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Structure, NMR and Electronic Spectra of [m.n]Paracyclophanes with Varying Bridges Lengths (m, n = 2 – 4) Taye B. Demissie, Helena Dodziuk, Jacek Waluk, Kenneth Ruud, Mariusz Pietrzak, Volha Vetokhina, Slawomir Szymanski, Jaros#aw Ja#wi#ski, and Henning Hopf J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b12168 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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The Journal of Physical Chemistry

Structure, NMR and Electronic Spectra of [m.n]Paracyclophanes with Varying Bridges Lengths (m, n = 2 – 4) Taye B. Demissie,†,‡ Helena Dodziuk,*,† Jacek Waluk,†,∥ Kenneth Ruud,‡ Mariusz Pietrzak,† Volha Vetokhina,† Sławomir Szymański,§ Jarosław Jaźwiński,§ Henning Hopf⊥ †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland;

‡

Centre for Theoretical and Computational Chemistry, Department of Chemistry, UiT – The Arctic University of Norway; §

Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland;

∥

Faculty of Mathematics and Natural Sciences, College of Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland; ⊥

Institut für Organische Chemie, Technische Universität Braunschweig, Hagenring 30, D38106, Braunschweig, Germany.

1 ACS Paragon Plus Environment


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ABSTRACT

Extending our earlier studies on cyclophanes, we here report the structure, chemical shifts, spinspin coupling constants, absorption and emission properties of [m.n]paracyclophanes, m, n = 2 – 4, obtained using a combination of experimental and computational techniques. Accurate values of proton chemical shifts as well as of JHH for the bridges are determined. The experimental chemical shifts, coupling constants, absorption and emission wavelengths are satisfactorily reproduced using density functional theory calculations, using both the B3LYP and ωB97X-D functionals. The geometries predicted using a functional that includes dispersion corrections (ωB97X-D) are in a better agreement with available experimental values than those obtained using the B3LYP method. Up to 8 UV-vis absorption/emission bands have been observed (or anticipated in the region below 200 nm) and assigned on the basis of quantum-chemical calculations. Optimized excited-state geometries showed that the distances between the aromatic bridgehead carbon atoms of all the [m.n]paracyclophanes in the excited state decrease compared to the ground-state geometries by ca. 0.2 - 0.9 Å, the largest being for [4.4]paracyclophane, though the rather large differences in the calculated emission wavelength compared to experiment cast some doubts on the accuracy of the excited-state geometries.

KEYWORDS: [m.n]paracyclophanes,

1

H and

13

C chemical shifts, coupling constants,

absorption, emission, DFT calculations


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1.

INTRODUCTION Benzene and its aromatic derivatives belong to the most important compounds in organic

chemistry. Among the many possible benzene derivatives, cyclophanes with short bridges hold a prominent place: they are, for instance, an important test-bed for understanding steric repulsions and π-π dispersion interactions.1-3 They also exhibit interesting conformational properties.2-6 With short bridge(s) connecting the aromatic ring(s) they are fixed in a very close distance relative to each other, leading to strain in the aromatic ring moieties and causing deformation of the rings, among others, breaking their inherent planarity.2-5,7-11 There are also considerable electronic interactions between the aromatic rings when they are close to each other, leading to additional significant changes in the geometry and electronic structure of the cyclophanes that is observed through unusual spectroscopic responses.1,3,9,12 Due to their unusual properties, cyclophanes have attracted the attention of both experimentalists and theoreticians for more than sixty years. Cyclophane derivatives have also found their use in diverse areas of research, such as asymmetric synthesis,13 supramolecular chemistry,14 polymer chemistry, and optoelectronic materials.10,15-18 They have also been shown to have anti-bacterial efficacy,19 to be inhibitors of HIV proteinase,20 and to act as catalysts and sensors.21,22 Until very recently, as discussed in detail in our previous studies,23 different experimental studies of [2.2]paracyclophane 1 gave conflicting structural information, with X-ray data measured at 100 K yielding an eclipsed D2h geometry11 and a D2 geometry at room temperature.24 Also the Csp2Csp3Csp3Csp2 twist angle for the D2 symmetry has been the subject of controversy in both experimental11,24 and theoretical studies.7,25-28 However, a recent study by Wolf et al.29 resolved the conflicting data by providing X-ray structural parameters measured at temperatures ranging from 15 K to 260 K. The authors reported that [2.2]paracyclophane has a



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twist angle of 12.83(4)o at 15 K, 10.7(3)o at 45 K and 0.0o above 55 K.29 This result thus supports the heat-capacity study of 1 by Andrews and Westrum,30 in which a hump in the Cp curve around 50 K was observed, suggesting the existence of a phase transition at this temperature.

1 13 14

7

2

3

6 8

5 9 10

12

15

13

16

14

12

11

13 14 15

7

2

4

5 9 10

12

11

4

2

3

6 7 8

5 9

1

15 13

16

14 17

11

12

16

13 17

18

14 15

3

2

1 7

6 8

12

4

15 16

10

17 18

11

3

2 3

6 8

4

5 9 10

6 78

1 1

3

2

1

4

4

16

13

5

17

14

9 10

18

15

11

19

1 2

3

6 8

5

7 16

5

4

9 10

17 18 20

19

11

12

6

Figure 1. Paracyclophanes 1 – 6 with the atom numbering used in this work.

The synthesis31 and a few static NMR31-33 parameters for the [m.n]paracyclophanes (m, n = 2 – 4) have been reported. Even though these molecules have interesting properties, they have not been studied intensely. In particular, no computational studies of the geometry and NMR parameters for the [m.n]paracyclophanes with different bridge lengths ([2.3]paracyclophane 2, [2.4]paracyclophane 3, and [3.4]paracyclophane 5) have been published. A computational stability analysis of [2.2]paracyclophane 1, [3.3]paracyclophane 4 and [4.4]paracyclophane 6 focusing on conformational analysis and strain energies was reported by Bachrach,7 who also obtained a twist angle of ca. 10o – 18o for 1 using different functionals. Previous studies of the structure and dynamics of 4 showed that in solution it exists as a mixture of cis- and trans4 ACS Paragon Plus Environment

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isomers,

4,34

with the cis prevailing, while in the solid state only the trans-isomer is present.8

Earlier, in our group4 using line-shape analysis combined with B3LYP computed NMR chemical shifts as well as previous studies in solution,34,35 the interconversion barrier between the cis and trans conformations of 4 was determined to be 12.29 ± 0.08 kcal/mol. We also recently reported36 an analysis of the dynamics of [2.3]paracyclophane 2 and [3.4]paracyclophane 5 using temperature-dependent NMR measurements, obtaining an Arrhenius activation energy of 11.6 ± 0.1 kcal/mol for 2. In the same report, two separate sets of Arrhenius parameters (activation energies 11.2 ± 0.5 kcal/mol for the propano and 9.7 ± 0.2 kcal/mol for the butano-bridges) were obtained for 5.36 In the case of 6, rotation of the rings, as well as conformational motions of the bridges, give four conformational isomers (vide infra) of which only the two lowest ones have significant population at room temperature.



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Figure 2. Conformers, enantiomers and topomers of 2 – 6, and their interconversion paths. The enantiomers are denoted by the unprimed and primed symbol of the compound. Topomers of the same enantiomer are distinguished by letters a and b. The interconversion processes for 5 and 5' denoted by 1 and 2 are nonequivalent.

