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J . Phys. Chem. 1991, 95,
1530-1532
specific heats K = CJCU than Ar); this effect opposes the more effective scavenging of H atoms from the sonolysis of water vapor with increasing O2concentration, and therefore a maximum in the yield is ob~erved.'~ In the case of the methanol solution, the temperature is strongly decreased by the high content of methanol vapor (low I( value), the relative concentrations of argon and oxygen (as components with high ratios K ) playing no role anymore. The increase in the yield of hydrogen peroxide with increasing oxygen concentration is therefore not attributed to a change in temperature of the bubbles but to an increase in the efficiency of methanol combustion.
sonolysis of pure water. However, in certain mixtures of the two liquids very much higher yields than in the sonolysis of the components can be observed. The phenomena are understood in terms of the hot-spot theory of sonochemical reaction^,'^ according to which the temperature reached in the adiabatic compression phase of cavitation bubbles depends on the heat content of the gases and vapors in the bubbles. As has been pointed out in many other reports from our laboratory over the past few years,'ss16 the products that are formed in the gaseous part of the hot spot are quite similar to the products in pyrolysis and combustion.
Final Remarks The present investigations show again that the sonochemical yields in a pure organic liquid are much smaller than in the
(14) Neppiras, E. A. Phys. Rep. 1980, 61, 159. (1 5) Henglein, A. Ultrasonics 1987, 25,6. (16) Henglein, A. In Aduances in Sonochemistry; Mason, T. J., Ed.; JAI Press Ltd.: London, Vol. 3, in press.
Photon Antennae Assembled by Nucleic Acid Base Pairingt Anthony Harriman,* Darren J. Magda, and Jonathan L. Sessler* Department of Chemistry and Center for Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 78712 (Received: November 15, 1990)
One or two free-base (HzP) or zinc (ZnP) porphyrins have been covalently attached to the nucleic acid bases cytosine and guanine. Upon mixing in CHCI,, the bases dimerize (-13.3 C AGO < -7.9 kJ mol-I), via multipoint hydrogen bonding, to produce flexible ensembles in which photons harvested by ZnP subunits are transferred to the complementary HzP. Energy transfer is observed from both singlet and triplet excited states of the ZnP subunit.
Spectroscopic investigations of artificial supramolecular systems exhibiting photoinduced energy or electron transfer between molecular subunits have contributed substantially toward our understanding of the natural photosynthetic apparatus. Many elegant models have been devised that favor long-range, vectorial electron transfer between covalently linked subunits'-3 and that resemble the structural features of bacterial photosynthetic reaction center c o m p l e x e ~ . Less ~ ~ ~ attention has been paid to imitating light-harvesting complexes where the pigments are more randomly dispersed and chromophore separation distances vary within the antenna.6 We can suppose that noncovalent molecular interactions may reproduce the requisite morphology for an artificial lightharvesting complex, although interactions stronger than hydrophobic forces may be needed. Complexation via multipoint hydrogen bonding can form regular arrays' that could be utilized to self-assemble photoactive subunits in an ordered structure. Here, we describe some porphyrin-ytosine and porphyrin-guanine conjugates as a first approach to producing hydrogen-bonded photon antennae. Porphyrin-base conjugates were prepared8 by alkylation of a base amine derivative with an electrophilic porphyrin to provide the protected intermediates 1-3 (Figure 1). Deprotection was accomplished by using trifluoroacetic acid or sodium methoxide in methanol for cytosine and guanine derivatives, respectively. The resulting free-base porphyrins 4-6 were metalated by using zinc acetate in methanol to provide zinc porphyrin analogues 7-9. Due to poor solubility in nonpolar solvents it was necessary to attach a solubilizing group at the N atom in the connecting chain for the monomeric porphyrins (4,5,7, and 8). All compounds gave satisfactory 'H NMR and mass spectral analyses. Dimerization of the nucleic acid bases at high concentration was confirmed by 'H NMR studies made with nonporphyrinic analogues in CDCI, solution, from which an association constant of 1220 f 200 M-I was derived for cytosine-guanine pairing. Dimerization of the bases brings the appended porphyrins into 'Presented in part at the 2nd International Chemical Congress of the Pacific Basin Societies, Honolulu, HI, Dec 1989.
