3312
J. Phys. Chem. 1996, 100, 3312-3316
Time-Resolved EPR Study of Photoexcited Triplet-State Formation in Electron-Donor-Substituted Acridinium Ions Hans van Willigen* Department of Chemistry, UniVersity of Massachusetts at Boston, Boston, Massachusetts 02125
Guilford Jones, II* and Mohammad S. Farahat Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215 ReceiVed: October 27, 1995; In Final Form: January 17, 1996X
The 10-methylacridinium ion displays an emission associated with a charge-shifted (CSH) species when substituted in the 9-position with a substituent having a relatively low ionization potential (naphthyl, biphenyl). Flash photolysis and time-resolved EPR (TREPR) measurements show that photoexcitation of these donoracceptor systems generates an acridinium-localized excited (LE) triplet state. While values of zero-field splitting parameters are virtually unaffected by the nature of the substituent, spin polarization patterns observed in the TREPR spectra display a striking dependence on substituent as well as orientation of donor ring system. Flash photolysis and TREPR data show that the LE triplet state is formed from the CSH singlet state. In these directly linked donor-acceptor molecules, in which the aromatic rings are near perpendicular because of steric hindrance, CSH singlet f LE triplet intersystem crossing (isc) is driven by spin-orbit coupling. This mechanism generates a unique dependence of isc spin selectivity on molecular structure which in one case even results in a temperature-dependent spin polarization pattern. The results demonstrate that TREPR can be a valuable source of information on molecules with twisted internal charge-transfer (TICT) states.
A mechanism for formation of excited triplet states that involves a photogenerated radical-ion pair intermediate has been discovered under varied circumstances. The “triplet recombination” mechanism that accompanies electron-transfer fluorescence quenching in polar media was recognized by Weller and coworkers some years ago.1 Photogenerated radical-ion pairs capable of intersystem crossing and population of a triplet state associated with one partner,
D* + A f 1(D+‚‚‚A-) f 3(D+‚‚‚A-) f 3D + A
(1)
have been the object of a number of magnetic resonance and magnetic field effect studies.2 Similar intermediates have been implicated in mechanisms of photosensitization (for isomerization and other reactions) for which CIDNP and related effects can be observed.3,4 Of special importance is the example of electron transfer in photosynthetic reaction centers that are modified by having their quinone acceptor sites rendered inactive. In this well-studied system, electron transfer between chlorophyll and pheophytin moieties follows a mechanism of early termination through a return electron transfer from the radical-pair state to local chlorophyll triplet.5 S-T0 mixing in this radical pair by Zeeman and hyperfine interactions gives rise to a unique spin polarization pattern in the time-resolved EPR (TREPR) spectrum of the chlorophyll “special pair” in reaction centers of photosynthetic bacteria.5,6 Very recently, Hasharoni et al. reported on a model system that mimics the characteristic features of the TREPR spectrum of the “special pair” triplet.7 The present report features a system in which an acridinium chromophore has been modified by single-bond attachment of aryl groups that serve as potential electron donors (structures X
1-4). The charge-transfer (charge-shift, CSH) nature of the
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-3312$12.00/0
excited state of these compounds has been reported previously.8-10 Crystallographic studies of the 9-aminophenyl derivative9 and molecular mechanics structure calculations for 1-4 indicate that the twist angles for the electronic ground state are ∼90°. Photophysical characteristics of these systems are affected by the twisting enforced at the point of fusion by nonbonded interactions between aryl rings.8,10 The work presented here is concerned with a flash photolysis and TREPR study of the role played by the CSH state in the formation of the acridinium-localized excited (LE) triplet state. It is found that the population of the LE triplet via the CSH singlet state exhibits a spin selectivity that is very sensitive to the relative orientation of donor and acceptor moieties. The results demonstrate that TREPR is a valuable tool for the study of molecules with TICT state characteristics. Experimental Section Compounds 1-4 were prepared using the Bernthsen reaction.10 Sucrose octaacetate (SOA, Aldrich) was purified by © 1996 American Chemical Society
