13132
J. Phys. Chem. 1995, 99, 13132-13140
Inter- and Intramolecular Oxidative Quenching of Mixed Ligand Tris(bipyridyl)ruthenium(IJ) Complexes by Methyl Viologen Lisa A. Kelly**’and Michael A. J. Rodgers Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: January 30, 1995; In Final Fomt: May 9, 1995@
The rate constants for photoinduced electron transfer, as well as thermal charge recombination, were measured in a series of [(4,4’-R2-2,2’-bipyridine)2Rut1(4’-(CH3)-2,2’-bipyridine-4-(CO~R’))] (R’ = (CH2),MV2+) systems, in which a tris(bipyridyl)ruthenium(II) chromophore was covalently linked to a 4,4’-bipyridinium (MV*+) electron acceptor. The nature of R(R = H, CH3, COO-, COOH, CONHCH(CHd2) and the number (x = 2, 3) of intervening methylene units were varied to tune the chromophore’s electronic properties, including the x* orbital energies of the 4,4’-R*-2,2’-bipyridine ligands and donor-acceptor separation distance, respectively. For a given donor-acceptor distance, x, and similar driving force, the rate constants for forward electron transfer were nearly 60 (x = 3) to 400 (x = 2) times smaller in complexes in which the two 4,4’Rl-2,2’-bipyridine ligands were R-substituted with electron-withdrawing functional groups (R = CONHCH(CH3)2). Charge recombination from the reduced viologen acceptor to the oxidized metal center occurs in the Marcus inverted region, with the rate constants (kb) decreasing with increasing magnitude of driving force. The kinetics of the bimolecular oxidative quenching of the electronically excited state of these mixed ligand tris(bipyridyl)ruthenium(II) complexes (R’ = CH(CH&) by methyl viologen was also characterized in homogeneous aqueous solution, and the escape efficiencies were measured for separation of the redox products from the solvent cage.
Introduction Considerable progress has been made in understanding the role of solvent, energetics, temperature, and distance dependencies of photoinduced, intramolecular charge transfer processes.’.2 In many cases, the bimolecular electron transfer kinetics become diffusion-limited at moderate driving forces, making elucidation of the thermodynamic effects difficult. However, the diffusional component of photoinduced electron transfer reactions can be eliminated by covalent attachment of the electron donor and acceptor. Such a strategy has been demonstrated with an increasing number of organic and inorganic donorlacceptor Tris(bipyridyl)ruthenium(II) complexes have been widely employed as photosensitizer components in a variety of energy and electron transfer scheme^.^.^ The major focus of such investigations has been on the utilization of these chromophores in photoinduced redox systems, owing to the powerful oneelectron-oxidizing and -reducing power of the lowest electronically excited states of these complexes. The complexes are attractive photosensitizers in that they are remarkably stable to irradiation with visible light. Furthermore, a wide array of synthetic methodologies is currently developing for functionalization of the 2,2‘-bipyridine ligands, thus allowing both the photophysical and redox properties to be readily tuned. Several recent reports have appeared in which transition metal diimine complexes, including tris(bipyridyl)ruthenium(II) complexes, linked to electron-accepting or -donating moieties, have been prepared and the kinetics characterized in homogeneous solution and heterogeneous environment^.^-'^ In all cases, the electronically excited state is quenched by the electron acceptor or donor, but the highly exergonic charge recombination generally occurs very rapidly, making the charge transfer state Current address: National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York. @Abstractpublished in Advance ACS Ahsrracrs, August 1, 1995.