0022-3654/91/2095-1530$02.50/0
TABLE I: Photophysical Properties Determined for the Various Porphyrin-Nucleic Acid Base Conjugates in Dilute (=lo4 M)CHCIs Solution compd
no. of porphyrins
4 5 6
I 1 2
I
1 1 2
8 9 H2OEP ZnOEP
base cyt gua
cyt cyt
gua
cyt
0.054 0.068 0.052 0.027 0.027 0.028 0.105 0.035
+s7b
rtra
ns
ps
6.0 5.8 5.4 1.5 1.5 1.4 8.9 1.9
0.38 0.45 0.32 0.56 0.60 0.58 0.58 0.69
4.7 3.7 4.0 5.0 4.0 4.3 200 95
o f l o % . b i O . l ns.
close proximity; space-filling models indicate that porphyrin center-to-center separation distances are 1-3 nm. Both singlet and triplet energy transfer is favorable from zinc porphyrins (ZnP) to free-base porphyrins (H,P) and has been demonstrated in covalently linked porphyrin dimersg and oligomers.1° Thus, excitation of ZnP within a base-derived dimer should result in energy transfer to the adjacent H2Pprovided the latter lies within ( I ) Gust, D.; Moore, T. A. Science 1989, 244, 3 5 . ( 2 ) Wasielewski, M. R. Photochem. Photobiol. 1988, 47, 923. (3) Mataga, N. In Photochemical Energy Conversion; Norris, J. R.,
Meisel, D., Eds.; Elsevier: New York, 1989, p 32. (4) Deisenhofer, J.; Epp, 0.;Miki, K.;H u h , R.;Michel, H. J. Mol. Biol. 1984, 180, 385. ( 5 ) Chang, C.-H.; Tiede, D.; Tang, J.; Smith, U.;Norris, J. R.; Schiffer, M. FEBS Lett. 1986, 205, 82. (6) Glazer, A . N.; Melis, A. Annu. Reu. Plant Physiol. 1987, 38, 11. (7) Lehn, J.-M.; Mascal, M.; DeCian, A.;Fischer, J. J . Chem. Soc., Chem. Commun. 1990, 419. (8) (a) Sessler, J. L.: Magda, D. J.; Hugdahl, J. J . Inclusion Phenom. 1989, 7, 19. (b) Harriman, A.; Magda, D. J.; Sessler, J. L. J . Chem. Soc.. Chem. Commun., in press. (9) Brookfield, R. L.; Ellul, H.; Harriman, A.; Porter, G . J . Chem. SOC., Faraday Trans. 2 1986, 82, 219. (IO) Davila, J.; Harriman, A.; Milgrom, L. R. Chem. Phys. Lett. 1987, 136, 427.
0 1991 American Chemical Society
Letters
4 X-&,
The Journal of Physical Chemistry, Vol. 95, No. 4, 1991
R-H
7 x-211, R - H
5 X-b,
1531
6 X-y,R-H
R-H
0 X-Zn, R-H
8 X-Zn, R-H
Figure 1. Structurcs of the porphyrin-nucleic acid base conjugates.
the critical distance for energy transfer. For singlet energy transfer occurring by the Forster mechanism the critical distance is 1.8 nm. At 5 mM, the average intermolecular separation distance is 8.6 nm and the translational distances over which porphyrins could diffuse within the ZnP excited singlet and triplet state lifetimes are 0.8 and 200 nm, respectively. Thus, bimolecular processes may occur from the triplet but not from the singlet state. Absorption and fluorescence spectra recorded for the porphyrin-base conjugates in ethanol-free CHCI, were indistinguishable from those of reference porphyrins not having the appended base. Fluorescence quantum yields (af),excited singlet ( T J , and triplet ( T J state lifetimes, and quantum yields for formation of the triplet state (at)were measured for each compound in dilute (=IO” M) CHC13 solution (Table I). Although the nature of the base had no effect on the photophysical properties of the porphyrins, the measured lifetimes and yields are reduced significantly compared to free-base octaethylporphyrin (H20EP) and its zinc complex (ZnOEP). This quenching effect is attributed to photoinduced electron abstraction from the tertiary amine in the bridge, although nanosecond laser flash photolysis studies in CHCl3 did not indicate the intermediate formation of the porphyrin r-radical anion.” The excited-state lifetimes were independent of concentration, at least below 5 mM, and absorption spectroscopy indicated the absence of exciton coupling effects between adjacent porphyrin rings in compounds 6 and 9. At the highest concentrations, where we expect partial dimerization of the bases,I2 energy hopping between porphyrin subunits can occur but this process does not affect the excited-state lifetimes. Aggregation of the porphyrin nuclei docs not occur under these conditions since there is no distortion of the Soret bands and Beer’s law is followed over the entire conccntration range. Separate experiments showed that neither cytosine nor guanine quenched the excited states of H20EP or ZnOEP, although complexation occurred at very high concentration (20.05 M) of base. Excited-state lifetimes were measured for solutions containing equimolar concentrations of both HIP- and ZnP-derived conjugates, concentrations being expressed in terms of base. Timeresolved fluorescence studies were made in air-equilibrated CHC13 using a mode-locked, synchronously pumped, cavity-dumped Rhodamine 6G dye laser with excitation at 570 nm with singlephoton-counting (fwhm 70 ps) detection. At this wavelength, approximately 85% of incident photons are absorbed by ZnP. Fluorescence from H2P, essentially free from fluorescence from ZnP, could be observed at 685 nm. In all cases, this fluorescence decayed via a single-exponential process having a fluorescence lifetime corresponding to that given in Table I. The measured lifetimes were independent of concentration within the range ( 1 1) Triethylamine quenches the singlet and triplet excited states of the reference porphyrins, albeit at slow rates, to generate the porphyrin r-radical anion as observed by laser flash photolysis. Our failure to observe intermediate species from the porphyrin-base conjugates is consistent with rapid intramolecular reverse electron transfer leading to re-formation of ground-state species. (12) Williams, L. D.; Chawla, B.; Shaw, B. R. Biopolymers 1987.26.591.