Letters column chromatography (Florisil) and repeated crystallization from ethanol. Glassy samples for spectroscopic measurements were prepared by thoroughly mixing the compound with SOA powder and heating the mixture to a melt (∼100 °C) in a 4 mm EPR sample tube. The sample was then slowly cooled to room temperature and sealed off. Absorption spectra were recorded on a Beckman DU-7 spectrophotometer and corrected fluorescence spectra were obtained with an SLM 48000S instrument. Absolute fluorescence quantum yields were determined for argon-bubbled samples with reference to 10-methylacridinium chloride in water (φf ) 1.011). Fluorescence lifetimes were measured with the phase shift and modulation technique12 using an SLM 48000S instrument with 370 nm as the excitation wavelength. Phosphorescence spectra were obtained with a cylindrical chopper unit installed in the cell compartment of the fluorimeter and adjusting the time delay to the minimum value of 4.5 ms. Nanosecond laser flash photolysis experiments were carried out with the instrument described previously.13 Solutions were prepared in 1 × 2.2 cm Pyrex cells (10-50 µM) and were bubbled 15-20 min with argon gas prior to measurements at room temperature (22 °C) using a Nd:YAG laser (355 nm, 8 ns pulses) for sample excitation. TREPR spectra were recorded with a VARIAN E9 X-band EPR spectrometer operating in the direct-detection mode (i.e., no field modulation/phase-sensitive detection) using a boxcar integrator.14 The output of a Lambda Physik dye laser (FL3001, 440 nm, 2 mJ/pulse, 40 Hz repetition rate) pumped by a Lambda Physik 103MSC excimer laser was used for sample excitation. In all experiments the EPR signal was detected ∼0.5 µs after laser excitation of the sample, the detection time window was about 300 ns and the microwave power was set at 10 mW. Solutions in SOA glass were measured from room temperature down to -170 °C. Solutions in THF were measured at about -170 °C. In some cases methyl iodide (MeI) was added to the solution to promote S-T intersystem crossing (isc) with the heavy-atom effect.15 TREPR spectra were simulated with a computer program based on the procedure outlined by Kottis and Levefbre.16 Molecular mechanics calculations provided potential energy curves as a function of dihedral angle between donor and acridinium moieties. Calculations were performed on a Silicon Graphics work station using the DISCOVER module in the INSIGHT environment (Biosymm Technologies). The geometries of ions were minimized using the CVFF forcefield with steepest descent and Newton-Raphson methods. Results Optical Spectra. The absorption spectra in the near UV and visible of compounds 1-4 display a dominant band at ∼360 nm ( ≈ 20 000 M-1 cm-1) associated with the long-axis polarized excitation of the acridinium chromophore. The weaker transition that is common to acridinium17 centered at ∼425 nm is altered in a subtle way on introduction of different substitutents at the 9-position.8,10 These changes depend on the ionization potential of the substituent and range from ca. 5 to 50 nm red shifts with naphthyl, biphenyl, and aminophenyl8 substitution. On introduction of these groups, the fluorescence emission spectrum of the acridinium ion is altered as well.10 The emission and fluorescence excitation spectra of 2 are shown in Figure 1. As the fluorescence data in Table 1 show, the position of the band that replaces the 500 nm emission of the local acridinium chromophore is dependent on the identity of the substituent and is somewhat solvent sensitive with respect to both position and
J. Phys. Chem., Vol. 100, No. 9, 1996 3313
Figure 1. Corrected excitation and emission spectra for 1 and 2 in acetonitrile (20 °C). Excitation spectra were monitored at the uncorrected emission peaks (500 and 640 nm, respectively).
