intro photochem

1

Introduction to photochemistry

J. C. Scaiano (Ottawa)

Basic principles of photochemistry including some selection rules, energy transfer processes and the properties of excited state reactions.

http://photo.chem.uottawa.ca and follow “teaching”

Much of the material in these classes is based on the books:

Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction; University Science Publishers: New York, N.Y., 2008.

Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Publishers: New York, N.Y., 2010.

1st NANOBIOPHOTONICS SUMMER SCHOOL UNIVERSITY OF OTTAWA

April 30-May 4, 2012

Co-organizers Hanan Anis (Engineering)- Tito Scaiano (Science)

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What Is Molecular Organic Photochemistry?

The field of molecular organic photochemistry can be conveniently classified in terms of:

•  the photophysics of organic compounds (the interactions of light and organic molecules resulting in net physical changes) and,

•  the photochemistry of organic compounds (the interactions of light and organic molecules resulting in net chemical changes)

Ground state reactants

Ground state products

hν

3

The overall reaction



hν

The overall reaction


Excited state reactants

Reaction Intermediates


hν

The next level of detail

The chemistry of excited states

4

A global paradigm

Reactive intermediate (I) that can typically be described as having the characteristics of a radical pair (RP), a biradical (BR), or a zwitterion (Z).

“F” for funnel, when a ‘real’ intermediate is not involved, will not be covered in any detail

When intermediates in the reaction are formed in an excited state, termed ‘adiabatic’

More than one type of *R excited state can be involved in a reaction

Simplification for a short course

5

Wavelength and color

Less energy, bonds vibrate More energy, bonds break

UV NIR & IR Visible region

Light absorption

6

The absorption of light is a pre-requisite for it to be able to cause chemical change.

This can be viewed as another expression of the laws of energy conservation.

Most molecules have all their electrons paired in their ground states, and the simplest (but not the only) effect of light absorption is the promotion of an electron from the HOMO to the LUMO.

Molecular oxygen and stable (or persistent) free radicals are exceptions of molecules that have unpaired electrons in their ground states.

A standard abbreviation:

Light absorption = hν

HOMO

LUMO

Light absorption

S0 S1

7

An Everyday Working Paradigm

8

Study questions

1. What are the electronic characteristics of the HO and LU involved in the R + hν →∗R process?

2. What is the electronic configuration of ∗R (i.e, the orbital occupancy of the HO and LU)?

3. What are the plausible primary photochemical and photophysical processes typical of ∗R based on its electron configuration (HO)1(LU)1?

4. What are the electronic natures of the NB orbitals of I?

5. What are the plausible secondary thermal reactions of I leading to P?

How much energy do photons carry?

• The energy of light depends on its wavelength, the shorter the wavelength, the higher the energy.

• When we count molecules we use Avogadro’s number. When we deal with light we measure photons in einsteins, which correspond to one mole of photons or 6.02 x 1023 photons. The energy associated with one einstein depends on the wavelength (or frequency) of light.

• When all the conversions are done its worth remembering a simple equation:

Relating the photon energy to the energetic requirements of the chemistry we want to initiate is essential to determine if the process is plausible.

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10





Singlet States

Triplet States

....

Sn

Tn

....

T1

S1

S0

Light absorption normally occurs with spin conservation.

For the vast majority of molecules this means the initially formed excited state is a singlet, not always the lowest one (S1).

hν

Can shorter wavelengths cleave a ‘photostable’ molecule?

electron spins"absorption"emission"non-radiative"

hν!

HOMO

LUMO

LUMO+1

LUMO+2

S0 S2 S1

In most cases upper excited states (higher than S1, such as S2, S3, ... Sn) relax rapidly to the lowest excited electronic state, S1 from which most photochemistry and photophysics (such as fluorescence) take place. This is known as ’s rule and is true for most molecules in solution.