Absorption and emission spectroscopic studies of 1 and 4 have been reported.37-42 Among these reports, Shen et al.40 published the absorption spectrum for the transition to the first excited state of 1 in the region between 307 and 326 nm. Melzer et al.42 studied the excited triplet state of 1 and 4 and found the maximum emission wavelength for both molecules to lie between 475 and 485 nm. The two-photon absorption spectrum of crystalline 1 has been reported by Fuke et al.,43 who assigned the two absorption bands in the region between 288.2 and 273.97 nm to two 6 ACS Paragon Plus Environment

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even-parity allowed lB1g ← 1Alg and 1B2g ← 1A1g transitions. Recently Shirai et al.44 reported an ab initio molecular orbital study on the excited states of 1 and 4. The authors showed that the equilibrium distances between the benzene rings in the first excited states were shorter than those in the ground states. The proximity of the rings in the paracyclophanes and the structural constraints in these molecules influence their spectroscopic properties. Among these, NMR chemical shifts and spinspin coupling constants, as well as UV spectral parameters, are very sensitive to the molecular structure. The excitation and emission spectra of the paracyclophanes are altered by the variation of the bridge length and deformation of the aromatic rings causing variations in the electronic interactions between the two aromatic rings. Because of these interesting properties and promising prospective applications of cyclophanes, a detailed analysis of the spectroscopic molecules is of value. We have previously reported a combined computational and experimental studies of NMR parameters in [2.2]-

5,23,37

and

[3.3]paracyclophanes,4 dynamic NMR spectra of [n.3]paracyclophanes (n = 2 – 4),36 as well as electronic and magnetic circular dichroism spectra of some cyclophanes37 (among them those of [2.2]paracyclophane 1), exploring the influence of nonparallel arrangement of the aromatic rings on their properties. In this paper we extend our investigations of the paracyclophanes to the study of the structure, NMR, absorption and emission properties of [m,n]paracyclophanes with m, n = 2 – 4. We were in particular interested in exploring the influence of the nonparallel arrangement of the aromatic rings and the bridge lengths on the spectroscopic properties and use experimental data to benchmark the computational studies. In cases where experimental data could not be obtained, our theoretical data shed insight into the trends of the spectroscopic properties and their relation to the structures of the cyclophanes.



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2. EXPERIMENTAL 2.1. Reagents [2.2]Paracyclophane, 1, 99% purity was purchased from Alfa Aesar. The synthesis of [2.3]paracyclophane

2,31

[2.4]paracyclophane

3,45

[3.3]paracyclophane

4,8,46

[3.4]paracyclophane 531 and [4.4]paracyclophane 647 have been reported earlier.

2.2. NMR measurements All NMR experiments at room temperature were performed in CDCl3 solutions on a Varian vnmr600 spectrometer using the XWIN NMR acquisition and processing program. The 5 mm triple broadband inverse probe equipped with a z-gradient coil was used for 1H and

13

C

measurements. All 13C, 1H and 2D correlation experiments were carried out using the inverse gradient technique and pulse sequences with adiabatic pulses (13C, 1H gHSQCAD and gHMBCAD Varian's pulse sequences). Signal assignments were carried out by means of 1D NOE, COSY and (13C, 1H)-HSQC and HMBC experiments. Variable-temperature 1H spectra were recorded on a Bruker 300 Avance II spectrometer equipped with a broadband inverse probe and a BVT 3200 temperature control unit. The solutions of 2 and 3 in CD2Cl2, and 5 and 6 in Freon R12/CD2Cl2 (10v/1v) were degassed and sealed under vacuum in 5 mm NMR tubes. The temperatures were measured using a methanol chemical shift thermometer down to 183 K. The values of chemical shifts are reported in ppm, and the coupling constants in Hz. For the chemical shifts, tetramethylsilane (TMS) was used as a reference.


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2.3. Analysis of NMR spectra 13

C chemical shifts, and proton-carbon and carbon-carbon coupling constants were read from

spectra. To determine the exact values of the proton chemical shifts and proton-proton coupling constants, the 1H spectra were analyzed numerically using a home-written FORTRAN program which performs iterative least-squares fits to the experimental line shapes, with the assumption that all the resonance lines have Lorentzian shape of the same width.

2.4. Absorption and Emission Measurements All the absorption spectra of the [m.n]paracyclophanes, (m, n = 2 – 4), were measured in nhexane (Aldrich, spectral quality) at 293 K on a Shimadzu UV-3100 spectrophotometer equipped with a variable-temperature chamber, allowing temperature control between 88 K and 333 K, with the accuracy of ±1 K. The fluorescence spectra were recorded on an Edinburgh FS 900 CDT fluorometer and corrected for the sensitivity of the instrument. Excitation spectra were measured for optical densities not exceeding 0.1 at the maxima of absorption bands in order to avoid nonlinear effects.

3. COMPUTATIONAL DETAILS The quantum chemical calculations were performed either using the Gaussian48 or Dalton49 quantum chemistry programs. The molecules were optimized using B3LYP,50-52 as well as the Head-Gordon and Chai long-range corrected functional,53 which includes damped atom-atom dispersion corrections, ωB97X-D. The augmented correlation-consistent polarized valence triple-



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ζ basis set of Dunning54,55 optimized by Jensen and coworkers for spin-spin coupling constant calculations, referred to as ccJ-pVTZ,56 and the basis set suggested for NMR parameter calculations by Schindler and Kutzelnigg57 based on the atomic basis sets of Huzinaga, HuzIV,58 were used. London atomic orbitals were used to ensure gauge-origin independence of the calculated shielding constants.59,60 For the TD-DFT absorption and emission calculations, the CAM-B3LYP61 and ωB97X-D functionals together with augmented correlation-consistent polarized valence triple-ζ basis set of Dunning (aug-cc-pVTZ)54 were used. Each optimized structure was conﬁrmed by frequency calculations to be the real minimum on the potential energy surface having no imaginary vibrational frequencies. The geometries optimized using B3LYP/ccJ-pVTZ level were used for all the NMR calculations using B3LYP, whereas those obtained from ωB97X-D/ccJ-pVTZ level of calculation were used for the NMR calculations using ωB97X-D. As is common when comparing with experimental results, TMS was used as reference for the chemical shift calculations. The ccJ-pVTZ basis sets were used for carbon and hydrogen atoms and since ccJ-pVTZ is not available, the cc-pVTZ basis set was used for silicon.

4. RESULTS AND DISCUSSION The results obtained for the ground-state interatomic distances between carbon atoms, ring deformation and twist angles of the two aromatic rings are collected in Table 1. 1H and

13

C

chemical shifts are presented in Figs. S2a - S2d and Figs. S3a - S3d, respectively, in the Supplementary Material, whereas the experimental and calculated results for 3JHH values together with torsional angles between the protons are given in Table 2. Full structural parameters as well as nJHH, nJCH and nJCC are given in the Supplementary Material. The interatomic distances between the carbon atoms, ring deformation and twist angles of the


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aromatic rings obtained from the excited-state calculations are collected in Table 3, while the absorption and emission spectra are shown in Fig. 3. 4.1. Geometry The calculated structures of 1 - 6 are displayed in Table 1 and Table S1 and Fig. S1. In this figure, compounds 3, 5, and the lowest-energy isomer of 6 are treated as chiral because for them the racemization processes either can be frozen or considerably slowed down on the time scale of liquid-phase NMR. The enantiomers are denoted by the unprimed and primed symbol of the compound. Topomers of the same enantiomer are distinguished by letters a and b. In the interconversion paths between isomers, enantiomers and topomers of paracyclophanes 2 - 6 simultaneous rearrangements/inversions of both aliphatic bridges in a single reaction step are excluded. The controversial issue of the equilibrium geometry of 1 was presented in Ref. 5 while other relevant structure questions in 2 - 6 are discussed in the rest of this Section.

