4.1

Photochemical techniques

Ground statereactants

Excited statereactants

ReactionIntermediates

Ground stateproducts

LASER TECHNIQUESLuminescence

Transient absorption (UV, Vis, IR)Time resolved diffuse reflectance

Photoacoustic spectroscopyTransient conductivity

L.I.F. of reaction intermediatesMirage spectroscopy

Time resolved light scatteringTime resolved resonance Raman

4.2

Pulsed lasers

As part of a method to study photoinitiated chemical reactions

To induce new chemistry, different

from that initiated by conventional sources

As a light source, to initiate the same chemistry as with

conventional sources

Ground statereactants

Excited statereactants

ReactionIntermediates

Ground stateproducts

4.3

Pulsed nanosecond lasers

LASER

Nitrogen

Excimer

Ruby

Nd/YAG

DiodeDye

WAVELENGTH, nm

337

193 (Ar/F)248 (Kr/F)

308 (Xe/Cl)351 (Xe/F)

694347 (x2)

1064532 (x2)

355 (x3)266 (x4)

> 700 nm> 300 nm

PULSE

8 ns, ≤ 10 mJ

5-50 ns, ≤300 mJ5-50 ns, ≤ 1J

5-50 ns, ≤ 500 mJ5-50 ns, ≤ 300 mJ

≥ 10 ns, 1 J≥ 10 ns, 300 mJ

5-10 ns, 0.5-5 J5-10 ns, ≤ 500 mJ

5-10 ns, ≤ 300 mJ5-10 ns, ≤ 150 mJ

low5-20 % of pump

4.4

Lindqvist's 1966 laser set-up

Lamp Filter

PMT

Monochromator

Laserat 337 nm

≤ 1 mJ

Amplifier

Scope

Lindqvist, L. (1966). Utilisation d'un Laser à Émission Ultraviolette Pulsé en Photolyse-Éclairs: Étude de l'état Triplet de l'Acridine. Hebd. Seances Acad. Sci., Ser. C. 263, 852-854.

4.5

Determining transient absorbancesPMT

output

0

Io

Signal

PMT o utput = Pm t = Io - S ig nal

T = Io

Pm t

∆O.D . = - log T = log 1 - Io

Signal

4.6

Laser flash photolysis technique

Lamp

Detector

LaserTypically266, 308

337, 355 nm

4.7

Laser photolysis sequence

-1010 -108 -106 -104 -102 100 102 104 106 108 1010

-1010 -108 -106 -104 -102 100 102 104 106 108 1010

A/D

LA

SER

DE

TE

CT

ION

MONITORINGSHUTTEROPENS

MONITORINGSHUTTERCLOSES

PMTOUTPUT(mV)

60 Hz AC SINE WAVE(From Line Synchronizer)

1 Hz LASERPULSES

PRE PULSE PULSE POST PULSE

LASER SHUTTER OPEN

TIME (ns)

TIME (ns)

SIG

NA

L

4.8

Conditions and capabilities of laser flash photolysis

FAST PRECURSOR PROCESSESSLOW DECAY OF PRODUCTS

Process under study

A B

final productsslow

B

10 ns - 500 µsBA

fasthνAReagents

4.9

Kinetic analysis for visible systems

kgrowth

= k0 + krxn[Ph

2CHOH]

krxn•

Bu tOH + Ph2 COHBu tO• + Ph2CHOH

k0

ButOH + (solvent)•Bu tO• + solvent-H

CH3COCH 3 + CH3•ButO•

PROBLEM: Most systems of interest do not involve reactants or products that can be readily detected in the UV-Vis region

E.g.:

•

Bu tOH +Bu tO• +

4.10

Some background on alkoxyl radicals

Alkoxyl radicals can be readily prepared by thermal or photochemical decomposition of the corresponding peroxide. Typical examples are di-tert-butyl peroxide and di-cumyl peroxide. While the excinction coefficients at λ > 300 nm tend to be small, it is usually possible to use enough peroxide to overcome this. Tert-butoxyl radicals are essentially invisible to laser flash photolysis, while cumyloxyl can be detected.

Alkoxyl radicals tend to abstract hydrogen readily; to a lesser extent, they also add to unsaturated systems. In addition, they undergo cleavage reactions which are hery sensitive ty the polarity of the medium (see below)

CH3H3C

O• H3C O

+ CH3•

4.11

An example of cumyloxyl reactivitytowards HP-136

0

0.02

0.04

0.06

0.08

0.1

300 400 500 600 700

1234

∆OD

wavelength, nm

O

O

CH3

CH3

H3C CH3

CH3

H3C

CH3H3C H

HP-136•

HP-136•

H3C CH3

O•

HP-136

4.12

The probe technique

kprobe

Bu tO•

krxn

• Bu tOH + Ph2 COHBu tO• + Ph2CHOH

k0

ButOH + (solvent)•Bu tO• + solvent-H

CH3COCH 3 + CH3•

ButO• + ButOH +

•

kgrowth

= k0 + k

probe[probe] + krxn[XH]

kgrowth

= k0' + krxn[XH]

The technique allows the determination of the absolute rate constant for a reaction where all the reagents and all the products are invisible to the technique employed

OH

•

signalcarrier

λmax 540 nm

4.13

Is there a catch ?

