Chap. 12 Photochemistry Photochemical processes Jablonski diagram 2nd singlet excited state 3rd triplet excited state 1st singlet excited state 2nd triplet excited state 1st triplet excited state Ground state 1. Light absorption: S 0 S 1, S 0 S 2 k~10 15 2. Vibrational Relaxation: k~10 12 /s, from high ν to low ν 3. Internal Conversion: to lower electronic state of same multiplicity (k>10 10 /s ) 4. Radiationless Decay: S 1 S 0, no emission, k<10 6 /s 5. Intersystem Crossing: k 10 6 ~10 10 /s, depends on molecules. (carbonyl fast; alkene slow) 6. Fluorescence: S 1 S 0, with emission. k 10 6-10 9. 7. Phosphorescene: T 1 S 0 with emission. k 10-2 10 4 8. Triplet Triplet Absorption 9. Singlet Singlet Absorption 10. Singlet Triplet Absorption
Photophysical Processes S 0 (ground st.) of 2 C= : [(1S ) 2 (1S C ) 2 (2S ) 2 (σ C- ) 2 (σ C- ) 2 (σ C- ) 2 ](π C- ) 2 (n ) 2 S 1 (1 st exc.state): [(1S ) 2 (1S C ) 2 (2S ) 2 (σ C- ) 2 (σ C- ) 2 (σ C- ) 2 ](π C- ) 2 (n )(π* C- ) S 2 (2 nd exc.state): [(1S ) 2 (1S C ) 2 (2S ) 2 (σ C- ) 2 (σ C- ) 2 (σ C- ) 2 ](π C- )(n ) 2 (π* C- ) C π* c-o n π c-o n π* π π*
UV Absorption and Emission Factors determining radiative transition: 1. Symmetry of electronic state (ini. final state) 2. Multiplicity of the spin Spin-orbit interaction (allows different spin transition mixing due to the mixing of magnetic moment of e - and the magnetic moment of the nucleus) eavy atom effect: higher rate of intersystem crossing Greater mixing if S and T are closer in energy, example carbonyl cpds. 3. Frank-Condon term, determined by overlap of nuclear coordinate of init. and final state Vertical transition Internuclear distance
Frank-Condon Principle At the instant of excitation, only electrons are reorganized, the heavier nuclei retain the ground state geometry The excited state has similar molecular geometry as ground state Anthrcene (1): Vib. energy diff. of S 0 (2): (0,0) transition (3): Vib energy diff. of S 1 (1) (2) (3)
The excited state geometry quite different from ground state geometry => large Stokes shift (anti-stokes shift: the fluorescence is at shorter wavelength) Fluorescence Stokes shift λ * planar geometry
Measurement of Absorption Beer-Lambert Law: log =A (absorbance) I 0 : incident light I t : transmitted light є: extinction coefficient c: concentration d: light path length I I ο t =єc d Quantum yield of emission: Ф f = # 1 of # photon emitted from S of photon absorbed
Phosphorescence Much reduced due to diffusional quenching with ground state species or 2 bserved in fixed matrix, such as liquid N2 temperature or surrounded by a host Br In aqueous solution with α-cd Concentration Dependent Fluorescence In heptane A*A AA* Excimer formation Excimer: A complex formed between an excited molecule with a ground state molecule of same compound
Geometric Requirement of Excimer Formation The molecular plane can stack together with interplanar distance less than 3.5 Å. C 2 (C 2 ) n C 2 n=0 no overlap of ring n=1 excimer formation n=2 strain of chain C 2 C 2 C 2 C 2 C 2 C 2 Excimer formation Partial overlap of ring plane
Exciplex: complex formed between an excited molecule with a ground state molecule of dissimilar molecule A*B AB* can give exciplex emission or quench emission N(C 2 C 3 ) 2 aromatics / amine aromatics/conjugate olefin quench by C 3 N C C 3 C 3 Fluorescence quencher Geometric requirement less stringent
Energy Transfer and Electron Transfer D*+A D+A* Pathways 1) Radiative energy transfer D* D+ A+ A* The rate depends on The quantum yield of emission by D* (Ф D e ) The concentration of (the # of) A in light path The light absorbing ability of A (extinction coefficient) The overlap of emission spectrum of D* and absorption of A (spectral overlap integral) 2) Förster energy transfer Long range (D*-A distance up to 100 Å) No radiation involved The dipole-dipole interaction of D* and A Coulomb interaction An interaction at a distance via electromagnetic field, induce a dipole oscillation in A by D*. Efficient transfer requires a good overlap of emission of D* with absorption of A.
