J. Phys. Chem. A 2006, 110, 7323-7333
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Spin Dynamics of Photogenerated Triradicals in Fixed Distance Electron Donor-Chromophore-Acceptor-TEMPO Molecules Qixi Mi, Erin T. Chernick, David W. McCamant, Emily A. Weiss, Mark A. Ratner,* and Michael R. Wasielewski* Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity, EVanston, Illinois 60208-3113 ReceiVed: February 26, 2006; In Final Form: April 21, 2006
The stable free radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO, T•) was covalently attached to the electron acceptor in a donor-chromophore-acceptor (D-C-A) system, MeOAn-6ANI-Phn-A-T•, having welldefined distances between each component, where MeOAn ) p-methoxyaniline, 6ANI ) 4-(N-piperidinyl)naphthalene-l,8-dicarboximide, Ph ) 2,5-dimethylphenyl (n ) 0,1), and A ) naphthalene-1,8:4,5bis(dicarboximide) (NI) or pyromellitimide (PI). Using both time-resolved optical and EPR spectroscopy, we show that T• influences the spin dynamics of the photogenerated triradical states 2,4(MeOAn+•-6ANI-PhnA-•-T•), resulting in modulation of the charge recombination rate within the triradical compared with the corresponding biradical lacking T•. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn+• and A-• is not altered by the presence of T•, which interacts most strongly with A-• and accelerates radical pair intersystem crossing. Charge recombination within the triradicals results in the formation of 2,4(MeOAn-6ANI-Phn-3*NI-T•) or 2,4(MeOAn-3*6ANI-Phn-PI-T•) in which T• is strongly spin polarized in emission. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, the rate at which the net spin polarization appears on T• closely follows the photogenerated radical ion pair decay rate. This effect is attributed to antiferromagnetic coupling between T• and the local triplet state 3*NI, which is populated following charge recombination. These results are explained using a switch in the spin basis set between the triradical and the three-spin charge recombination product having both T• and 3*NI or 3*6ANI present.
Introduction Controlling the lifetimes of photoinduced radical ion pairs is important for developing molecular materials for electronics, photonics, and spintronics.1-16 We have developed several methods for controlling charge transport in organic donorchromophore-acceptor (D-C-A) triads using multiple ultrafast laser pulses to manipulate the course of electron-transfer reactions.17-19 Building on this work, we are now exploring the use of spin dynamics to control charge and spin transport properties within D-C-A molecules. Photogenerated radical pairs are capable of exhibiting coherent spin motion over microsecond time scales,20,21 which is considerably longer than coherent phenomena involving photogenerated excited states. This affords the possibility that coherent spin motion can provide the basis for novel organic computational devices.22-31 A greater fundamental understanding of the factors controlling spin dynamics in complex organic donor-acceptor systems is necessary to achieve this goal. The rate of radical pair intersystem crossing (RP-ISC) between photogenerated singlet and triplet radical pairs (RPs) has been shown to increase in the presence of stable free radicals and triplet state molecules.32-36 In some of these systems, the photoinduced RP is covalently linked to the stable radical using a flexible connection, which results in a broad distribution of distances and spin-spin interaction strengths that make the analysis of the spin dynamics difficult.37-40 We have addressed this problem by rigidly attaching a stable radical to a donoracceptor system having restricted distances and orientations between all molecular components. We recently showed that * To whom correspondence
[email protected].
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covalently attaching a 2,2,6,6-tetramethylpiperidinoxyl (TEMPO, T•) stable free radical to the acceptor of a rigid D-C-A molecule perturbs charge recombination rates via an enhanced intersystem crossing (EISC) mechanism similar to that observed for intermolecular systems,32,33,35 while not altering the spinspin exchange interaction within the photogenerated radical ion pair.41 Given these encouraging results, we have expanded our studies of how the spin dynamics of D-C-A triads are influenced by the presence of a third spin attached specifically to the electron acceptor at a relatively close distance. The spin density distribution of T• is localized largely on its N-O group and not on the molecule to which T• is appended,42,43 so that attachment of T• to A within the triad does not result in significant spin delocalization onto A itself. We have modified the well-characterized D-C-A triad system, MeOAn-6ANI-Phn-NI (or PI), 1a-4a,41,44,45 where MeOAn ) p-methoxyaniline, 6ANI ) 4-(N-piperidinyl)naphthalene-1,8dicarboximide, Ph ) 2,5-dimethylphenyl (n ) 0, 1), NI ) naphthalene-1,8:4,5-bis(dicarboximide), and PI ) pyromellitimide to probe the spin dynamics of the photogenerated threespin system (Scheme 1). We have demonstrated that photoexcitation of D-C-A triads 1a-4a with 416-nm light results in a two-step charge separation (CS) to form a singlet radical ion pair (RPS), which undergoes radical pair intersystem crossing (RP-ISC) to the corresponding triplet radical ion pair (RPT).41,45-47 Charge recombination (CR) can occur either from RPS to the ground state or from the RPT to the lowest neutral triplet state. We have now synthesized analogues of this system: MeOAn6ANI-Phn-NI(or PI)-T•, 1b-4b, in which the stable T• radical is attached to the terminal N-imide of either the NI or PI acceptors. The resulting molecules position T• at a fixed distance and orientation relative to each electron donor and acceptor
10.1021/jp061218w CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006
7324 J. Phys. Chem. A, Vol. 110, No. 23, 2006
Mi et al.
