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Article pubs.acs.org/JPCA

Elucidation of Photoisomerization-Related Structural Changes in an Acrylamide-Bridged Binaphthalene−Diazene Macrocyclic Chiroptical Switch by Experimental Electronic Circular Dichroism Spectra Simulation: Role of Dispersion Corrections Lukás ̌ Kerner,† Anna Kicková,†,§ Juraj Filo,† Stanislav Kedžuch,‡ and Martin Putala*,† †

Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovak Republic ‡ Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovak Republic S Supporting Information *

ABSTRACT: Nondestructive readout of light-driven molecular memory devices can be achieved by monitoring the alterations in the chiroptical properties of 1,1′-binaphthalene as a conformationally responsive chiral group. In our system, this signaling unit is connected via acrylamide linkers to the receiving diphenyldiazene fragment, which undergoes significant geometrical changes upon (E)/(Z)-photoisomerization. The compound functions as a stable photochromic switch by alternating irradiation at 365/465 nm, with fully reversible modulation of circular dichroism (CD) signal intensity (up to 1:3) and extended thermal stability of the (Z)-isomer. According to molecular modeling, the acrylamide spacers are due to the imposed cyclic strain upon photoisomerization forced to switch amide conformations, which is markedly reflected in the CD spectra, whereas binaphthalene conformational changes are mostly neglected both by theory and by experiment. In CD simulation by TD-DFT, CAM-B3LYP outperforms B3LYP and M06 by means of similarity analysis, whereas the last mentioned functional also delivers satisfactory performance qualitatively. The inclusion of dispersion corrections during geometry optimization was crucial to retain consistency with the measured spectra. By carefully considering all relevant conformations of this 20-membered macrocycle, reasonable agreement with the experiment is reached not only for the CD simulation of the individual conformers but also of the photoisomerization process of their admixture.

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or 7 and 7′ (5)6,7 via relatively flexible (oligomethylene)dioxy or tighter methyleneoxy linkers (3, 4, 5b)6 (Chart 1). These cyclophanes exhibit increased thermal stability of the (Z)-form, resulting in their extended lifetimes up to 53 days at room temperature for compound 2.8 In the case of di-metaphenyldiazene derivative 4 with a tight spacer, the (Z)-form possesses even higher stability than the (E)-form. Although it has been shown that the switching of macrocyclic cyclophane 1a induces changes in its nematic liquid-crystalline host,9 the variations in the binaphthalene dihedral angle were not significant and, therefore, rather low relative and absolute changes of ellipticity (θ) were found in the corresponding region of its electronic circular dichroism (ECD) spectra (typically in tens of millidegrees). Higher relative difference in ellipticity was observed at longer wavelengths, which was attributed to the unlike induced diphenyldiazene chirality of the (E)- and (Z)-forms.10,11,7

INTRODUCTION The explosive increase in information demands development of new recording technologies and materials for high density data storage.1 With the aim of downsizing these technologies to the molecular level, light-driven molecular machines have received considerable attention.2,3 Several alternative strategies have been developed for the nondestructive readout of such devices, mostly based on the change of molecular properties induced by distortions in molecular geometry associated with photochemical switching. One of these approaches utilizes accompanying alterations in the chiroptical properties of a conformationally responsive chiral group.4,5 In particular, the significant change in the molecular geometry of photochromic diaryldiazenes upon photochemical (E)/(Z)-isomerization has been explored for the construction of chiroptical switches in combination with a chiral binaphthalene unit as the conformationsensitive moiety. One group of such derivatives is represented by macrocyclic cyclophanes consisting of a di(ortho or meta)-phenyldiazene unit attached to binaphthalene at positions 2 and 2′ (1−4)6−13 © 2015 American Chemical Society

Received: April 10, 2015 Revised: July 2, 2015 Published: July 14, 2015 8588

DOI: 10.1021/acs.jpca.5b03474 J. Phys. Chem. A 2015, 119, 8588−8598

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The Journal of Physical Chemistry A

response of the photoisomerization process, which allows us to discuss the observed switching behavior in terms of the structural differences between the (E)- and (Z)-diphenyldiazene containing isomers.

Chart 1. Structure of the Reported Macrocyclophanes with Diphenyldiazene Unit Attached to Chiral Biaryls via Various Linkers (MOM = Methoxymethyl)