A consequence of Kasha’s rule is that most molecules show wavelength-independent behavior. Sometimes molecules show wavelength dependence simply because different isomers or conformers can be excited at different wavelengths; this can be seen as a trivial case: different species showing different behavior.

relax fast

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12





Singlet States

Triplet States

T1

S1

S0

Kasha's Rule

hν

Light absorption normally occurs with spin conservation.

For the vast majority of molecules this means the initially formed excited state is a singlet, not always the lowest one (S1).

Note that upper states (S2 … Sn, T2 … Tn) are absent; this is because their lifetimes are usually very short and they relax to S1 or T1, which in most cases are the states that are responsible for chemical and spectroscopic properties.

13

Azulene and violation of Kasha’s rule

Kasha’s rule (a reminder from Wikipedia) Kasha's rule is a principle in the chemistry of electronically excited molecules. The rule states that photon emission (fluorescence or phosphorescence) occurs only from the lowest-energy excited electronic state of a molecule.

14





Singlet States

Triplet States

ki sc

Intersystem crossing from the first excited singlet state to the lowest triplet state

The importance of spin

Triplet states play a very important role in photochemistry, but they are usually derived from the initially formed singlet state.

In the first few slides we assumed that electron spin is conserved in all cases. This is not always the case, and quite frequently one of the fates of S1 is for one of the electron spins to change to produce a lower energy state called a triplet state, T1.

Let the electron spin change

S1 T1

The T1 state is at lower energy

(think Hund’s rule)

A different representation, the Jablonski diagram

S1

T1

S0

Note electron spin change

Changing electron spins is of course a forbidden process. For the moment we just need to know that processes are slowed down for this reason, and that there are mechanisms by which the total angular momentum can (and

must) be conserved. The change from S1 to T1 is called intersystem crossing (ISC).!

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16





T1

S1

S0

T1

S1

S0

small big

n,π* π,π*

hν

The singlet-triplet gap, ΔEST, is an important property of a molecule, small gaps are encountered for n,π* states and large ones for π,π* states.

17

The state energy diagram is a fundamental paradigm of modern molecular photochemistry

Jablonski diagram

The Jablonski diagram-everyday version

A remarkably useful tool, the Jablonski diagram lets us see the energetic, multiplicity and dynamic connections between electronic states of a molecule. Solid lines are always radiative processes color coded here as:

• Absorption • Fluorescence • Phosphorescence

Wavy lines always show radiationless processes, such as intersystem crossing and internal conversion (IC). The horizontal axis is used to show multiplicity. Only states of the same multiplicity are shown vertically aligned.

S1

T1

S0

IC

Multiplicity axis

The simplicity of the Jablonski disgram is mostly due to Kasha’s rule that most of the time lets us ignore all those upper electronic states. Sometimes we may find that even if a molecule obeys Kasha’s rule the upper

states do play an important role. Benzophenone and anthracene will be such examples.

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20





EXAMPLE: Butyrophenone: putting a face to R, I and P

krxn

H-atom transfered

Biradical generated

in the triplet state

Butyrophenone

O

CH3

O*

CH3

OH

CH2•

hν

21





Triplet biradical

Singlet biradical Enol

krxn

O

CH3

O*

CH3

OH

CH2•

OH

CH2•

OH

CH2CH2

EXAMPLE: Butyrophenone: Norrish Type II reaction

22





Product forming reactions

Final product

O

CH 3

OH

Ph

krxn

O

CH3

O*

CH3

OH

CH2•

OH

CH2CH2

O

EXAMPLE: Butyrophenone: the complete reaction

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EXAMPLE

Product forming reactions

Final product

O

CH 3

OH

Ph

krxn

O

CH3

O*

CH3

OH

CH2•

OH

CH2CH2

O

Back reaction reduces the quantum yield

24

Quantum yield

The number of molecules of I or P produced per photon absorbed is called the quantum yield () of the formation of a reactive intermediate (I) or a product (P).

