butadiynyl-bridged binuclear complexes is consistent with the results of HMO calculations made on the various ditopic ligands from which it is concluded that the energy of HOMOs localized on the ligand decreases with increasing length, over short distances. Relative to the parent complex, it is clear that the metal centers in these substituted complexes are somewhat more difficult to oxidize, presumably due to the presence of an electron-withdrawing ethynyl substituent. Interestingly, for those binuclear complexes having one or two ethynyl group(s) in the ditopic bridging ligand
J. Phys. Chem., Vol. 100, No. 44, 1996 17477 the E1/2ox values do not seem to depend on the site of attachment of the bridge. In each case, there is a small decrease in E1/2ox upon insertion of a second RuII cation into the ditopic ligand (Table 2). A series of peaks was observed on reduction scans, each corresponding to addition of an electron to one of the 2,2′bipyridyl ligands.24 For the mononuclear complexes, including the parent,25 three one-electron reduction processes were observed. Comparison of the derived half-wave potentials for the initial reduction step (E1/2red) shows that the first electron is added to the ditopic ligand (Table 2). The second electron is most likely added to a parent ligand while addition of the third electron occurs at the uncomplexed 2,2′-bipyridyl ligand present in the mononuclear RuII complexes. One-electron reduction of the other coordinated parent ligand cannot be properly resolved in the cyclic voltammograms due to the onset of irreversible waves at very negative potentials (
