Article pubs.acs.org/JPCB
Photogenerated Quartet State Formation in a Compact Ring-Fused Perylene-Nitroxide Scott M. Dyar, Eric A. Margulies, Noah E. Horwitz, Kristen E. Brown, Matthew D. Krzyaniak, and Michael R. Wasielewski* Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: We report on a novel small organic molecule comprising a perylene chromophore fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or PerNO•. This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited PerNO• and utilize X-ray crystallography to probe the molecular structure and stacking motifs of PerNO• in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective molecule-based spintronics.
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INTRODUCTION In the pursuit of molecule-based spintronics, control of the spin dynamics of multispin systems is crucial.1−4 It is well-known that photoexcitation of organic molecules results in well-defined spin states5−10 that can be examined with electron paramagnetic resonance (EPR) spectroscopy.3,11−14 The spinselective intersystem crossing of photoexcited molecules to their triplet states is one such example.15 These triplet states can interact with stable radicals, resulting in the formation of transient three-spin systems that in some cases form quartet states and in nearly all cases transfer spin polarization to the radical.16−23 Polarization of the stable radical in these systems has been attributed primarily to two complementary mechanisms: the radical-triplet pair mechanism (RTPM) and the electron spin polarization transfer (ESPT) mechanism, both of which are predicated on diffusive encounters between the triplet excited state of a molecule and a stable radical in solution. In the RTPM, the triplet and radical species diffuse toward each other and their spin states mix via the spin−spin exchange interaction (J) to produce excited doublet and quartet states.19,20,24−36 The decay rate of the excited doublet state to the ground state is more rapid than that of the excited quartet state due to conservation of spin multiplicity. The polarization (enhanced absorption or emission) is determined by the sign of J. On the other hand, an initially polarized triplet state is required for the ESPT mechanism.17,28,37 The non-Boltzmann spin sublevel populations of the triplet state are directly transferred to the radical, resulting in quenching of the triplet state to the ground state singlet.37−41 In some systems,19,20,22 excited doublet and quartet states are observed in which time© 2015 American Chemical Society
dependent polarization inversion occurs (emission to absorption and vice versa), which has been explained by the so-called reversed quartet mechanism (RQM).22 The presence of excited doublet and quartet states results from having a constant J value, which is a consequence of fixing the distance between the triplet chromophore and the stable radical. Spin polarization in this mechanism derives from reversible transitions from the doublet manifold to the quartet manifold due to mixing by the zero-field splitting interaction. Selective depletion of the excited doublet state to the ground state leads to reversed transitions from the quartet to the doublet states, resulting in polarization inversion. In general, stable free radicals are well-known to quench singlet excited states in a wide variety of noncovalent and flexibly linked covalent systems largely through spin exchange driven enhanced intersystem crossing (EISC);42−54 however, there have been far fewer studies of the structural and electronic basis of such quenching in rigid systems, and these studies have often been limited to examinations of fluorescence quantum yields and lifetimes.31,55,56 In contrast, there have been several reports of triplet excited state molecules bound to stable radicals by either covalent bonds or metal−ligand coordination,19,20,22,23,31,37−41,57−63 two of which focused on a perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophore covalently attached to a nitroxide radical.56,64 An important aspect Special Issue: Wolfgang Lubitz Festschrift Received: March 11, 2015 Revised: May 24, 2015 Published: May 26, 2015 13560
DOI: 10.1021/acs.jpcb.5b02378 J. Phys. Chem. B 2015, 119, 13560−13569
Article
The Journal of Physical Chemistry B
