1,3,5–Hexatriene is known to undergo light–induced cyclization to give 1,3–cyclohexadiene. The photo — Analytical Chemistry Chemistry Question
Photochemical Ring Closure and Opening
1,3,5–Hexatriene is known to undergo light–induced cyclization to give 1,3–cyclohexadiene. The photochemical reaction is reversible and stereospecific. Thus, irradiation of (2E,4Z,6E)-octatriene (A) with UV-light gives cyclohexadiene (B). The choice of the wavelength of light depends on the absorption maximum of the compound to be irradiated, and the absorption maximum is related to the number of conjugated double bonds in a chain.
[VISUAL]
What is the chemical name of the starting triene (C) for the related reaction shown below?
irradiation
[VISUAL]
Model Answer
(2E,4Z,6Z)–octatriene
A similar reaction mechanism is involved in the synthesis of biologically active molecules. For example, in the presence of sunlight, 7–dehydrocholesterol (E) undergoes an electrocyclic ring opening reaction to give provitamin D3 (F), which can be further transformed through a [1, 2]–hydrogen shift to yield vitamin D3 (G).
[VISUAL]
Of the two compounds 7–dehydrocholesterol (E) and vitamin D3 (G), which would you expect to absorb light with the higher energy? (E or G)
Model Answer
E
What is the chemical structure of F?
Model Answer
[VISUAL] (An organic structural formula representing provitamin D3, with a cyclohexenol ring linked to a conjugated triene system and a steroid side chain).
This principle has been elaborated to develop photochromic materials. For example, irradiation of colorless compound H with UV light gives colored compound I. The color change is reversed upon exposure to visible light.
[VISUAL]
Give the structure of colored compound I.
Model Answer
[VISUAL] (Skeletal structure representing colored compound I with three carbonyl/ether-like oxygen atoms and methyl substituents in a fused polycyclic ring system, showing the closed-ring form).
Aromatic hydrocarbons are usually highly fluorescent. However, a neighbouring amino substituent may quench the fluorescence. This quenching mechanism is due to Photoinduced Electron Transfer (PET), which can be clearly illustrated by the molecular orbital diagrams shown below. Upon irradiation with a light of suitable wavelength (step 1), the initial aromatic chromophore (state a) will pump an electron from the highest occupied molecular orbital (HOMO) up to the lowest unoccupied molecular orbital (LUMO) (state b). In the presence of a neighbouring amino group, one of the lone–pair electrons at the nitrogen atom moves to the HOMO of the excited chromophore (step 2), and thus blocks the normal fluorescent pathway (state c). Coordination of the amine lone–pair electrons to proton or metal ions efficiently inhibits the PET process, and retrieves the fluorescence of the aromatic chromophore (step 3).
[VISUAL]
Many interesting and sensitive proton or metal ions fluorescent sensors have been developed based on the manipulation of the PET process. For example, compound J is used as a pH–sensor.
[VISUAL]
Do you expect that compound J is fluorescent in an alkaline solution (pH = 10.0)?
Model Answer
No.