TABLE 1: Photophysical Properties of Acridinium Ions in Methylene Chloride and Acetonitrile at 20 °C CH2Cl2 compd 1 2 3 4
λaa (nm)
λfb (nm)
362 430 360 430 360 432 361 437
CH3CN
φfc
τfd (ns)
λaa (nm)
λfb (nm)
φfc
τfd (ns)
500
0.085
2.5
496
0.063
1.5
609
0.015
5.2
656
0.0039
3.0
597
0.052
7.3
662
0.0032
1.4
561
0.13
5.2
360 424 360 423 359 428 360 428
628
0.020
1.2
a Position of the absorption maxima for the two lowest energy bands. Position of fluorescence maxima for 370 nm excitation. c Fluorescence quantum yields. d Fluorescence lifetimes from phase-shift and modulation measurements. b
intensity. Fluorescence quantum yields and lifetimes of the acridiniums in two solvents are summarized in Table 1. The optical data are consistent with a charge-shift process in which a localized excited (LE) state is converted into an excited state in which charge is displaced to the 9-aryl group.10
Ac+-D f 1Ac+*-D f 1Ac-D+ f Ac+-D
(2)
Consistent with this assignment, picosecond transient absorption measurements show bands due to the acridinyl radical (500 nm) and aryl cation radical (∼680 nm for oxidized naphthyl and biphenylyl species). These transients decay with the same time constant as the CSH fluorescence.10 For 2-4, excitation in a glassy medium results in dominant LE fluorescence at the expense of the red-shifted emission from the CSH state. In addition, emission spectra obtained at low temperature are structured and have both fluorescence and phosphorescence components. On use of a chopper for the resolution of bands with lifetime >1 ms, spectra virtually identical with that assigned to phosphorescence of the acridinium triplet (λmax ≈ 650 nm)10 are obtained. Clearly, the chargetransfer process that leads to emission from the CSH state requires some solvent and/or intramolecular reorganization and is attenuated as the viscosity of the solvent medium is increased. Nanosecond laser flash photolysis spectra of compounds 2-4 in methylene chloride show a common transient absorption band with λmax ) 510 nm (cf. Figure 2). This transient is attributed to the LE triplet state of the acridinium cation rather than a CSH triplet state because of the close correspondence with the spectrum of the photo transient reported for the 10-methylacri-
3314 J. Phys. Chem., Vol. 100, No. 9, 1996
Letters
Figure 4. TREPR spectrum of 2 in SOA at -90 °C. The calculated spectrum is given by the solid line.
Figure 2. Transient absorption spectra given by argon-purged solutions of (top) 1 (40 µM with 0.8 M CH3I) and (bottom) 2 (40 µM) in methylene chloride after 355 nm laser excitation.
Figure 3. TREPR spectrum of 1 with CH3I in SOA at -170 °C. The calculated spectrum is given by the solid line.
dinium cation.18 A similar transient was observed for 1 only when isc was enhanced by the presence of methyl iodide or butyl iodide in solution. For 2-4 the transients observed are weak, consistent with the dominance of singlet excited state decay via the CSH state (eq 2). However, the 1-naphthyl derivative (2) gave a noticeably higher yield than 4 (3×) and 3 (2×). The triplet yield for 3 and 4 could be increased as well by the presence of a heavy-atom solvent (CH3I, ∼1 M). It is noted that transient absorption measurements did not show the formation of a CSH triplet state which would display bands characteristic for the acridinyl radical (500 nm) and radical cation (∼680 nm for naphthyl and biphenylyl). TREPR Spectra. Solid solutions of 1 and 4 in SOA or THF gave (weak) TREPR spectra of photoexcited triplets only at low temperatures and upon enhancing isc with MeI. The spectrum given by 1 is shown in Figure 3. It can be simulated reasonably well (cf. Figure 3) with the zero-field splitting (zfs) parameters D ) 0.068 cm-1 and E ) 0.0062 cm-1. The low-field emission/ high-field absorption (eeeaaa) pattern can be accounted for in terms of spin-selective isc to the |x〉, |y〉, and |z〉 zero-field spin
Figure 5. TREPR spectrum of 3 in SOA at 20 °C (top) and -165 °C. Calculated spectra are given by the solid lines.
