Photophysical and Electrochemical Characterization of a Helical

Letter pubs.acs.org/OrgLett

Photophysical and Electrochemical Characterization of a Helical Viologen, N,N′‑Dimethyl-5,10-diaza[5]helicene Xiaoping Zhang, Edward L. Clennan,* and Navamoney Arulsamy Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States S Supporting Information *

ABSTRACT: The first helical viologen (4,4′-bipyridinium salt) has been prepared and characterized. Its reduction to the radical cation at −0.22 V vs SCE makes it the most easily reduced redox-active helicene known. It exhibits absorption at 397 nm for the S1 ← S0 transition, and it is luminescent allowing measurement of both its singlet (59.3 ± 0.1 kcal/mol) and triplet (54 ± 1 kcal/mol) energies. In contrast to neutral helicenes, it is not aromatic π-stacked in the crystal and has a shortest interdication distance of 4.977 Å. Its racemization barrier is calculated to be a sensitive function of its redox state.

V

iologens are rare examples of robust organic dications with reasonable thermal and hydrolytic stabilities.1 For many years,2 they were only the focus of academic curiosity, but that changed abruptly in the mid-1950s when it was discovered that Diquat (DQ2+) was a potent herbicide. Subsequently, viologens have found extensive use as electron-transfer mediators,3 as partners in supramolecular assemblies,4 as DNA photocleaving agents,5 as redox indicators,6 and as acceptor components of host−guest complexes.7 These electrochromic and photochromic applications take advantage of the strong electron-accepting character of the viologens and the stability and vivid color of their radical cations. In contrast, applications that use the anticipated potent redox behavior of their excited states are nearly nonexistent. This omission is due in part to the very short excited state lifetime of *1MV2+ (1.00 ± 0.04 ns).8 We have recently embarked on an effort to broaden the viologen structural landscape by embedding the 4,4′-bipyridinum functional group into nonplanar9 polyaromatic hydrocarbons (PAHs). A highly desirable outcome would be formation of a series of new redox addressable compounds that encompass the attractive well-established material properties of PAHs. We report here the successful formation of the first of these new materials, N,N′-dimethyl-5,10-diaza[5]helicene, 12+. This compound becomes the newest member of a very small but growing group of redox active helicenes that are attracting attention because of their potential use as electron transfer triggered ultrasensitive chiroptical switches.10

The viologen was purified by trituration with warm 95% ethanol. Attempted synthesis with other common alkylating agents such as methyl iodide or dimethyl sulfate failed, giving 3+ as the only observable product. This is in stark contrast to 4,4′-bipyridine which successfully reacts with all of these alkylating agents to give MV2+.1 We attribute the reduced reactivity of 3+ in comparison to monomethylated 4,4′-bipyridine to its greater planarity that leads to an enhanced incipient destabilizing electrostatic interaction between the two positive charges and to a developing peri interaction in the transition state for viologen formation. The same alkylation reactivity pattern was observed with 3,10diazaperylene, which is also planar and has a developing peri interaction in the TS for viologen formation.12 Heating of 12+ in water also resulted in demethylation to form the monomethylated helicene, 3+. The 1H and 13C NMR spectra (Figures S31−S33, Supporting Information) are in complete accordance with the structures of 12+, 2, and 3+. Furthermore, the proton chemical shifts of H6 and H9 are diagnostic for the amount of positive charge on the helical framework, decreasing from 10.07 ppm in 12+ to 9.90 and 9.68 ppm in 3+ to 9.50 ppm in 2. The structure of 12+ was also confirmed by X-ray crystallography. The X-ray structures of 12+ and the previously published X-ray structure of its bipyridine precursor,13 2, exhibit remarkable similarity. The only substantial differences are in the fjord region of the helicene. The dihedral angles that define the boundaries of this region, Θ14,14a,14b,14c, Θ14a,14b,14c,14d, and Θ14b,14c,14d,1 change upon methylation from 20.0°, 28.6°, and 14.0° to 17.2°, 33.8°, and 17.2°, respectively. The increase in the size of the fjord region, defined for the most part by the >5° increase in the internal dihedral angle, appears to be a fundamental characteristic of the viologen rather than a result of crystal packing forces. This is supported by B3LYP/6-311+G(2d,p) calculations that show that a similar, but somewhat attenuated, increase (16.9°, 27.9°, and 16.9° to 17.1°, 29.8°, and 17.0°) is observed in the gas phase.

The helical viologen, 12+, was synthesized by treating 5,10diaza[5]helicene, 2,11 with trimethyloxonium tetrafluoroborate.