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EXPERIMENTAL AND THEORETICAL METHODS The synthesis of the title compound in a blend with its dimer has been described elsewhere.15 The separation of (R)-7 from this mixture was achieved after prolonged standing (2 weeks) in deuterated acetone at room temperature (RT) and subsequent filtration. The resolution of its (Z)- and (E)-isomers on the analytical scale was carried out on a CC 125/4 Nucleosil C8 HPLC column using isocratic mode (methanol/water = 60:40; flow rate 0.5 mL/min). The HPLC system was coupled to a CD spectrometer specified below, which allowed on-the-fly measurements of ECD spectra of the individual mixture components employing the stop-flow technique. Retention times: 19.77 min (Z); 38.34 min (E). UV/vis and ECD spectra were recorded on a JASCO J815 instrument at 25 °C in spectroscopy-grade methanol or 1,4-dioxane (Sigma-Aldrich). Irradiation was effected with 365 nm (2.4 mW) and 465 nm (8.5 mW) light-emitting diodes (Thorlabs) in a standard UV/vis spectroscopic cell (light path length = 1 cm). The (E)/(Z)-ratio in the initial and photostationary states was determined by integrating amidic proton signals in 1H NMR spectra obtained before and immediately after irradiation at 365 nm in deuterated acetone. For thermal stability measurements, the irradiated sample in 1,4-dioxane was kept in the dark while being thermostated at 20.00 ± 0.02 °C. The CD spectra were recorded at this temperature in respective time intervals. A random-walk conformational search was performed as implemented in the HyperChem 8.0 software20 utilizing the MM+ force field21 with an energy window of 6 kcal/mol relative to the best conformer. Subsequent geometry optimizations and vibrational analyses were conducted in Gaussian 0922 at the DFT/B3LYP level of theory,23 optionally including empirical corrections for dispersion interactions24 as implemented in the A.02 revision of the software. Solvent effects (1,4dioxane) were evaluated with the self-consistent reaction field (SCRF) method employing the polarizable continuum model (PCM).25 Ab initio molecular dynamics (NVT ensemble, CSVR thermostat, 400 K) on selected conformers was performed in the CP2K (Quickstep)26 software using GTH-BLYP potential27 with D3 empirical dispersion correction28 and DZVP-MOLOPTGTH basis set.29 MP2 (Møller−Plesset second-order perturbation theory)30 single-point calculations employed the haug-ccpVTZ basis set,31 i.e., cc-pVTZ for H’s and aug-cc-pVTZ for heavy atoms. The Boltzmann distribution was calculated for RT (298.15 K). Electronic excitations based on the optimized ground-state geometries (i.e., vertical transitions or the Franck− Condon states) were predicted with time-dependent (TD)DFT32 and processed in the GaussSum program.33 The ECD spectra were simulated by overlapping Gaussian functions for particular transitions with peak broadening of 0.48 eV (full-width) at (1/e) times peak height,34 which corresponded to averaged experimentally determined values. We used Pople’s split valence basis set35 of double-ζ (6-31G**) and triple-ζ (6-311G**) quality with polarization functions on all atoms for geometry optimization and TD-DFT calculations, respectively. Plots of noncovalent interactions were calculated with the NCIplot software using SCF electronic density (B3LYP/6-31G**) and default settings.36 Similarity factors for simulated vs experimental spectra were obtained with the SpecDis program.37

Significant response upon diazene isomerization up to sign inversion was found for optical rotation at 589 nm, especially for derivative 1b bearing benzyloxy groups at positions 3 and 3′.12 A similar structural concept was used for the construction of chiroptical switch 6 representing a chiral macrocycle composed of three meta-diphenyldiazene units with chirality arising from a configurationally stable substituted biphenyl moiety.14 Although this switch can exist in the form of up to seven (E)/(Z)-isomers in different photostationary states, it exhibited inversion of the CD signal sign at 275 nm upon irradiation. Recently, we have communicated the synthesis and preliminary characterization of macrocyclic cyclophane 7 consisting of a photoresponsive meta-diphenyldiazene unit attached to binaphthalene via acrylamide linkers.15 The two moieties provide the receiving and signaling functions to the switch, respectively, with the acrylamides anticipated as mediators between the former and latter. Successful separation of (R)-7 from its dimer enabled the investigation of its switching performance reported here, which was tracked by electronic circular dichroism (ECD) spectroscopy. This in turn motivated us to uncover the details of the photoisomerization process with the help of in silico calculations. As is usually the case for flexible molecules, their CD spectra in solution are governed by the contributions of individual conformers.16,17 Both the conformer relative population and the extent to which different chromophores interact with each other must be taken into account when relating experimental CD data to structural features. With this goal in mind, we first perform a detailed conformational analysis of both isomers of (R)-7, followed by the comparison of their TD-DFT-predicted CD spectra18,19 with the experiment. Next, we simulate the CD 8589


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RESULTS AND DISCUSSION Conformational Analysis. To find all possible conformers and due to abundant degrees of freedom of the macrocyclic 20-membered ring present in (R)-7, we decided to search its conformational space by means of torsional variations depicted in Figure 1. The parameters which were held fixed correspond