TABLE II: Rate Constants for Energy Transfer (k)within the Nucleic Acid Base Dimers and Association Constants ( K )Measured in Chloroform at 22 OC no. of ~,,(1 r2.a k,,llO*, ktt/lOS, K,b dimer bases DorDhvrins ns ns s-I s-’ M-’ 4-7 4-9 6-1
6-9 5-8 4-8 5-7 6-8 5-9
2 3 3 4
2 2 2 3 3
1.58 1.37 1.60 1.42 1.41 1.47 1.52 1.44
1.16 0.80 0.77 0.61 0.82 0.87 0.86 0.63 1.51 0.65
2.3 5.2 6.8 9.4 5.1 4.7 5.1 9.0 8.7
nd 8.4 4.9 92
nd 16.5 12.8 32.5 34.0
51 41 48 40 24 225 190 200 215
‘ f 0 . 0 5 ns. b f 2 0 % .
(1-50) X M. Fluorescence emitted by ZnP could be monitored at 600 nm where H2P does not emit. At low M) concentrations, fluorescence decay could be described in terms of a single-exponential process having a lifetime similar (Le. f0.2 ns) to that shown in Table I. As the concentration increased, however, the decay profiles became increasingly dual exponential due to the appearance of a faster decaying component in addition to the regular lifetime (Table 11). The fractional amplitude of this shorter lived species increased with increasing concentration but the measured lifetime remained constant (fO.l ns) for a particular porphyrin conjugate pair. Similar behavior was observed for systems in which the concentration of ZnP was fixed at 2 mM and the concentration of HIP was varied over the range 0-5 mM. Experiments performed with ZnOEP or H,OEP replacing the appropriate porphyrin-base conjugate gave no indication of the shorter lived species. Similarly, addition of ethanol (10% v/v), which will break hydrogen bonding between bases, removed the shorter lived component causing the fluorescence decay profiles to become single exponential with a lifetime within the range of (1.5 f 0.2) ns. We assign the shorter lifetime ( T ~ to ) a ZnP subunit within a base-derived dimer that possesses both ZnP and H2Psubunits such that electronic energy transfer occurs between porphyrins. This proposal, which is depicted in Figure 2 for a cytosine-guanine complex possessing mixed porphyrins, was supported by the observation that fluorescence due to the H2P subunit grew in on a time scale longer than the instrument response function. The rate of appearance of H2P fluorescence corresponded to the fast decaying component in the ZnP fluorescence profiles. Assuming that the longer lifetime ( T , ) is due to ZnP for which the base is either noncomplexed or complexed to an identical ZnP-base, rate constants for singlet-state energy transfer (kSJ were calculated ( k , = ( ( 1 / ~ * )- ( l / ~ ~ )and ] ) are collected in Table 11. The derived rate constants are not markedly dependent on the nature of the base but there appears to be an increase in the rate of energy transfer with increasing number of porphyrins within an ensemble. This is attributed to the increased probability of finding a porphyrin acceptor molecule having the preferred orientation and
Letters
1532
7. A-
-*
@.ITFigure 2. Rcprcscntation of the
hydrogen-bonded dimer 5-8 and subsequent energy transfer from ZnP to H2P.
separation distance for efficient Forster energy t r a n ~ f e r . ~ * l ~ Related experiments were made in which the mixed porphyrin conjugates in N2-saturated CHCl3were excited with a IO-ns laser pulse at 532 nm and the triplet states of the porphyrin subunits were monitored by transient absorption spectroscopy. Unfortunately, the strongly overlapping triplet spectra9 do not favor accurate kinetic analysis. At 487 nm the differential triplet absorption spectrum of H2P has an isosbestic point whereas that of ZnP absorbs with modest extinction coefficient. Monitoring at this wavelength, it was observed that decay of ZnP triplet could be described satisfactorily in terms of a single-exponential process for which the rate constant increased linearly with increasing concentration of HzP. This is due to intermolecular triplet energy transfer between noncomplexed porphyrin conjugates. Similar behavior was observed for mixtures of ZnOEP and HzOEP. To avoid the bimolecular process, experiments were made using a high concentration (2 mM) of a ZnP-derived conjugate and adding low concentrations (0-50 fiM) of an HzP-derived conjugate. Under such conditions, all incident photons are absorbed by the ZnP subunit. Measurements made at 585 nm, an isosbestic point for the differential triplet absorption spectrum of ZnP, showed that the triplet of the HzP subunit grew in after the laser pulse by first-order kinetics. The rate of formation was independent of HIP concentration (