Note the use of capital Φ for quantum yields

25

Quantum yields

Rate at which a process occurs of rate of formation of a product, or of disappearance of a reactant

Intensity of light, i.e. rate of light absorption

Φ =

From standard analytical techniques

Chemical systems or physical devices called actinometers allow the determination of the number of photons in an excitation beam

Note that the denominator refers to absorbed photons, not incident photons

26

Norrish I vs. II. Which one when?

Norrish II, γ hydrogens are essential

Norrish I, weak α C-C essential

Substitution at the α and γ position can determine competitive processes

27

Ground and excited states are different

IR frequency is 1665 cm-1 in the ground state

1222 cm-1 in the triplet state

O

Different geometries and different vibrational properties make them spectroscopically different

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Excited state energies The energy required to produce an electronically excited state

(R + hν → *R)

is obtained by inspecting the absorption or the emission spectrum of the molecule in question, as well as applying Einstein’s resonance condition for the absorption of light.

h is Planck’s constant (1.58 × 10−34 cal s = 1.58 × 10−37 kcal s), ν is the frequency (commonly given in units of s−1 = Hz), λ is the wavelength at which absorption occurs (commonly given in units of nanometers, nm), c is the speed of light (3× 108 cm s−1)

HOMO

LUMO

Light absorption

S0 S1

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Einstein as a unit

€

ΔE =28600

λ(in nm)= energy in kcal /mol

e.g. 28600400 nm

= 71.5 kcal /mol

Worth remembering!

Einstein = energy of a mole of photons

N0 = Avogadro’s number

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Relationship among energy, wavelength & frequency

Visible region = 400 to 700 nm

31

Energies, energies, energies

33

Time scales

short

long

34

Born-Oppenheimer approximation The BO approximation states that the motions of electrons in orbitals are much more rapid than nuclear vibrational motions. This approximation assumes that the light mass, rapidly moving electrons can immediately adjust their distribution to the positive potential of slowly moving, heavy massive nuclei.

The important consequence of this approximation is that it allows electronic and nuclear motions to be treated independently mathematically and makes it possible to compute a good first guess of, the “true” molecular wave functions of a molecule

€

Ψ ≈ Ψ0 •χ •S

Electrons Nuclei Spin

This approximation breaks down whenever there is a significant interaction between the electrons and the vibrations (called vibronic coupling) or between the spins and the orbiting electrons (called spin–orbit coupling).

35

Transitions in formaldehyde"

Transitions at relatively low (UV/Vis) energies involve:! (πCO

)2(n

O)2(π

CO* )

0

π!

n!

π*!

ground!state!

n,π*! π,π*!

standard abbreviations

36

Excited state properties determined by type of excitation

n,π* π,π*

Half-filled orbital localized on oxygen: species resembles an alkoxy radical

Transitions involve only the π system, no free radical properties expected. In aromatic ketones the aryl π system is usually involved

π

n

π*

The other labeling

S0

S1 (n,π*)

S2 (π,π*)

37

Formaldehyde orbital occupancy Recognizing standard state labels

38

Energies of the excited state of formaldehyde

Coulombic term (classical)

Electron exchange (Pauli)

Comparing acetone and benzophenone

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40





T1

S1

S0

T1

S1

S0

small big

n,π* π,π*

41





P h2C=O

CH2C=O

CH2=CH2

∆E(S-T)Type

π,π*

π,π*

π,π*

n,π* n,π*

70 40

35

10 7

kcal/mol

Each electronic state can be described in terms of a characteristic electronic configuration which in turn can be described in terms of HOMO and LUMO and in terms of a specific spin configuration, either a singlet or a triplet state.