In comparison, the inclusion of dispersion corrections (Table 2) led to a clear distinction between the two conformers of the (E)-isomer by exclusively preferring (E)-II′ (>99%) over (E)-I′ ( (E)-I′. Therefore, we decided to include dispersion corrections also in the case of (Z)-7. Interestingly, from these computations we obtained only one dominant conformer (Z)-I′ as without the corrections, along with an additional minor conformation (Z)-IV′ (7%). We discuss this point later. In (Z)-II and (Z)-II′ there is an indication of a feeble intramolecular H-bond between the amide groups on the opposite sides of the molecule, with the O···H−N distance of 3.6 and 3.2 Å, respectively. Under PCM solvation, this value increases to 3.8 and 3.4 Å for (Z)-II and (Z)-II′, respectively, resulting in a similar effect on their population within the regular and dispersion-corrected approaches. Overall for both methods, geometry optimization and frequency analysis in solution (1,4-dioxane) gave trends that were in accord and consistent with the gas-phase calculations. It can be therefore concluded that the conformer distribution is not significantly influenced by this solvent, at least not within the framework of the solvation model employed (PCM). We decided to proceed only with the conformers optimized in the gas phase. All structures exhibited real vibrational frequencies only. In addition to the conformational search, we also performed ab initio molecular dynamics on the best dispersion-corrected conformers of both configurations. This process revealed only one new significantly populated (16%, Table S1, Supporting Information) conformer (Z)-I′a based on (Z)-I′ geometry, containing a weak intramolecular H-bond (Figure S2, Supporting Information). However, when (Z)-I′a was reoptimized under PCM solvation, it converged to the parent structure of (Z)-I′/PCM, in the same way as the initial molecular dynamics frame. Due to this fact, we decided not to include (Z)-I′a in the computed population-weighted ECD spectra. Tables 3 and S1 (Supporting Information) list relevant geometric parameters of the optimized conformers obtained both with the regular and with the dispersion-corrected B3LYP functional. Note that both optimization methods were fed with the identical initial MM+ geometries of the individual conformers.

Figure 1. Torsional variations employed in the conformational analysis of (R)-7.

to the CNNC diazene torsional angle (0° and 180° for the (Z)- and (E)-configuration, respectively) and to the geometry of the (E)-double bonds (180° for HCCH) which was elucidated from NMR measurements. Although the value for the naphthyl−naphthyl dihedral angle was neither constrained nor specifically set up for variation, we did obtain energetically equivalent pairs of enantiomers in the resulting list of MM+ conformers in both cases of diazene configurations. Also, redundant identical conformers resulting from the C2 symmetry of binaphthalene (with the correct axial chirality, though) had to be eliminated. Geometry optimization at the DFT/B3LYP level of the shortlisted 231 MM+ conformers followed by vibrational analysis allowed us to determine their Boltzmann distribution both in the gas phase and in solution (Table 1 and Figure S1, Supporting Information, plain notation). At this stage, on the basis of subsequent ECD simulations we decided to employ empirical corrections for dispersion interactions (Table 2 and Figure 2, prime denoted) for comparison and with the aim to enhance the simulated spectra (vide inf ra). These interactions have been recognized as a weak point in the conventional DFT.38,39 Dispersion-corrected functionals deliver enhanced performance40−42 and have been successfully applied in the analysis of binaphthalene-containing systems.10,43,44 Indeed, we obtained different results with the two approaches. Identical roman numerals are used to denote conformers which shared the same initial MM+ geometry, separately for the (E)- and (Z)-diazene configurations. With the dispersion-uncorrected global hybrid B3LYP functional we found two prevailing (E)-configured conformers in the ratio of 57:42 and only one major conformer with the (Z)-configuration populated at 94%.

Table 1. Calculated Relative Values of Electronic Energy, Gibbs Free Energy, and Corresponding Population of (E)-7 and (Z)-7 Conformers with the B3LYP/6-31G** Method in the Gas Phase and Solutiona B3LYPb

B3LYPb with SCRFc

MP2d

conformer

ΔE (kJ/mol)

ΔG (kJ/mol)

population (%)

ΔE (kJ/mol)

ΔG (kJ/mol)

population (%)

ΔE (kJ/mol)

(E)-I (E)-II (E)-III (Z)-I (Z)-II (Z)-III

0.0 0.2 13.3 0.0 9.6 14.4

0.0 0.7 10.9 0.0 7.1 11.9

57.0 42.3 0.7 93.9 5.3 0.8

0.0 0.4 13.4 0.0 9.1 15.2

0.0 0.4 10.4 0.0 6.1 12.0

53.7 45.5 0.8 91.5 7.8 0.7

15.3 0.0 21.4

Only the first conformer of population lower than 1% is included for both isomers. bGeometry optimization of MM+ conformers followed by frequency analysis. cPCM, solvent: 1,4-dioxane. dSingle-point calculation on the gas-phase optimized B3LYP geometries. a

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Table 2. Calculated Relative Values of Electronic Energy, Gibbs Free Energy, and Corresponding Population of (E)-7 and (Z)-7 Conformers with the B3LYP/6-31G** Method in the Gas Phase and Solution, Including Dispersion Correctionsa B3LYPb with dispersion corrections

B3LYPb with SCRFc and dispersion corrections

MP2d

conformer

ΔE (kJ/mol)

ΔG (kJ/mol)

population (%)

ΔE (kJ/mol)

ΔG (kJ/mol)

population (%)

ΔE (kJ/mol)

(E)-II′ (E)-I′ (Z)-I′ (Z)-IV′ (Z)-II′ (Z)-V′

0.0 15.7 0.0 9.6 7.2 13.5

0.0 14.1 0.0 6.3 7.3 12.5

99.7 0.3 88.0 6.8 4.6 0.6

0.0 15.6 0.0 9.7 7.2 13.0

0.0 13.8 0.0 6.3 6.4 13.6

99.6 0.4 86.2 6.8 6.6 0.4

0.0 29.3

Only the first conformer of population lower than 1% is included for both isomers. bGeometry optimization of MM+ conformers followed by frequency analysis. cPCM, solvent: 1,4-dioxane. dSingle-point calculation on the gas-phase optimized B3LYP geometries. a