Femtosecond transient absorption experiments were performed using a previously described regeneratively amplified Ti:sapphire laser system (Spectra Physics, Spitfire Pro XP) operated at a 1 kHz repetition rate.69,74 Samples were excited at 298 K in 2 mm glass cuvettes at 400 nm with 0.5−1.0 μJ, 110 fs pulses. Femtosecond transient absorption spectra were analyzed using home-written Matlab75 singular value decomposition (SVD) programs. SVD deconvolutes the two-dimensional spectra to produce an orthonormal set of basis spectra, which describe the wavelength dependence of the species, and a corresponding set of orthogonal vectors which describes the time dependent amplitude of the basis spectra.76 A speciesassociated first-order kinetic model77 was fit to a linear combination of the time-dependent amplitude vectors, and the same linear combination of basis spectra was used to construct the spectra for the chemical species. Nanosecond transient absorption measurements were performed using a previously described frequency-tripled Nd:YAG laser (Continuum, Precision II 8000), coupled to an optical parametric oscillator (Continuum, Panther).78 Kinetic traces were collected from 430 to 800 at 5 nm intervals, and spectra were constructed by merging the kinetic traces together (binned to 12 ns steps). Each kinetic trace used to construct the spectra is representative of an average of 100 laser shots. In order to obtain decay rates, single wavelength kinetics were fit with a Levenberg−Marquardt nonlinear least-squares fit to a sum of exponentials convoluted with a Gaussian instrument response function. EPR Spectroscopy. EPR measurements at both the X-band (9.5 GHz) and W-band (94 GHz) were made using a Bruker Elexsys E680-X/W EPR spectrometer outfitted with a variable Q dielectric resonator (ER-4118X-MD5-W1) at the X-band and a cylindrical resonator (EN-680-1021H) at the W-band. For EPR measurements at the X-band, toluene solutions (∼10−4 M) were loaded into quartz tubes (4 mm o.d. × 2 mm i.d.), subjected to four freeze−pump−thaw degassing cycles on a vacuum line (10−4 Torr), and sealed using a hydrogen torch. For EPR measurements at the W-band, toluene solutions (∼10−3 M) were loaded into quartz tubes (0.84 mm o.d. × 0.6 mm i.d.) in a N2 filled glovebox to a height of ∼8 mm and sealed with a clear ridged UV doming epoxy (IllumaBond 607160RCL). The EPR samples were stored in a dark freezer when not in use. Continuous wave (CW) EPR spectra were measured at the X-band using 2 mW microwave power and 0.01 mT field modulation at 100 kHz. The temperature was controlled by an Oxford Instruments CF935 continuous flow cryostat using liquid N2. TREPR measurements were performed at the Xband following photoexcitation with 7 ns, 3 mJ, 416 nm pulses using the output of an optical parametric oscillator (SpectraPhysics Basi-scan), pumped with the output of a frequencytripled Nd:YAG laser (SpectraPhysics Quanta-Ray Pro 350). Transient CW EPR (TCW-EPR) spectra were collected following photoexcitation, and the kinetic traces of the transient magnetization were acquired in quadrature under CW irradiation (2−20 mW). Sweeping the magnetic field gave 2D spectra versus both 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 magnetic field values off-resonance were considered background signals, whose average was subtracted from all kinetic traces. SVD global analysis was applied to the processed two-dimensional data set
of these PDI-radical systems relates to PDI itselfit is a robust chromophore that is easily tailorable, particularly at the imide positions.65−67 In previous work on these rigid, covalent PDIradical systems, it was shown that the lowest excited singlet state (1*PDI) is rapidly quenched by EISC in both the weak and strong coupling regimes.65−67 Metal−ligand coordination often introduces complexity into triplet-radical systems as a result of coordination equilibria,31 while even in some covalent systems, for example, those incorporating a π-conjugated oxoverdazyl radical,39 the stable radical may decompose by disproportionation over time and might be unsuitable for scaled spintronic applications.68 In this investigation, we report a novel small organic molecule comprising the thoroughly characterized perylene chromophore,69−73 which is fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or PerNO•. This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited PerNO•, and utilize X-ray crystallography to probe the molecular structure and stacking motifs of PerNO• in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective moleculebased spintronics.