42

Electronic Energy Difference between Molecular Singlet and Triplet States of ∗R

Hund’s rule

For organic photochemistry, Hund’s rule can be rephrased for MOs as follows: For molecules possessing two half-filled orbitals, one a HO and the other a LU, the triplet state (↑↑) is always of lower energy than the energy of the corresponding singlet state (↑↓) derived from the same electronic (HO)1(LU)1 configuration

43

Exchange interaction, J

CH2O

TRIPLET STATE: the average repulsion energy will be less than the repulsion computed from the classical model because of the tendency of electrons with parallel spins to avoid each other, and thus reduce electron–electron repulsions

44

Singlet triplet gap

The energy gap between a singlet and a triplet state of the same electronic configuration of half-filled orbitals (i.e., the same orbital occupancy) is purely the result of electron exchange and is responsible for the observation that the energy of a triplet state is generally lower than that of a singlet state of the same electronic configuration (the same orbital occupancy) for organic molecules.

n,π* for formaldehyde

45

Singlet-triplet splittings

46

Basics of carbonyl photochemistry

Basics of carbonyl photochemistry!

380 420 460 500 540Wavelength, nm

So

S1

T1

T2 Benzophenone: n,π* p-MeO-ketones: π,π*

Benzophenone!phosphorescence!

T1 state!

O

77 K glass

π,π*

n,π*

47

More on benzophenone

Jablonski diagram at 77 K

100%

~1012 s- 1

<107 106 s- 1

1011 s- 1

1.8 x 102 s- 1 (90%)

S0

T1 (n,π*)69 kcal

T2 (π,π*)S1 (n,π*)74 kcal

100 kcalS2 (π,π*)

380 420 460 500 540Wavelength, nm

77 K

48

Franck-Condon (FC) principle

The FC principle states that because nuclei are much more massive than electrons (the mass of a proton is ∼1000 times the mass of an electron), an electronic transition from one orbital to another takes place while the massive, higher-inertia nuclei are essentially stationary.

Expressed in quantum mechanical terms, the FC principle states that the most probable transitions between electronic states occur when the wave function of the initial vibrational state (χ1) most closely resembles the wave function of the final vibrational state (χ2).

ver

tica

l

Vibrational energy storage in FC transitions

49

When the ground and excited state geometries differ extensively, the original nuclear geometry of the ground state is a turning point of the new vibrational motion in the excited state, and that vibrational energy is stored by the molecule in the excited state. This vibrational energy is released as the excited states relaxes to a lower vibrational state. A

gain

, ver

tica

l

50

Colored objects A green object, such as a leaf, reflects only those wavelengths that create the visual effect of green. Other colors in the incident light are absorbed by the leaf.

Red light is absorbed by the chlorophylls, blue light is absorbed by carotenoids.

Source: CHM220 notes, Univ. of Toronto

A chromophore (“color bearer”) is defined as an atom or group of atoms that behave as a unit in light absorption. A lumophore (“light bearer”) is an atom or group of atoms that behave as a unit in light emission (fluorescence or phosphorescence). Typical organic chromophores and lumophores are the common organic functional groups, such as ketones (C O), olefins (C C), conjugated polyenes (C C C C), conjugated enones (C C C O), and aromatic compounds (benzene ring and condensed benzene rings).

51

Oscillator strength

The oscillator strength f is defined as a measure of the intensity or probability of an electronic transition that is induced by light.

Bottom line: big extinction coefficient → short radiative lifetime, possibly high fluorescence quantum yield

52

Electronic spectra Examples Atoms Molecules Gas phase Molecules solution

53

S0 → S1 transitions

The ‘real’ lifetime is frequently much shorter, as a result of radiationless transitions that

contribute to excited singlet decay.

54

More on Franck-Condon principle

The anthracene case

Absorbance Fluorescence

Wavelength, nm

S1 T2

S0

IC T1

Vibrational states play an important role in absorbance and in emission

The absorption and emission spectra of anthracene reveal features beyond the “one line state” Jablonski diagram; these are due to the vibrational states of anthracene (the same ones we see in an IR spectrum).

Anthracene is unusual in thet the 0,0 bands for absorption and emission coincide almost exactly.

55

Fluorescence

Visible light

Cyclohexane with (right) and without (left) anthracene

UVA light

How does fluorescence look?

