Photochemistry Chemical reactions involving photons This course material is supported by the Higher Education Restructuring Fund allocated to ELTE by the Hungarian Government Ernő Keszei Eötvös Loránd University, Institute of Chemistry

IR UV ~ 700 nm ~ 400 nm ~ 200 nm VUV Electromagnetic spectrum relevant for photochemistry wavelength energy / frequency ~ 1.77 eV ~ 3.1 eV ~ 6.2 eV ~ 170 kJ/mol ~ 300 kJ/mol ~ 600 kJ/mol ~ cm –1 ~ cm –1 ~ cm –1

Electromagnetic spectrum 3/2 RT at 25°C wavelength, nm frequency, s –1 energy, kJ/mol – 2 10 – v i s i b l e infraredmicrowaveultravioletX-ray γ radiation

Unit conversions in photochemistry λ : wavelength ν : frequency (per second) E : energy from → to eVcm –1 kcal/molkJ/molHz (s –1 ) eV x cm – x 10 – x kcal/mol x kJ/mol x Hz (s –1 ) x 10 – x 10 – x 10 – x 10 –13 1

Excited states and related bond strengths S1S1 T1T kJ/mol VACUUM ULTRAVIOLET INFRARED V I S I B L E 700 nm 400 nm 300 nm adapted from Turro 1991

Basic law of photochemistry spectroscopic transitions are quantized: line spectra (in gas phase) Only absorbed radiation induces chemical reaction wavelength absorbance wavelength absorbance band spectra due to broadening (in condensed phase) adapted from:

Lambert – Beer law Transmitted intensity: I = I 0 10 –εcℓ ε : decadic absorption coefficient usual unit: dm 3 mol –1 cm –1 c : concentration; dm 3 mol –1 ℓ : optical path length; cm Transmittance: T = I / I 0 I 0 : incident intensity Absorbance: A = –log 10 (I / I 0 ) = log 10 (1/T ) A = εcℓ

S = 0 singlet (S i ) S = ½doublet (D i ) S = 1triplet (T i ) Multiplicity of electronic states S : spin quantum number Terms derived from: 2 S + 1

E S1S1 S2S2 T1T1 T2T2 photon absorption or emission no photon involved Jablonski diagram S0S0 Transitions:

E S1S1 S2S2 T1T1 T2T2 Absorption S0S0 S → T : forbidden transition (  10 8 times smaller probability)

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

E S1S1 S2S2 T1T1 T2T2 Vibrational relaxation S0S0 always radiationless (”dark”) typically to vibrational ground state

E S1S1 S2S2 T1T1 T2T2 Singlet state deactivation channels S0S0 Let us excite first… the excited state typically relaxes

E S1S1 S2S2 T1T1 T2T2 Fluorescence: spin conserving emission S0S0 from the vibrational ground state to several vibrational states

E S1S1 S2S2 T1T1 T2T2 Internal conversion ( IC ) S0S0 IC IC spin conservation

E S1S1 S2S2 T1T1 T2T2 Intersystem crossing ( ISC ) S0S0 ISC ISC spin change

E S1S1 S2S2 T1T1 T2T2 Phosphorescence: emission with spin change S0S0 from the vibrational ground state to several vibrational states

ISC & phosphorescence: potential energy curves absorption & relaxation ISC : S 1 → T 1 relaxation phosphorescence: T 1 → S 0 S1S1 → InterSystem Crossing: InterSystem Crossing: transition at the crossing of S 1 and T 1 adapted from Keszei 2006

Quenching Any process which deactivates the excited state of a given substance – thus avoiding the possibility of radiative transition. In a broader sense, fluorescence quenching is any process which decreases or completely extinguishes the fluorescence intensity. Example: naphtalene*(S 1 ) + phenanthrene(S 0 ) → naphtalene(S 0 ) + phenanthrene*(S 1 ) Phosphorescence quenching is a similar but less common process.

