This little light of mine...

Photoisomerization

The two electrons are now in different orbitals. Hund's rule says that electrons in two different orbitals want to be as far apart as possible. When the molecule is excited, the two halves twist around the sigma bond so the two p-orbitals are orthogonal. There no longer is a p bond between the two carbons, but each electron is in an orthogonal p-orbital.

transition	compound
s Æ s *	alkanes
n Æ s *	alcohols, amines, ethers, halides
p Æ p *	alkenes, alkynes, carbonyl compounds
n Æ p *	carbonyl compounds

Absorption data for typical chromophores

Chromophore	Example	Solvent	lmax (nm)	emax	transition
		heptane	180	12,500	p Æ p*
		vapor	172	4,500	p Æ p*
		vapor	289 182	12.5 10,000	n Æ p* p Æ p*
		cyclohexane	275 190	22 1,000	n Æ p* p Æ p*
		hexane	295	14	n Æ p*
æCO2H	CH3CO2H	ethanol	204	41	n Æ p*
æCOCl	CH3COCl	heptane	240	34	n Æ p*
æCO2R	CH3CO2CH2CH3	water	204	60	n Æ p*
æCONH2	CH3CONH2	methanol	205	160	n Æ p*
æNO2	CH3NO2	hexane	279 202	15.8 4,400	n Æ p* p Æ p*
		ethyl ether	417	7	n Æ p*
		water	204	25	n Æ p*
		water	254 203.5	205 7,400	p Æ p* p Æ p*
		water	261 206.5	225 7,000	p Æ p* p Æ p*

UV/VIS Spectrum:

Exercise: Predict what the spectrum of 2-cyclohexenone would look like.

Jablonski diagrams:

Radiationless processes (energy losses by collisions with other molecules):

• vc = vibrational cascade within an energy level
• ic = internal conversion from one singlet level to another singlet level
• isc = intersystem crossing from singlet to triplet

Radiative processes (energy losses by emission of light)

• hn_f fluorescence
• hn_p phosphorescence

Primary Photochemical Reactions of Excited Molecules

(a)	Simple cleavage into radicals
(b)	Decomposition into molecules
(c)	Intramolecular rearrangement
(d)	Photoisomerization
(e)	Hydrogen atom abstraction
(f)	Photodimerization
(g)	Photosensitization

Example of photochemical intramolecular rearrangement:

Example of photodimerization:

Simple cleavage.

Free Radical Halogenation

Exercise: Write the mechanism showing how each product is formed. Label each step as initiation or propagation. What might be possible termination steps? (See Ege, Chapter 21.)

Decomposition into molecules. Norrish Type II cleavage of aldehydes and ketones with g-hydrogen atoms.

Exercise: Write the mechanism showing how the products are formed.This reaction involves homolytic cleavage of sigma and pi bonds in an excited state.

Simple hydrogen atom abstraction.

The formation of benzpinacol, which is the objective in Experiment 49 in Pavia, can also take place in the presence of a protic solvent and light.

Exercise: What experimental conditions would you expect to be important to favor the formation of benzhydrol instead of benzpinacol in the above two reactions? How would you design the experiments?

Photoisomerization.

Photosensitization.

In the photodimerization of benzophenone, one part of the experiment uses naphthalene to quench the reaction. In other words, the presence of naphthalene prevents the photodimerization from taking place.

Exercise: See if you can figure out what this experimental observation tells you about the nature of the photodimerization of benzophenone. Don't look in Pavia until you have tried to figure it out for yourself.