Fluorescence is light emitted when an electronically excited state relaxes to a lower state (usually the ground state) of the same multiplicity.

56

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Mirror image rule Emission spectrum is typically a mirror image of the absorption spectrum of the S0-S1 transition, but shift to longer wavelengths

•  Same electronic transition being involved in both absorption and emission and the similarities of the vibrational energy levels of S0and S1

•  In many molecules vibrational energy levels are not significantly altered by the different electronic distributions of S0 and S1

Source: Ajayaghosh web notes

58

Stokes shift

Owing to a change in dipole moment of the molecule in its excited state compared to that of the ground state, the energy difference between S0 and S1 is lowered prior to fluorescence emission (in about 0.1 to 100 ps). This is the Stokes’ shift.


G.G. Stokes (1819-1903)

Not a Jablonski diagram

Excited singlet relaxation

59

Stokes shift: example


When electrons go from the excited state to the ground state there is a loss of vibrational energy.

60

The example of terphenyl

Source: Ajayaghosh notes on web

The absorption spectrum of p-terphenyl is devoid of structure, but the emission spectrum shows vibrational structure The deviation from the mirror image rule usually indicates a different geometric arrangement of nuclei in the excited state and the ground state

In the case of p-terphenyl, the individual rings become more coplanar in the excited state. As a result, the emission spectrum is more highly structured than the absorption spectrum. This is unusual, the opposite is generally observed.

61

Chromophore and substituent: who’s who?

62

Absorption and emission

63

Extinction coefficient measuring “how allowed” a transition is

64

Excitation spectra

The wavelength at which emission is monitored should always be reported with excitation spectra

65

Excited state character

66

Vibrational structure

It is rare to observe this level of detail at room temperature

Energy gap law

The decay rate of excited electronic states in a large molecule or of an impurity in a solid is calculated for a model of a large number of displaced harmonic oscillators. The rate depends exponentially on the energy difference (‘gap’) between the initial and final electronic states.

Englman R & Jortner J. The energy gap law for radiationless transitions in large

molecules. Mol. Phys. 18:145-64, 1970.

In the absence of a Zero Order surface crossing between S1 and S0, an S1→ S0 internal conversion must occur via a "Franck-Condon forbidden” mechanism, i.e., the nuclei in one state must undergo a rather drastic change in position and momentum as a result of the transition, since the net overlap of vibrational wave functions in both states is small. For such situations, the S1 → S0 internal conversion is generally rate-limited by the Franck-Condon factor, < χ |χ>2 = fν.

67

Energy gap law applies to radiationless transitions"

ki c

= 1013

e- α ∆E

(s ec- 1

)

Singlet States Triplet

States

Ground Singlet State

ISC

Frank Condon factor: "Proportional to the overlap"of the wavefunctions for"the initial and final states"

fv ≅ exp(- α∆E)

68

Fig. 7.1: Energetically favorable conditions for the energy-transfer process;

∗R +M→R + ∗M The darker lines indicate the lowest vibrational level for each electronic state, and the lighter lines indicate the excited vibrational levels for each electronic state. Some vibrations Of M are excited in order to conserve energy during the energy-transfer step.

Energy Transfer

69

Energy transfer schematically

This example involves electron exchange

70

Trivial energy transfer also known as radiative energy transfer

D* D + hν

A*A + hν*D A A

*D A B

•  no electronic interaction between D* and A

•  D* emits a quantum of light which is absorbed by A

A physical encounter between A and D* is not required, the photon must only be emitted in an appropriate direction and the medium must be transparent in order to allow transmission.

71

donor*! acceptor!

