Elementary Photophysical Transitions and Photochemical ...hilinski/reu/REU... · From Photochemistry of Organic Compounds by Peter Klán and Jakob Wirz, 2009 Photochemistry’s Icons

Elementary Photophysical Transitions and Photochemical Reactions

NSF REU June 25, 2019

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

Photochemistry’sIcons

Why I am here.

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