0022-365419512099-13132$09.00/0
elusive. However, using picosecond transient pump-probe spectroscopy, Yonemoto et al. have been able to directly observe the charge transfer state in certain tris(bipyridyl)ruthenium(II) viologen dyad systems6a-band have demonstrated the utility of heterogeneous zeolite systems for achieving charge separation.l 4 We have prepared a series of novel tris(bipyridy1)ruthenium(11) complexes, in which a viologen electron acceptor is chemically attached via an amide linkage at the 4-position of one of the 2,2’-bipyridine ligands. The donor-acceptor separation distance was varied with the number of intervening methylene subunits between the amide functional group and viologen acceptor. Furthermore, the nature of the R substituent on the “remote” (that not covalently attached to the viologen) 4,4’-R2-2,2’-bipyridine ligands was varied. This functionalization affects both the energy of the ligand-centeredz* molecular orbital and the stabilization of the dn metal-centered orbital, both of which influence the energetics of the charge transfer process to the viologen acceptor. The net charge transfer, from the Ru” dn orbital to the viologen acceptor, occurs subsequent to photon absorption to form the metal-to-ligand charge transfer (MLCT) excited state of the metal complex. Following rapid nonradiative decay, the optically promoted electron is resident in the lowest energy n* molecular orbital, associated with the most easily reduced 2,2’-bipyridine ligand. 4,4’-R2-substitution of the 2,2‘-bipyridine ligands alters their x* orbital energy, via electronic perturbation, thus affecting the electron density distribution among the three 2,2’-bipyridine ligands in the vibrationally relaxed MLCT excited state. The effects of these substitutions on the forward and back electron transfer kinetics were investigated, and the results are presented in this report.
Experimental Section Materials. RuCly3H20 (99.9%) and SeO2 (99.4%) were obtained from Alfa and used as received. Isopropylamine, 4,4‘dimethyl-2,2’-dipyridyl (recrystallized from ethyl acetate prior to use), and 2,2’-bipyridine were obtained from Aldrich
0 1995 American Chemical Society
Mixed Ligand Tris(bipyridyl)ruthenium(II) Complexes
J. Phys. Chem.,Vol. 99,No. 35, 1995 13133
neutral alumina column. 1-x-MV were eluted with 5050 H20/ EtOH, following the successive elution of a yellow impurity band and purple band attributed to the bis(bipyridy1)ruthenium(11) complex. 2-x-MV were separated from impurities by first eluting the yellow impurity band with acetone and then eluting the purple and, finally, the orange tris(bipyridyl)ruthenium(II) band with EtOH. The purification of 5-x-MV was analogous, eluting the yellow impurity band first with 1% H20 in acetone, followed by 5050 HzOIacetone to remove the purple bis(bipyridyl)ruthenium(II) starting material. The desired orange band was eluted with EtOH. All of the complexes were recrystallized from water as their PFs- salts. Methods. All measurements were conducted at ambient temperature (22 f 2 "C). Ground-state UV/vis absorption spectra were measured using a Perkin-Elmer, Lambda Array 3840 single-beam spectrophotometer. Nanosecond transient absorption measurements employed the technique of laser flash photolysis. The second harmonic (532 nm) of a Q-switched (4-(((34 l'-Methyl-4,4'-bipyridinediium-l-yl)propyl)amino)- Nd:YAG laser (Quantel YG-660, pulse width ca. 8 ns) was used for laser flash excitation. Pulse energies of 50 & were typically carbonyl)-4'-methyl-2,2'-bipyridine)(PFs-)2(LMVj2') and (4employed at the excitation wavelength. Measurement of (((2-(l'-Methy1-4,4'-bipyridinediium-l -yl)ethyl)amino)carbonyl)luminescence lifetimes less than ca. 20 ns employed the 4'-methyl-2,2'-bipyridine)(PFb-)2(LMVz2'). Synthesis and technique of time-correlated single-photon counting.22 Typicharacterizationof the covalently linked 