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EXPERIMENTAL SECTION Materials and Synthesis. The synthesis and characterization of PerNO• is described in detail in the Supporting Information. All solvents were spectrophotometric grade, unless specified otherwise. Final compounds were purified by preparative thin layer chromatography on a 20 μm silica plate prior to characterization. Electrochemistry. Electrochemical measurements were performed on a CH Instruments model 660A electrochemical workstation. Samples were measured in a solution of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) in dichloromethane purged with Ar to remove oxygen. A 1.0 mm diameter platinum disk electrode, platinum wire counter electrode, and silver wire reference electrode were used. The ferrocene/ferrocenium couple was used as an internal reference. Optical Spectroscopy. Ground-state absorption measurements were made on a Shimadzu UV-1601 spectrophotometer. The optical density of all samples was maintained between 0.4 and 0.8 at 400 nm for both femtosecond and nanosecond transient absorption. Low temperature experiments at 85 K were performed using a Janis VNF-100 N2 vapor-cooled variable temperature cryostat and held at the desired temperature throughout the duration of the experiment. Samples were prepared in a nitrogen-filled glovebox and suspended between two quartz windows in a 2 mm path length home-built sample holder and then sealed. 13561
DOI: 10.1021/acs.jpcb.5b02378 J. Phys. Chem. B 2015, 119, 13560−13569
Article
The Journal of Physical Chemistry B
Scheme 1. (a) 2,6-Diisopropylaniline, Zinc Acetate Dihydrate, Imidazole, H2O, 190 °C, 24 h, 61%; (b) KOH, t-BuOH, 100 °C, 3 h, 64%; (c) Benzylamine, Imidazole, 105 °C, 3 h, 99%; (d) CH3MgI, Xylenes, Reflux, 4 h, 11%; (e) (i) H2, Pd/C (10 wt %), AcOH, 4 h, (ii) NaHCO3, sodium tungstate dihydrate, H2O2, THF, MeCN, 72 h, 7%
Crystallography. Single crystals of PerNO• were grown by slow diffusion of n-pentane vapor into PerNO• in a chloroform solution. The crystal was mounted on a glass capillary with Paratone oil, and data were collected at 100 K on a Bruker Kappa APEX II CCD diffractometer equipped with a Cu Kα IμS microfocus source with Quazar Optics. The data was absorption corrected using SADABS. The structure was solved using SHELXS and refined using SHELXL.
to yield the principal absorption and dispersion spectrum and their corresponding kinetic components. The CW-EPR and TCW-EPR spectra were fit in Matlab using Easyspin v4.5.5.79 The absorptive part of the TCW-EPR spectrum was fit as an S = 3/2 quartet using the following spin Hamiltonian:80,81 ⎡ 2 1 2 ⎤ E 2 Ĥ = βegSẑ B + D⎢Sẑ − S(S + 1) + (Sx̂ − Sŷ )⎥ ⎦ ⎣ 3 D
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RESULTS Synthesis and Steady-State Characterization. The synthesis of PerNO• is summarized in Scheme 1, and the details are given in the Supporting Information. Briefly, perylene-3,4,9,10-tetracarboxylic dianhydride (PDA) is decarboxylated and condensed with 2,6-diisopropylaniline to form N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide (PMI),87 which is hydrolyzed and then condensed with benzylamine to form benzyl PMI. Reaction with methylmagnesium iodide in refluxing xylenes yields PerBn.88 Debenzylation of PerBn with Pd/C and H2 followed by oxidation with H2O2 catalyzed by Na2WO4·2H2O yields PerNO•. Figure 1 shows the UV−vis absorption spectrum of PerNO• (blue), which is red-shifted relative to that of perylene (red).