Figure 3. Top view of the resulting structures of geometry optimization performed with (red) and without (blue) dispersion corrections in the gas phase, with both methods starting from the same initial structures: (a) (E)-II′ and (E)-II; (b) (Z)-I′ and (Z)-I. Hydrogen atoms not shown.

binaphthalene fragment (adopting the transoid form) and by adjusting the conformation of both amide groups to (E) so as to relieve the imposed strain. On the contrary, in (Z)-I′ the attachment points of diphenyldiazene are only 4.3 Å far away from each other, which results in a narrower (cisoid) binaphthalene accompanied by the preference for the (Z)-amide conformation. Following this model, the interconversion between (E)- and (Z)-diphenyldiazene invoked on the one side of the molecule, is supposed to have considerable influence on the CD-active binaphthalene unit, which in turn should lead to substantial alterations in its CD response.9,10,12 Next, we investigate the effect of dispersion corrections on geometry optimization. As anticipated, neither amide conformations nor diphenyldiazene helical chirality were altered by the corrections and in the case of (Z)-I′, the C2 symmetry was retained as well (Figure 3b). On the contrary, a notable

Figure 2. Geometries of significantly populated conformers obtained by B3LYP/6-31G** with dispersion corrections. For clarity, H atoms except N−H’s have been removed. Overlapped bonds are colored light-gray.

First, we compare the (E)- and (Z)-diazene configurations. The most striking feature is the difference in the naphthyl− naphthyl dihedral angle, with that of (E)-configured conformers being 19−40° larger than in the (Z)-conformers. This can be readily understood by inspecting the corresponding geometries (Figure 3a vs 3b). For example, to accommodate the (E)-diphenyldiazene moiety in (E)-II′, where its meta attachment positions are separated by 7.5 Å in space, the rest of the macrocycle adapts both by increasing the span of the

Table 3. Selected Geometric Parameters of the Gas-Phase Optimized (B3LYP/6-31G**), Significantly Populated Conformer Structures, and Their Summed Contributions to the Simulated Population-Weighted ECD Spectra of the Isolated Admixture

a

C(2)C(1)C(1′)C(2′). bSee Figure S9, Supporting Information. cAs determined from 1H NMR spectra of the initial and photostationary states; only conformers with population higher than 1% were taken into account 8591


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phase, respectively. PCM solvation has little effect on this value as well (29.2 with vs 29.7 kJ/mol without the corrections). These numbers, however, are significantly lower than that reported for diphenyldiazene, in which the (E)-form is favored over (Z) by ca. 60 kJ/mol.46,47 We ascribe this discrepancy to the macrocyclic strain existing in (E)-7, which in part negatively compensates for the effect of disrupted conjugation and steric hindrance present in the diphenyldiazene moiety of (Z)-7.6 The contribution of more stable (Z)-amides over (E)-amides should also be taken into account. For dipeptides, this difference has been reported to be 10−13 kJ/mol in favor of the former.48 Electronic Spectroscopy and ECD Prediction. Comparison of UV/vis spectra of both isomers of (R)-7 (Figure 5a)

change can be seen in the binaphthalene dihedral angle in the (Z)-configured conformers, becoming smaller by additional 8° in the dispersion-corrected structures. We speculated that as opposed to the (E)-configuration, intramolecular dispersion interactions in (Z)-diazenes are manifested at the more packed diphenyldiazene moiety, which is accompanied by adjustments in the remaining part of the molecule. Plots of noncovalent interactions (NCI) confirmed this presumption, revealing such interactions between the two benzene rings of the (Z)-diphenyldiazene fragment (Figures 4a and S3, Supporting

Figure 4. Top views of NCI plots of (a) (Z)-I′ and (b) (E)-II′. Magenta shapes denote relevant regions discussed in the text. Only N−H’s and hydrogen atoms involved in noncovalent interactions are shown. Figure 5. UV/vis (a) and ECD spectra (b) of both isomers of (R)-7. Conditions: c((R)-7) = ca. 10−6 M, methanol/water, 25 °C.

Information). The influence of dispersion corrections becomes most significant in the case of (E)-II′: although the binaphthalene dihedral is virtually unaffected (105° vs 103° in (E)-II), the (E)-diphenyldiazene seems to be pulled toward the binaphthalene core (Figures 3a and S4, Supporting Information). This outcome can be ascribed to the interactions taking place between the two fragments,39,45 which is again illustrated by the corresponding NCI plots (Figures 4b; S5 vs S6, Supporting Information). To further support this conclusion, we performed geometry optimization on the dispersionuncorrected structure of (E)-II, this time incorporating the corrections. The optimized geometry matched exactly the one of (E)-II′ coming from an MM+ conformer. The same holds true also for (E)-I′, but in this case the effect of dispersion interactions is negligible (Figure S7, Supporting Information). We explain the evident distinction as follows. Whereas in (E)-I the diphenyldiazene group is perpendicular to and well away from binaphthalene (Figure S8, Supporting Information), (E)-II possesses a skewed, more compact, and hence more congested geometry suited for dispersion interactions to such an extent that they profoundly affect the conformer distribution. Importantly for CD spectra simulation, in this way (E)-II′ exhibiting (M)-diphenyldiazene helicity is exclusively favored. To sum up, in our system, dispersion interactions surely play a role in geometry optimization, but only in conformers with a suitable arrangement of the constituent fragments. Nevertheless, the difference in Gibbs free energy between the best conformers with (E)- and (Z)-diazene configuration appears to be unaltered by dispersion corrections, being 31.4 kJ/mol for the corrected and 31.7 kJ/mol for the regular gas