4.1.1. Symmetry, torsional angles in bridges and conformations The detailed analysis of the structural properties of 1 has been reported in our previous paper,5 and here we only summarize the main features of its structure. The X-ray data measured at 297 K by Hope et al.24 gave a disordered structure with D2 symmetry, whereas the X-ray structure studied at 100 K reported by Lyssenko et al.11 resulted in the D2h structure without any disorder. Wolf et al.29 reported that 1 has a twist angle of 12.83(4)o at 15 K, 10.7(3)o at 45 K and 0.0o above 55 K,29 thus providing very clear experimental evidence for the change in structure as a function of temperature, and thus resolving the apparent discrepancy between different experimental investigations. As observed by Grimme,25 Bachrach7 and our group,23 the optimized geometry depends strongly on the functional used. The ωB97X-D functional gives an



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optimized structure with D2 symmetry, whereas B3LYP gives the D2h geometry. The energy barrier between the two minima obtained from the calculation using ωB97X-D functional is 0.2 kcal/mol. The C1C13C14C7 angle that defines the twist of the aromatic rings of 1 calculated using B3LYP is -0.5o (close to D2h symmetry) and with ωB97X-D 14.9o (D2 symmetry). Test calculations were also performed to see the effect of dispersion correction to B3LYP. The results show that adding the D3 version of Grimme’s dispersion with the original D3 damping function62 to the B3LYP functional (B3LYP-D3) does not lead to any significant change in twist angle, giving actually a twist angle of 0.0o. However, adding the D3 version of Grimme’s dispersion with Becke-Johnson damping63 (B3LYP-D3BJ) functional gives a twist angle of 9.4o. Keeping in mind that the comparison of experimental and computed geometries is often very difficult (since the latter refer to isolated molecules in the gas phase while the former to the solid state), we note that the functionals that correctly reproduce the interring separations (vide infra) do not necessarily give reliable values of the bridge torsional angles. For instance, our reported bond lengths and bond angles of 1 (studied together with other cyclophanes with unsaturated bridges)23 calculated using ωB97X-D are in excellent agreement with the experimental geometry obtained by Lyssenko et al.11 The C1C13C14C7 twist angle of 1 determined using the same functional is 14.9o (Table 1) while the experimental twist angle obtained by Lyssenko et al. at 100 K is 0.0o11 and that by Hope et al. is 6.4o at room temperature.24 However, this calculated value agrees well with the very low-temperature twist angle reported by Wolf et al., 12.83(4)o at 15 K.29 Also Shirai et al.44 and Bachrach7 on the basis of MP2 and ωB97X-D calculations reported values of 21.8o and 15.4o, respectively, for the C1C13C14C7 twist angle. From the inspection of the data collected in the Cambridge Structural Database, CSD,64-66 the main conclusions for cyclophanes with multiple ethano bridges are: (i) there is considerable disorder at


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room temperature in the observed structures and (ii) the Csp2Csp3Csp3Csp2 torsional angles are less than 6o. When we come to the calculated values, the large differences in the torsional angles obtained using the different functionals illustrate the complexity of the problem of finding an appropriate functional to describe the structure of highly strained molecules such as [2.2]paracyclophane 1. An added complexity in the analysis arises from the strong temperature sensitivity of the structure, making a direct comparison with experiment difficult. The C1C13C14C7 and C4C1C7C10 twist angles of 1 – 6 are collected in Table 1. As the bridge lengths increase, there is a possibility for the existence of conformational isomerism (Fig. S1a-d). 3, 5, and the most stable conformer of 6 (vide infra) have two enantiomers of C2, C1, and D2 symmetries, respectively. The C1C13C14C7 twist angle determined using the ωB97X-D functional for 2 is 0o while that of 3 is 28.5o. Similar results are obtained with the B3LYP functional, but for these geometries the two aromatic rings of 1 and 2 are eclipsed, in contrast to 3, whereas only those of 2 are eclipsed in the ωB97X-D calculations. Unfortunately, no experimental data are available to allow us to choose the best functional. As already noted, 4 exists in solution as a mixture of the more stable C2v cis and C2h trans conformers. However, in the solid state, 4 crystalizes in its trans form because of favorable crystal packing. The X-ray structure of 4 has S2 symmetry (with a C1C13C15C7 torsional angle of 2.8o),9 whereas all calculations in our study lead to a structure with C2h symmetry (with this angle equal to 0o). All the calculations performed for 5 resulted in a structure with C1 symmetry. Rotating one of the bridges of 6 while keeping the rings parallel, gives rise to two conformers with D2 and C2h (1.61 kcal/mol higher than D2) symmetries, with 94% population of the D2 structure at room temperature, which is in agreement with the experimental result.45 Rotating one of the rings, while keeping the other fixed, gives two additional conformers of much higher energies. In



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conclusion, although there are differences for 1, the B3LYP and ωB97X-D functionals provide very similar torsional angles for all the other cyclophanes studied here. Nevertheless, considering the importance of intramolecular interactions in these systems, we strongly recommend using functionals that include dispersion for these systems, in agreement with the recommendations of Grimme et al.25,67

4.1.2. Interring separation The available experimental and calculated distances between the corresponding carbon atoms of the aromatic rings together with ring deformation and twist angles are listed in Table 1. The average planes of the rings in 2, 3 and 5 are not parallel unlike those of 1, 4 and 6, which mainly is due to the equal number of carbon atoms in the alkyl chains in the latter group of molecules. The C1…C7 interring separation distance of 1 – 5 is less than the sum of the van der Waals radii of two carbon atoms (ca. 3.4 Å) unlike that of 6. Understandably, the C4…C10 distances of 3, 5 and 6 are longer than 3.4 Å, unlike those of 1, 2 and 4. In our previous study23 as well as the study by Grimme et al.,67 the dispersion-uncorrected B3LYP functional has been found to overestimate the interring separation distance, as well as the Csp3-Csp3 bond length of 1, due to its over-repulsive nature. In contrast, dispersion-corrected functionals (e. g. ωB97X-D) give results close to the experimental values.7,23,25 Overall, the ωB97X-D functional gives somewhat shorter interring distances than B3LYP, with larger differences for the molecules with the longest bridges. This is due in part to the increased flexibility in the structures with long bridges, but also because of the increased importance of dispersion interactions compared to strain energy. In general, the ωB97X-D interring distances are closer to experiment, and this is particularly the case for 6 where the differences between the two functionals are the largest.


