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Hyperfine Interaction, Spin Polarization, and Spin Delocalization as Probes of Donor-Bridge-Acceptor Interactions in Exchange-Coupled Biradicals† Martin L. Kirk,*,‡ David A. Shultz,*,§ Diana Habel-Rodriguez,‡ Robert D. Schmidt,§ and Ubie Sullivan§ Department of Chemistry and Chemical Biology, The UniVersity of New Mexico, MSC03 2060, 1 UniVersity of New Mexico, Albuquerque, New Mexico 87131-0001, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: May 12, 2010
Computations and EPR spectroscopy are used to probe the spin distribution of donor-bridge-acceptor (D-B-A) biradical complexes: TpCum,MeZn(SQ-NN) (1), TpCum,MeZn(SQ-1,4-Ph-NN) (2), TpCum,MeZn(SQ2,5-TP-NN) (3), and TpCum,MeZn(SQ-2,5-Xyl-NN) (4) (SQ ) orthosemiquinone and NN ) nitronylnitroxide). These complexes are ground-state analogs of the charge-separated excited states formed in photoinduced electron transfer reactions. The intraligand magnetic exchange interaction (J) in these complexes is mediated by the bridges and has been found to stabilize the triplet ground states of 1 and 2. Detailed spectroscopic and bonding calculations have been used to elucidate the role of the bridge fragment (B) and its conformation relative to donor (SQ) and acceptor (NN) on spin density distributions. The computed results correlate well with experimental nitrogen hyperfine coupling constants. Introduction Static and dynamic distortions that result in nonplanar geometries for π-delocalized donor-bridge-acceptor (D-B-A) systems can have a dramatic impact on the ability of the bridge fragment to facilitate electron transfer/transport over long distances.1-8 For example, in photoinduced electron transfer (PET) processes, photoexcitation of the D yields a D*-B-A excited state that can undergo facile electron transfer to form a spin singlet biradical D+-B-A- charge-separated state.2,9 The singlet-triplet gap (2J, where J is the exchange parameter) of the charge-separated excited state is determined by the effect of applied magnetic fields on decay rates.10,11 The exchange parameter is then used to evaluate the electronic coupling matrix element (Hab) for a given D-A or D-B-A system.2,8,10,11 During our study of magnetic exchange interactions and adiabatic electronic coupling, we have developed covalently linked, nondisjoint heterospin triplet D-B-A biradicals that can be described as ground-state analogues of these photogenerated triplet charge separated states.2,7,8,12,13 In these studies, a valence bond configuration interaction (VBCI) model that utilizes spectroscopic and magnetic observables was used to understand the electronic origin of both Hab and the magnetic exchange coupling in the semiquinone-nitronylnitroxide (SQB-NN) D-B-A biradical series, TpCum,MeM(SQ-B-NN).2,8 Wasielewski has suggested that a potentially important mechanism for controlling or modulating charge recombination dynamics in D-B-A charge-separated states occurs through the spin-density distribution on the bridge fragment.9 Some of the key questions in molecular electron transfer/transport focus on the magnitude of the electronic coupling in the D-B-A fragment, molecular distortions that are coupled to the charge separated state, and the intimate electronic structure of the bridge †
Part of the “Michael R. Wasielewski Festschrift”. * Corresponding authors. E-mail:
[email protected];
[email protected]. ‡ The University of New Mexico. § North Carolina State University.
