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