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Table 1. Distances (in Å) between the corresponding non-bonded carbon atoms, ring deformation angles (in degrees), and twist angles (in degrees) of the two aromatic rings of 1 - 6: experimental values, no brackets; and calculated values using B3LYP/ccJ-pVTZ, in parenthesis; and ωB97X-D/ccJ-pVTZ, in square brackets. 1

C1C7

2

trans-4

3

cis-4

5

6-C2h

6-D2

Exp

Calc

Calc

Expc

Calc

Expd

Calc

Calc

Calc

Calc

Expe

Calc

2.786a, 2.782b

(2.826)

(2.831)

2.790

(2.847)

3.137

(3.238)

(3.242)

(3.274)

(4.156)

3.960

(4.160)

2.7837(8)f,

[2.797]

[2.785]

[3.205]

[3.209]

[3.168]

[3.891]

3.099a, 3.097b

(3.156)

(3.156)

(3.410)

(3.328)

(3.489)

(4.216)

3.0990(9)f,

[3.112]

[3.094]

[3.372]

[3.294

[3.355]

[3.892]

(3.156)

(3.357)

(3.427)

(3.328)

(3.817)

(4.216)

[3.112]

[3.283]

[3.387]

[3.294]

[3.674]

[3.892]

(2.826)

(3.256)

(3.238)

(3.242)

(3.936)

(4.156)

[2.797]

[3.186]

[3.205]

[3.209]

[3.814]

[3.891]

(2.826)

(3.444)

(3.409)

(3.512)

(3.921)

(4.216)

[2.797]

[3.351]

[3.372]

[3.469]

[3.765]

[3.892]

3.099a, 3.097b

(3.156)

(3.222)

(3.426)

(3.512)

(3.606)

(4.216)

3.0990(9)f,

[3.112]

[3.140]

[3.387]

[3.469]

[3.458]

[3.892]

-14.4a, 12.6b

(-14.9)

(-10.9)

(-8.7)

(-8.2)

(-4.5)

(-2.8)

12.5f, 12.4g

[-14.8]

[-11.7]

[-8.5]

[-8.0]

[-4.9]

[-3.1]

(-0.4)

(0.0)

-

-

-

-

-

-

-

[-14.9]

[0.0]

0.0a, nab

(-1.0)

(0.0)

0.0

(0.0)

(0.0)

(5.4)

(13.8)

-

(0.0)

4.52f, 0.0g

[-5.2]

[0.0]

[0.0]

[0.0]

[5.2]

[14.0]

[2.798]

[4.046]

2.7812(12)g C2C8

-

(3.303)

3.289

[3.223]

-

(4.229) [4.080]

3.0956(11)g C3C9

3.099a, 3.097b f

3.0990(9) ,

-

(3.847)

3.310

[3.751]

-

(4.229) [4.080]

3.0956(11)g C4C10

2.786a, 2.782b f

2.7837(8) ,

3.860

(3.959)

3.137

[3.878]

3.990

(4.160) [4.046]

2.7812(12)g C5C11

3.099a, 3.097b f

3.0990(9) ,

-

(3.847)

3.289

[3.751]

-

(4.229) [4.080]

3.0956(11)g C6C12

-

(3.303)

3.310

[3.223]

-

(4.229) [4.080]

3.0956(11)g C2C1C6C5h

C1C13C14C7

0.0a, 6.4b f

12.83(4) , 0.0 C4C1C7C10

g

-

(-7.7)

-8.2

[-7.9] -

(25.9)

-

(-3.3) [-3.4]

[28.5] -

(11.5) [12.9]

[0.0]



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a

Ref. 11 at 100 K, b Ref. 24 at 297 K, c Ref. 45, d Ref. 8, e Ref. 47, f Ref. 29 at 15 K, g Ref. 29 at 120 K, The difference in sign is immaterial. h

4.1.3. Bond lengths and angles The bond lengths and angles of 1 – 6 are collected in Figs. S1a – S1d and Table S1, respectively, in the Supplementary Material. In general, the calculated bond lengths reproduce experimental trends, with the ωB97X-D/Huz-IV calculations giving in most cases the best agreement with the available experimental results although they still are not fully satisfactory. B3LYP predicts in most cases somewhat longer bond lengths than the ωB97X-D functional. As expected, the largest distortions from a standard geometry are those of the Csp3-Csp3 bond lengths of the short C13-C14 bridge of 1 – 3 which are longer than the standard Csp3-Csp3 bond length of 1.540 Å.68 The Csp2-Csp3 bonds of 1 – 3 along the bridges are slightly longer than the standard exocyclic bond length of 1.478 Å, whereas the length of the Csp2-Csp2 bonds (Fig. S1a) is close to that of benzene. The Csp3Csp3Csp3 bond angles are larger than the regular tetrahedral bond angle of 109.5o, with a significantly larger, close to 120o, angle for 2. The Csp2Csp3Csp3 and Csp2Csp2Csp3 bond angles are close to the corresponding regular bond angles. The same is also observed for the Csp2Csp2Csp2 bond angles, except those at the bridgeheads that are less than 120o due to the ring strain. The Csp2Csp2Csp3 angles, such as C1C4C15 in 1, and the analogous ones in the other molecules of this study, which describe the deviation of the Csp3 atom from the approximate plane of the aromatic rings, are less than 160o in 1 and increasing up to 177o for 6-C2h, in agreement with expectations and available experimental results. The C2C1C6C5 torsional angle, which characterizes the non-planar distortions of the aromatic rings, decreases as the bridge length increases. This torsional angle for the cyclophanes with non-parallel rings, 2, 3 and 5, is smaller on the side of the shorter bridge, suggesting that the influence of the π-π electron 16 ACS Paragon Plus Environment

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repulsion between the two aromatic rings is stronger than the energy gained by increasing the length of the bridges. Overall, no significant differences in the bond angles are predicted by the two functionals.

4.2. 1H Chemical Shifts The proton chemical shifts of 15 and 44 were reported previously, but in view of the extension of the functionals/basis sets applied, they are also given in Figs. S2a and S2c of the Supplementary Material, and the complete lists of proton chemical shifts of 2, 3, 5 and 6 are presented in Figs. S2a – S2d. The chemical shifts of 1 are somewhat dependent on the optimized structure, where those obtained from the structure optimized using ωB97X-D, which includes empirical dispersion,53 are in better agreement with the experimental results compared to those obtained from the B3LYP optimized geometry, thus improving on our previous estimates.5 It should be stressed that for the conformationally labile compounds 2 - 6 the experimental values reported in this and earlier papers4,5,35 involve data measured at low temperatures at which the mentioned dynamics are frozen. The values for protons were obtained from iterative fits to the experimental line shapes. In all subsequent discussions, differences less than 0.05 ppm are not considered. In most cases, the calculated chemical shifts values for the aromatic protons are larger than the experimental ones and reproduce the experimental trends. Exceptions do occur, however. For instance, the chemical shifts of the H2 and H12 protons of 2 and the H12 proton of 5 calculated with B3LYP/Huz-IV and the H11 proton calculated using ωB97X-D/Huz-IV are smaller than experiment, and the H2 and H12 the chemical shifts for 2 calculated with B3LYP/Huz-IV and that for H8 calculated using ωB97X-D/ccJ-pVTZ for 3 do not follow the experimental trends.



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Page 18 of 44

There is a stronger dependence of the chemical shifts on the positions of the aliphatic protons relative to the aromatic rings than for the position of the aromatic protons in the ring system. Therefore, no simple relation between the experimental and calculated chemical shift values could be found for the aliphatic protons. These protons also show a greater dependency on the choice of functional and basis sets than the aromatic protons. Taking into account the complicated spectra of coupled dynamic spin systems, highly sensitive to the position of protons with respect to the aromatic rings, the reproduction of experimental trends by the calculations is in most cases satisfactory, although the differences between the calculated and experimental values in three cases are greater or equal to 0.9 ppm (H15b of 2 calculated using ωB97X-D/HuzIV, and for H13a and H17o of 3 (where ‘a’ refers to protons pointing to the viewer, and ‘b’ to those pointing to the back of the figure plane, see figures in Supplementary Information. On the other hand, “o” refers to protons pointing outside of the molecule), both calculated with B3LYP/ccJ-pVTZ). No clear conclusion on the performance of the two basis sets can be reached since lengthening the bridges affects the chemical shifts of the aliphatic protons in a complicated way. In most cases, as the length of the bridge increases, the chemical shifts of the bridge protons neighboring the aromatic rings are larger than those further away from the rings. The exceptions are some calculated values for H16o for 3, and some of those for H17o and H18o for 5. Temperature effects on the proton chemical shifts have also been studied experimentally, with the chemical shifts of the ethano-bridge protons of 236 and 34 decreasing when the temperature decreases from 303 K to 223 K.