moiety in a D-B-A system. In this study, we address the nature of the spin density distribution in the triplet state of D-B-A biradical ligand complexes using a combination of EPR spectroscopy and detailed calculations that probe the effects of the bridge and its conformation. These studies provide detailed insight into the relationship between the degree of D f A charge transfer, and the nature of the bridge fragment, key geometric distortions and the spin density distribution in the D, B, and A fragments. We find that direct measurement of the specific isotropic hyperfine interaction (hfi) parameters in D-B-A biradicals that employ an NN acceptor provide key insight into the electronic origin of their spin density distributions and serve as important ground-state probes of excited state contributions to spin delocalization, spin polarization, charge transfer, and nonplanar D-B-A distortions. Experimental Section Electron Paramagnetic Resonance Spectroscopy. Fluid solution X-band EPR spectra were recorded at room temperature on an IBM-Bru¨ker E200SRC continuous-wave spectrometer at room temperature. Samples were dissolved in CH2Cl2 solution (ca. 0.2 mM) and spectra collected in quartz sample tubes. The resultant isotropic EPR spectra were simulated using the programs XSophe14 and EasySpin15 to obtain accurate isotropic nitrogen hyperfine coupling constants (aN). The complexes TpCum,MeZn(SQ-NN) 1 and TpCum,MeZn(SQ-1,4-Ph-NN) 2, (TpCum,M ) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate; NN ) nitronyl nitroxide; SQ ) semiquinone; 2,5-TP ) thiophene; 1,4-Ph ) phenylene; 2,5-Xyl ) para-xylyl; Scheme 1) possess ferromagnetically exchange-coupled biradical ligands that possess a total spin of ST ) 1. Molecules TpCum,MeZn(SQ-2,5-TPNN) 3 and TpCum,MeZn(SQ-2,5-Xyl-SQ) 4 are also nondisjoint biradicals and are thus anticipated to possess ST ) 1 ground states. The nitronyl nitroxides NN-Ph (5) and NN-Ph(OMe) (6) are monoradicals and possess ST ) 0.5. Spectra were fit assuming a hyperfine splitting due to two, equivalent I ) 1 N nuclei and a line shape model possessing an angular dependence
10.1021/jp102955j 2010 American Chemical Society Published on Web 06/16/2010
D-B-A Interactions in Exchange-Coupled Biradicals
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14713
SCHEME 1: Radical and Biradical Molecules
on g and Gaussian line shapes. The magnitudes of the isotropic hyperfine splitting as well as the line widths were adjusted accordingly to fit the experimental spectra. For the S ) 1 donor-acceptor systems, the relationship
aN(S ) 1) ) aN /2 has been used to relate the measured N hyperfine coupling constant of the S ) 1 pair state (aN(S ) 1)) in the strong exchange limit (J . aN(S ) 1)) to the intrinsic NN isotropic hyperfine coupling constant (aN). Electronic Structure Calculations. Spin-unrestricted gasphase geometry optimizations for all compounds were performed at the density functional level of theory using the Gaussian 03W software package.16 Calculations for molecules 1-4 were performed on the ST ) 1 spin triplet ground state, whereas those for monoradicals 5 and 6 were performed on the spin doublet ground state. All calculations were performed on neutral molecules and employed the B3LYP hybrid exchange-correlation functional. A 6-31G(d,p) split valence basis set with added polarization functions was used for all atoms. Input files were prepared using the molecule builder function in the Gaussview software package. Ground-state optimized geometries were calculated with dihedral angles between the nitronylnitroxide and phenyl rings or the phenyl and semiquinone rings of 2 varying between 0 and 180° in steps of 15° to determine the effects of bond torsions on spin density distributions. In these calculations, the SQ-B and B-NN dihedral angles were held fixed, and all remaining coordinates were allowed to optimize to the lowest energy geometry using the Berny algorithm. Mulliken spin populations were extracted from the resulting output file. Results and Discussion Electron Paramagnetic Resonance Spectroscopy. The room-temperature solution EPR spectra for molecules 1-6 are presented in Figure 1 (red) along with their spectral simulations (blue). Here molecule 1 is a D-A biradical, molecules 2-4 are D-B-A biradicals, and molecules 5 and 6 are B-A monoradicals. The biradical molecules 1 and 2 have been shown to possess high-spin ST ) 1 ground states due to ferromagnetic
exchange coupling between the SQ and NN radicals in the solid state and in solution.2,8,13 All biradical spectra are observed to be in the strong exchange limit with the isotropic HeisenbergDirac van Vleck (HDvV) exchange parameter, J, being much greater than the N hyperfine interaction (hfi), as evidenced by the line spacings of aN/2. (See the Experimental Section.) The resonance line for 1-6 is centered at the free electron g value (ge) and split into five lines due to the hfi with the two equivalent or very nearly equivalent N nuclei associated with the nitrogen atoms of the nitronylnitroxide acceptor radical. The isotropic N hyperfine parameters obtained by spectral simulation for molecules 1-6 are listed in Table 1. The experimentally determined aN values for molecules 1-6 reflect the effects of D f A charge transfer, spin polarization, and spin delocalization on the spin density distribution in these molecules. We have performed full DFT geometry optimizations on molecules 1-6 and the optimized dihedral angles between the D-B and B-A are given in Table 2. Note the remarkable B-A planarity calculated for all of the molecules except for xylyl-bridged 4, indicating a strong electronic driving force for a high degree of B–A interaction in the absence of steric constraints. Interestingly, for D-B-A molecule 3, which possesses a thiophene bridge, the calculated ground-state geometry is planar, whereas the phenylene bridge in 2 is quite distorted from planarity with a donor-bridge dihedral angle of ∼32 to 33°. To develop a greater understanding of the spin density distributions in these molecules, we have calibrated the results of DFT spin density calculations at these optimized geometries to the experimental aN hfi parameters. This is shown in Figure 2, where the calculated N spin density on the NN fragment (FN) is plotted as a function of the experimental isotropic N hfi parameter, aN. The observed linear behavior follows the expected relationship aN ) QFN,17,18 which was first described by McConnell, and suggests that these calculations can adequately predict the proper trends in spin density distribution for the triplet states of molecules 1-4 and provide strong support for the calculated gas phase geometries dominating in solution. It is clearly evident from Figure 2 that donor interactions, the nature of the bridge fragment, and D-B/B-A ring torsions conspire to modulate the N hfi on the NN acceptor, indicating that the N hfi can serve as a sensitive probe of D f
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Figure 1. Room-temperature solution isotropic EPR spectra of molecules 1-6. Experimental data are in red, and the spectral simulations are in blue. Note that the line splitting of the biradical (1-4) triplet states are half (aN(S ) 1) ) aN/2) that observed for monoradicals 5 and 6.