reveals the typical relationship between the (Z)- and (E)-forms of the diphenyldiazene unit.49 Namely, the n → π* transition centered at 448 nm is more intensive in (Z)-7, whereas the π → π* absorption band discernible at ca. 368 nm as a low energy shoulder is more pronounced in (E)-7. Bands at 310, 274, and ca. 210 nm could be presumably ascribed to the 1Lb (long), 1 La (short), and 1Bb (long axis) transitions of naphthalene,50,51 respectively, but the variable extent of conjugation between individual chromophores due to conformational flexibility makes such assignments disputable. Asymmetric features of (R)-7 originating in the axially chiral binaphthalene assisted by the presence of additional chromophores in the macrocycle render it active in terms of ECD spectroscopy.52 The hallmark negative bisignate couplet (Figure 5b) of (R)-binaphthalene arising from the 1Bb transition coupling is clearly visible at 200−230 nm (λext = 209 and 221 nm). The peaks at ca. 240, 274 (negative), 310, and 460 nm could also be matched to the corresponding UV/vis transitions. Features between 290 and 400 nm are markedly different for the (E)- and (Z)-isomers and will be discussed below. To understand the intrinsic relationship between structure and measured CD response, we have simulated entire ECD spectra53 of particular isomers (Figures 6, 7, and S10−S12, Supporting Information) as well as of the initial and UV photostationary states of their admixture. Three functionals were employed in the framework of TD-DFT:32 the commonly used global hybrid exchange−correlation functional of Becke, 8592


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(short) and 65% (long-range) for CAM-B3LYP. On the basis of subsequent similarity analyses of computed UV spectra, all transitions predicted with B3LYP and M06 were uniformly blue-shifted by 12 or 5 nm, respectively, whereas in the case of CAM-B3LYP they were red-shifted by 16 nm. In general, the spectra simulated with B3LYP and M06 exhibit very similar features, with the diazene n → π* peak shifted slightly to the longer wavelength for the latter and thus further away from the experimental value of 460 nm. On the contrary, B3LYP fails to correctly describe the binaphthalene couplet within the same number of excitations as the other two functionals. Undisplaced spectra predicted with the rangeseparated CAM-B3LYP were the most blue-shifted of the three, which initially led to better agreement with the experimental data above 300 nm but produced a significant shift for higherenergy bands. Moreover, this functional predicts the most notable difference between conformers with distinct geometry ((E)-I vs (E)-II in Figure S10, (Z)-I′ vs (Z)-IV′ in Figure S12, Supporting Information). The experimental band at 240 nm seems to have a complex structure difficult to reproduce theoretically, which is supported by the presence of its low-energy shoulder appearing at ca. 260 nm. We note here that consistently throughout our work we always obtained a positive or negative lowest-energy band (n → π* transition) for the (M)- and (P)-diphenyldiazene handedness, respectively, as defined in Figure S9 (Supporting Information, and related discussion). This relationship has been disclosed previously and has been advantageously utilized by means of ECD spectroscopy.7,10,47 In the case of the (E)-diazene configuration, all methods based on dispersion-uncorrected conformer distribution erroneously predict the n → π* transition of diminished intensity and with negative sign, in contrast with the experiment (Figure 6a). As shown in Figures 6b and S10 (Supporting Information), this is due to the population averaging of spectra with opposite signs in this region: the contribution of (E)-I possessing a negative n → π* band prevails over that of (E)-II with a positive band. Interestingly, for both these conformers the n → π* transition is calculated at the same λ, which points to the similar extent of conjugation in the (E)-diphenyldiazene fragment. Dispersion corrections lead to the single conformer (E)-II′, whose n → π* band agrees with the experiment both qualitatively and quantitatively (Figure 6c). Because this transition represents a crucial spectral feature for diphenyldiazene-containing systems, in our case dispersion corrections are a key factor for reaching agreement between theory and experiment, at least for the (E)-isomer. On the contrary, the areas under the n → π* bands are very small relative to the whole spectra, so similarity analysis like the one described below cannot fully reveal the importance of this region. However, due to a much better match with the experimental spectrum especially below 300 nm, the dispersioncorrected conformer distribution (i.e., single conformer (E)-II′) leads to significantly improved results for all three functionals (vide inf ra). Because the conformer population of the (Z)-isomer is not influenced by dispersion corrections significantly, both sets of geometries fare equally well and provide CD spectra consistent with the experimental data (Figure 7a,b). However, CAM-B3LYP evidently fails to reproduce the band at 240 nm faithfully. As illustrated for dispersion-corrected structures in Figure 7c, similarly populated (Z)-II′ and (Z)-IV′ cancel each other’s contribution out at 450 nm, which leaves the n → π* band of (Z)-I′ at 500 nm. We relate this difference in λ to the unequal extent of conjugation in the (Z)-diphenyldiazene

Figure 6. Comparison of experimental and Boltzmann-averaged simulated ECD spectra of (E)-7 based on geometries optimized without (a, b) and with (c) corrections for dispersion interactions in the gas phase. All spectra in plots (a) and (c) were normalized by division with their respective maximal positive values. x-Axis has the same scale throughout (a)−(c). Applied shifts for simulated spectra: −12 nm (B3LYP), +16 nm (CAM-B3LYP), −5 nm (M06). Experimental conditions: methanol/water, c((E)-7) = ca. 10−6 M, 25 °C.