Exchange energy transfer also known as orbital overlap, collisional, and Dexter energy transfer

donor! acceptor*!

electron clouds of D* and A overlap in space and electron exchange occurs in the region of overlap

72

Electron exchange processes •  Energy transfer

•  the only mechanism in some cases

•  Triplet-triplet annihilation

•  Charge transfer

•  Charge translocation

•  K is related to specific orbital interactions like overlap dependence on specific orientations of D* and A

•  J is the normalized spectral overlap integral, where normalized means that both the emission intensity (ID) and extinction coefficient (εA) have been normalized to unit area on the wavenumber scale

•  J, by being normalized does not depend on the actual magnitude of εA

•  rDA is the donor-acceptor separation relative to their van der Waals radii, L

•  By being defined in this manner rDA corresponds to the edge-to-edge separation

A theory of energy transfer by electron exchange was worked out by Dexter:

€

kET (exchange) = KJe−2rDAL

#

$ %

&

' (

73

Coulombic energy transfer also known as dipole-dipole, resonance, and Förster energy transfer

donor*! acceptor! donor! acceptor*!

A transmitter-antenna mechanism for energy transfer

74

Coulombic energy transfer The dipole-dipole interaction represents a classical Coulombic interaction so we can make a classical analogy

D* A

the electric field around an excited

molecule D* behaves like a field generated

by a classical oscillating dipole

the electrons in the ground state of A are assumed not to be

oscillating at all Coulombic interaction

•  the oscillating field of D* causes the excitation of nearby electronic systems (provided certain resonance conditions are met)

•  this is analogous to absorption of a photon by A to generate A* as a result of coupling between Ae- and and the oscillating electric field of the light wave

∴ This mechanism will be most plausible for S-S energy transfer because multiplicity conserving transitions have large transition dipoles

75

Coulombic or Förster mechanism

electrostatic interaction energy (E) between 2 dipoles is directly related to their magnitude and the distance between them...

€

E(dipole − dipole)∝ µDµA

RDA

3

* in this case we use the center-to-center distance, RDA

Förster related the dipole moments to the oscillator strengths of the transitions:

•  quantification of E in terms of measured oscillator strengths, ƒ

•  ƒ is a measured property of a real system (electronic, vibrational, and spin factors) and is

related to the inherent radiative lifetime and the extinction coefficient

rate of energy transfer is related to the interaction energy according to:

€

kET dipole − dipole( )∝ E 2 ≈µDµA

RDA

3

%

& '

(

) *

2

=µD

2µA

2

RDA

6

76

Coulombic or Förster mechanism

Rate constant for any separation

€

kET ∝ kDRDA

0

RDA

#

$ %

&

' (

6

=1τ D

RDA

0

RDA

#

$ %

&

' (

6

Efficiency for any separation

€

φET ∝RDA

0

RDA

$

% &

'

( )

6

The rate constant and efficiency of ET can be easily related to the separation distance if we first define RDA0 as

the critical distance where the rate of ET is equal to the inherent rate of deactivation...

€

kETA[ ] = kD at RDA = RDA

0 RDA0 = 6.5[A]1/3

taking into account geometry and assuming D* and A are

spherical

77

The diffusion limit Debye’s equation provides an easy approximation

An upper limit for kET

The units of kET are usually M-1s-1

A typical value for kdif in a fluid solvent at room temperature is around

1010 M-1s-1

78

A few useful numbers, not a lot to say

Based on Debye’s equation

79

Light in biology

Wavelength ranges are labeled in photobiology as UVA, UVB and UVC

•  UVA : 315 to 400 nm

•  UVB : 280 to 315 nm

•  UVC : below 280 nm.

In some cases the boundary between UVA and UVB is placed at 320 nm.

80

Redox properties of excited states

"vacuum"

LU

HO

Reduction Oxidation

groundstate

groundstate

Excitedstate

Excitedstate

EA IP

E*

81

Redox properties of excited states

Excited states of diamagnetic molecules with

closed shell ground states are always better

oxidizing and reducing agents than their

corresponding ground states

This is not necessarily true of species with open shell ground states such as radicals

Important take home message

82

intro photochem

Documents

energy of light

r excited state

absorption of light

ground states

interactions of light

effect of light absorption

r h r process

exceptions of molecules