levels with relative probabilities px ) 0.6, py ) 0.3, pz ) 0.1.19 In labeling the EPR transitions it has been assumed that D > 0, as is normally the case for ππ* triplets, and that the relative order of triplet spin levels is |x〉 > |y〉 > |z〉.19 The spectrum of 4 is virtually identical with that of 1. Furthermore, TREPR spectra obtained upon photoexcitation of the naphthyl-substituted acridiniums (2 and 3) in solid solutions containing MeI exhibit the same emission/absorption pattern as that given by the phenyl- and biphenyl-substituted compounds and show only minor changes in D and E values. In contrast with the results obtained with 1 and 4, compounds 2 and 3 in glassy SOA, with no MeI added, gave triplet spectra over the entire temperature range covered in this study (-170 °C to room temperature). Spectra from 2 at -90 °C and 3 at 20 and -165 °C are shown in Figures 4 and 5, respectively. While the D and E values derived from these spectra are identical with those found for the spectra given by 2 and 3 in
Letters
J. Phys. Chem., Vol. 100, No. 9, 1996 3315
TABLE 2: Zero-Field Splitting Parameters and Relative Rates of Population of Zero-Field Spin States compd
D (cm-1)
E (cm-1)
px
py
pz
1a
0.068 0.0644 0.0607 0.0605
0.0062 0.0053 0.0055 0.0048
0.6 0.15 0.25 0.46
0.3 0.63 0.10 0.0
0.1 0.22 0.65 0.54
2b 3c 3d
a In SOA with MeI recorded at -170 °C. b In SOA at -90 °C. c In SOA at room temperature. d In SOA at -165 °C.
the presence of MeI, spin polarization patterns differ dramatically. In the case of 2 the pattern is eaeaea, which can be simulated with relative population rates px ) 0.15, py ) 0.63, pz ) 0.22 (cf. Figure 4). The spectrum of this compound was found to be independent of temperature except for a gradual increase in signal intensity with decreasing temperature which can be attributed to an increase in spin-lattice relaxation time. The spectrum from 3 recorded at room temperature shows an aaaeee pattern which is simulated with px ) 0.25, py ) 0.10, pz ) 0.65 (cf. Figure 5a). Contrary to what is found for 2, the relative isc rates are temperature dependent which shows up in the form of a change in the spin polarization pattern. At -165 °C the polarization pattern has changed to aeaeae simulated with px ) 0.46, py ) 0, pz ) 0.54 (cf. Figure 5b). Zero-field splitting parameters and isc probabilities extracted from the TREPR spectra are summarized in Table 2. Discussion The data in Table 2 show that D and E values of the 9-substituted acridinium triplets show little variation with change in substituent. Furthermore, they are very similar to those reported for the anthracene triplet, D ) 0.0704 and E ) 0.008 cm-1.20 Therefore, it can be concluded that photoexcitation of 1-4 results, to varying extents, in formation of a ππ* triplet state localized on the acridinium ring system. Phosphorescence and flash photolysis data are in agreement with this conclusion. TREPR spectra show that the 3LE state can be generated via two routes. One, common to all four compounds, involves promotion of isc with the heavy-atom effect (i.e., MeI). In the case of 1, which shows no photoinduced internal charge transfer, MeI promoted triplet formation must be direct 1LE f 3LE isc. Judging from the similarity of polarization patterns in the EPR spectra, triplet formation in 2-4 in the presence of MeI must involve the same direct process. By contrast, 3LE generation in 2 and 3, without assistance of MeI, is governed by spin selection rules that differ markedly from those that apply for the direct process (cf. Table 2). Femtosecond flash photolysis measurements10 show that in these compounds 1LE f 1CSH electron transfer occurs with a rate of ∼1012 s-1 . Hence it is likely that 1LE f 3LE isc cannot compete with photoinduced electron transfer. It follows that the triplets must be formed instead as a result of back charge transfer from the CSH state. That the CSH state is involved in triplet state formation is confirmed by the strong dependence of phosphorescence intensity on substituent. The remarkable dependence of phosphorescence intensity and polarization patterns in TREPR spectra on substituent linkage (i.e., 1-naphthyl versus 2-naphthyl), which clearly does not affect the electronic structure of the 1LE and 3LE states, also provides evidence for the role of back charge transfer. TREPR spectra of triplets formed by back electron transfer have been observed for prereduced photosynthetic reaction centers5,6 and a covalently linked donor-acceptor system.7 In these cases, 3LE formation involves S f T0 spin state evolution in the radical pairs driven by Zeeman and hyperfine interactions.