(1)
βe and B are the Bohr magneton and magnetic field, respectively; g is the g-value of the quartet and was taken to be isotropic; and D represents the zero field interaction. The coupling between triplet perylene and doublet nitroxide, J, was assumed to be much greater than the Zeeman splitting. In this case, at the X-band, transitions between the doublet and quartet spin manifolds are not expected; therefore, the exact magnitude of J is unnecessary, and not included in the simulation. In addition, TREPR measurements utilizing field swept electron spin echo (ESE) detection, where the echo intensity was integrated as the field was swept, were performed. For these pulsed experiments, a 1 kW TWT amplifier (Applied Systems Engineering 117X) was employed to generate highpower microwave pulses. The resonator was partially decoupled to maximize echo intensity. ESE data was obtained with a π/2τ-π pulse sequence with π/2 = 8 ns, π = 16 ns, and τ = 128 ns. The integrated echo intensity is recorded as a function of the magnetic field to yield the spectrum. If the sample is photoexcited at some time t prior to the π/2-τ-π pulse sequence, spectra can be collected of the transient species as a function of t. Computational Methods. The geometry of the groundstate doublet PerNO• was initially relaxed using molecular mechanics with the MMFF294 force field, as implemented in the Avogadro 1.1.0 software.82 Starting from this geometry, the ground-state doublet and lowest excited-state quartet PerNO• were subsequently relaxed using density functional theory (DFT), as implemented in the TeraChem 1.5K software.83 All DFT calculations made use of the unrestricted B3LYP (UB3LYP) exchange-correlation functional with an augmented correlation-consistent double-ζ basis set (aug-cc-PVDZ). Images of the optimized structures were generated with PyMol 1.2r1.84 Molecular orbitals were printed through single point energy calculations with TeraChem, using restricted open-shell DFT (ROB3LYP/aug-cc-pVDZ), and visualized with VMD 1.9.2a27.85 Time-dependent DFT (TD-DFT) was utilized to calculate the singlet excited state energies of bare perylene and PerNO•, making use of the DFT conditions mentioned above (B3LYP/aug-cc-pVDZ), as implemented in QChem 4.1.86
Figure 1. UV−vis spectra of PerNO• (blue) and perylene (red) in toluene.
The molar absorptivity of PerNO• is 38 800 M−1 cm−1 (toluene, 451.5 nm), similar to that reported for perylene at λmax = 435.8 nm.89 The weak absorption in the PerNO• spectrum from 300 to 350 nm and, to an even lesser extent, from 480 to 500 nm that appears to differ from the perylene spectrum is due to absorption of the nitroxide (see the Supporting Information). The one-electron redox potentials of PerNO• were obtained using cyclic voltammetry. The nitroxide undergoes reversible oxidation and reduction at 0.89 and −1.26 13562
DOI: 10.1021/acs.jpcb.5b02378 J. Phys. Chem. B 2015, 119, 13560−13569
Article
The Journal of Physical Chemistry B
O bonds of the nitroxides are slightly bent out of the plane of the molecule, allowing the cyclohexyl ring to adopt a chairlike conformation. The lengths of the two N−O bonds are 1.294 and 1.283 Å, which are both consistent with TEMPO-like nitroxide radical N−O bond lengths obtained from the Cambridge Structural Database90 as opposed to bond lengths upward of 1.4 Å for N−OH bonds. Each asymmetric set of PerNO• molecules packs with a herringbone motif on the short axis of the molecule. These pairs of molecules also pack end to end with the N−O bond in alternating directions. These herringbone dimers extend in the a−c plane to the larger assembly, and the long axis of every molecule lies approximately in the b-axis of the unit cell. The PerNO• geometry obtained via DFT closely matches that of the crystalline system in most aspects: the lengths of the average π-bond, average σ-bond, and N−O bond all differ by less than 1% (see the Supporting Information). The DFT structure does not, however, account for the core twist present in the XRD geometry. Transient Absorption Spectroscopy. The ultrafast dynamics of PerNO• in toluene were studied using femtosecond transient absorption spectroscopy (fsTA). The transient spectra are given in Figure 3A. Immediately following selective photoexcitation of the perylene chromophore at room temperature with a 400 nm, 1 μJ, 110 fs pulse, its transient absorption spectrum is characterized by a strong absorption at 730 nm, which is assigned to the lowest excited singlet state of perylene, Per S1. Within 1 ps, the Per S1 absorption decays, and is replaced by an absorption between 430 and 500 nm, characteristic91 of the perylene triplet excited state, Per T1. The Per T1 absorption persists for >7 ns. The transient spectra were subjected to SVD and global analysis (see the Supporting Information) and fit to a pair of coupled first-order reactions, an A → B → C model, where C is the ground state, to obtain the species-associated spectra (Figure 3B). Femtosecond transient absorption data was also obtained for PerNO• in glassy 2-methyl tetrahydrofuran (MTHF) at 85 K (Figure 4A). The dynamics of the system were very similar to those at room temperature, other than a significantly slower decay rate of the TN ← T1 absorption (Figure 4B) that persists past the experimental window (7 ns). The decay of the longlived triplet absorption at 85 K (Figure S4, Supporting Information) is τ = 100 ± 24 ns, as measured using nanosecond transient absorption (nsTA) spectroscopy following excitation with a 1.5 mJ, 7 ns, 416 nm pulse (see the Supporting Information).