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 ur MA or M + + A – k br +A

Deactivation channels of the excited triplet state 3M3M M + h  ′′ k ph M k ISC′ M (+ Q or Q*)k q +Q+Q M iso or M′ + M′′k ur MA or M + + A – k br +A

Quantum yield The number of times a specific event occurs per photon absorbed by the system

Quantum yield Example: fluorescence yield

Quantum yield etc… exceptions…

Stern-Volmer plot 1 [Q] A method to use if our only ”computer” is a straightedge ruler

Fluorescence lifetime Pseudo-first order reaction (no quencher): Pseudo-first order reaction (with quencher): Solutions for t = 0 → [S 1 ] = [S 1 ] 0 : no quencher: quencher:

Stern-Volmer plot substitute Equivalent formulations: ∑k : fluorescence decay constant; τ 0 : fluorescence lifetime; k q : quenching rate constant

Suppose σ 2 (I 0 ) = σ 2 (I ) = σ 2, thus corr (I 0, I ) = σ 2, Problems with the Stern-Volmer plot Error propagation: =1

Suppose σ 2 (I 0 ) = σ 2 (I ) = σ 2, thus corr (I 0, I ) = σ 2, Problems with the Stern-Volmer plot Let us choose I 0 = 1, σ = 0.03 → σ 2 = ; I = 0.1…1

, Problems with the Stern-Volmer plot I fl I fl,0 = 1 error of I fl and I fl,0 = 0.03 Steep increase of error below I fl = 0.6 → above I fl,0 / I fl = 1.67 I fl,0 / I fl Keszei 2015

, Problems with the Stern-Volmer plot 1 [Q] Steep increase of error above I fl,0 / I fl = 1.67 How to avoid this large distortion? Do not use Stern-Volmer plot ! Estimate the parameters of the original I fl − [Q] function: Parameters: α k fl, ∑k and k q We more frequently use computers than straightedge rulers. Keszei 2015

Problems with the Stern-Volmer plot Estimate the parameters of the original I fl − [Q] function: Parameters: α k fl, ∑k and k q I fl [Q] Here, the error of I fl is the same as that of the experimental error of measuring the intensity I fl — no distortion occurs Keszei 2015

Resonance energy transfer (RET) Via radiation a real photon is emitted and absorbed between molecules further apart than the wavelength –radiative transfer from donor to acceptor Without radiation no real photon is emitted and absorbed between molecules closer than the wavelength of light –long range (Coulomb) interaction (Förster) –short range, electron exchange (Dexter)

Radiative energy transfer D* + A → D + A* A photon emitted by a molecule D (donor) is absorbed by a molecule A (acceptor). D*→ D + hν The emission spectrum of the donor and the absorption spectrum of the acceptor must overlap. mechanism: hν + A → A* Turro 1991

Radiative energy transfer The emission spectrum of the donor and the absorption spectrum of the acceptor must overlap. overlap adapted from Keszei 2006

Non-radiative energy transfer Non-radiative transfer occurs without emission of (real) photons, although, as any electromagnetic interaction, it is still mediated by so-called virtual photons Singlet-singlet transfer: all types of interactions 1 D* + 1 A → 1 D + 1 A* Triplet-tiplet transfer: only orbital overlap 3 D* + 1 A → 1 D + 3 A*

Long-range dielectric interaction The initially excited electron on the donor D returns to the ground state orbital on D, while simultaneously an electron on the acceptor A is promoted to the excited state. Being a dipole-dipole interaction, the rate is proportional to the inverse 6 th power of the intermolecular distance Non-radiative transfer by dipole-dipole interaction acts at distances up to nearly 20 nm (Förster Resonance Energy Transfer — FRET) 1 D* 1 A 1 D 1 A* adapted from Turro 1991

Short-range electron exchange interaction Non-radiative transfer by orbital overlap interaction The exchange corresponds to an energy transfer process associated with an exchange of two electrons between D and A D* A D A* adapted from Turro 1991

Photosensitisation: triplet-triplet energy transfer nm 315 nm Turro 1991 benzophenone naphtalene

Optically induced relaxation A system in chemical equilibrium is subject to a temperature jump; the subsequent rate of reaching the new equilibrium is mesured Measuring the rate of protein folding: The fluorescence of tryptophane is influenced by the polarity of the medium. (Fluorescence response is at ns time-scale.) Protein folding tends to exclude water from the globule. 1. The protein is defolded by cooling 2. A laser pulse returns the temperature to physiological conditions 3. The fluorescence of a tryptophane in the protein is measured on the ms timescale 4. The change in fluorescence indicates the change of polarity near the tryptophane, thus the rate of folding

A tricky old method: TCSPC time-correlated single photon counting TCSPC: a trigger pulse excites the probe and starts a decaying electric signal. detector signal in single cycles collected counts after many cycles Best time resolution : clocking cycle of computer processors ~ 10 Ghz, equivalent to 10 –10 s = 100 ps Thus, between ~ 10 ps and 1 ns, alternative time measurement is needed When the detector counts the first photon, it stops detecting and the decaying signal is measured. The signal is converted to time of the detection of the photon. After a few 10s of thousands of measurement, counts vs. time is displayed.