The technique allows the determination of the absolute rate constant for a reaction where all the reagents and all the products are invisible to the technique employed

• The method provides no information on the nature of the reaction; for example the mode or site of attack cannot be established by this method.

• The signals observed get weaker as the reactant is added. The rates are largely derived from conditions where the growth is fast and the signal weak.

It is essential to select probes that overcome the second problem by giving intense, readily detectable signals.

OH

•

4.14

The significance of growth rates

All products from a reaction grow-in with a lifetime that is identical to the decay lifetime of their precursor. It is this characteristic that makes the probe technique possible

This characteristic also implies that when two spectral bands grow-in with the same kinetics they have the same precursor.

It does not mean that the two bands necesarily belong to the same species.

4.15

hν

Bromine atoms can be readily generated, but are invisible to the technique of

laser flash photolysis

•Br• + RCH-CH2BrRCHBr-CH2Br

HBr + R•Br• + RH

RCH-CH2Br Br• + RCH=CH2•

Studies of atomic species in solution

4.16

∆ O

.D.

340.0 460.0 580.0 700.0

Wavelength, nm

Time

∆ O.D

.

0.2 µs

Br2•Br• + Br–

growth or Br2•

4.17

2

4

6

8

0.05 0.1 0.15

kobs, 10

6s

-1

0.2 0.25 0.3 0.35 0.4

kQ= 1.6x10

10M

-1s

-1

[Et4NBr], mM

kgrowth = k0 + krxn[Rt4NBr]

Analysis of the growth kinetics for Br2•

4.18

kgrowth

= k0 + k

Br–[Br–] + kX[X]

2

3

4

5

6

7

kobs, 10

6s

-1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

[2-propanol], M

kX= 3.9x107M-1s-1

[Br-]= 50µM

The kinetics for invisible reactions can be determined by

using the absorption from Br2• as a probe

4.19

Reate constants for reactions of bromine atoms

Methanol 0.93Ethanol 16

1-Pentanol 11 1-Octanol 12

2-Octanol 35

2-Propanol 393-Pentanol 12

2-Methil-1-propanol 17

Dioxane 1.2 Ether 17

Toluene 66

Triethilamina 29000 p-Cresol 30000

Quencher k , 10 M s6 -1 -1Q

4.20

One step further in the probe techniqueExamining triethylsilyl radicals

(k=5.7x106M -1s-1)

Et3 SiBr + R•Et 3Si• + R-Br

Signal carrierEt3 Si• + Probe kprobe

Bu tO•

krxn

Bu tOH + Et3Si•Bu tO• + Et3SiH

k0

Bu tOH + (solvent)•Bu tO• + solvent-H

CH3 COCH3 + CH3 •

O

OSiEt3•

Signal carrierderived from benzil

4.21

Time resolved diffuse reflectance

an alternate approach for opaque samples

Lamp

PMT Detector

Digitizer

Computer

Monochromator

Laser

Sample

4.22

Decay of diphenylmethyl on silicagel

+++++++++++

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++

+++++++++++

+

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Time

∆J/J

0

5 µs

100 µs

A

B

Diffuse reflectance

under nitrogenO

Ph

Ph

Ph

Ph

Hhν

-CO

•2

4.23

Complex kinetics are common in heterogeneous systems

+

OCH 3

OCH 3H3CO

TMT+

0.07

0.08

0.09

0.1

0.11

0.12

0.13

0.1 1 10 100 1000

∆ J/J

time, µs

1 10-3

2 10-3

3 10-3

4 10-3

5 10-3

6 10-3

7 10-3

0.1 1 10 100 1000 104

ab

un

da

nce

log (τ /µs)

(LEFT) Decay trace in different time domains of TMT+/HY film. The traces were monitored at 350 nm and the 2, 5, 10 and 20 µs time scales were used. Note the logarithmic time scale. (RIGHT) Distribution analysis of the TMT+/HY film.

LIFETIME DISTRIBUTION

4.24

Pulsed lasers

As part of a method to study photoinitiated chemical reactions

To induce new chemistry, different

from that initiated by conventional sources

As a light source, to initiate the same chemistry as with

conventional sources

Ground statereactants

Excited statereactants

ReactionIntermediates

Ground stateproducts