J. Phys. Chem. A 2010, 114, 6701–6704
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Unusually Large Coupling Constants in Diradicals Obtained from Excitation of Mixed Radical Centers: A Theoretical Study on Potential Photomagnets Ujjal Bhattacharjee, Anirban Panda, Iqbal A. Latif, and Sambhu N. Datta* Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai-400076, India ReceiVed: April 01, 2010; ReVised Manuscript ReceiVed: May 03, 2010
Three sets of heterosubstituted, interconvertible, cyclophanediene (CPD), and dihydropyrenes (DDPs) and one such set involving dinitrilepyrenes were examined by UB3LYP broken-symmetry methodology with 6-311++g(d,p) bases. Nitronyl nitroxide and oxoverdazyl (with both N and C terminals) are monoradical centers, whereas CPD and DDP moieties serve as couplers. The photoexcited CPD converts to DDP. The calculated exchange coupling constant (J) for o-VER(N)-DDP-NN is surprisingly high, 6412 cm-1, and much larger than 28.9 cm-1 for the CPD species, but the unsubstituted DDP is known to transform readily into pyrene, with the loss of reversibility. Nevertheless, o-VER(N)-(15,16-dinitrile)DDP-NN also has a large J value, 589.4 cm-1. The corresponding CPD species has J ) 53.3 cm-1. We predict that the latter CPD and DDP diradicals are potential molecules to synthesize photomagnetic materials. The o-VER(N)-DDP-NN can also be an excellent photomagnetic switch at a considerably low temperature. Introduction The reversible photochemical conversion of a chemical species between two different forms having different absorption spectra is called photochromism. By irradiating photochromic materials, geometries and physical properties can be changed. This is important for designing photoswitchable species. If a photoswitchable molecule is used as a spin coupler between two magnetic units, then the magnetism of the resulting species can change upon irradiation.1 Perfluorocyclopentene has been widely studied as a photochromic spin coupler. In many nitronyl nitroxide diradicals with perfluorocyclopentene, the intramolecular exchange interaction is extraordinarily weak with the coupling constant J often of the order of hyperfine coupling constants (hfcc). Interest in photomagnetic properties has led to the investigation of photoexcited states of diradicals.2,3 Ali et al. have theoretically predicted photoswitching magnetic properties of four substituted dihydropyrenes and shown that the magnetic exchange coupling constants vary up to 9.44 cm-1.4 A photoinduced antiferromagnetic-to-ferromagnetic crossover involving the conversion of substituted trans-azobenzene to cisazobenzene has been investigated by Shil and Misra.5 The substituted pyrene molecule exists in two different forms, namely, cyclophanediene (CPD) and dihydropyrene (DDP), as shown in Figure 1. Recently, Latif et al. have shown quite large and positive intramolecular magnetic exchange coupling constants for coupled diradicals constructed from nitronyl nitroxide (NN) and oxoverdazyl (o-VER).6 The present work stems from the natural expectation that the mixed diradical systems coupled via CPD and DDP will result in high positive J values. In this work, we have investigated the ground states of diradicals of NN and o-VER. Out of many possible pairs of isomers, we focus our attention on only three pairs of hydrogen isomers (R ) H) and 1 pair of nitrile isomer (R ) CN). These are shown in Figure 2. Pairs a and b with N linkages are chosen in accordance with the rule of spin alternation in the unrestricted formalism7 so that the resulting diradicals would be ferromagnetically coupled. (See the Supporting Information.) The other * Corresponding author. E-mail:
[email protected].