TABLE 1: Isotropic NN nitrogen hfi parameters for molecules 1-6a |aN| (10-4 cm-1) line width (10-4 cm-1) a
1
2
3
4
5
6
7.60 (0.05 2.1
7.20 (0.05 1.1
7.38 (0.1 1.9
6.74 (0.05 1.7
7.06 (0.03 0.9
7.07 (0.03 0.9
g ≈ gfree electron.
A charge transfer, spin polarization, and spin delocalization in D-A and D-B-A molecules. Molecular Orbital Description of Donor-Acceptor Interactions and Spin Delocalization. To develop greater insight into the electronic and geometric origins of the experimental isotropic N hfi, the calculated spin density distributions on the
Figure 2. Calculated N spin density on the NN fragment as a function of the isotropic N hfi (aN) for molecules 1-6. The blue line is a bestfit line through the six data points.
TABLE 2: Calculated Dihedral Angles for Molecules 1-6 (Full Unconstrained Geometry Optimization) dihedral angle (deg)
NNSQZnTp (1)
NNPhSQZnTp (2)
NNThioSQZnTp (3)
NNPh(CH3)2SQZnTp (4)
NNPh (5)
NNPh(MeO)2 (6)
bridge-acceptor donor-bridge
3a n/a
1 31-32
0 0
46-51 47-48
6 n/a
6 n/a
a
Donor-acceptor dihedral.
D-B-A Interactions in Exchange-Coupled Biradicals
Figure 3. Qualitative molecular orbital diagram for 2 describing interactions between SQ donor, phenylene bridge, and NN acceptor.
donor, bridge, and acceptor, and the spin populations on the NN nitrogen and bridging carbon atoms, we will utilize a qualitative molecular orbital (MO) diagram for phenylene bridged 2 that is based on the results of our bonding calculations. The MO diagram is presented in Figure 3 and displays key SQ, Ph, and NN frontier MO interactions. The phenylene e1 orbital possesses the proper symmetry to mix with the SQ(π) singly occupied molecular orbital (SOMO) and the NN(π*) lowest unoccupied molecular orbital (LUMO). Strong orbital mixing between the NN(π*) and Ph(e1) orbitals results in an energetically stabilized NN-Ph(π*-e1) bonding MO that allows for greater interaction with the SQ SOMO. From this Figure, it is easy to see how occupation of the SQ(π)SOMO allows for positive spin density to be transferred from the SQ donor to the phenylene bridge and the NN nitrogens via a spin delocalization mechanism. A planar D-B-A bridge geometry, as calculated for the triplet state of thiophene bridged 3, will result in a markedly stronger transfer of positive spin density from the SQ donor to the bridge and NN acceptor fragment. This effect is observed experimentally for 3, which possesses the largest aN value for any of the three D-B-A systems detailed in this study (Figure 2 and Table 1). However, bond torsions that reduce the D-B and B-A π conjugation will dramatically reduce SQ(π)Ph(e1) and Ph(e1)-NN(π*) overlap, aN, and the transfer of positive spin density to the bridge and acceptor. This is observed experimentally for phenylene-bridged 2 and xylyl-bridged 4, which possess markedly reduced aN values compared with 3. Donor-Bridge-Acceptor Bond Torsions. Our results also allow us probe D f A charge transfer interactions in 1-4 and
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14715 to make predictions about conformation-dependent spin densities. We have calculated the spin densities on the NN, phenylene, and SQ fragments of models for molecule 2 in limiting geometries to ascertain the relative effects of conformationdependent D-B and B-A interactions in SQ-B-NN biradicals, and the results are presented in Table 3. In the first geometry, the phenylene bridge is orthogonal to both the SQ j -A; overbar indicates 90° donor and the NN acceptor (D-B rotation from planar geometry) with the expected result that the now isolated NN and SQ radicals possess spin densities that j -B j -A geometry, the phenylene bridge approach unity. In the D is orthogonal to the NN acceptor resulting in ∼9% of the SQ spin density being transferred to the phenylene bridge, whereas the planar D-B-A geometry yields increased B and A spin density with a remarkable 20% reduction in positive spin density on the SQ donor, underscoring the strong D f A interaction in the fully planar geometry that facilitates spin