2,2'-bipyridine/viologen cally, 310 nm light (pulse width ca. 5 ps) was employed for ligands have been reported e1se~here.I~ excitation, with a red band-pass filter (transmittance = 50% at [Ru(bpy)z(4-(CH3)-bpy-4'-(CONHCH(CH3)2))l2'(PF6-)2 (11, 1 > 580 nm) positioned between the sample and the photon[Ru(4,4'-( CHj)~-bpy)2(4-( CH~)-~~Y-~'-(CONHCH(CH~)~))]~+counting photomultiplier tube to filter scattered excitation light. (PFs-)~ (~),[RU(~,~'-(COO-)~~~~)~(~-(CH~)-~~Y-~'-(CONHCHThe fwhm of the instrument response function was ca. 300 ps. (CHd2))l2-(Na+)z (3), [Ru(4,4'-~COOH)zbpy)z(4-(CH~)-bpy-4'(CONHCH(CH3)2))]2+(PF6-)2 (4), and [Ru(4,4'-(CONHCH- The instruments for nanosecond time-resolved kinetic spectroscopy and time-correlated single-photon counting have been (CH~/~)~~~Y)~(~-(CH~)-~~Y-~'-(CONHCH(CH~)~))I~'(PF~-)~ (5). previously d e ~ c r i b e d . The ~ ~ . double ~ ~ diode array spectrograph The mixed ligand tris(bipyridyl)ruthenium(II) complexes were utilized for picosecond pump-continuum probe absorption prepared by refluxing (for 2-13 h under argon, depending upon experiments has been previously described.25 Sample excitation the ligand employed) the appropriate bis(bipyridy1)rutheniumemployed the frequency-doubled (532 nm, 25 ps pulse width) (11) complex with a stoichiometric amount of N-isopropyl-4light from a mode-locked, Q-switched Nd:YAG (Quantel methyl-2,2'-bipyridine-4'-carboxamidein a minimal amount of YG571) laser operating at 5 Hz. Typical excitation energies 1:l H20EtOH (for 1 and 2), 1:l EtOWacetone (for 5), or 1:l were 5-10 &/pulse. Kinetic measurements were carried out H20DMF (for 4) and monitored by UV/vis spectroscopy. The as in our previous work.19 For the nanosecond flash photolysis bis(bipyridyl)ruthenium(II) complexes ([Ru(4,4'-R2-bpy)~C12]* and picosecond pump-probe experiments, the ground-state xH20) were prepared using methods described previously.20,2' optical density of the aqueous solutions at 532 nm was ca. 0.1 Complexes 1 and 2 were purified on a neutral alumina column. (k0.02). A yellow impurity band was first eluted with acetone, followed by elution of the purple bis(bipyridyl)ruthenium(II) starting The one-electron potentials were measured by the techniques material with 1:l acetonelEtOH. The desired orange product of cyclic voltammetry and differential pulse polarography. All was eluted with EtOH. Complex 3 was prepared from complex of the measurements employed a BAS lOOA electrochemical 4 by titration with NaOH. The conjugate base was purified on analyzer controlled with the commercial BAS lOOW (version a silica gel column, as previously r e p ~ r t e d . ' Complex ~ 5 was 1.0) software package for Windows 3.1. The measurements eluted from a silica gel column using 1% water in acetone as were made in either H20 or CH3CN solvent medium containing the eluent. The desired orange fraction containing the triseither 0.10 M Na2S04 or 0.10 M TEAP, respectively, as (bipyridyl)ruthenium(II) complex was eluted as the second band supporting electrolyte. To obtain deprotonated and protonated following a band of a yellow impurity and preceding the purple 3, 3-x-MV and 4, 4-x-MV, respectively, the pH was adjusted bis(bipyridyl)ruthenium(II) band. All of the complexes were to 10.0 with NaOH or > kf, the maximum concentration of viologen radical cation can be expressed in terms of a simple ratio of the forward and back electron transfer rate constants, as shown in eq 9. The excited-state minus groundstate extinction coefficient for the ruthenium chromophore at 600 nm was measured using the appropriate model complex, MVis known (13 700 M-' cm-'),** and the initial concentration of excited state ([Ru*]o) was determined in the usual manner using an aqueous solution of Ru(bpy)s*+ of matched optical density. Equation 9 was solved for kb and used to estimate these charge recombination rate constants.
, , , ,
0.10
ui s
