Photochemistry Reactions involving photons. (Radiation-induced chemical processes: chemical transformations induced by high energy photons. Radiochemistry (nuclear chemistry): processes in the nuclei of atoms.) Tamás Vidóczy Institute of Structural Chemistry Chemical Research Center, HAS

Electromagnetic spectrum important for photochemistry IR E UV ~700 nm ~400 nm E = hν = hc/λ ~200 nm VUV

Excited states and related bond strength

Multiplicity Name redived from: 2S + 1 S = 0 singlet S = ½doublet S = 1triplet

The Jablonski diagram E SS1S1 S2S2 T1T1 T2T2 involving a photon without photons singlet – triplet splitting

The basic law of photochemistry: only absorbed radiation can cause chemical change spectroscopic transitions are quantized - line spectra (in gas phase at low pressure), band spectra (in condensed phases)

Absorption E SS1S1 S2S2 T1T1 T2T2

Lambert – Beer law I = I εcl ε: decadic absorption coefficient unit: dm 3 mol -1 cm -1 T = I/I 0 T(%) = 100 I/I 0 A = -lg T = lg (1/T) = lg I 0 /I = εcl

Typical absorptions n →  *carbonyls, tiocarbonyls, nitro-, azo- and imino- group containing compounds  →  * alkenes, alkynes, aromatics n →  * amines, alcohols, haloalkanes  →  * alkanes

Absorption SS1S1 S2S2 T1T1 T2T2

Vibrational relaxation E SS1S1 S2S2 T1T1 T2T2

Deactivation channels of the singlet state E SS1S1 S2S2 T1T1 T2T2 ?

Fluorescence: emission without change of spin state E SS1S1 S2S2 T1T1 T2T2

IC: internal conversion E SS1S1 S2S2 T1T1 T2T2

ISC: intersystem crossing (spinváltó átmenet) E SS1S1 S2S2 T1T1 T2T2

Phosphorescence: emission with change of spin state E SS1S1 S2S2 T1T1 T2T2

Quenching Deactivation of an excited state with the help of another species. We investigate the process from the point of view of the excited species, the state of the quencher is irrelevant.

Deactivation channels of the excited singlet state 1M1M M + h ` k fl M k IC 3 M k ISC M (+ Q or Q*)k q +Q+Q M iso or M` + M``k mr MA or M + + A - k br +A

3M3M M + h `` k ph M k ISC` M (+ Q or Q*)k q +Q+Q M iso or M` + M``k mr MA or M + + A - k br +A Deactivation channels of the triplet state

Quantum efficiency  = number (rate) of chosen process number (rate) of photons absorbed

Quantum efficiency

1M1M M + h ` k fl M k IC 3 M k ISC M (+ Q or Q*)k q +Q+Q M iso or M` + M``k mr MA or M + + A - k br +A

Stern-Volmer plot 1 I 0 /I [Q]

Energy transfer Through radiation (trivial) Without radiation –long-range, coulomb-interaction (Förster) –short-range, electron-exchange (Dexter)

Trivial energy transfer Condition: the emission spectrum of the donor and absorption spectrum of the acceptor must overlap.

Long-range dielectric interaction The rate is proportional to the -6th power of the distance between donor and acceptor

Short-range, electron exchange interaction The rate is proportional to (e -r/l ) 2, r: the distance between donor and acceptor, l: van derWaals distance

Triplet-triplket energy transfer PHOTOSENSITIZATION