Thermal back reaction of sterically constrained systems

Known systems CP-DHA, CHex-DHA and CHept-DHA show different rates of VHF to DHA thermal back reaction.

In case of CP-VHF at room temperature it has a lifetime of more than 6 h.⁹⁴ But this is the only example of sterically constrained dihydroazulene systems that consistent with results obtained from other experiments.⁹⁸ Other systems show totally different behaviour from the point of view of thermal back reaction. CHex-VHF has drastically smaller lifetime in comparison to other vinylheptafulvenes. At room temperature it could be detected only in non-polar solvents and clearly seen under lower temperature. CHept-DHA shows similar to CHex-DHA behaviour with a slightly slower thermal back reaction speed.

According experimental result achieved for A1 system that showed extremely fast thermal back reaction. Unlike photochromic reaction where A1 could be compared with A2-A4 in case of thermal reaction of vinylheptafulvene it shows comparable properties to CHex- and CHept-DHA/VHF sterically constrained system.90-92, 95, 100

Preliminary studies with cooperation of Riedle group in Munich showed that the results of measuring A1a are consistent with obtained for C4a and CP-DHA.90^,94^,96 The thermal back reaction have been resolved at 22°C in cyclohexane and CH3CN. The lifetimes of vinylheptafulvene A1b are 1.12 s in cyclohexane and 0.09 s in CH3CN.

Several factors should be discussed that are important for description of thermal back reaction VHF DHA and influence the rate of this reaction:

• Stabilization by transformation of VHF from s-cis conformation to more energetically favourable s-trans conformation

• Structural aspects of constrained systems

• Energetics of s-cis-VHF DHA process, value of energy barrier of process

100 T. Mrozek, Dissertation, Universität Regensburg, 2000.

Stabilization due to s-cis – s-trans conformation have been proposed before and described several times. The stability of s-trans form is postulated. It is not possible in case of discussed systems.

This option is prohibited by structure of molecules.

If not to take into account CP-DHA system, introduction of hindrance to DHA/VHF system destabilizes its open form. The CP-DHA case might be explained from the point of view of geometrical properties and will be discussed later.

Sterical hindrance due to different linking pattern might play a great role in molecule open form stabilization. But by this factor it is harder to explain why system with less flexible 5-membered ring is more stable than 6-membered. The answer might be comparing of geometry of cyclic fragments in molecule with geometry of similar model compounds that are free from hindrances caused by interaction of heptafulvene and dicyanoethylene fragments. For described systems CP-, CHex- , CHept-VHF, and A1b this model compounds could be proposed, Scheme 3.6.

A1a-ml CP-ml CHex-ml CHept-ml

Scheme 3.6: Model compound of constrained systems.

All vinylheptafulvenes and corresponding model compounds were calculated with Gaussian 98W.

By DFT method B3LYP/6-31Gd for all structures optimized geometries were found. Calculated data presented below:

Table 3.1: Calculated structures of constrained systems and corresponding model compounds.

Structure Calculated structure¹⁰¹ C=C-C=C angle

0.00

32.69

47.64

39.02

CN

CN 30.52

101 Calculated structures are visualized with MOLVIEW 3.0.

CN

CN ^42.43

CN CN

60.45

Comparison of model systems with corresponding VHFs shows only higher geometrical changes in CP-system due to hindrances caused by overlapping of cyano- and tropilium substituents. Also clear that larger ring systems are more flexible in comparing to five-membered. It might be assumed that the geometrical perturbations upon thermal reaction should be smaller for bigger ring systems and have lower energy barrier. Also should be noticed that partial charges of methylene and 2’

carbon atom of tropilium moiety are comparable for all observed systems and might be assumed that they are not play a great role in such a big differences of thermal back reaction rates.

Taking into account results of time measurements of the thermal back reactions of vinylheptafulvenes described before and geometrical structures of calculated open forms might be proposed such a explaining of this reaction.

CN CN

n

δδδδ⁺⁺⁺⁺

δδδδ⁻⁻⁻⁻

1' 2'

3'

5' 4' 6'

7'

Scheme 3.7: Vinylheptafulvene with showed positions of orbitals that form – bond.

Thermal back reaction proceeds on ground state reaction pathway and according to frontier orbital theory leads to bond formation between 2’ carbon atom of heptafulvene moiety and methylene atom and requires disrotatory motion.

CN NC

Figure 3.6: Higher occupied molecular orbital (HOMO) orbitals of CP-VHF.

Taking in account previous assumption it might be conclude that thermal back reaction rate will depends mostly on geometrical changes needed for thermal rearrangement and this rearrangements in most rigid CP-VHF system should be largest.

3.5 Conclusions

Annulation of A2a as indicated in Scheme 3.8 has a two-fold effect on the structure and dynamics of the DHA/VHF system. At first, the rotation around C12a-C12b in A1a is restricted and second, the rotation around C1-C2 bond in A1b is blocked.

CN CN CN

CN

CN CN

12a 6a

12b

6b 4a

11a

A1a A2a

hνννν

∆∆∆∆

1

2

A1b Scheme 3.8: Structure of dihydroazulenes A1a and A2.

Six-ring annulation as shown in A1 leads to an increase of the rate of the thermal back reaction (VHF → DHA) and as a consequence the photochromism cannot be observed at room temperature under the normal experimental conditions. However, lowering the temperature the colouring occurs on irradiating the DHA form.

Future photophysical investigations, especially by the resolved methods will provide more information on the excited states of this sterically restricted system.