two pairs (c and d) contain C-linkages instead and are chosen to test the contention of Koivisto and Hicks that there is not any spin delocalization and very small spin polarization onto the carbon joining a substituent (in the present case, the coupler), thereby leading to only a few percent of spin population on the same carbon atom.8
Methodology Borden, Davidson, and Feller9 discussed that the ROHF calculations provide qualitatively correct molecular orbitals but in general fail to produce the correct molecular geometry. They suggested the use of the UHF methodology for a reasonably correct description of triplet (T, S ) 1) and open-shell singlet (S, S ) 0) geometries. We have used spin-polarized unrestricted density functional theory (DFT), (more specifically, the UB3LYP method), at first with 6-31G bases and finally with 6-311G (d,p) basis set for the optimization of geometry of the triplet diradicals. We then carried out single-point runs using 6-311G++(d, p) basis. Single-point broken symmetry (BS) calculations were based on the final triplet geometries and the latter basis set. The optimized geometries for the N-linked, C-linked, and the 15,16-dinitrile diradicals are illustrated in the Supporting Information. The relatively greater planarity of o-VER(N)-DDP-NN indicates a high positive J value and a potential organic ferromagnet. The optimized triplet energies, the mean dihedral angles, and C15-C16 (Figure 1) distances for the systems are also given in the Supporting Information. All computations were performed using Gaussian 03 software.10 The average value of the square of spin angular momentum is ideally 2.0 in the triplet (T) state and 1.0 in the BS state. In actual calculations, however, these ideal values are approximately obtained, showing spin deviation. Therefore, the coupling constant has been calculated using the Yamaguchi expression11
10.1021/jp102939m 2010 American Chemical Society Published on Web 05/27/2010
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Figure 1. Conversion of CPD to DDP.
Figure 2. Photochemical interconversion of (a) o-Ver(N)-CPD-NN and o-Ver(N)-DDP-NN, (b) o-Ver(N)-(15,16-dinitrile)CPD-NN and o-Ver(N)(15,16-dinitrile)DDP-NN, (c) o-Ver(C)-CPD-NN (linear) and o-Ver(C)-DDP-NN (linear), and (d) o-Ver(C)-CPD-NN (bent) and o-Ver(C)-DDPNN (bent).
JY )
(DFTEBS - DFTET) 〈S2〉T - 〈S2〉BS
(1)
Results and Discussion The single-point UB3LYP/6-311++G (d, p) energies of the triplet and BS states for the o-VER-Coup-NN systems are given in Table 1. With the N-linkage systems, we can see a large change in J value as the species undergoes conversion from open ring to closed ring isomer (from 29 to 6412.1 cm-1 for the 15,16-dihydrogen species and from 53.3 to 589.4 cm-1 for the 15,16-dinitrile species). The dihedral angle between DDP coupler and two magnetic centers is somewhat smaller, and C15-C16 distance in the DDP diradical is much less than that in CPD species, owing to the near-planarity of the DDP coupler. The high degree of conjugation facilitates the migration of spin waves and the JY value is very large. For the C-linkage systems, the J values are comparatively small for both open and closed ring isomers. It is mainly due to the much less planarity of the diradical. In 2(d), the o-VER moiety is joined through the 14th carbon of the coupler and because of steric interaction with hydrogen
