טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLavy Tali
SubjectHomogeneous Photochemical Reactions in the Solid-State
DepartmentDepartment of Chemistry
Supervisor Professor Emeritus Menahem Kaftory


Abstract

Homogeneous photochemical reactions are often called single-crystal to single-crystal photoreactions because the crystal maintains its crystallinity throughout the reaction until full conversion is reached. Heterogeneous photochemical reactions are characterised by the disintegration of the single-crystal into smaller crystals and the loss of the single-crystal attributes that it possessed. An interesting manner of obtaining homogeneous photochemical reactions was proposed by Enkelman, its principle being to irradiate the crystal in the tail of an absorption spectrum in which the intensity of the absorbed light is relatively less than the intensity of the light at the peak of absorption.

In this work, the tendency of inclusion compounds to undergo homogeneous photochemical reactions was studied. The inclusion compounds studied in this work are based on hosts that have a hydroxyl group and guests that have a carbonyl oxygen so that the interaction between the guest and the host is a hydrogen bonding interaction. The molecule that is intended to undergo the photochemical reaction is a guest molecule, while the host molecule creates a kind of cage around the guest molecule. A description of the guest and host molecules and the combinations between them that were studied in this work, are given below:


  Host-guest ratio guest host Inclusion compound  
  1:2 1 a I  
  1:2 2 a II  
  1:1 3 c III  
  1:1 4 c IV  
  1:1 5 d V  
  1:1 6 e VI  
  1:2 7 b VII  
  1:2 1 b VIII  
  1:2 2 b IX  

(c)
(R,R)-(-)-2,3-bis(hydroxydiphenylmethyl)-1,4-dioxaspiro[4.5]decane


  (b)
2,5-diphenylhydroquinone


 
(a)
1,1,6,6-Tetraphenylhexa-2,4-diyne-1,6-diol

 (e)
1,6-bis(o-chlorophenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol


(d)
(R,R) -(-)-trans- 4,5-bis(Hydroxydiphenylmethyl)-2,2-dimethyl-1,3-dioxolane


(1)
Benzylideneacetophenone
(Chalcone)

(2)
2-pyridon

(3)
4(oxophenylacetyl)morpholine

(4)
1- (Benzoylcarbonyl)piperidine

(5)
tropolone methyl ether

(6)
tropolone ethyl ether


4’-Methoxychalcone
 (7)

 

 


 

The crystal structure of these inclusion compounds was determined, though the structure of some of them (I, II and VI) was known. It was concluded on the basis of the crystal structure that inclusion compounds VIII and IX are not photoreactive because the mutual orientation and the distances between the atoms which have the potential to react do not satisfy the geometric criteria for photodimerization determined previously by Schmidt and Cohen whereas an examination of the crystalline structure of compounds I-VII showed that there was a chance of their undergoing a photochemical reaction. These compounds absorb light in the UV range (approximately 200-300nm). They were irradiated with a Xe lamp. Compounds I and II underwent a homogeneous dimerization reaction. Compounds III and IV underwent homogeneous, enantioselective, photochemical reactions that gave oxazolidin-4-one products. The reason for the enantioselectivity is the induction of the asymmetric environment of the chiral host. The crystal structure of the product of the irradiation in all these four cases was also determined. Compounds V and VI underwent a heterogeneous photochemical reaction (the crystals were destroyed in the course of the irradiation) and finally compound VII did not undergo any photochemical reaction at all.


The case of inclusion compound II was of special interest. At a certain stage of the reaction it became apparent, according to a peak in the Fourier difference map, that a foreign molecule was penetrating the crystal and was filling the space created in the course of the reaction without destroying the structure of the crystal. This peak was suspected to be the oxygen of a water molecule. An attempt was made to perform the irradiation under dry conditions but it was unsuccessful and the atom still penetrated the crystal. In order to determine when this molecule penetrated the crystal, the irradiation was carried out in stages. For each stage the crystal structure was solved. It was found that up to the stage of full conversion, the crystal contained both the monomer and the dimer and after further irradiation the foreign molecule penetrated the crystal. This led to the conclusion that the molecule penetrated the crystal at final stage of the conversion. The structure of the product of the irradiation that contains the foreign peak that was assigned as oxygen was also solved.