This radical pair (RP) isc mechanism gives rise to a unique (aeeaae) polarization pattern in the 3LE EPR spectra.5-7 S-T0 mixing can play a role only in cases where the singlet-triplet splitting (2J) in the radical pair is of the order of the hyperfine interactions (∼0.001 cm-1). If J is of the order of the Zeeman splitting (∼0.3 cm-1), S-T-1 (or T+1) mixing can be an isc mechanism in the radical pair. In that case, the TREPR spectrum of 3LE will either be completely in absorption or emission. Polarization patterns found in the spectra of the naphthyl-substituted acridiniums evidently do not match those associated with these RP isc mechanisms. This is not surprising because in these systems 2J is expected to far exceed magnetic interactions that can mix singlet and triplet RP states. Two alternatives can be considered to account for the spectroscopic data. One involves RP isc driven by spin-orbit coupling followed by back charge transfer:
Ac-D+ f 3Ac-D+ f 3Ac+-D
1
(3)
If this mechanism applies, spin-selective isc from 1CSH to 3CSH generates spin polarization which in the subsequent back-chargetransfer step is projected onto the 3LE state. The observed polarization pattern then depends on the relative orientation of the two aromatic ring systems and is expected to change going from 2 to 3. As was noted earlier, there is no spectroscopic evidence that the 3CSH state is actually formed as a precursor of 3LE which raises doubt that (3) plays a role in 3LE formation. An alternative mechanism is a direct 1CSH f 3LE transition induced by spin-orbit coupling: 1
Ac-D+ f 3Ac+-D
(4)
This mechanism is favored by the large dihedral angle between the aromatic planes which ensures that π-electron transfer from acridinium (Ac) to naphthyl cation (D+) generates the necessary torque to flip the electron spin. Spin-orbit coupling matrix elements linking 1CSH to individual spin states of the 3LE manifold are distinct and depend on the relative orientation of the aromatic planes.21 Hence, this mechanism will give rise to an orientation-dependent isc spin selectivity which can account for the pronounced difference in px , py , pz (cf. Table 2) upon going from 2 to 3. It is also noted that molecular mechanics calculations show a single, broad, energy minimum for 2 over a dihedral angle range of 80-100° with a steep increase in energy as the twist angle decreases further.10 On the basis of this result, one predicts that the aromatic rings in 2 can assume a relatively broad range of relative orientations but that this distribution will not be affected significantly by changes in temperature. Consistent with this conclusion, it is found that the spin polarization pattern in the triplet spectrum from 2 shows no temperature dependence. For 3, on the other hand, our calculations show energy minima for dihedral angles of 72° and 108° with an energy barrier of ∼47 cm-1 between the two. That in 3 a twist angle less than 90° is favored is consistent with the fact that steric hindrance is less than in 2. For 3 calculations predict that the distribution in geometric structure is dependent on temperature. This can account for the observed temperature dependence of the TREPR polarization pattern. Finally, spinorbit coupling should be a function of spin density at the carbon atom in the naphthyl group that is bound to acridinium.21 Consistent with this it is found that 3LE formation in the 1-naphthyl-substituted derivative (C1 spin density in the cation radical ∼0.1822) is significantly higher than in the 2-naphthyl derivative (C2 spin density ∼0.0722). It can be concluded that the experimental data are consistent with a 1CSH f 3LE mechanism of triplet-state formation.