SCHEME 1
within the triad. The spin dynamics of the photogenerated triradicals are studied using magnetic field effects on the triplet yields following CR as well as time-resolved electron paramagnetic resonance spectroscopy (TREPR). Experimental Section The synthesis and characterization of compounds 1a-4a, 1b, and 3b have been reported previously,41,45 and those of 2b and 4b can be found in the Supporting Information. Samples were purified for spectroscopy by preparative thin-layer chromatography and stored for at most 7 days at 4 °C until measurements were performed. All solvents were spectrophotometric grade or distilled prior to use. Cyclic voltammetry measurements were performed in butyronitrile solutions containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) electrolyte using a CH Model 622 electrochemical work station. A 1.0-mm diameter Pt disk electrode, Pt wire counter electrode, and Ag/AgxO reference electrode were employed. The ferrocene/ferrocinium couple (Fc/Fc+, 0.52 V vs SCE) was used as an internal reference for all measurements. Ground-state absorption measurements were made on a Shimadzu (UV-1601) spectrophotometer. Sample solutions were prepared in toluene, transferred to a sealable cuvette, degassed by three freeze-pump-thaw cycles and then kept under vacuum throughout each spectroscopic run. The optical density at the pump wavelength was 0.4-0.6 per 2-mm path length for femtosecond transient absorption (TA) experiments and 0.81.0 per 1-cm path length for nanosecond transient absorption. Absorption spectra taken after each experiment indicated that there was no degradation of the samples over the course of the transient absorption runs. The apparatus for femtosecond transient absorption has been described in detail previously.48,49 Briefly, a home-built Ti: sapphire regenerative amplifier operating at a 2-kHz repetition rate generates 250-µJ, 130-fs pulses at 828 nm. A portion of this output is split off and focused with all reflective optics through a 3-mm sapphire plate to generate a white-light probe extending from 450 nm through the near-IR region. The remainder of the amplifier output is frequency doubled to generate the 414-nm pump pulse. After passing through the sample, the visible portion of the probe is isolated, dispersed, and detected with a fiber-optic coupled charge-coupled device
(CCD, Ocean Optics). Before the sample, the pump is attenuated to 1.0 µJ/pulse, focused to a 200-µm spot within the sample, polarized at 54.7° relative to the probe, and chopped at 50 Hz. Sequential 10-ms pump-on and pump-off exposures of the detector are measured to generate the transient absorption spectrum. The temporal instrument response was measured using the optical Kerr effect (OKE) and was wavelength dependent because of the group-velocity mismatch between the pump and probe wavelengths.50 OKE cross-correlations ranged from 230 fs at 450 nm to 480 fs at 700 nm. Transient absorption kinetics were fit to a sum of exponentials with a Gaussian instrument function using Levenberg-Marquardt least-squares fitting. Samples for nanosecond transient absorption spectroscopy were placed in a 10-mm path length quartz cuvette equipped with a vacuum adapter and subjected to five freeze-pumpthaw degassing cycles. The samples were excited with 5-ns, 1-mJ, 420-nm laser pulses generated using the frequency-tripled output of a Continuum 8000 Nd:YAG laser to pump a Continuum Panther OPO. The excitation pulse was focused to a 5-mm diameter spot and matched to the diameter of the probe pulse generated using a xenon flash lamp (EG&G Electrooptics FX-200). The signal was detected using a photomultiplier tube with high voltage applied to only 4 dynodes (Hamamatsu R928). The total instrument response time is 7 ns and is determined primarily by the laser pulse duration. The sample cuvette was placed between the poles of a Walker Scientific HV-4W electromagnet powered by a Walker Magnion HS-735 power supply. The field strength was measured by a Lakeshore 450 gaussmeter with a Hall effect probe. Both the electromagnet and the gaussmeter were interfaced with the data collection computer, allowing measurement and control of the magnetic field to (1 × 10-5 T during data acquisition. Kinetic traces were recorded over a range of 1 µs. Between 50 and 80 shots were averaged at each field strength with a LeCroy 9384 digital oscilloscope and sent to a microcomputer, which calculated the ∆A. Due to the length of the sample runs (>3 h), a small amount of sample degradation was observed, resulting in a decrease in the triplet yield at zero field, ∆A(B ) 0), over the course of the experiments. To compensate for this, the magnetic field was reset to B ) 0 mT every 3 kinetic traces and ∆A(B ) 0) was plotted and fit with a polynomial or a series of polynomials. These functions were used to calculate the relative triplet yield
Electron Donor-Chromophore-Acceptor-TEMPO Molecules
J. Phys. Chem. A, Vol. 110, No. 23, 2006 7325