delocalization. j -B-A geometry, positive spin density Interestingly, in the D j -A), but on the NN acceptor increases (compared with D-B now negative spin density resides on the phenylene bridge. To better understand the origin of negative spin density on the j -B-A, we now focus on the calculated bridge fragment in D spin populations for the NN nitrogen atoms (FN) and the NN bridging carbon atom (FC) as a function of donor, bridge, and acceptor rotations from planarity (Figure 4). Here it is observed that FN and FC basically track one another with individual NN acceptor and phenylene bridge rotations away from the planar geometry (in particular, Figure 4B,C are nearly identical). The j -A and D j -B j -A similar tracking occurs because both D-B conformations disrupt B-A π-communication. The behavior of FN and FC can be explained with the help of Figure 3. In a planar geometry, D f A charge transfer is maximized, and there is maximal overlap among the SQ(π), Ph(e1), and NN(π*) orbitals resulting in a SQ(π)SOMO-based MO delocalized over all three fragments. This effectively increases the positive spin density on the NN N and C atoms due to the large atomic orbital coefficients of these atoms in the NN(π*) orbital. Conversely, when the phenylene bridge and NN acceptor are j -A and D j -B j -A, Figure 4B,C, respectively), orthogonal (D-B the spin populations on NN resemble those of the isolated NN radical, with a large negative NN bridging carbon FC due to spin polarization and a less positive FN due to the reduced spin delocalization from the SQ donor. Finally, when the donor is j -B-A; red lines in Figure 4A), spin rotated out of the plane (D delocalization via D f A charge transfer is minimized. The negative spin density on the phenylene bridge and the increased positive spin density on the NN acceptor is a result of enhanced spin polarization that results from a NN(π) f NN-Ph(π*-e1) charge transfer state. The strong interaction between the NN(π*) and Ph(e1) orbitals when these groups are planar stabilizes the
TABLE 3: Calculated Spin Densities for Molecule 2 as a Function of D-B and B-A Bond Torsions
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Kirk et al. detailed bonding calculations, allow the NN hfi to be used in the evaluation of D-B-A electronic interactions in SQ-B-NN biradicals. In addition, the computed spin density distributions were used to probe molecular conformation and the degree of D f A charge transfer. Our results indicate that NNLUMO-bridgeLUMO interactions are most critical for strong D-B-A interaction in 1-4. Further studies are underway to experimentally evaluate these predictions. Acknowledgment. M.L.K. acknowledges the National Science Foundation (NSF CHE-0616190) for financial support. DAS thanks the National Science Foundation (CHE-0910585) for financial support. Supporting Information Available: Synthetic and spectroscopic details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 4. Calculated spin populations on the NN nitrogen (dashed lines) and bridging C (solid lines) for a triplet state computational model j -B-A: of 2. Calculations were performed with the SQ donor (A; D j -A: green), and the NN acceptor red), the phenylene bridge (B; D-B j -B j -A: blue) individually rotated from the planar D-B-A (C; D geometry.
NN-Ph(π*-e1) orbital, leading to a smaller ∆E term in the perturbative configuration interaction expression that leads to a negative FC on the NN bridging carbon atom. These results indicate that direct measurement of the isotropic N and C hfi in D-B-A biradicals that employ a NN acceptor will provide considerable insight into the electronic origin of the spin density distribution in D-B-A biradicals, and serve as a ground-state probe of excited state contributions to spin delocalization, spin polarization, charge transfer, and nonplanar D-B-A distortions. Conclusions The excellent correlation of computed FN and the experimentally determined nitrogen hfi, evaluated in the context of
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