Figure 7. Comparison of experimental and Boltzmann-averaged simulated ECD spectra of (Z)-7 based on geometries optimized without (a) and with (b, c) corrections for dispersion interactions in the gas phase. All spectra in plots (a) and (b) were normalized by division with their respective maximal positive values. The x-axis has the same scale throughout (a)−(c). Applied shifts for simulated spectra: −12 nm (B3LYP), +16 nm (CAM-B3LYP), −5 nm (M06). Experimental conditions: methanol/water, c((Z)-7) = ca. 10−6 M, 25 °C.

Lee, Yang, and Parr (B3LYP),54 its range-separated variation CAM-B3LYP55 and hybrid meta-generalized gradient approximation (GGA) Minnesota functional M06.56 Besides other features, they differ in the content of Hartree−Fock exchange as follows: 20% for B3LYP, 27% for M06, and between 19 8593


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The Journal of Physical Chemistry A moieties in these conformers. Indeed, whereas in (Z)-II′ and (Z)-IV′ the angle of the phenyl rings and the plane defined by the CNNC atoms ranges between 54° and 76°, its value is only 42° for C2-symmetric (Z)-I′, hence promoting conjugation (Table S1 and Figure S9c, Supporting Information). In an attempt to remove any bias in the assessment of the individual functionals’ performance, we turned to similarity analysis described initially by Bultinck et al.57 and implemented by Bruhn et al.37 The enantiomeric similarity indices (Δ) for the computed population-weighted ECD spectra are summarized in Table S2 (Supporting Information). These values were obtained by subtracting the similarity factor for the enantiomeric (i.e., x-axis mirrored) computed spectrum from that for the original calculated spectrum. In this way, the statistical level of agreement between theory and experiment was determined. The dispersion-corrected set of geometries leads to a better match with the experiment in comparison with their uncorrected counterparts, especially in the case of the (E)-isomer (68% vs 23% on average). The lowest values of Δ (2 and 13%) were obtained for the uncorrected (E)-conformers with B3LYP and M06, which was caused mostly by a notable mismatch of the large-area peaks below 300 nm and for M06 also by the difference in λ in the binaphthalene region (Figure S10, Supporting Information). In contrast, CAM-B3LYP predicts spectra of the (E)-isomer with the highest Δ in both sets (86 and 52%). Agreement between theory and experiment in the case of the (Z)-isomer is generally much less fluctuating (Δ = 31−50%), but in the dispersion-corrected set also less convincing (46% on average) than for the (E)-isomer: with a Δ value lower by as much as 41% in the case of CAM-B3LYP (due to the mismatch in the 240 nm band) and by 9 and 16% for B3LYP and M06, respectively. Although the latter two functionals benefit from dispersion corrections for the (Z)-isomer by 9−12% (leading to the highest Δ value for B3LYP), CAM-B3LYP’s performance is unchanged. The relative intensity of the individual ECD spectral bands remains challenging for all investigated functionals for both diazene configurations. To retain an objective stance, it has to be noted that in our case the adopted similarity analysis was due to a number of alternating positive and negative ECD regions rather sensitive to the applied wavelength shift which we determined from UV spectra simulation and applied consistently for a particular functional. Because such a uniform displacement of the whole spectrum cannot sufficiently compensate for the differences in the relative positions of individual bands predicted with different functionals, conclusions drawn solely from Δ can be somewhat misleading. Nevertheless, these values provide a good and quick quantitative measure of the level of agreement between theory and experiment. Further comparison between calculated and experimental spectra also reveals that in our system the axially chiral (R)binaphthalene unit induces the (M)-helicity of diphenyldiazene in both diazene configurations (Figure S9, Supporting Information). This feature arises from the binaphthalene’s twisted yet adjustable geometry and is supported by the tension present in the macrocycle. Because ECD simulation should be supported by comparison of experimental and theoretical electronic absorptions, we simulated also UV/vis spectra for both sets of geometries (Figures 8a,b and S14−S16, Supporting Information) and obtained corresponding similarity factors (Σ; Table S3, Supporting Information). Briefly, on a par with ECD simulation, dispersion corrections lead to very little change in

Figure 8. Comparison of experimental and Boltzmann-averaged simulated UV spectra of (E)-7 (a) and (Z)-7 (b) based on geometries optimized with corrections for dispersion interactions in the gas phase. The x-axis has the same scale for (a) and (b). Applied shifts for simulated spectra: −12 nm (B3LYP), +16 nm (CAM-B3LYP), −5 nm (M06). Experimental conditions: methanol/water, c((E)-7, (Z)-7) = ca. 10−6 M, 25 °C.