Page 19 of 44

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4.3. 13C Chemical Shifts Complete lists of

13

C chemical shifts are presented in Figs. S3a – S3d including the

13

C

chemical shifts of 15 and 44 reported previously, in view of the extension of the functionals/basis sets applied. With the exception of the B3LYP carbon shielding constants for 1, no significant basis set dependence is observed. Overall, the calculated values reproduce the experimental trends. In particular, both experimental and calculated (with one small exception) values of chemical shifts of the aromatic bridgehead carbon atoms are larger than those of the other aromatic carbons atoms. However, a few exceptions to the reproduction of the experimental trends exist, such as the carbon chemical shifts of C13 for 2 calculated using B3LYP/ccJ-pVTZ and ωB97X-D/ccJ-pVTZ, of C5 for 3 calculated using B3LYP/ccJ-pVTZ, of C4 for cis-4 calculated using ωB97X-D/ ccJ-pVTZ, and of C6 for 5 calculated using B3LYP/ccJ-pVTZ, ωB97X-D/ccJ-pVTZ, and B3LYP/Huz-IV. The calculated values for the chemical shifts for both aromatic and aliphatic hydrogen atoms are almost without exception larger than the experimental ones, the only exceptions being the aliphatic C14 carbon atoms of both isomers of 4 for the B3LYP functional and both basis sets used, where instead values smaller than experiment are obtained. Interestingly, very small errors < 1 ppm in the calculated values of chemical shifts were obtained for C14 in cis-4 using B3LYP/Huz-IV and for C6 in 3 using B3LYP/ccJ-pVTZ. In contrast, most of the shielding constants for the aromatic carbon atoms differ by more than 10 ppm reaching 18.5 ppm for C6 of 6–D2, for which the experimental spectrum of 6 with the frozen conformations could not be obtained. Inspection of the data in Figs. S3a – S3d shows that no basis set or functional used in this work reproduces the experimental values significantly better than the others.



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Page 20 of 44

The chemical shifts of the carbon atoms bonded to an aromatic ring are larger by ca. 5 ppm than chemical shifts of other aliphatic carbon atoms. Also in agreement with expectation, the chemical shifts of the ethano-bridge are only slightly influenced by the length of the second bridge. For instance, the ωB97X-D/ccJ-pVTZ calculated values δ(C1) of 1, 2 and 3 are 153.1, 150.4 and 150.5 ppm, respectively. Slightly larger differences were observed for the central atom of the propano-bridge, e.g. 32.6 ppm, 29.2 ppm, 29.7 ppm and 30.3 ppm for C16 in 3, C14 in trans- and cis-4, and C14 in 5, respectively. For the non-bridgehead carbon atoms of 2 and 3 (C5 and C6 for example), both the experimental and calculated chemical shifts decrease with an increase in the neighboring bridge length.

4.4. nJHH Coupling Constants Selected 3JHH coupling constants are listed in Table 2. The complete list of nJHH values is presented in Table S2 of the Supplementary Material. With the exception of 5, experimental trends are reproduced satisfactory both for 2JHH and 3JHH. However, some exceptions still exist for the 2JH15aH15b (where, as described before, ‘a’ refers to protons pointing to the viewer, and ‘b’ to those pointing to the back of the figure plane, see figures in Supplementary Information) for 2 and trans-4. In agreement with expectations, the calculated 2JHH values are negative, whereas the values of nJHH are positive for n = 3 as are those for n = 4. Interestingly, of the functionals/basis sets used, the best agreement with experiment was obtained with ωB97X-D/ccJ-pVTZ, except for 5. The 3JHH coupling constants fall into two categories: couplings across aromatic or aliphatic carbon atoms, respectively. Those involving the first group (e.g. H2H3) are about 8 Hz. In agreement with the cosinusoidal Karplus relation,69 the 3JHH coupling constants involving the aliphatic carbon atoms are largest (ca. 12 Hz ) for the torsional angle θ values close to zero or


Page 21 of 44

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180o. On the other hand, the smallest 3JHH (less than 2 Hz) values are obtained for the calculated θ values of ca. ±75o. Also for 3JHH, ωB97X-D/ccJ-pVTZ give results closest to the experimental values, but these differences are smaller than was found for the 2JHH values.

Table 2. 3JHH of 1 – 6 (in Hz): experimental values, no brackets; calculated values using B3LYP/ccJ-pVTZ in parenthesis and ωB97X-D/ccJ-pVTZ in square brackets. Torsional angles (θ, in degrees) values are extracted from the ωB97X-D/ccJ-pVTZ optimized structures. “nm” stands for “not measured”. 15 c

H13aH14a

3

JHH

a

2 θ

b

10.65

H13aH14b

[12.12]

JHH

(9.11) [0.0]

[9.54]

4.15

3.51

nm

(2.22)

(7.98)

[98.2]

[-117.0]

5.03

(2.81) [6.34]

[2.89]

[5.24]

[9.85]

[9.85]

10.65

2.61

nm

(2.81)

(9.11)

[17.2]

[2.62]

θ

-

JHH

θ

JHH

θ

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

[66.2]

[9.54]

-

-

-

-

-

(2.80)

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

3.10(5)

(2.62) [-65.6]

[2.89]

1.83

(2.89) [-65.9]

[2.99]

(1.31) [-64.4]

[1.59]

4.36

4.81

3.90(5)

5.14

(4.65)

(4.83)

(4.68)

(6.64)

[48.5]

[5.28]

[48.2]

13.13

(11.84) [12.82]

H13bH14o

-

2.94

12.77 -

θ

[31.9]

[5.09]

H13bH14i

JHH

[147.8]

[3.08]

H13aH14o

3

[147.8]

3.24 H13aH14i

6-D2

5 3

[31.9]

(7.98) [117.0]

(10.22) [11.09 ]

JHH

3

nm

(4.73) [132.0]

θ

3

cis-44

nm

(10.35) [17.2]

4.15

H13bH14b

Θ

(2.33) [2.89]

H13bH14a

JHH

3

12.31

(10.22) [11.09]

3

trans-44

3

(11.73) [179.7]

[12.71]

[4.84]

[50.5]

13.32(5)

[12.36]

(10.36) [179.0]

[12.09]

2.42

2.64

2.69(5)

1.63

(2.46)

(2.31)

(2.69)

(1.54)

[2.69]

[-66.2]

[2.53]

[-66.6]

[2.93]

[37.6]

12.67

(11.23) [179.3]

[6.78]

[-75.8]

[-64.4]

[1.55]

[166.0]

[-75.8]

a

Extracted from 13C satellite spectra (AA'BB'X spin system); b θ is the dihedral angle between the proton-carbon bonds. c For 5 and 6 ‘a’ refers to protons close to the region between the rings, and ‘b’ to those outside of the rings; whereas the “i’ and “o” refer to protons pointing toward 21 ACS Paragon Plus Environment


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Page 22 of 44

inside and outside of the molecule, respectively. For details of the orientation of the protons and all nJHH values, see the Supporting Information.