as a function of applied field strength. The relative triplet yield is thus
∆A(B) T ) T0 ∆A(B ) 0) The results presented are an average of two or more experiments conducted on separate days with freshly prepared samples. Samples for EPR measurements were prepared in toluene (0.2-2 mM), loaded into quartz tubes (4 mm OD × 2 mm ID), and subjected to several freeze-pump-thaw degassing cycles on a vacuum line (10-4 mBar). The tubes were then fused with a hydrogen torch and kept in the dark when not being used. The samples were excited at 416-nm, 1-mJ, 7-ns laser pulses from the H2-Raman shifted output of a frequency-tripled Q-switched Nd:YAG laser (Quanta Ray DCR-2). Steady-state EPR spectra, transient CW EPR spectra and pulse EPR spectra were measured using a Bruker Elexsys E580 X-band EPR spectrometer with a variable-Q dielectric resonator (Bruker ER 4118X-MS5) at room temperature. Steady-state CW EPR spectra were measured under the conditions of 0.2-2 mW microwave power and 0.01-0.05 mT field modulation at 100 kHz. The g values of the spectra were calibrated with a crystalline 2,2-diphenyl-1-picrylhydrazyl (DPPH) standard (g ) 2.0036). Transient CW EPR measurements were carried out under CW microwave irradiation (typically 2-20 mW) by accumulating kinetic traces of transient magnetization following photoexcitation. The field modulation was disabled to achieve a response time τ ) Q/πυ ≈ 30 ns51 and microwave signals in emission (e) and/or absorption (a) were detected in both the real and the imaginary channels (quadrature detection). Sweeping the magnetic field gave 2D complex spectra vs time and magnetic field. For each kinetic trace, the signal acquired prior to the laser pulse was subtracted from the data. Kinetic traces recorded at off-resonant fields were considered background signals, whose average was subtracted from all kinetic traces. The spectra were subsequently phased into a Lorentzian part and a dispersive part, and the former, also known as the imaginary magnetic susceptibility χ′′, is presented. High-power microwave pulses were generated by a 1-kW TWT amplifier (Applied Systems Engineering 117X). The typical length of a π/2 pulse was 8 ns. The resonator was fully over-coupled to achieve Q < 200 and a dead time of ∼72 ns. All the quadrature-detected spectra were properly phased, and χ′′ is presented. In the pulse EPR kinetic measurements, the first microwave pulse was initially timed 200 ns earlier than the laser pulse and was delayed incrementally relative to the laser pulse. A kinetic trace was formed by measuring the intensity of the free induction decay (FID) at different time delays of the microwave pulse with respect to the laser pulse. Results Steady-State Properties. The photophysics of the 6ANI chromophore have been previously described in detail.44,52 Briefly, the electronic absorption spectra of ground-state 1a and 3a in toluene have a broad charge-transfer band centered at 397 nm due to 6ANI. The NI acceptor has ground-state absorptions due to π-π* transitions at 343, 363, and 382 nm. The PI acceptor and the T• radical have no appreciable ground-state absorption at λ > 350 nm. Steady-state UV-vis spectra of 1b4b, 5, and NI in toluene are shown in Figure 1. The redox potentials for 6ANI, MeOAn, NI, and PI within 1a-4a have been used to calculate the energies of the chargeseparated states generated following photoexcitation of 6ANI.45,53
Figure 1. Ground-state UV-vis spectra of the triads 1b/2b, 3b/4b, the dyad 5, and the NI acceptor in toluene. 1b and 2b exhibit essentially the same spectrum and so do 3b and 4b. The λmax for PI is 318 nm. T• has very little absorption at λ > 300 nm.
The redox potentials of the corresponding donor and acceptor components within 1b and 3b were reported earlier,41 and those of 2b and 4b measured here are unchanged by attachment of T•. Hence, substitution of T• onto the NI or PI acceptors does not change significantly the energetics of CS and CR within 1b-4b relative to those of 1a-4a. Relevant energy level diagrams are given in Figure 2 and numerically in Table S2. The lowest neutral triplet states energies of NI, 6ANI, and PI are 2.03, 2.05, and 2.45 eV, respectively.41 Ultrafast Transient Absorption Spectroscopy. Charge separation and recombination time constants for 1a-4a were measured previously and are reported in Table 1.44,45,54 Charge separation time constants for 1b-4b were determined after direct excitation of 6ANI with 414-nm light by monitoring the femtosecond transient absorption kinetics at λmax of the acceptor radical anions (NI-• at 475 nm in 1b and 478 nm in 2b; PI-• at 709 nm in 3b and 723 nm in 4b)53 and at 510-540 nm, where the initial 1*6ANI stimulated emission and subsequent MeOAn+• absorption lie.44 Representative transient spectra at selected times are shown in Figure 3, while kinetic traces are given in the Supporting Information (Figure S1). In 1b, the signals at 510 and 475 nm both rise in