3316 J. Phys. Chem., Vol. 100, No. 9, 1996 Conclusion. Systems consisting of directly fused aromatic rings with twisted geometries constitute an important class of structures for observation of intramolecular electron transfer (generally, the formation of twisted intramolecular charge transfer, or TICT, states).23,24 Triplet intermediates have only rarely been investigated for these systems. As demonstrated in this work, spin selectivity of the isc process is a sensitive function of the relative orientation of donor and acceptor rings. As a consequence, EPR measurements can provide valuable information on the structure of these systems and charge-transfer dynamics associated with these systems. Acknowledgment. We thank the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy for financial support to the University of Massachusetts and to Boston University. Dr. Igor Koptyug of the International Tomography Center, Novosibirsk, Russia, is thanked for providing us with a copy of his triplet EPR simulation program. References and Notes (1) Schulten, C. K.; Staerk, H.: Weller, A.; Werner, H. J.; Nickel, B. Z. Phys. Chem. 1976, 101, 371. (2) For a recent review and leading references see: Steiner, U. E.; Ulrich, T. Chem. ReV. 1989, 89, 51. (3) Roth, H. D. Acc. Chem. Res. 1987, 20, 343. (4) (a) Jones, G., II; Schwarz, W.; Malba, V. J. Phys. Chem. 1982, 86, 2286. (b) Schwarz, W.; Dangel, K. M.; Jones, G., II; Bargon, J. J. Am. Chem. Soc. 1982, 104, 5686. (5) Dutton, P. L.; Leigh, J. S.; Seibert, M. Biochem. Biophys. Res. Com. 1972, 46, 406. Thurnauer, M. C.; Katz, J. J.; Norris, J. R. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 3270. (6) Levanon, H.; Norris, J. R. Chem. ReV. 1978, 78, 185.
Letters (7) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.; Svec, W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1995, 117, 8055. (8) Jonker, S. A.; Ariese, F.; Verhoeven, J. W. Recl. TraV. Chim. PaysBas 1989, 108, 109. (9) Jonker, S. A.; van Dijk, S. I.; Goubitz, K.; Reiss, C. A.; Schuddeboom, W.; Verhoeven, J. W. Mol. Cryst. Liq. Cryst. 1990, 183, 273. (10) Jones,G., II; Farahat, M. S.; Greenfield, S. R.; Gosztola, D. J.; Wasielewski, M. R. Chem. Phys. Lett. 1994, 229, 40. (11) Weber, G.; Teale, F. W. J. Trans. Faraday Soc. 1957, 53, 646. (12) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (13) Malba, V.; Jones, II, G.; Poliakoff, E. D. Photochem. Photobiol. 1985, 42, 451. (14) van Willigen, H.; Vuolle, M.; Dinse, K. P. J. Phys. Chem. 1989, 93, 2441. (15) Turro, N. J. Modern Molecular Photochemistry; Benjamin Cummings Publishing: Menlo Park, 1978; Chapter 6. (16) Kottis, P.; Levebvre, R. J. Chem. Phys. 1963, 39, 393. (17) Acheson, R. M., Ed. Acridines, 2nd ed.; Interscience Publishers: New York, 1973; Chapter 11. (18) Kasama, K.; Kikuchi, K.; Nishida, Y.; Kokubun, H. J. Phys. Chem. 1981, 85, 1291, 4148. (19) A detailed discussion of spin selectivity in formation and decay of photoexcited triplets of aromatics and its effect on EPR signals can be found in: Sixl, H.; Schwoerer, M. Z. Naturforsch. 1970, 25A, 1383. (20) Clarke, R. H. Chem. Phys. Lett. 1970, 6, 413. (21) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92. (22) According to a simple HMO calculation: Coulson, C. A.; Streitwieser, Jr., A. Dictionary of π-Electron Calculations; Pergamon Press: New York, 1965. (23) Rettig, W. Top. Curr. Chem. 1994, 169, 253. (24) Jones, G., II; Farahat, M. S. In AdVances in Electron Transfer Chemistry; Mariano, P. S., Ed.; JAI Press: Greenwich, CT, 1993; Vol. 3, p 1.
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