the shape of absorption bands for the (Z)-isomer, with more notable differences in the case of CAM-B3LYP. Interestingly, for the (E)-isomer this functional predicts population-averaged UV/vis spectra almost neglecting the effect of dispersion corrections, in contrast with B3LYP and M06. The similarity factors are very high (over 94%) due to unresolved and overlapping experimental bands and hence possess limited differentiating ability. Nonetheless, Σ values are in agreement with qualitative visual comparison lower for CAM-B3LYP. In summary, all three functionals describe essential UV/vis absorptions satisfactorily, which validates their ECD prediction capacity. Photochromism. On the preparative scale, compound (R)-7 was isolated as a blend of its (E)- and (Z)-isomers. The composition of this mixture before (in which (E) prevails over (Z) as 72:28) and immediately after irradiation (where (E):(Z) = 41:59) was determined from the integration of amidic proton signals in respective NMR spectra. The irradiation wavelength of 365 nm was chosen to minimize possible interference with the transition bands of other chromophores present in the molecule, and it also resulted in the most significant changes in the ECD spectra. Upon photoisomerization, the (E)-form of diphenyldiazene is converted49 to the (Z)form and this is accompanied by conformational changes in the rest of the macrocycle. In Figure 9 we show the CD spectra of (R)-7 in 1,4-dioxane after consecutive irradiation of the initial mixture. The UV photostationary state is reached after 15 min, with two isosbestic points at 324 and 422 nm that are maintained throughout the irradiation. This process can be completely reversed by irradiation at 465 nm (diazene n → π* transition band maximum). Study on switching behavior showed that the CD signal intensity can be controlled repeatedly by alternating irradiation at 365 for 7 min and 465 nm for 35 s (Figure 10). No fatigue was observed within 10 repetition cycles, with the highest absolute change in intensity determined by reading at 307 nm (14.8 ± 0.7 mdeg) and the highest relative change at 338 nm 8594


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Figure 9. ECD spectra of (R)-7 in 1,4-dioxane upon consecutive irradiation at 365 nm. Conditions: c((E)-7) = 1.12 × 10−5 M, c((Z)-7) = 4.38 × 10−6 M, 25 °C, light path length = 1 cm, 0.8 mW/cm2, irradiation time (total): 0, 0.5, 1, 1.5, 2, 3, 4, 5, 7, 10, 15 min.

Figure 11. Simulated ECD spectra (a, b) of photoisomerization of the mixture of (E)-7 and (Z)-7 based on dispersion-corrected conformer geometries compared to the experiment (d). (c) Differential spectra showing change after irradiation of the mixture. For comparison, these were normalized by division with their respective maximal positive values. The x-axis has the same scale throughout (a)−(d). Applied shifts for simulated spectra: +16 nm (CAM-B3LYP), −5 nm (M06). Experimental conditions: methanol, c((E)-7) = 5.6 × 10−6 M, c((Z)-7) = 2.19 × 10−6 M, 25 °C, light path length =1 cm, irradiation wavelength 365 nm (0.8 mW/cm2, 15 min.).

Figure 10. Switching of the CD signal intensity of (R)-7 in 1,4-dioxane. Conditions: c((E)-7) = 1.12 × 10−5 M, c((Z)-7) = 4.38 × 10−6 M, 25 °C, light path length = 1 cm, irradiation wavelengths: 365 nm (0.8 mW/cm2, 7 min) and 465 nm (11 mW/cm2, 35 s).

(E)- and (Z)-isomers’ spectra with CAM-B3LYP; (iv) the inability of B3LYP to accurately describe the binaphthalene couplet. Comparison of the calculated most abundant conformers’ geometries of the (E)- and (Z)-isomers suggests that the most significant changes upon photoisomerization observed between 290 and 400 nm are, besides diphenyldiazene (E) → (Z) conversion (π → π* transition), apparently related to the switching of the amide conformation: from (E,E) in the (E)-diphenyldiazene containing conformers to (Z,Z) in the (Z)-diphenyldiazene containing ones. This is induced by the severe structural distortions originating in the (E) → (Z) diazene transformation.6,10,58 In this way, the cyclic strain controls the conformation and interactions of the individual chromophores that are reflected in the CD spectra. Considering the difference in the binaphthalene dihedral angle between the conformers of the (E)- and (Z)-isomers, there is interplay between the binaphthalene and diphenyldiazene moieties on the opposite sides of the molecule. This happens at the expense of the acrylamide spacers, which in fact adopt the role of the signaling unit, because no significant changes were observed in the binaphthalene couplet region (200−230 nm; Figure 11d). It is somewhat surprising, because the CD couplet of 2,2′-disubstituted binaphthalenes has been predicted to decrease in intensity and eventually invert the sign upon increasing the naphthalene planes C(2)−C(1)−C(1′)− C(2′) angle.50 The limit value has been calculated to lie between 100 and 120°, very close to that in the conformers of the

