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MMP+ Chapter 5

MMP+ Chapter 5. Photophysical Radiationless Transitions. Kathy-Sarah Focsaneanu November 28, 2002. 6.2 A Classical Interpretation of Radiationless Electronic Transitions as Jumps between Surfaces. radiationless “jumps” occur at critical nuclear geometries, r c

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MMP+ Chapter 5

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  1. MMP+ Chapter 5 Photophysical Radiationless Transitions Kathy-Sarah Focsaneanu November 28, 2002

  2. 6.2 A Classical Interpretation of Radiationless Electronic Transitions as Jumps between Surfaces • radiationless “jumps” occur at critical nuclear geometries, rc • probability of surface jump @ rc is P ~ e-(∆E/ s)

  3. 6.3 Wave Mechanical Interpretation of Radiationless Transitions between States • adiabatic (Born-Oppenheimer) approximation: simplifies to motion of nuclei only • treat nuclei classically; electrons as waves 2 1 2  1 Initial  mixing near rc  Final • mixing is needed to produce the “jump”; otherwise, the point will continue along original surface • frequency of “resonance” called electronic tautomerism, where •  = ħ/∆E ~ 10-13/∆E s <<< ∆ “resonance region”

  4. point passes through unperturbed • if E of 1 2 coupling is < Evib, may consider E’~0 • typical for  or  bond breaking • no Z.O. linkage: • no dynamic coupling near rc • jump probability varies inversely with how strongly the crossing is avoided • occur most readily when there is little geometry change

  5. 6.4 Formulation of a Parameterized Model of Radiationless Transitions • processes must be isoenergetic • radiationless transitions enduced by: • mixing of n and  orbitals by out-of-plane vibrations (see Fig 6.6) • spin-orbit coupling, where a force is required to change the spin; this force must act while the point is near rc Selection Rules 1. 1n, *3, * allowed 2. 1n, * 3n, * not allowed 3. 1, * 3n, * allowed 4. 1, * 3, * not allowed El-Sayed’s Rules for S1T n,*  n, * Forbidden n, * , * Allowed ,*  , * Forbidden T1 S0 n,*  n2 Allowed ,*  2 Forbidden

  6. 6.5 The Relationship of Rates and Efficiencies of Radiationless Transitions to Molecular Structure Vibrational “promoters” of radiationless transistions: -Loose bolt:strong vibration in another part of the molecule -Free Rotor: twisting of a bond; efficiency  constraint within molecule and within the environment Matching Surfaces: -no intersection means no opportunity to mix -probability is poor, e.g. S1S2dris very small

  7. 6.6 Factors that Influence the Rate of Vibrational Relaxation • transfer of excess energy to the environment (solvent) is fast because the solvent behaves as a heat bath • electronic motion and position change • local excited vibration • electronic-vibrational radiationless transition • excess energy is transferred through the molecule to surrounding solvent molecules

  8. process = kprocess kprocess + kcompeting processes 6.7 The Evaluation of Rate Constants for Radiationless Processes from Quantitative Emission Parameters • measurement of lifetimes and quantum yields allows calculation of rate constants

  9. Sn Tn kSS IC kTT IC S1 T1 kIC phosphorescence fluorescence S0 6.8 Internal Conversion (Sn S1, S1 So)  absorption (S0  Sn) krad S0  Sn F knonrad Sn  S1 kST Zero Order crossings are common above S1  IC from Sn is easy! (Kasha’s rule) Ermolev’s Rule: F + IC + ST = 1 or 1 – (F + ST) ~  Deuterium Effect: -switching C-D for C-H  wavenumber -as a result,  thus IC  and F & S 

  10. 6.9 Intersystem Crossing from S1 to T1 • the S1 to T1 transition can occur via: • -direct S1 coupling to upper vib’l levels of T1 • -coupling of S1 to Tn, followed by rapid Tn to T1 IC • variation in size of kST from • -amount of electronic coupling between S and T • -size of energy gap between S and T • -amount of spin-orbit coupling between S and T • Temp dependence • -kraddoes not vary with temp, but knonraddoes • kST obs = kSTo + Ae-E/RT • -F and S thus vary with temp, but not at T < 100 K (energy term is less significant) • Triplet Sublevels • -ISC occurs from an individual sublevel • -processes from different sublevels have different rate constants

  11. 6.10 Intersystem Crossing (T1 So) • Size of kTS varies with E(T1) • Excess energy dissipated through C-H vibrations • Deuterium effects: • -more significant than in the singlet • -large T1 to S0 gap: smaller frequency for C-D stretch means that many more vibrational quanta are needed • -inhibition of ISC (enhancement of phosporescence?) • Temp effects: kTS relatively independent of temp • Triplet sublevels: k(T+S0), k(T0S0), k(T-S0) may be resolved at 4K T1 phosphorescence kTS S0

  12. 6.11 Perturbation of Spin-Forbidden Radiationless Transitions • Heavy Atom effect: • -kST, kTS, kP increased by adding a heavy atom, kF, kIC unchanged • -again, phosphorescence is a trade-off between kTS and kP • -i.e. who wins? P or TS? • External Perturbation: • -outside influence on spin-orbit coupling and energy transfer • -kST obs = kST + kST-X[X] (pure + perturbation by X)

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