3892
J. Phys. Chem. 1991,95, 3892-3894
Rh(NN),)+-DQ’+ Rh(~hen)AMe2bpy)~+ Mebpy-DQ2+
-0.62
-0.85 -0.84
-0.60
#In CH,CN/O.I M[TEA]TFB, glassy carbon electrode, vs SCE. bRevenible potential, AEp = 80 mV. Clrreversibleprocess, sweep rate 200 mV/s.
that of the analogous process of the R h ( ~ h e n ) ~ ( M e ~ b pmodel y)~+ compound). The energetics of the intramolecular electron-transfer process can be deduced from the electrochemistry of the dyad. The cyclic voltammetry of Rh(NN)$+-DQ2+ is practically the superposition of the corresponding curves for the Mebpy-DQ2+ and Rh(~hen)~(Me~bpy)’+ model compounds (Table I). The first two features are an appreciably reversible wave corresponding to one-electron reduction of the DQ2+ component (El,: = -0.62 V vs SCE) and an i r r e v e r ~ i b l e ~wave ~ - ~associated ~ with the Rh(NN)33+reduction (E, = -0.85 V vs SCE). At more negative potentials, further reductions of the DQ2+and Rh(NN)d+ components ( E l l 2= -0.95 and E, = -1.05 V vs SCE) take place. Within the uncertainty caused by the irreversible nature of the rhodium-centered reduction, the driving force for the intramolecular electron-transfer step can be estimated as A G O = -0.2 eV. (33) The irreversible redox behavior of the Rh(II1) polypyridine complexes is known in the lit~rature.~*’~ (34) Kew, G.; Hanck, K.;De Armond, K. J . Phys. Chem. 1975,79, 1829. ( 3 5 ) Creutz, C.; Keller, D.; Sutin, N.; Zipp, A. P. J . Am. Chem. Soe. 1982, 104. 3618.
It may be noticed that, in photoinduced-electron-transfer experiments with some related Ru(II)-W+ dyads,Mintercomponent electron-transfer processes with similar driving forces are faster by 1 order of magnitude than that observed here. The relative slowness of this process can be explained by a larger internal reorganizational energy for the Rh(III)/Rh(II) couple relative to the Ru(III)/Ru(II) one, a feature consistent with the irreversible redox behavior of the former species. Of course, arguments based on a different degree of adiabaticity for the two processes cannot be ruled out. The work reported in this Letter shows that bimolecular electron-transfer quenching can be used as a convenient means to trigger and study intercomponent charge-shift processes in covalently linked supramolecular systems. The information so obtained may be used in the design of more complex systems (triads, etc.) for photoinduced multistep charge ~eparation.~’ Finally, a possibility of some practical interest is suggested by this type of work. For the use of Rh(II1) polypyridine complexes (and of related inorganic excited-state one-electron o x i d a n t ~ ~ as ~J~) sensitizers in cyclic photocatalytic systems, the main limitation is constituted by the kinetic instability of the reduced form. Covalent coupling of the photosensitizer to a suitable charge shifter component may be used as a way to circumvent this pr~blem.~’ ’
Acknowledgment. We thank L. Righetti for the drawings. This work has been supported by the Minister0 della Universit3 e della Ricerca Scientifica e Tecnologica and by the Consiglio Nazionale delle Ricerche (Progetto Finalizzato Chimica Fine). (36) &ley, L. F.;Headford, C. E. L.; Elliott, C. M.; Kelley, D. F. J. Am. Chem. SOC.1988, 110,6673. (37) Work is in progress in this direction. (38) Indelli, M. T.; Bignozzi. C. A.: Marconi. A.: Scandola. F. J . Am. Chem. SOC.1988, 110, 7381.
Fluorescence LHetlmes of Diphenyihexatriene in Fiat and Bent Bllayer Lipid Membranes David Yogev, Angelio T. Todorov, and Janos H. Fendler* Dgpartement de Chimie. UniversitC de MontrCal, C.P. 6128, succursale A, MontrCal, QuCbec, Canada H3C 357 (Received: February 12, 1991) Fluorescence lifetimes ( T values) of diphenylhexatriene (DPH) incorporated into glyceryl monooleate (GMO), egg yolk phosphatidylcholine (PC), and L-a-dioleoylphosphatidylcholine(DOPC) bilayer lipid membranes (BLMs) bathed in 0.10 M KCI have been determined to be 7.48,8.23, and 7.90 ns. The T value of DPH in GMO BLMs bathed in H 2 0was observed to be 8.85 ns. Applying hydrostatic pressure to one side of GMO, PC, and DOPC BLMs (bathed in 0.10 M KCl) decreased the fluorescence lifetimesof DPH to 6.65, 7.52, and 7.54 ns, respectively. Reorganization of the surfactant molecules upon bending the BLM to allow the penetration of electrolytesin the bilayer was proposed to be responsible for changes in T values of the DPH probe.
