59th Annual Report on Research 2014

The original objective of the proposed research was to use spiropyran (SP) photoisomerization to reversibly alter the physical properties of organic-inorganic hybrid systems. We successfully prepared photo­chromic nanoparticles in accord with the original research proposal and have measured kinetics of pho­toisomerization for SP surface-immobilized on silica nanoparticles. We observed that thermal relaxation of the MC back to SP was slower by a factor of 2 compared to kinetics in ethanol solution.

In this report, we report results on the conformational dynamics of cis-azobenzene that has fluorine substitution at the ortho position of the aromatic rings. The use of azozbenzene (AB) isomerization as a molecular probe of cavity dimensions and polarity is an impetus for this work.   AB is arguably the most studied organic chromophore in chemistry. AB exists as two geometric isomers with the trans conformation 12 kcal/mol more stable than the cis isomer. We have observed through-space (TS) ¹⁹F-¹⁹F coupling in the cis-conformation of ortho-fluoro azobenzenes. We argue that this coupling provides a sensitive probe of conformation of cis-AB in hybrid systems through the use of ¹⁹F NMR.

Scheme 1. Cis-trans (E-Z) isomerization of ortho-substituted azobenzene; library of compounds.

1 (F₄OMe): A,A’,B,B’ = F; R=OMe

2 (Fa₁Fb₂OMe): A,A’,B’ = F; B = H; R=OMe

3 (Fb₂OMe): A,A’ = F; B,B’ = H; R=OMe

4 (Fa₁OMe): A,A’,B’ = H; B = F; R=OMe

5 (Fa₂Fb₁OMe): A,B,B’ = F; A’ = H; R=OMe

6 (Fb₁OMe): A,A’ = F; B,B’ = H; R=OMe

7 (Fa₁Fb₁OMe): A,A’,B’ = H; B = F; R=OMe

8 (Fa2Fb1): A,B,B’ = F; A’ = H; R=H

Azobenzene compounds 1–8 (Scheme 1) were prepared via the diazonium salt for 1-7; 8 was prepared from 2-fluoronitroso benzene and 2,6-difluoroaniline. As prepared, the configuration of these compounds is principally Z, which can be converted to a mixture of Z and E in min using UV irradiation. The thermal Z to E isomerization is sufficiently slow so that isomer separation is possible by flash chromatography (the more polar Z elutes after E).

The ¹⁹F{1H} spectrum of (E)-1-(2,6-difluoro-4-methoxyphenyl)-2-(2,6-difluorophenyl)diazene is shown in Figure 1. Z1 exhibits two triplets with J_obs = 5 Hz. Table 1 lists the ¹⁹F chemical shifts for 1-8. All ¹⁹F spectra were collected with broadband proton decoupling. The fluorines on the methoxy-substituted ring are designated Fa. With the exception of 1 (Fa shift upfield by 0.2 ppm), the fluorines are shifted downfield upon photoisomerization to cis. No through-space coupling was observed for the trans isomer. We confirmed that the observed coupling is due to ¹⁹F-¹⁹F interaction via COSY and 2-D J-exchange experiments.

Variable temperature NMR was conducted to investigate dynamic behavior. No significant change was observed in toluene-d₈ up to 98°C. At low temperatures, 1-3 and 7 displayed a decrease in the coupling with the multiplet coalescing near -100°C. We have assigned phenyl rotation as the dynamic process responsible for this diminution of through-space coupling.

Table 1. ¹⁹F NMR chemical shifts for 1-4, CDCl₃, 28°C^a

Compound	δ, ppm trans		19F NMR δ, ppm cis
	A,A’ (Fa)	B,B’(Fb)	A,A’ (Fa)	B,B’(Fb)	JFF, Hz
1	-120.3	-125.2	-120.1	-122.6	4.82
2	-123.4	-125.0	-121.8	-123.0	4.08
3		-125.2		-123.0
4	-124.4		-120.9
5	-120.5	-127.5	-120.3	-125.46	4.53
6		-127.3		-125.4
7	-124.0	-128.0	-120.8	-125.0	2.22
8b	-126.8	-124.4	-125.6	-122.4	5.90

^a δreferenced to internal standard, C₆F₆ (-164.9 ppm). ^b For 8, Fa = methoxy substituted ring.

X-ray Analysis

We were able to obtain X-ray structures for Z1 and Z2 (Figure 2). The structural features of the optimized structure (b3ylp/cc-pVTZ) compared closely to the X-ray structure with bond lengths and angles all within 2%.

Density Functional Theory

The conformational behavior of Z1, Z2, Z5, and Z7 was investigated by density functional theory (DFT) using Gaussian09. It is has been noted that DFT on conjugated systems requires electron correlation. The most consistent and sensible results were obtained using the b3ylp method and the cc-pVTZ basis set. Calculations were performed without solvent fields; for 1, the four possible conformers were energetically equivalent and two transition states were found with a bisected geometry (see representation for 8 in TOC). For 1, the energy of the bisected transition state is 4.7 kcal above the ground state which compares favorably with the experimental value of 3 kcal (taken from the coalescence temperature in the NMR spectrum). This is the first experimental measurement of phenyl rotation in cis-azobenzene.

Figure 1. ¹⁹F{¹H} NMR spectrum of E/Z  in CDCl₃ at 28°C for azobenzene 1.

Figure 2.  a. X-ray structure of Z1, b. overlap of X-ray and DFT structures for Z1.