Fluorescence and Phosphorescence from Higher Excited States of

Organic MolecuelsItoh, T. Chem. Rev. 2012, 112, 4541-4568.

Alexander J. Kendall

Tyler Lab Group Meeting

5/21/2014

Outline

• Fundamental principles • Born-Oppenheimer approximation

• Frank-Condon principle

• Kasha’s rule

• Breaking the rules• Identification of higher state luminescence

• Fluorescence from higher states

Born–Oppenheimer Approximation

• ΨTotal= ψelectronic x ψnuclear

• ψ electronic = electron derived wave function

• ψ nuclear = nuclear derived wave function (vibrational, rotational, translational)

• ĤΨ = EΨ

• ETotal= Eelectronic + Evibrational + Erotational + Etranslational

Photochemical Rules

• Grotthus-Draper Law• A photon must be absorbed to do photophysics or photochemistry.

• Stark-Einstein Law• The primary photochemical act involves absorption of one photon by one

molecule.

4Coyle, J. D. Introduction to Organic Photochemistry; Wiley: Great Britian, 1991.

Organic Absorption

• Frank-Condon Principle• Electrons move fast compared to

nucleons

• Coupled electronic- and vibronic-transitions

5Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Organic Absorption and Emission

• Frank-Condon Principle• Electrons move fast compared to

nucleons

• Coupled electronic- and vibronic-transitions

• Morse potential

6Coyle, J. D. Introduction to Organic Photochemistry; Wiley: Great Britian, 1991.

Organic Fluorescence• Jablonski diagram

• Selection rules• ∆S = 0• g → u or u → g• ν → ν

• Kasha’s rule• In solution, only

observable luminescence• S1→S0

• T1→S0

• Vavalov’s Rule• ΦF independent of

excitation wavelength

7Kasha, M. Faraday Discuss. 1950, 9, 14. ; Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Turro, N. J.; Scaiano, J. C.; Ramamurthy, V. Principles of Molecular Photochemistry: An Introduction; 1st edition.; University Science Books: Sausalito, Calif, 2008.

Kasha’s Rule (Fluorescence)

S2

S1

S0

• Photon absorption

Kasha, M. Faraday Discuss. 1950, 9, 14. ; Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Kasha’s Rule (Fluorescence)

Kasha, M. Faraday Discuss. 1950, 9, 14. ; Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

S2

S1

S0

• Vibrational relaxation

• Internal Conversion

Kasha’s Rule (Fluorescence)

Kasha, M. Faraday Discuss. 1950, 9, 14. ; Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

S2

S1

S0

• ν0 of S2 close to ν0 S1

ν0

ν0

Kasha’s Rule Napthalene

Kasha, M. Faraday Discuss. 1950, 9, 14. ; Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

S2

S1

S0

Non-Radiative Decay

Turro, N. J.; Scaiano, J. C.; Ramamurthy, V. Principles of Molecular Photochemistry: An Introduction; 1st edition.; University Science Books: Sausalito, Calif, 2008.

Non-Radiative Decay

Turro, N. J.; Scaiano, J. C.; Ramamurthy, V. Principles of Molecular Photochemistry: An Introduction; 1st edition.; University Science Books: Sausalito, Calif, 2008.

Rates of Relaxation• Radiative decay• Fluorescence (10-

12 to 10-6)

• Phosphorescence (10-6 to101)

• Non-radiative decay

• Internal conversion

• Vibrational relaxation

• Intersystem crossing

• Energy transfer

• Photochemistry

Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Identifying Higher State Luminescence • Abnormal or unidentified emissions

• Fluorescence or Phosphorescence• Si → S0

• Tj → S0

(i,j > 2)

Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Turro, N. J.; Scaiano, J. C.; Ramamurthy, V. Principles of Molecular Photochemistry: An Introduction; 1st edition.; University Science Books: Sausalito, Calif, 2008.

Napthalene and Azulene

Multiple Fluorescence

• So far, single fluorescence dominates photophysics

• Now, multiple fluorescence states compete!

Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

• Dual fluorescence

• ΦF(S2)/ΦF(S1)• Dependent on temp.

• ρ(S2)/ ρ(S1)

• S2 → S1

• k21 = (2π/ħ)V2ρ(S1)

• V = coupling constant

• ρ(S1) = density of vibronic states

Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

• Dual fluorescence

• ΦF(S2)/ΦF(S1)• Dependent on temp.

• ρ(S2)/ ρ(S1)

• S2 → S1

• k21 = (2π/ħ)V2ρ(S1)

• V = coupling constant

• ρ(S1) = density of vibronic states

Poly-enes• Dual fluorescence ΦF(S2)/ΦF(S1)

• S1→S0

• S2→S0

• n = 3,4• Temperature dependence

• Thermal population of S2

• n = 5• Slightly temp. dependent

• n = 6,7• Temp. independent

• Prompt fluorescence S2

n = 3,4 n = 6,7

n = 5

Summary

• Photophysics is complicated• Fundamental rules derived from quantum mechanics

• Explain the majority of observed photophysics in molecules

• Multiple excited state fluorescence is uncommon• Leads to higher energy emissions (fluorescence and

phosphorescence)

• Difficult to purify materials

• Difficult to identify and study higher energy emissions

• Difference in electronic and vibronic overlap determines rates of conversions

• Use temperature, pressure, lasers, mirrors, photon counters, etc.

Questions

Itoh, T. Chem. Rev. 2012, 112, 4541-4568.

Alexander J. Kendall

Tyler Lab Group Meeting

5/21/2014