V vs SCE, respectively. These potentials are close to the oxidation and reduction potentials of the similar TEMPO radical, 0.82 and −1.25 V. The second one-electron oxidation of PerNO• occurs at 1.14 V, which is only slightly more positive than that of perylene at 1.05 V. Computational Modeling. DFT (UB3LYP/aug-ccpVDZ) was used to minimize the geometries of the doublet ground state and lowest quartet excited state of PerNO•. Frontier orbitals were obtained via single point DFT calculations (ROB3LYP/aug-cc-pVDZ), and are shown in Figure S1 (Supporting Information). Spin density distributions were also calculated and visualized (ROB3LYP/aug-cc-pVDZ), and are given in Table S3 and Figure S3 (Supporting Information), respectively. TD-DFT (B3LYP/aug-cc-pVDZ) was used to calculate the energies of the first singlet excited state and first and second triplet excited states of perylene relative to the singlet ground state (Table S1, Supporting Information), as well as the first quartet excited state and first and second doublet excited states of PerNO• (Table S2, Supporting Information). While the calculated optical transitions of perylene and PerNO•, respectively, are both lower in energy than those measured with UV−vis, the PerNO• absorption is indeed redder than that of perylene (Figure S2, Supporting Information), as seen experimentally. The first quartet excited state closely matched the energy of the first triplet excited state of bare perylene (1.21 eV vs 1.25 eV, respectively). X-ray Crystallography. PerNO• crystallizes in the monoclinic crystal system in the P21/c space group with two molecules in the asymmetric unit (Figure 2). The unit cell
Figure 2. Molecular dimer of PerNO• which constitutes the asymmetric unit of the crystal structure.
contains a total of eight molecules and has the following dimensions: a = 11.6367, b = 27.4387, c = 11.7856, β = 95.732°. The two inequivalent perylene cores exhibit a core twist between naphthalene subunits of 8.41 and 7.45°. The N−
Figure 3. (A) Visible femtosecond transient absorption spectra recorded in toluene (λex = 400 nm, 1 μJ/pulse) of PerNO• presented as a line plot. (B) Species associated spectra obtained through SVD and global analysis clearly shows the rapid decay from S1 to T1. 13563
DOI: 10.1021/acs.jpcb.5b02378 J. Phys. Chem. B 2015, 119, 13560−13569
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The Journal of Physical Chemistry B
Figure 4. (A) Visible femtosecond transient absorption spectra recorded at 85 K in MTHF (λex = 400 nm, 1 μJ/pulse) of PerNO•. (B) Species associated spectra and kinetics obtained through SVD and global analysis are similar to those obtained at room temperature.