4.5. nJCH Coupling Constants The values of the 1JCH coupling constants are listed in Figs. S4a – S4e, and the nJCH values for n = 2, 3 are listed in Tables S3. The experimental 1JCH values involving Csp2 carbon atoms are about 155 Hz, whereas those involving the aliphatic ones are about 126 Hz, with those involving atoms more distant from the bridge larger by ca. 1 Hz. The calculated 1JCH values are always smaller than the experimental ones, but in most cases they reproduce the experimental trends. The errors are largest for ωB97X-D/Huz-IV calculations. Interestingly, in many cases the smallest errors were obtained using the B3LYP functional with the same basis set. Since no experimental values of nJCH for n > 1 could be easily measured, the trends of the calculated values will be discussed only briefly. In agreement with expectations, all calculated 2JCH are negative, with those involving Csp2 atoms larger than the others by ca. 2 Hz. All calculated 3JCH are positive. The largest values were obtained for the bridge protons pointing inside, as for instance is the case for 3JC1H14b (with the coupling constant for the other geminal proton H14a exhibiting the lowest value). 3JCH in the benzene rings, such as 3JC1H3, are the second largest. In most cases, the values calculated using different functionals/basis sets differ by less than 1 Hz.

4.6. nJCC Coupling Constants The 1JCC coupling constants are listed in Figs. S5a – S5e, whereas the nJCC couplings for n = 2, 3 are listed in Tables S4. For 1JCC, most calculated values reproduce the trends observed for the available experimental data. However, an inspection of the results obtained for different functionals and basis sets reveals that none of the combinations yields considerably better results 22 ACS Paragon Plus Environment

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than the others despite the fact that the ccJ-pVTZ basis set has been designed for the calculation of coupling constants. Nevertheless, the agreement with experiment is in all cases rather good. Independent of the basis sets and functionals used, the calculated values of the 1JCC and 3JCC constants are positive, whereas most of the coupling constants through two and four bonds are negative. As for the 2JCC coupling constants, those involving Csp2Csp2Csp3 carbon atoms are positive, whereas those involving only the aromatic carbon atoms are negative. Comparing the 1

JCC values involving Csp2 and Csp3 carbon atoms shows that those on the side of the ethano-

bridge are larger than those on the side of the propano- and butano-bridges of 2 and 3, respectively. Similarly, the 1JCC values of Csp2Csp3 of 5 on the side of the shorter propano bridge are larger than those on the side of the butane bridge (e.g. the experimental value for 1JC7C15 is 43.8 Hz, whereas 1JC10C19 is 38.1 Hz). 1JCC values between the bridgehead carbon atoms and the bridge sp3 carbon atoms have larger values than the aromatic ring carbon-carbon coupling constants. For instance, the ωB97X-D/ccJ-pVTZ calculated value of 1JC2C3 is 57.2 Hz, whereas 1JC7C15 is 42.5 Hz for 5. The coupling constants nJCC values for n = 2, 3 could not be measured experimentally, therefore only the trends in the calculated values will be briefly discussed. The 2JCC values exhibit an interesting trend: those involving Csp2Csp2Csp3 hybridized carbon atoms are positive and slightly larger in absolute values, while others are negative. The all positive 3JCC values fall into three groups equal to ca. 9 Hz, 3.5 Hz or smaller than 2 Hz for the atoms involving Csp2Csp2Csp2Csp2, Csp2Csp2Csp2Csp3, and Csp2Csp2Csp3Csp3 hybridized carbon atoms. In the first group, one can approximately differentiate between the smaller coupling constants between the



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Page 24 of 44

bridgehead carbon atoms and the other. Two small negative values of ca. 1 Hz were obtained for 4

JCC.

4.7. Electronic Spectra and Excited-State Structures 4.7.1. Excited state interring separation and twist angle The interring separation distances, ring deformation and twist angles in the excited state are collected in Table 3. The distances between the aromatic bridgehead carbon atoms of 1 – 6 in the excited state decrease compared to the ground-state geometries by ca. 0.2-0.9 Å, the largest change being observed for 6. The aromatic ring distortion angle (defined by C2C1C6C5) of 1 decreases to -13.6o compared to the ground state structure (-14.8o). The same torsional angle of 2 decreases to -9.5o compared to that of the ground state structure, being -11.7o. This angle for 3 in the excited state is -6.3o. For 5 the ring deformation angle in the excited state decreases to -1.2o compared to the ground state (-4.9o), similarly as for the other paracyclophanes. Interestingly, the rings of both conformers of 6 are bent in opposite directions, differently than in the case of the other molecules. For instance, the C2C1C6C5 torsional angle of the two conformers of 6 has a positive value, while those of 1 – 5 have negative values (Table 3). On the other hand, the trends of this deformation angle observed for the ground state are also observed in the excited state. The ring twist angle (C1C13C14C7) of 1 decreases to -11.6o compared to the ground-state geometry (-14.9o), whereas that of 2 remains constant (0o). Unlike in 1 and 2, the C1C13C14C7 twist angle is much larger in 3, being 28.5o in the ground state and 15o in the excited state. Similar values have been obtained for the C4C1C7C10 angle in the first excited state in 4 and 6-D2 and also to a smaller extent in 5, with an exception of 6-C2h where it has considerably larger value.


Page 25 of 44

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Table 3. Calculated distances between the corresponding non-bonded carbon atoms, ring deformation angle, and twist angles of the two aromatic rings of 1 – 6 in the excited state calculated using B3LYP/ccJ-pVTZ, in parentheses; and those calculated using ωB97X-D/ccJpVTZ, in square brackets.

C1C7

C2C8

C3C9

C4C10

C5C11

C6C12

C2C1C6C5

1

2

3

trans-4

cis-4

5

6-C2h

6-D2

(2.643)

(2.636)

(2.600)

(2.891)

(2.956)

(2.875)

(3.212)

(3.170)

[2.615]

[2.604]

[2.573]

[2.839]

[2.861]

[2.821]

[3.186]

[3.132]

(2.943)

(2.945)

(2.927)

(3.130)

(3.069)

(3.053)

(3.142)

(3.136)

[2.906]

[2.900]

[2.907]

[2.989]

[3.018]

[2.963]

[3.108]

[3.101]

(2.943)

(3.077)

(3.311)

(3.138)

(3.069)

(3.272)

(3.142)

(3.136)

[2.906]

[3.030]

[3.293]

[3.003]

[3.018]

[3.173]

[3.108]

[3.101]

(2.643)

(2.980)

(3.412)

(2.891)

(2.956)

(3.365)

(3.212)

(3.170)

[2.615]

[2.931]

[3.394]

[2.939]

[2.861]

[3.289]

[3.186]

[3.132]

(2.943)

(3.135)

(3.310)

(3.130)

(3.108)

(3.272)

(3.142)

(3.136)

[2.906]

[3.061]

[3.220]

[2.989]

[3.102]

[3.172]

[3.108]

[3.101]

(2.943)

(2.944)

(2.926)

(3.138)

(3.108)

(3.020)

(3.142)

(3.136)

[2.906]

[2.887]

[2.903]

[3.003]

[3.102]

[2.954]

[3.108]

[3.101]

(-13.9)

(-9.9)

(-6.5)

(-6.1)

(-6.1)

(-1.8)

(3.2)

(2.1)

[-13.6]

[-9.5]

[-6.3]

[-6.2]

[-6.2]

[-1.2]

[3.1]

[2.1]

(0.0)

(-15.1)

-

-

-

-

-

[-11.6]

[0.0]

[-17.6]

(0.0)

(0.0)

(7.1)

(0.0)

(0.0)

(-0.9)

(-12.6)

(0.0)

[-4.9]

[0.0]

[10.7]

[0.0]

[0.0]

[-0.4]

[-13.3]

[0.0]

C1C13C14C7 (0.0)

C4C1C7C10

All distances are in angstrom and all angles are in degrees.



