Elementary Photophysical Transitions and Photochemical Reactions

NSF REU June 25, 2019

From Photochemistry of Organic Compounds by Peter Klán and Jakob Wirz, 2009

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HC C

H

H

HCH3-CH3

H H

H H

Average bond energiesC-C 83 kcal/molC=C 146 kcal/molπ bond ~ 63 kcal/mol

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.

h

S = 1/2 - 1/2 = 0 S = 1/2 - 1/2 = 0 S = 1/2 + 1/2 = 1

0 1 1

Spin States

T T T S

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

EWG

HOMO LUMO

EWG

HOMO Donor

LUMO Acceptor

Frontier Orbital Control of Diels-Alder Reaction

CH3CH3 175 oCh

CH3

CH3

CH3CH3175 oC h

Electrocyclic reactions are stereospecific

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.

Havinga, E.; Schlatmann, J. L. M. A Tetrahedron 1961, 16, 146‐152.NEER Havinga

NEER PRINCIPLE

NPE

conformersc-NPEBc-NPEA

t-NPEA t-NPEB

NPE Conformers

Conformer specific photochemistry in c- and t-NPE

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

Heptane - - -Water

Benzophenone State Diagram

El-Sayed’s Rules for Intersystem Crossing

Triplet Sensitization

5 kcal/mol

62 kcal/mol

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

The Saltiel Plot for Stilbene Triplet Photoisomerization

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

Saltiel Plots

1,3-butadiene dimerizationdashes

1,3-pentadiene E,Z isomerizationsolid line

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