From Photochemistry of Organic Compounds by Peter Klán and Jakob Wirz, 2009
Photochemistry’sIcons
Why I am here.
π bond ~ Eact = 66 kcal/mol
cis- and trans-2-butenes are stable separable isomers. They have the same connectivity and are stereoisomers. Rotation about a double bond occurs only at very high T or following light absorption.
n λmax, nm εmax x 10‐3, M‐1cm‐1
1 294.1 282 328 533 348 804 404 86
max max)170 nm 217 (21,000) 263 (52,500) 239 (3,400) 256 (8,000)
( )n
Effect of Conjugation
Absorbance
Molecular Orbital Analysis of the Diels-Alder Reaction
The diene is the electron donor and the dienophile is the electron acceptor. The concerted nature of the D-A reaction can be understood by considering the interaction of the HOMO of the diene with the LUMO of the dienophile (Fukui/Kyoto, Woodward/Harvard, Hoffmann/Harvard now Cornell---Nobel prize in Chemistry)
1965 1981
CH3
CH3
CH3
CH3
Thermal reaction
conrotatory
Rotation in the same direction leads to a bonding interaction (+ lobe on + lobe or - lobe on – lobe). The reaction passes through the same TS in both directions.
CH3
CH3
CH3CH3
Photochemical reaction
disrotatory
h
Rotation in opposite directions leads to a bonding interaction (+ lobe on + lobe or - lobe on – lobe). The reaction passes through the same TS in both directions.
330 350 370 390 410 430 450 470 490 510
, nm
0.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity exc = 320 nm
exc = 345 nmexc = 350 nm
Fluorescence Emission Spectra of t-NPE as a Function of exc in Ar Outgassed Benzene at 20 oC
t‐NPE/Bz
TransposedMatrix
Spectral InputMatrix
CovarianceMatrix
Diagonalized Matrix
Eigenvector Matrix
Eigenvectors
EigenvaluesNumber ofComponents
Experimental Spectra
Si = iV + iV
i = V Si, i =VSiSi = iV + iV
Pure Component Coefficients
A, A and B, B
Pure Component Spectra
SA = AV + AV
SB = BV + BV
x =
Summary of PCA-SM Mathematical Operations
t‐NPE/PCA‐SM
330 350 370 390 410 430 450 470 490 510, nm
‐0.175
‐0.115
‐0.055
0.005
0.065
0.125
Norm
alized Intensity
Principal Eigenvectors
0.0500 0.0775 0.1050
-0.009
-0.003
0.003
0.009
0.015
Plot
0.0 0.1 0.2 0.3Standard Deviation
‐0.0004 0.0013 0.0030 0.0046 0.0063 0.0080[O2]/M0.0
1.0
2.0
3.0Global Stern‐Volmer Plots
o
= 1 + KSV[Q]Stern-Volmer equation:
F0(/F(= 1 + KSVA[Q]
Global Stern-Volmer equation:
330 350 370 390 410 430 450 470 490 510, nm
0.0
0.2
0.4
0.6
0.8
1.0
Norm
alized Intensity
t‐NPEBt‐NPEA
Conformer Spectra
t‐NPEAt‐NPEB
S‐V Kinetics
Stern-Volmer Quenching
Equation Rate
1A + hexc→ 1A* Ia
1A* → 1A + hf kf[1A*]
1A* → 3A* kis[1A*]
1A* → P kp[1A*]
1A* → 1A + Q* kq[Q][1A*]
SV2
A*][kdt
]νd[ 1f
f h
0 A*])[ (IdtA*]d[ 1
pisfa
1
kkk
pisf
a1
I A*][
kkk
pisf
aff
I
dt]νd[
kkkkh
τ dt
]d[I1 f
pisf
ff
a
of k
kkkkh
τ[Q] 1τ
[Q] q
f
qpisf
ff k
kkkkk
k
[Q] 1 [Q] 1 / SVqfof Kk
The Stern-Volmer Plot
where Ia is the rate of light absorption
The Photostationary State
1t-St + 3S* 3t* + 1S*3t* 3p*3p* 1t + (1-)1c1c-St + 3S* 3c* + 1S*
13 1 1
t d
13 1 1
t d
3 1 1t d
3 1 1c d
1t
1c
d[ t] [ S*][ t] [ p*] 0dt
d[ c] [ S*][ c] (1 ) [ p*] 0dt[ S*][ t] [ p*][ S*][ c] (1 ) [ p*]
[ c] [ ] (1 )[ ][ t] [ ]
PSS
k k
k k
k kk k
kk
1,3-Diene Triplets: Conformer Specific Photoisomerization and Photodimerization
Fluorenone sensitized dimerization
1,3-Diene Triplets: Conformer Specific Photoisomerization and Photodimerization
Fluorenone sensitized photoisomerization
57 kcal/mol 52 kcal/mol
59 kcal/mol Not present
Multidimensional Isomerization – Conical Intersections – Trapping Twisted Intermediates
B. G. Levine, T. J. Martínez. Annu. Rev. Phys. Chem. 2007, 58, 613-34.
H. E. Zimmerman. J. Am. Chem. Soc. 1966, 88, 1566-1567.Michl and Bonacic-Koutecky 1990. Electronic Aspects of Organic Photochemistry.
*
OCH3
H
H
h
hH
H
-h
CH3OH
HCH3OH
OCH3
H