(1:2.96 ± 0.02). There are also less pronounced effects at 350 and 475 nm. On the basis of the determined isomer content in the initial and UV photostationary states (Table 3), we were able to simulate the ECD spectra before and after irradiation of the mixture (Figures 11a,b and S13a−c, Supporting Information). Essential trends are reproduced mainly correctly with all three functionals, with similarity indices slightly higher (by 4−11%) for the initial spectra when compared to the irradiated state in the case of B3LYP and M06 and significantly higher (by 19%) for CAM-B3LYP (Table S2, Supporting Information). This behavior naturally reflects the 72% content of the more reliably simulated (E)-isomer in the initial mixture as opposed to its 41% contribution after irradiation. Overall, B3LYP and M06 fare equally well (Δ = 55−66%), whereas the performance of CAM-B3LYP is superior (65−84%). However, in the light of certain shortcomings of the Δ analysis described above for our case, we point out the following facts when evaluating the performance of the individual functionals: (i) comparison of differential spectra, particularly the extent of relative changes between the initial and irradiated states, favors B3LYP and M06 over CAM-B3LYP (Figures 11c and S13d, Supporting Information); (ii) inferior ability of CAM-B3LYP to reproduce the shape of experimental UV/vis spectral bands (Figure 8); (iii) a significant discrepancy between Δ values for the simulation of the 8595



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(E)-isomer. This trend is to some extent theoretically reproduced with CAM-B3LYP (which predicts clearly recognizable binaphthalene couplets), whereas for the other functionals it cannot be readily identified. However, the photoisomerization of the mixture is simulated realistically in all cases. Kinetic Analyses. We also performed first-order kinetic evaluation of the photoisomerization processes upon irradiation at 365 nm (Figure S17, Table S4, Supporting Information). The rate constant for the (E) → (Z) transformation was determined to be (5.68 ± 0.05) × 10−3 s−1, whereas that for the reverse process equals (3.97 ± 0.03) × 10−3 s−1. The thermal stability of the irradiated mixture was investigated in dark at 20 °C. This yielded a rate constant of (4.27 ± 0.14) × 10−7 s−1, which corresponds to a rather long half-time of 18.8 ± 0.6 days. Presumably, considering the significant conformational changes in the rigid macrocycle upon the (Z) → (E) isomerization, there is a relatively high barrier for this thermal process at 20 °C.

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AUTHOR INFORMATION

Corresponding Author

*M. Putala. E-mail: [email protected]. Telephone: 00421 2 60296 323. Present Address §

Synkola Ltd., Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovak Republic. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Slovak Research and Development Agency, Grant APVV-062212; Slovak Grant Agency for Science, Grant No. 1/1032/12; Research & Development Operational Programme of the Slovak Republic funded by ERDF, projects ITMS 26230120002 and 26210120002 (Slovak infrastructure for high-performance computing).

CONCLUSIONS

Notes

We have characterized the photochemical behavior of binaphthalene containing macrocyclic diazene (R)-7 as a blend of its (E)- and (Z)-isomers. It functions as a durable and reversible photochromic switch by alternating irradiation at 365 and 465 nm. The competitive thermal (Z) → (E) isomerization occurs only slowly, with the half-time of 18.8 ± 0.6 days at 20 °C. The switching process was monitored by means of ECD spectroscopy, with CD signal amplitudes up to 14.8 ± 0.7 mdeg (readout at 307 nm) and the highest relative change 1:2.96 ± 0.02 (338 nm). A thorough conformational analysis of the individual isomers and comparison of their CD spectra have shown that for our system, the inclusion of empirical corrections for dispersion interactions is crucial to obtain results consistent with the experiment, especially in the case of the (E)-isomer. Reliable theoretical predictions enabled us to realistically simulate the CD response of the photoisomerization process. The cyclic strain is the controlling force behind the performance of the switch. The major changes in the CD signal are most likely related to the swapping of the amide conformation induced by the diphenyldiazene (E)/(Z)-isomerization. In contrast, the change in the binaphthalene dihedral angle as calculated for the (E)- and (Z)-geometry interconversion is mostly neglected both in the experimental and in simulated spectra. On the basis of the similarity analysis of ECD spectra, in the framework of TD-DFT, the range-separated CAM-B3LYP outperforms the hybrid metaGGA M06 and the common global hybrid B3LYP. In addition, this functional correctly reproduces some important features of the binaphthalene couplet. From the qualitative point of view, M06 also delivers reasonable overall performance supported by the satisfactory agreement between the calculated and experimental UV/vis absorptions.

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Article

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Michal Pitoňaḱ (Faculty of Natural Sciences, Comenius University in Bratislava, Slovakia) for conducting ab initio molecular dynamics and reference MP2 calculations. This work was supported by the Slovak Research and Development Agency (Grant APVV-0622-12) and the Slovak Grant Agency for Science (Grant No. 1/1032/12). Part of the calculations was performed in the Computing Centre of the Slovak Academy of Sciences using the supercomputing infrastructure acquired in project ITMS 26230120002 and 26210120002 (Slovak infrastructure for high-performance computing) supported by the Research & Development Operational Programme funded by the ERDF.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Full listing of computed structural parameters, additional conformer geometries and comparisons, detailed simulated ECD and UV/vis spectra, kinetic analysis details and Cartesian coordinates. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b03474. 8596


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