Cis–trans photoisomerisation Turro 1991 Vertical state: Franck-Condon transition (no nuclear movement)

Photostationary state cis  trans Continuous irradiation

Photochemistry of vision

Additive colours Blue Green Red Magenta Yellow Cyan White Mix in colours to get white

Subtractive colours Filter out colours to get black (complementer colours) Cyan Magenta Yellow Green Red Blue Black

Colour schemes (print / screen ) Cyan Magenta Yellow Green Red Blue Key Blue Green Red Magenta Yellow Cyan White screen Screen : Screen : additive; R G B 3 codes: Red – Green – Blue Projector: 3 colour lights Monitor: 3 LEDs at each pixel Print: subtractive; CMYK 4 codes (4 inks): Cyan – Magenta – Yellow – Key Key = black; 3 colours aligned to the black key

Sensors in the human eye σ  = darkness

Sensors in the human eye

Sensors in the canine eye Bichromat view Trichromat view

Photochemical electrocyclisation Pericyclic rearrangement where one  -bond being converted into one σ -bond or vice versa e. g. hν

Example: synthesis of vitamin D Physiological process: Industrial process:

Industrial synthesis of vitamin D 2 Turro 1991

Stereochemistry of cycloaddition Turro 1991

Cycloaddition within DNA Turro 1991

Relative weight of DNA photodamage damagerelative weight thymine dimer 1 cytosine hydration 0.5 DNA-protein crosslink DNA helix crosslink strand break W. J. Schreier, P. Gilch, W. Zinth Annual Review of Physical Chemistry, 66, (2015)

Photochemistry of proteins Amino acid Absorbance ε at 254 nm Quantum efficiency, Φ effectiveness 10 4 x ε Φ Cystine(-S-S-) 2700,1335,1 Tryptophane 28700,00411,5 Phenylalanine 1400,0131,8 Tyrosine 3200,0020,6 Turro 1991

Enzyme photoinactivation and cystine Turro 1991

fluid Human eye and its damage by UV radiation T. Sarna, M. Rozanowska

Main photoproduct of lens damage lens damage: 295–390 nm formation of 3HKG E. R. Gaillard, L. Zheng, J. C. Merriam, J. Dillon, Invest. Ophthalmol. Vis. Sci. 41, (2000)

Global daily erythemal dose June 2007, cloud corrected, in kJ mole –1 day –1 ἐρύθημα : reddening, inflammation (of skin)

European daily erythemal dose June 2007, cloud corrected, in kJ mole –1 day –1 ἐρύθημα: reddening, inflammation (of skin)

P. J. Aucamp Photochem. Photobiol. Sci. 6, Shading harmful UV radiation

Thiol production induced by light Beer photochemistry D. De Keukeleire Qímica Nova 23, (2000) e. g. riboflavin (vitamin B 2 )

Beer photochemistry D. De Keukeleire Qímica Nova 23, (2000) Addition of isohumulons: diminished reactivity → no mehyl-buthane-thiol enhanced hydrophobicity → creamy foam-head

Light transmittance spectra of bottles Different bottles transmit light differently Natural sunlight B. E. Sturgeon 2008

Light transmittance spectra of bottles Fluorescent bulb Different bottles transmit light differently B. E. Sturgeon 2008

Keszei Ernő: Femtokémia; a pikoszekundumnál rövidebb reakciók kinetikája, Akadémiai Kiadó Budapest, 1999 Acknowledgements N. J. Turro: Modern molecular photochemistry, University Science Books 1991 Figures are reproduced/adapted from the following sources: M. J. Pilling, P. W. Seakins: Reaction Kinetics, Oxford University Press, 1995 M. J. Rosker, M. Dantus, A. H. Zewail, J. Chem. Phys. 89, 6113 (1988) Keszei Ernő: Fizikai kémiai fejezetek, in : Kémia, Akadémiai kézikönyvek sorozat, szerkesztő: Náray-Szabó Gábor, Akadémiai Kiadó, Budapest, 2006 Figures marked as „Keszei 2015” are constructed by the author T. Sarna, M. Rozanowska: Phototoxicity to the eye. In: G. Jori et al (eds), Photobiology in Medicine. New York, Plenum Press, 1994 B. E. Sturgeon, Data to Accompany April 2nd, 2008 Basic Brewing Radio Podcast with James Spencer

END of the lecture Photochemistry Thank you for your attention ! This course material is supported by the Higher Education Restructuring Fund allocated to ELTE by the Hungarian Government