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Page 26 of 44

4.7.2. Absorption Spectra In the absorption spectrum shown in Fig. 3, five absorption bands are observed for 1; a detailed assignment of the absorption spectrum for this molecule has been discussed in our previous work.37 Iwata et al.38 observed an additional band at 188.7 nm. We have calculated the six lowest excited states, of which one with an excitation wavelength shorter than 200 nm. The assignments of the absorption bands (labeled by Roman numbers) based on the calculated absorption spectra, are collected in Table S5. The shoulder on the low-energy side of band IV corresponds to band III. Band V is a lower-energy shoulder to band VI. Of all the calculated bands, band VI has the highest intensity, but the calculated excitation energy is outside the range of our spectrophotometer. Because of the D2h symmetry of 1, only B1u, B2u, and B3u transitions are allowed, whereas for the D2 symmetry, also B1, B2 and B3 transitions are symmetry-allowed and can exhibit non-zero intensity.


Page 27 of 44

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Figure 3. Absorption spectra of 1 – 3 (a), and 4 – 6 (b); and emission spectra of 1 – 3 (c) and 4 – 6 (d), measured in n-hexane. The emission spectra were measured at λex(1) = 244 nm, λex(2) = 275 nm, λex(3) = 282 nm, λex(4) = 268 nm, λex(5) = 265 nm and λex(6) = 274 nm. The Roman numbers refer to the transitions in 1. For the sake of clarity, the absorption spectra of 2 and 3 are shifted up with respect to that of 1 by 1x103 and 4x103, respectively; and similarly that of 5 and 6 are shifted up with respect to that of 4 by 2x103 and 4x103, respectively.



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From the experimental spectra of 2, it is found that five electronic transitions exist in the frequency range of the spectrophotometer, listed in Table S5. Band IV is the most intense transition, located at 216 ± 2 nm, whereas band I is the weakest one, observed at 303 ± 2 nm in the experimental spectrum. The calculations predict that the low-energy transition is the least intense one, and that the most intense transition is the higher-energy transition band labeled as VII, located at 177 nm, outside the experimentally accessible frequency range. In agreement with the experimental result, the most intense absorption band above 200 nm of the calculated spectrum is located at 217 nm. With regard to the absorption spectrum of 3, the experimental results listed in Table S5 show four electronic transitions, among which band IV is the most intense transition above 200 nm, being 215 ± 2 nm. The calculated results obtained from CAM-B3LYP and ωB97X-D are consistent, even though there are deviations in the low-energy region. The intense transition in experiment is reproduced well by both functionals, at 214 nm and 215 nm, using CAM-B3LYP and ωB97X-D, respectively. In molecules 1 - 3, the calculated absorption spectra show the presence of multiple transitions below 200 nm. As discussed in the Introduction, 4 has two non-equivalent conformers, one with C2h (trans-4) and the other with C2v (cis-4) symmetry in which the cis conformer is dominant in solution.4 In the calculations, both conformers were studied and comparisons made between the calculated spectra of each conformer with the experimental data. In the experimental absorption spectrum of 4, listed in Table S5, there are two bands at 269 ± 2 nm and 261 ± 2 nm, however, on the basis of the calculations, these two bands are assigned to a single electronic transition, exhibiting a vibronic structure. This allowed transition does not exist in the calculated spectrum of trans-4, but appears in cis-4. These differences support the identification of the cis-4 conformer as the 28 ACS Paragon Plus Environment

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dominating conformer in solution, as we reported earlier4 based on a temperature-dependent NMR study. The most intense transition at 220 ± 2 nm is reproduced by the calculations at 217 nm in the cis-4 case, whereas it is not allowed in the case of trans-4, see Table S5. There are also two other similarly allowed transitions in the calculated spectra of both conformers, neither of which is observed in the experimental spectrum. In the higher-energy regions, there are transitions predicted to lie between 175 nm and 200 nm, among which those located at 177 nm for both trans-4 and cis-4 are the most intense ones in this region. The calculated absorption spectrum obtained for cis-4 is in best agreement with experiment. As was pointed out in the Geometry section, 5 has two energetically equivalent conformers, but there is no difference in the calculated absorption spectra for the two. The experimental spectrum plotted in Fig. 3 shows a maximum absorption at 214 ± 2 nm which agrees well with the calculated spectra using the CAM-B3LYP functional, where the most intense peak is found at 214 nm and at 210 nm in the ωB97X-D calculated spectrum. The other intense absorption is observed at 208 ± 2 nm in the experimental spectrum; and 211 nm at the CAM-B3LYP and 208 nm at the ωB97X-D calculated spectra. Below 200 nm, the calculations show an intense transition at 178 nm using CAM-B3LYP and 176 nm using ωB97X-D. Comparison of the absorption spectrum of 5 with that of 4 also shows the effect of increasing the bridge length on only one side of the paracyclophane. For 4, the intense absorption band is located at 220 ± 2 nm, whereas it is blue-shifted to 208 ± 2 nm for 5. The experimental spectrum of 6 shows an absorption band maximum at 217 ± 2 nm and the CAM-B3LYP calculated absorption band for the C2h conformer is 214 nm (Au) and 212 nm (B3) for the D2 conformer, see Table S5. Among these bands for the two conformers, the one calculated for the C2h structure is more intense than that of the D2 conformer. The D2 conformer 29 ACS Paragon Plus Environment


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has two additional allowed absorption bands that are not allowed for the C2h conformer. In the experimental spectrum, band IV is located at 226 ± 2 nm. This absorption band (Ag) is not allowed for the C2h conformer but allowed for the D2 conformer (B3). This indicates that in solution, the D2 conformer is dominant. This observation is in excellent agreement with the optimized geometry analysis in this work, as well as with the X-ray structure reported by Jones et al.47 The spectra plotted in Fig. 3 highlight the effect of the bridge length on the absorption spectrum of the molecules. Both functionals reproduce the absorption wavelengths of all the paracyclophanes studied fairly well. Since the low-lying transitions are weak and overlapping with each other, we focus on the strongest observed absorption band discussed above. The experimentally observed transition energies shift to the blue from 226 and 225 nm in 6 and 1, respectively, to practically the same value (214-216 nm) in 2, 3, and 5. An intermediate band position is observed for 4 (220 nm). These values can be correlated with the orbital energies and their contributions to the most intense absorptions shown in Figs. S6 and S7 and Table S6 of the Supplementary Material. In 1 the dominant configuration of the most intense absorption is mainly from HOMO-2 (31 % and energy gap of 7.94 eV) to the 1st LUMO and from HOMO-2 (26 % and energy gap of 8.80 eV) to the 9th LUMO, whereas in 4 from HOMO (29 % and energy gap of 7.63 eV) to the 4th LUMO, and in 6 from HOMO (37 % and energy gap of 9.26 eV) to the 10th LUMO and HOMO-1 (17 % and energy gap of 9.78 eV) to the 20th LUMO; also note that in these three molecules the two aromatic rings are parallel. On the other hand, in 2, 3 and 5, non-parallel aromatic rings, different orbitals contribute to the most intense absorption band, all without considerable dominant configurations compared to the former three molecules.