attached to carbon 11 goes out of the ring plane and subtends a mean angle of 32.3°. The numbering of carbon atoms is taken from Williams et al.13 A similar situation does not exist with 2(c) in which the coupling is linear. There is a large change of J values (-15.4 to -219.4 cm-1) in 2(c). The systems are antiferromagnetically coupled. The systems 2(d) are ferromagnetically coupled. This phenomenon can be explained by the generalized spin alternation rule due to Ali et al.4 When there is more than one nonbonded electron in π orbitals of heteroatoms like N, O, S, and so on and the bonding of these atoms is otherwise satisfied, all nonbonded π electrons should be considered in the spin alternation rule. That is, the nitrogen atom in sp2 hybridization offers two electrons for spin alternation. The spin alternation has been illustrated in Figure S1 of Supporting Information, and it correctly predicts the nature of the coupling: ferromagnetic or antiferromagnetic. Koivisto and Hicks8 concluded that the odd electron in verdazyl resides in a π* singly occupied molecular orbital (SOMO) that spans the four nitrogen atoms of the ring. Because of the orbital symmetry, there is no spin delocalization on the carbon atoms, and spin polarization effects lead to a small spin density on the carbon atom. If the substituent is aromatic, then
Theoretical Study on Potential Photomagnets
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TABLE 1: Properties of the Studied Diradicals Calculated from Single-Point UB3LYP Calculations Using 6-311++G(d,p) Basis Seta system o-VER(N)-CPD-NN o-VER(N)-DDP-NN o-VER (N)-(15,16-dinitrile)CPD-NN o-VER (N)-(15,16-dinitrile)DDP-NN o-VER(C)- CPD -NN (linear) o-VER(C)-DDP-NN (linear) o-VER(C)-CPD-NN (bent) o-VER(C)- DDP-NN (bent)
ET in au ()
EBS in au ()
-1600.10256 (2.08548) -1600.13631 (2.19630) -1784.62141 (2.09587) -1784.63857 (2.21659) -1600.10728 (2.08379) -1600.13858 (2.08870) -1600.10159 (2.08516) -1600.13103 (2.14194)
-1600.10242 (1.07971) -1600.10710 (1.00171) -1784.62117 (1.08651) -1784.63550 (1.07524) -1600.10735 (1.08839) -1600.13954 (1.18009) -1600.10156 (1.08296) -1600.13069 (1.10430)
JY (cm-1) 28.9 6412.1 53.3 589.4 -15.4 -219.4 5.1 71.4
a BS calculation has been achieved by using ROHF wave functions for the optimized triplet structure and then using the corresponding molecular orbitals as an initial guess. We have used 1 au of energy ) 27.2114 eV and 1 eV ) 8065.54 cm-1.
Figure 3. Dehydrogenation of unsubstituted DDP.
the spin population on the carbon atom is ∼1% or less, and there is also vanishingly small spin on the opposite carbon atom. Whereas their discussion is of some qualitative merit, our calculations show the following Mulliken spin populations on the o-VER carbon that is coupled to the spacer. In the triplet states, spin population on this carbon atom are -0.1767 and -0.1753 for 2(d) CPD and 2(d) DDP, respectively. For 2(c) species, spin population on this carbon in the BS state are -0.1778 in CPD and 0.1850 in DDP. These numbers are not negligibly small, and arranging spin alternation by assuming almost zero spin on this carbon atom would be wrong. The spin population on the coupling C atom of o-VER is substantially large, which indeed agrees with spin alternation rule. The spin populations on all atoms of ferromagnetically coupled species are given as Figure S2 in the Supporting Information. In passing, we note that the spin alternation shown for the o-VER(C) containing diradicals in ref 6 has been somewhat incorrect. The spin alternation rule in Figure S2 in the Supporting Information shows a slight deviation from spin alternation on atoms 1 and 8 in CPD of 2(a). This indicates a reduced FM interaction, accompanied by a J value much smaller than that of the DDP isomers. The extremely large change in J value of 2(a) upon photoexcitation is a good indication of the applicability of such systems as good photomaterials. But there is a problem of thermal instability. Although the DDP form can be kept intact indefinitely in degassed cyclohexane at 0 °C, at higher temperature and in the presence of air, it undergoes a rapid conversion to pyrene (Figure 3).12 This disqualifies the criteria of reversibility that is a fundamental requirement of photomagnetic coupler. When R is H (Figure 1), the CPD form is expected to have a restricted lifetime because hydrogen cannot pose enough hindrance to the rapid conversion to DDP. Therefore, to have a pragmatic application as molecular switches, R should be so chosen that the activation barrier for CPD f DDP conversion