Received: July 23, 2014

© XXXX American Chemical Society

A

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Table 1. Photophysical and Electrochemical Data for 12+ and 2 12+

2

λmax (nm) (ε × 10 ) (M cm )

465 (0.40 ± 0.02) 441 (0.38 ± 0.02)

λF (nm) λP (nm) τP (s) Stokes shift (nm) ΦF τF (ns) E(S1) (kcal/mol) E(T1) (kcal/mol) Egapa E1/2(1)b E1/2(2)b

504 623 0.028 ± 0.002 39 0.066 ± 0.004 5.8 ± 0.6 59.3 ± 0.1 54 ± 1 3.03 (2.48) −0.22 −0.52

397 (0.299 ± 0.005) 377 (0.256 ± 0.005) 358 (0.147 ± 0.004) 408 (1.0),c 431 (0.80), 455 (0.37) 527 1.7 11 0.14 ± 0.01 6.3 ± 0.6 71.0 ± 0.1 61 ± 1 4.01 (2.92)

−4

a

−1

−1

Calculated [B3LYP/6-311+G(2d,p)] (optical) in eV. bVolts vs SCE. cRelative intensity.

[E(S1) = 69.6 kcal/mol and ΔES−T = 15.1 kcal/mol].15 Finally, the very small Stokes shift observed for 2 (11 nm) and slightly larger value observed for 12+ (39 nm) are a reflection of the fairly rigid helical backbones and the lack of any substantial geometry change during excitation from S0 to S1. Viologen 12+ is reduced in CH3CN at a glassy carbon electrode in a quasi-reversible one-electron cyclic voltammetry (CV) step at −0.22 V, and an irreversible step at −0.52 V versus SCE. MV2+ undergoes more difficult (less extended π-system) but chemically and electrochemically reversible one-electron reductions in CH3CN at −0.50 and −0.92 V versus SCE.16 In contrast, the helquat, 42+ (Scheme 1), is reduced to the radical cation

Comparisons of the photophysical properties of 12+ and its bipyridine precursor, 2, are shown in Table 1. The low energy UV−vis bands exhibit vibronic progressions with an energy separation of approximately 1200 and 1300 cm−1 for 12+ and 2, respectively, corresponding to a C−H bend. TD-DFT calculations at the B3LYP/6-311+G(2d,p) computational level faithfully reproduce the experimental absorption wavelengths within 12 nm for both 12+ (477 nm) and 2 (375 nm). They also reveal that the low energy envelope of bands in both compounds are for the S1 ← S0 excitations and are primarily due to the HOMO → LUMO transitions. The extinction coefficient for the low energy band is approximately 25% larger in 12+ (4,000 M−1cm−1) than in 2 (3,000 M−1cm−1), which is also correctly predicted by the magnitudes of the computed oscillator strengths for these bands in the viologen, 12+ (f = 0.0519), and in bipyridine 2 (f = 0.0428). The S1 ← S0 band in 12+ is 208 nm (49.8 kcal/mol) bathochromic of the S1 ← S0 band in MV2+. This is consistent with the dramatic decrease in the HOMO−LUMO energy gap (5.41 to 3.03 eV) that occurs as the viologen backbone host is changed from biphenyl to [5]helicene, respectively. Viologens 12+ and 2 both fluoresce with quantum yields of 0.066 ± 0.004 and 0.14 ± 0.01 and lifetimes of 5.8 ± 0.6 and 6.3 ± 0.6 ns, respectively. The fluorescence from 2, but not from 12+, exhibits vibrational fine structure that is nearly the mirror image of that observed in its absorption spectrum. The lack of a vibronic progression in the 12+ emission is likely due to a larger density of rotational states in S0 as a result of the presence of the methyl rotors. Phosphorescence was also observed at −196 °C in an ethanol glass for both 12+ (λmax = 623 nm) and 2 (λmax = 527 nm). The singlet energies for 12+ (E(S1) = 59.3 ± 0.1 kcal/mol) and 2 (E(S1) = 71 ± 0.1 kcal/mol) were determined from the crossing point of the absorption spectra and the normalized emission spectra. The triplet energies for 12+ (E(T1) = 54 ± 1 kcal/mol) and for 2 (E(T1) = 61 ± 1 kcal/mol) were determined from the onset of the phosphorescence spectra. The shorter fluorescence lifetime and smaller quantum yield for 12+ in comparison to 2 (vide infra) can be attributed to the slightly smaller singlet−triplet energy gap, ΔES−T, in 12+ (5 kcal/mol) than in 2 (10 kcal/mol) and the concomitant increase in intersystem crossing rate. It is noteworthy that both of these ΔES−T are significantly smaller than the 20−30 kcal/mol observed for many aromatic hydrocarbons.14 On the other hand, E(S1) and ΔES−T for 2 are comparable to those reported for [5]helicene [E(S1) = 71.2 kcal/mol and ΔES−T = 14.9 kcal/mol] and [6]helicene

Scheme 1. Redox Couples for 42+ and MV2+

at a potential (−0.87 V vs SCE)10d very close to that of N-methylpyridinium perchlorate (−0.81 V vs SCE),17 indicative of its dipyridinium rather than viologen core structure. In the case of MV2+ the structures of the radical cation, MV•+, and neutral, MV0, redox partners are well-established and are depicted in Scheme 1. The most dramatic structural changes accompanying electron transfer are decreases in the interpyridinium-ring dihedral angle and in the length of the interconnecting bond. At the B3LYP/6-311+G(2d,p) computational level, the dihedral angle ∠344′3′ decreases from 42.04° in MV2+ to 0.16° in MV•+ and to 0.26° in MV0. Concomitantly, the 4−4′ bond length decreases from 1.487 to 1.428 Å to 1.380 Å in MV2+, MV•+, and MV0, respectively. Similar changes are evident as well in the B3LYP/ 6-311+G(2d,p) geometries of the 12+ redox partners. The interpyridinium-ring dihedral angle decreases from 29.80° to 25.89° to 23.9°, and the interconnecting bond length, d14b−14c, decreases from 1.449 Å to 1.411 Å to 1.381 Å in 12+, 1•+, and 10, respectively. It is this f ramework-enforced inability to achieve idealized B