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4.7.3. Emission Spectra The emission spectra of 1 – 6 are plotted in Fig. 3. The results show that the experimental emission band of 1 is located at 351 ± 2 nm, whereas the corresponding calculated value using ωB97X-D is 314 nm. That of 2 is located at 355 ± 2 nm in the experimental spectra, whereas the calculated value using the same functional as that of 1 is 313 nm. 3 has an emission wavelength of 330 ± 2 nm, which is blue-shifted compared to 1 and 2. The corresponding calculated emission of 3 using ωB97X-D functional is 286 nm. For 4, the measured emission band is located at 381 ± 2 nm. The ωB97X-D calculated emission wavelength of trans-4 is 324 nm while that of cis-4 is 330 nm. Similarly, the measured emission bands of 5 and 6 are observed at 357 ± 2 nm and 331 ± 2 nm, respectively. The corresponding calculated value of 5 is 303 nm, whereas that of 6-D2 is located at 293 nm and that of 6-C2h is at 306 nm. Comparing the emission spectrum of 4 with those of 1 – 3 shows the effect of increasing the bridge length in these paracyclophanes systems. For example, the emission band of 2 is located at 355 ± 2 nm, whereas that of 4 is red-shifted to 381 ± 2 nm. Comparing the emission spectrum of 5 with that of 4 also shows the effect of the bridge length variation, with the emission of 5 being blue-shifted compared to that of 4. Compared to the emission bands of 4 and 5, that of 6 is blue shifted, showing the effect of increasing the bridge length on the emission spectra of the molecules studied. Due to a very weak intensity of the transition to the lowest excited singlet state, the analysis of the absorption is difficult. In general, the first absorption band lies at similar wavelengths for all compounds. On the other hand, the differences observed in the emission spectra are readily distinguishable. In particular, the Stokes shift observed for 4 is very large. 1, 2, and 5 reveal similar values, whereas for 3 and 6 the excitation-emission energy separation is the smallest. These trends can be correlated with the structural differences discussed in sections 31 ACS Paragon Plus Environment


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4.1.2 and 4.7.1 between the S0 and S1 states calculated for the series. For instance, the C1-C7 interring separation distances of 1 and 2 obtained from the ωB97XD/ccJ-pVTZ calculations are 2.797 Å and 2.785 Å in the ground state, whereas 2.615 Å and 2.604 Å in the excited state, respectively.

5. CONCLUSIONS The structures of [m.n]paracyclophanes (m, n = 2 – 4) were optimized using two different functionals and two basis sets. In general, only small differences are observed for the calculated chemical shifts in the different methods. The 1JHH coupling constants were best estimated using ccJ-pVTZ basis and the ωB97XD functional, but also here differences are small. Similar observations can be made for the structures predicted, ωB97X-D in general giving somewhat shorter bond and interring distances, in particular in the case of the interring distances of 6, where ωB97X-D gives results in much better agreement with experiment than B3LYP. Due to the unequal alkyl chain lengths, the average planes of the rings in [2.3]-, [2.4]-, and [3.4]paracyclophanes are not parallel, unlike those of [2.2]-, [3.3]-, and [4.4]paracyclophanes. Except for [4.4]paracyclophane, the C1…C7 interring separation distance is shorter than the sum of van der Waals radii between two stacked carbon atoms, 3.4 Å. On the other hand, the C4…C10 interring separation distance of only the [2.2]-, [2.3]-, and [3.3]paracyclophanes are shorter than 3.4 Å. NMR spectra of all the [m.n]paracyclophanes were measured and very accurate values of chemical shifts and proton-proton coupling constants have been determined. With several exceptions, most of which are within the limits of errors, the calculated values of chemical shifts and coupling constants reproduce the experimental trends. Substantial


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temperature effects on the proton chemical shifts have been observed for the ethano-bridge protons. The shifts decrease when the temperature decreases. It is interesting to note that, whereas the different basis sets and functionals largely give similar results for the paracyclophanes studied here, the much studied [2.2]paracyclophane remains a challenge, B3LYP leading to a C1C13C14C7 angle close to 0o (D2h symmetry) while that obtained using the ωB97X-D functional is equal to 14.9o (D2 symmetry). The most recent experimental result at around room temperature has the D2h structure with no disorder.29 Also the D2 structure cannot be excluded, since it exists at very low temperatures.29 Of the two functionals, ωB97X-D gives structural parameters that are in fair agreement with the very lowtemperature experimental results.29 Moreover, these latter results show the importance of including dispersion corrections when dealing with molecules of this kind. The experimental and computational study of the absorption and emission properties of the paracyclophanes considered also revealed the dependence of these electronic properties on the bridge lengths. Up to eight UV-vis absorption/emission bands have been measured (or anticipated in the region below 200 nm where they could not be observed) and assigned on the basis of quantum chemical calculations. Optimized excited-state geometries showed that the distances between the aromatic bridgehead carbon atoms of all the [m.n]paracyclophanes in the excited state decrease compared to the ground-state geometries by ca. 0.2 - 0.9 Å. The largest differences have been found for [4.4]paracyclophanes. However, the rather large differences in the calculated emission wavelength compared to experiment cast some doubts on the accuracy of the calculated excited-state geometries.



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ACKNOWLEDGMENTS The authors would like to thank the Interdisciplinary Centre for Mathematical and Computational Modeling of Warsaw University for the computational grant (G28-7) and the Norwegian Supercomputing Program for a grant for computer time (nn4654K). T.B.D. acknowledges the International PhD Projects Programme of the Foundation for Polish Science, co-financed from European Regional Development Fund within Innovative Economy Operational Programme "Grants for innovation". K.R. has also been supported by the Research Council of Norway by research grants No. 179568/V30 and 177558/V00.

ASSOCIATED CONTENT Supporting Information Full citation for ref 48, the structural and NMR parameters as well as experimental and calculated wavelengths of 1 - 6 are provided as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * Helena Dodziuk, e-mail: [email protected]

Notes The authors declare no competing financial interest.


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Toc graphic:



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Figure 1. Paracyclophanes 1 – 6 with the atom numbering used in this work. 318x155mm (96 x 96 DPI)

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Figure 2. Conformers, enantiomers and topomers of 2 – 6, and their interconversion paths. The enantiomers are denoted by the unprimed and primed symbol of the compound. Topomers of the same enantiomer are distinguished by letters a and b. The interconversion processes for 5 and 5' denoted by 1 and 2 are nonequivalent. 278x208mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Absorption spectra of 1 – 3 (a), and 4 – 6 (b); and emission spectra of 1 – 3 (c) and 4 – 6 (d), measured in n-hexane. The emission spectra were measured at λex(1) = 244 nm, λex(2) = 275 nm, λex(3) = 282 nm, λex(4) = 268 nm, λex(5) = 265 nm and λex(6) = 274 nm. The Roman numbers refer to the transitions in 1. For the sake of clarity, the absorption spectra of 2 and 3 are shifted up with respect to that of 1 by 1x103 and 4x103, respectively; and similarly that of 5 and 6 are shifted up with respect to that of 4 by 2x103 and 4x103, respectively. 226x193mm (96 x 96 DPI)


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Structure, NMR and Electronic Spectra of [mn ... - ACS Publications

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