is high. Proper substitutions can increase the activation barrier to hinder the thermal conversion. Indeed, Williams et al.13 have found that when R is CN, the activation barrier is maximum (E# ) 25.314 kcal mol-1) among the related species. Therefore, we have taken 15,16-dinitrile pyrene as coupler and found a sufficiently high positive J value for the closed ring dimer (J ) 589.4 cm-1), whereas in the CPD form, the J value is small (53.3 cm-1). Therefore, we predict o-VER (N)-(15,16-dinitrile)DDP-NN as a potential photomagnet. At room temperature, J/KbT ≈ 0.25 for the CPD species that would behave as a paramagnet, whereas J/KbT ≈ 3 for the DDP species that can behave ferromagnetically if properly aligned. These molecules have not been synthesized so far, but a suggestion can be made. The synthesis of 1,5-dimethyl-6oxoverdazyls typically proceeds through the reaction of methyl hydrazine with phosgene to form a bis-hydrazide, followed by condensation with an aldehyde to form a tetrazene and subsequent oxidation to give the verdazyl (scheme 1 of ref 14). Nitronyl nitroxide is routinely synthesized starting from aliphatic aldehydes and 2,3-dimethyl-2,3-bis(hydroxylamino)-butane. Direct treatment of the reaction mixture with sodium perchlorate or lead dioxide forms a nitronyl nitroxide derivative.15 Mitchell et al. have discussed the synthesis of dinitrile CPD (scheme 1 in ref 16). Mitchell and Boekelheide12 prescribed the method of synthesizing CPD derivatives with hydrogen atoms at position 15 and 16 (Figure 1). One has to now attach o-Ver to positions 1 or 14 and NN to position 8 of CPD. It may not be easy. The CPD diradical may turn out to be quite unstable, but the preparation of a stable derivative may be feasible. If the derivative contains an electron-withdrawing group, then it is likely to have a decreased absolute value of J, whereas an electron-donating group can lead to a larger J value. The real utility of the systems as photomagnetic materials requires fitting o-VER(N)-(15,16-dihydrogen)DDP-NN and o-VER(N)-(15,16-dinitrile)DDP-NN in suitable geometric patterns in a matrix, the former at a low temperature.
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Acknowledgment. We gratefully acknowledge Council of Scientific and Industrial Research for financial support. Supporting Information Available: Optimized triplet geometries, mean dihedral angle, spin alternation diagram, and spin population on all the diradicals. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Matsuda, K.; Irie, M. Polyhedron 2005, 24, 2477. (b) Tanifuji, N.; Matsuda, K.; Irie, M. Polyhedron 2005, 24, 2484. (c) Matsuda, K. Bull. Chem. Soc. Jpn. 2005, 78, 383. (d) Tanifuji, N.; Irie, M.; Matsuda, K. J. Am. Chem. Soc. 2005, 127, 13344. (e) Tanifuji, N.; Matsuda, K. M.; Irie, M. Org. Lett. 2005, 7, 3777. (f) Matsuda, K.; Irie, M. J. Photochem. Photobiol., C: Photochem. ReV. 2004, 5, 69. (g) Matsuda, K.; Matsuo, M.; Irie, M. J. Org. Chem. 2001, 66, 8799. (2) Teki, Y.; Toichi, T.; Nakajima, S. Chem.sEur. J. 2006, 12, 2329. (3) Huai, P.; Shimoi, Y.; Abe, S. Phys. ReV. B 2005, 72, 094413. (4) Ali, Md. E.; Datta, S. N. J. Phys. Chem. A 2006, 110, 10525. (5) Shil, S.; Misra, A. J. Phys. Chem. A 2010, 114, 2022. (6) Latif, I. A.; Panda, A.; Datta, S. N. J. Phys. Chem. A 2009, 113, 1595. (7) (a) Trindle, C.; Datta, S. N. Int. J. Quantum Chem. 1996, 57, 781. (b) Trindle, C.; Datta, S. N.; Mallik, B. J. Am. Chem. Soc. 1997, 119, 12947. (8) Koivisto, B. D.; Hicks, R. G. Coord. Chem. ReV. 2005, 249, 2612. (9) (a) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587. (b) Borden, W. T.; Davidson, E. R.; Feller, D. Tetrahedron 1982, 38, 737. (c) Feller, D.; Davidson, E. R.; Borden, W. T. Isr. J. Chem. 1983, 23, 105. (d) Kato, S.; Morokuma, K.; Feller, D.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 1983, 105, 1791.
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