EPR Spectroscopy. The continuous wave (CW) EPR spectrum of PerNO• at room temperature in toluene obtained at the X-band (9.5 GHz) shows a characteristic nitroxide spectrum (Figure 5, black trace). This three-line spectrum was
density distribution (∼1%) and by the lack of any further splitting of the PerNO• three-line spectrum. The CW EPR spectra of PerNO• in toluene at 85 K were obtained at both the X- and W-band (94 GHz, see the Supporting Information). The W-band CW EPR spectrum of PerNO• in frozen toluene (Figure S5, Supporting Information) was fit to determine the anisotropic g-factors (gx = 2.0107, gy = 2.0070, gz = 2.0027) and nitrogen hyperfine coupling constants (Ax = 18.1 MHz, Ay = 15.1 MHz, Az = 97.9 MHz). It should be noted that the isotropic g-factor giso = (gx + gy + gz)/3 = 2.0068 differs slightly from giso = 2.0062 measured by the X-band CW EPR spectrum (Table 1) due to the larger error bars in the Xband measurement. The field-swept, echo-detected (FS-ED) spectrum of PerNO• in frozen toluene (85 K) shows a characteristic nitroxide powder spectrum (Figure 7, blue trace). The CW EPR spectrum of a polycrystalline sample of PerNO• at room temperature, measured at the X-band, shows a broad, featureless spectrum, indicative of strong intermolecular spin exchange,93 while the corresponding spectrum collected at 77 K shows only slight broadening (Figure S6, Supporting Information), which remains essentially unchanged down to 5 K. Figure 6A shows the deconvoluted TREPR spectra using CW microwaves and the corresponding time dependence of the PerNO• spectrum after excitation with a 3 mJ, 7 ns, 416 nm pulse (Figure 6B). A wide, complex feature with both emissive and absorptive features grows in after the laser flash and decays over the course of ∼450 ns at which point the signal inverts (seen in Figure 6B as a negative amplitude). The TREPR spectrum was fit as an S = 3/2 system (3J ≫ D) with g = 2.0031 and a zero-field splitting of D = 750.4 MHz and E/D = 0.1942. Table 1 gives a summary of all EPR fitting parameters. The TREPR spectrum of a polycrystalline sample of PerNO• shows a single spin-polarized absorptive feature, ∼8 mT wide, that appears within the instrument function of the spectrom-
Figure 5. CW EPR spectrum (9.59 GHz, 0.02 mW, 1.0 G/100 kHz) of PerNO• at room temperature in toluene (0.1 mM, deoxygenated).
simulated with an isotropic g-value of 2.00618 and a single nitrogen with an isotropic hyperfine coupling constant of 43.3 MHz (Figure 5, red trace). There is an additional set of lines observed that appear as low-intensity wings, that are due to a small, distinct population (4%) of PerNO• in which the nitroxide radical has a hyperfine coupling of 14.1 MHz to a spin 1/2 nucleus, most likely naturally abundant 13C.92 Table S5 (Supporting Information) gives the Hamiltonian parameters obtained by fitting the spectra. The unpaired spin density distribution calculated via DFT is primarily localized on the nitroxide nitrogen or oxygen (>90%), but ∼8% is distributed between the six closest carbons (see the Supporting Information). The frontier orbitals of PerNO• (Figure S1, Supporting Information) suggest that there is little spin leakage onto perylene, which is supported both by the calculated spin Table 1. Summary of EPR Fitting Parameters system PerNO• (doublet) PerNO• (doublet) PerNO• (quartet)
measurement CW (RT) 14
N
CW (85 K) 14
N
TCW (85 K) ZFS
giso = 2.0062 Aiso = 43.3 MHz gx = 2.0107 Ax = 18.1 MHz giso = 2.0031 D = 750.4 MHz 13564
gy = 2.0070 Ay = 15.1 MHz
gz = 2.0027 Az = 97.9 MHz
E/D = 0.1942 DOI: 10.1021/acs.jpcb.5b02378 J. Phys. Chem. B 2015, 119, 13560−13569
Article
The Journal of Physical Chemistry B
Per T1 (τ = 100 ns); this is related to the T1 relaxation time of the nitroxide spins, which must still reach equilibrium after decay of the transient species.
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DISCUSSION Sub-picosecond Intersystem Crossing. The fsTA spectra and kinetics of PerNO• show that Per S1 decays very rapidly,