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planar radical cation and neutral redox partner geometries that we suggest is the source of 1•+ and 10 instability on the CV time scale.18 Consequently, we have the interesting situation in which the helical extend π-system in 12+ thermodynamically enhances reductive formation of 1•+ and 10 while its rigid framework provides the stress necessary to kinetically enhance their decompositions. The redox partners 12+, 1•+, and 10 are all helically chiral molecules and exist as a mixture of P and M enantiomers. The transition states, TSs, for racemization of these redox isomers and for [5]helicene, 5,10-diaza[5]helicene, and N-methyl-5,10diaza[5]helicene were all located (Figure 1) with the synchronous

Figure 2. Several perspectives of 12+ crystal packing. Figure 1. Three views from different directions of the calculated transition state for racemization of 12+.

to the centro symmetric orthorhombic space group, Pbcn, and is achiral. Parts a−c of Figure 2 are different projections along different crystalline axes of an array of 16 molecules of 12+. Projection b shows that the dications are arranged in three columns with alternating stereochemistry from top to bottom as shown to the left of the projection. The 2l screw axis present between the columns indicates that each horizontal row is a single enantiomer sandwiched between two horizontal rows of the opposite enantiomer with the open fjord regions pointing in the opposite direction. In each horizontal row there are two BF4− associated with each 12+. They are actually located above the π-cloud of the terminal rings on opposite faces of the molecule, as can be seen in projection c or more clearly in d, at a B−N distance of 4.58 Å. However, the closest B−N approach distance (3.45 Å) is between 12+ and a BF4− in an adjacent horizontal row as shown by the hashed line in projection b. A comparison of the packing of 12+ with those of a series of neutral diaza[5]helicenes12 reveals that in contrast to the neutral helicenes, 12+ dications do not exhibit any π-stacking of their aromatic rings. The shortest interdication distance is 4.977 Å and the corresponding distances in the neutral helicenes are ca. 3.5 Å. In summary, we have described the synthesis and characterization of the first helical viologen. We anticipate that helically chiral viologens will find applications in a large number of areas including supramolecular chemistry, catalysis, and electrontransfer chemistry.

transit-guided quasi-Newton (STQN) method in conjunction with the B3LYP/6-311+G(2d,p) theoretical method/basis set. The TS for racemization of 12+ (12+-TS) adopts a saddle-like geometry with Cs symmetry reminiscent of TSs previously located for some carbo- and heterohelicenes.19 Similar TSs were also located for the other helicenes including for the redox isomers 1•+ and 10. The B3LYP/6-311+G(2d,p) activation barriers for racemization are collected in Table 2. Examination of the barriers reveal Table 2. Racemization Barriers for Helicenesa,b Ea c 2 3+

23.4 19.9 20.0

ΔG⧧calc 22.5 19.9 20.2

ΔG⧧exp 24.1

d

2+

1 1•+ 10

Ea

ΔG⧧calc

21.2 18.8 14.8

21.4 19.3 14.8

In kcal/mol. bEa potential energy and ΔG⧧calc = Eelec + ZPE + Evib + Erot + Etrans + RT − TS; T = 298.15 K. c[5]Helicene. dReference 23.

a

the following: (1) replacement of C5 and C10 in [5]helicene with nitrogen lowers the activation barrier for racemization by 3.6 kcal/mol; (2) stepwise alkylation of the two nitrogens in 5,10-diaza[5]helicene sequentially increase the barrier by 0.3 and 1.2 kcal/mol, respectively; and (3) reduction of 12+ by one and two electrons lowers the barrier by 2.1 and 4.5 kcal/mol, respectively, consistent with decreasing loss of aromatic resonance energy in the TSs for racemization. Experimental decreases in the activation barriers for racemization of chiral phosphines20 and sulfoxides21 upon one-electron oxidation and changes in racemization barriers of of the cationic, radical, and anionic redox states of alkyl-substituted cyclobutadiene−Fe(CO)322 have previously been reported. However, the result depicted in Table 2 is the first suggestion of this phenomenon in helically chiral substrates. The activation barriers for the charged helicenes in Table 2 can be considered intrinsic barriers that can be compared to experimental values to help understand the effect of counterion movement that must occur in order to compensate for the new electrostatic topology in the TSs. It is well established that crystal packing plays an important role in determining the bulk properties of materials.24 Viologen 12+ has C2 molecular symmetry and forms a crystal that belongs

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ASSOCIATED CONTENT

* Supporting Information S

Experimental procedures, characterization data, CV, absorption and emission spectra, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1147542) for their generous support of this research. C

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REFERENCES

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