3) Collisional energy transfer (Dexter energy transfer): exchange of electron between the donor and acceptor electron exchange The exchange of electron via overlap of electron clouds require physical contact between the interacting partners. Spectral overlap integral also required This process allows triplet state to be generated D* + A 0 D 0 + A* A short-ranged interaction Electron Transfer The photo excited state is a better donor (lower oxid. potential) as well as a better acceptor (lower reductive potential) relative to ground state
Acidity and Basicity in Excited States - A- A - + + [A-]* [A - ]* + +
Δ*+ = Δ+ Δ*-Δ = + - ΔG*-ΔG (if ΔS* ΔS for ionization) ΔG = 2.303RTpK G *- G " pk*-pk= 2.303RT ' 2.303RT (1) (2) (3) (1):more acidic (2):less acidic (3):more acidic
When photochem. excited, electron from M LUM, and change the e - density
LUM M LUM C M
M LUM 3 C N C 3 N + 3 C π n π* C
Bond angle, Dipole moments of Excited state * C C LUM M C C C π* n π
(allowed, strong) (forbidden, Weak) S 0 S 1 (n-π*) excited state less polar than the ground state hypsochromic (blue) shift with polar solvent S 0 S 2 (π- π*) excited state more polar than the ground state bathochromic (red) shift with polar solvent
Photochemical Reactions of Carbonyl Compounds Sat d Ketone 3 * 1 * n π* * C diradical character or C C Norrish Type I Cleavage (α- cleavage) R R R R * C R + R R R + C
gas phase more energetic lose C readily C 3 (C 2 ) 2 C=C 2 +C disproportionation + C 2 C = C 2 + 3 CC = C 2 +C fragmentation S 1 T 1 at vibrationally excited state of T 1 2 C=C(C 2 ) 3 C solution C C Ketene R R Less energetic; no C loss 2 C acid
ydrogen Abstraction Reaction C * -A + A Ph Ph Ph Ph Ph Ph 3 C + + Ph Ph 3 C Photoreduction of benzophenone in ipa Ph Ph Ph Ph 3 C 3 C Norrish Type II Cleavage(β-Cleavage) C 2 R C 2 C 2 α β R C 2 C 2 α γ C 2 β R Stable radical + C 2 C 2 6-membered ring (T.S.) R C 3
R C 2 R If the S.M. is retrieved, the γcarbon may loose stereochem. (if chiral), so not exactly the same original S.M. R C 3 α-β unsaturated ketone Absorb at longer wave length Photo-driven De-conjugated absorb at shorter wave length
xetene Formation (Paterno-Buchi Reaction) * + Ar Ar + + Photochemical Reactions of Alkene and Dienes Isomerization 1. Trans compound has longer absorption C C wavelength C C π π* λ 2. Both cis and trans max < 200nm 265 nm give the same higher є excited state species Cis trans => twisted geometry with 90 o rotation of lower є p-orbital relative to each other λ Ar
R 1 R 3 C C R 2 R 4 R 1 R 1 R 3 R R 4 2 R 2 R 4 R 3 3. A photostationary state will be reached from either side (cis photostationary state; trans photostationary state) [ C] [ T ] pss pss εt εc kc kt k c =formation constant of cis from the excited twisted state k t =formation constant of trans from the excited twisted state
(C 2 ) n xylene (C 2 ) n * strained - + + R R, - + R For n=4,5,6 (C 2 ) n - + (C 2 ) n (C 2 ) n + -catalyzed hydration + + 89% 2% + -catalyzed hydration
For cyclopentene, the cis-trans isomerization doesn t occur. C 3 C 2 by abstraction of the diradical species + + 2 The photochemically allowed reaction by symmetry rule may be only one of many reaction pathways 193nm thermally allowed other mechanism + + Symmetry-allowed dis. rot. product