Introduction Bimolecular thick (or bilayer lipid) membranes (BLMs), separating two aqueous compartments, have been used extensively as membrane models.’** Much of our current understanding on ion-transport and impulse transmission has resulted from careful electrical measurements across BLMs in the absence and in the presence of ionophores.M In spite of its importance, only a few reports have appeared on the mechanical properties of BLMs7+ (I )
Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience:
New York, 1982. ( 2 ) Tien H. T. Bilayer Lipid Membranes (ELM). Theory and Practice; Marcel Lkkker: New York, 1974.
(3) Tien, H. T. Prog. Surf. Sci. 1989, 30, 1. Miller, C. Ion Channel Reconstifurion;Plenum: New York. 1986. (5) Hille, B. Ionic Channels of Excitable Membranes; Sinaver Associates: Sunderland. MA, 1984. ( 6 ) Sackmann, B.; Ncher, E. Single-Channel Recording, Plenum: New York, 1983. (4)
and on the effect of mechanical changes on ion transport.loJ’ We recently reported measurements of ultrasmall-pressure-induced curvature changes by two-exposure interferometric holography1* and optical interferometry.” The results obtained allowed the assessment of interfacial tensions and elastic moduli. Attention is focused in the present work on microenvironmental changes of lipid molecules that accompany BLM curving. Advantage has been taken of diphenylhexatriene (DPH) as an environmentally (7) Tien, H. T. J. Phys. Chem. 1967, 71, 3395. (8) Cater, H. G. L.; Simons, R. Elmhim. Eiophys. Acta 1968,163,234. (9) Wobschall, D. J. Colloid Interface Sci. 1971, 36, 385.
(IO) Petrov, A. G.; Ramsey, R. L.; Usherwood, P. N. R. Eur. Eiophys. J.
1989, 17, 13. (I 1) Petrov,
Submitted for
A. G.; Miller, B. publication.
A.; Hristova,
K.;Usherwood, P. N. R.
(12) Picard,G.; Schneider-Henriquez, J. E.; Fendler, J. H. J. Phys. Chem. 1990, 94, 510.
( I 3)
Picard, G.; Denicourt, N.; Fendler, J. H.J . Phys. Chem., in press.
0022-3654/91/2095-3892$02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. IO, I991 3893
Letters
1500
BS
II
0.1
-
0
a\
10
10
ma.
SO
40
yu
I
0
Figure 1. A schematic representation of the system used for the simultaneous electrical and spectroscopicmeasurementsof BLMs: BS, beam splitter; L, lens; P, polarizer; F, filter; M,monochromator; MCP,Hamamatsu multichannel plate; RLC. GENRAD Model 1689 M digibridge; CD, cavity dumper; PD, photodiode; DYE, dye laser.
sensitive fluorescence probe. Fluorescence lifetimes of DPH, incorporated in high dilution into glyceryl monooleate (GMO), egg yolk L-a-phosphatidylcholine (PC), and L-a-dioleoylphosphatidylcholine (DOPC)BLMs, have been shown to markedly decrease upon subjecting the membranes to hydrostatic bending. Experimental Section Glyceryl monooleate (GMO; Sigma), egg yolk phosphatidylcholine (PC; Sigma), L-a-dioleoylphosphatidylcholine (DOPC; Sigma), 1,6-diphenyl-l,3,5-hexatriene(DPH; Molecular Probes, Inc.), and decane (Aldrich) were used as received. Water was purified by a Millipore Milli-Q system containing a 0.4-pm Millistack filter at the outlet. The resistivity of this water was monitored and kept at 18 MQ cm. BLMs were formed actoss a 1.00 f 0.01 mm conical hole drilled in a black Teflon or a black DELRIN film (Curbell, Inc.). The film was pressed diagonally between two compartments with a total internal volume of 5.0 mL. Two Ag/AgCl electrodes were introduced into the two separate compartments containing 0.10 M KCI. BLMs were made by smearing the BLM-forming solution (40 mg of GMO, PC, or DOPC per milliliter of decane) across the aperture. The needle was rinsed with methanol after each dipping. Thinning of the membrane to a BLM was monitored by both capacitance measurements and reflectivity. Capacitance values were measured with a GENRAD Model 1689 M Digibridge. Specific capacitances of GMO, PC, and DOPC BLMs were determined to be 0.387 f 0.010,0.391 f 0.010, and 0.381 f 0.010 pF/cm2. Stringent requirements were placed on BLM experiments. Measurements were performed only on BLMs that had relatively small Plateau-Gibbs borders (
