Chapter 11. Basics in spin-orbit couplings

Transcription

1 1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission, excitation spectra and electroluminescence 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15-2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region Chapter 11. Basics in spin-orbit couplings What is spin-orbit couplings? It is the coupling (also called mixing) of the spin and electron wavefunctions. What is the consequence of spin-orbit couplings? The excited states are not pure singlets or triplets but they are mixed, so that the selection rules for multiplicity are no longer rigorous (larger S-T absorptivities, more intense T 1 emissions shorter triplet lifetimes).

2 What promotes spin-orbit couplings? Heavy elements promote more couplings. Magnitude of the spin-orbit couplings: Measurements of the spin-orbit splittings in photoelectron spectra. Molecule(g) + energy Molecule + (g) + 1 electron F F + 2 π g,1/2 2 π g,3/2 Spin-orbit constant Stretching frequency of the cation Spin-orbit constant F cm -1 Cl cm -1 Br cm -1 I cm -1

3 Heavy atom effect on the S 0 T 1 absorption spectra Internal heavy atom effect External heavy atom effect I Cl I Cl. J. Turro, Modern Molecular Photochemistry, Benjamen/Cummings, Menlo Park, M = Zn Fluo Fluo M = Zn M S Fluo M Phosphorescence M = Pd

4 1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission, excitation spectra and electroluminescence 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15-2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region Chapter 12: The measurements of some of the photophysical parameters Excited state lifetimes Radiative (emission decay traces) on-radiative (transient absorption decay traces) Quantum yields (Chapter 14) Radiative (absolute or comparison with a standard) on-radiative (intersystem crossing & photochemical)

5 Measurements of the singlet (and triplet) lifetimes from the emission decay traces -d[s 1 ]/dt = Σ k i [S 1 ], with: Σ k i = k F + k c +k isc + k RS k isc (order 1 reaction i.e.: ln[s 1 ] = Σ k i ) and +d[s 1 ]/dt = I a With [S 1 ] α I F, then one can monitor I F vs time: Excitation pulse then: a graph of ln(i F ) vs time gives a slope = Σ k i. Decay trace τ F = 1/Σ k i.

6 then: a graph of ln(i F ) vs time gives a slope = Σ k i. residual (raw data fit), also considered as a criteria of quality Small problem There! ln (I F ) calculated fit τ F = 1/Σ k i. excitation pulse raw data time (ns) Modified image from: The residual is evenly distributed all along the decay trace: This is a good fit of the experimental data Picture from:

7 Sometime there are multiple decays! B short component α B x e -t/τ` A long component α A x e -t/τ Deconvolution of very short emission lifetimes Picture from a technical report:

8 Possible techniques a) laser spectroscopy, b) photocounting Photon counting is based on the fact that there are «slow» and «fast» photons. The instrument counts the number of photons it sees at a given time and so the decay grows from bottom to top. sample 1 Photon Count (In scale) sample 2 flash pulse Emission quantum yields There is the absolute and comparative methods. Definition: Φ e = number of emitted photons number of absorbed photons sample Laser beam Spherical detector

9 Exemples of fluorescence quantum yields (Φ F ): n π* Benzene 0.2 aphtalene 0.2 Antracene 0.4 9,10-diphenylanthracence 1.0 Pyrene 0.7 Perylene 1.0 Stilbene 0.05 Biacetyl Diaza bicycloheptane light Photochemical isomerization (non-radiative) There is also the comparative method. Quantum Yield Standards [%] Conditions Excitation [nm] Cy3 4 PBS 540 9,10-DPA 100 toluene 380 Cresyl Violet 53 Methanol 580 Fluorescein M ah 496 PPP 97 Cyclohexane 300 Quinine Sulfate M H 2 S Rhodamine Ethanol 450 Rhodamine 6G 95 Water 488 Rhodamine B 31 Water 514 Tryptophan 13 Water, 20 C 280 L-Tyrosine 14 Water 275 PBS = phosphate-buffered saline PPP = 1, 4-bis(5-phenyloxazole-2-yl)benzene

10 The comparative method; conditions of application: 1- The standard must be strongly luminescent. 2- The standard must absorb and emit at the same place as the sample (n). 3- The absorbtivity of both standard and sample must be below 0.05 abs (B&L law). Wavelength (nm) 4- The excitation wavelength must be the same for both also. 5- The spectra should corrected for instrumental responses. 6- The spectra must be recorded in S/E (for lamp fluctuations). 7- The spectra must be plotted in energy scale (cm -1 ) before calculating the area under the curve. The standard should absorb and emit in the same region as the sample (to minimize the effect of the difference of the refractive index) Φ sample = Φ standard x Area(sam) x Absorbance(sta) Area(sta) x Absorbance(sam) From corrected spectra in linear energy scale (cm -1 ). At the same wavelength of excitation with identical absorbance and < 0.05 (linear region of the Beer-Lambert law

11 9,10-diphenylanthracene Φ F = 1.0 Quinine sulfate, 0.1 M H 2 S 4 Me H H H H 2 S 4 ΦF = 0.54 H CH 2 Photo from Zn Φ F = 0.30

12 Φ F = 0.14 Chlorophyll A Tryptophan, water ph = 7.2 H Mg H C 20 H 39 CH 3 Φ F = 0.32 The photophysical parameters are temperature dependent L C Pt C L Au Au L C Pt C L M = Pd gives activation energy M = Pt Wavelength (nm) Harvey & Gray Polyhedron, 1990, 9, Harvey & coll. Inorg. Chem., 1999, 38, 4928

13 Some useful relationships k nr = k c + k i + k isc + k RS k F Φ F = τ k F + k F = nr 1 k F + k nr Φ F = k F τ F k nr = (k F /Φ F )(1 Φ F ) Quantum yields Radiative (absolute or comparison with a standard) on-radiative (intersystem crossing & photochemical) Φ isc = k isc k F + k nr k Φ RS RS = k F + k nr k Φ RT RT = k F + k nr

14 Measurements of the photochemical quantum yields: actinometry Example: photolabilisation of L (C-2,6-(i-Pr) 2 C 6 H 3 ) t = 0 s t = 60 s L L L Cr L L L + hν L L Cr L L L + L The actinometer: K 3 [Fe(C 2 4 )] K 3 [Fe(C 2 4 ) 3 ] + H 2 S 4 "[Fe(C 2 4 )] + " + "[Fe(C 2 4 ) 2 ] - + Fe Fe C C 2 Pale green complex (d-d transition) Φ reaction = 1.0 at 436 nm. ne can measure the exact time that is necessary for that 10% of the materials disappears upon irradiation for both the sample and the standard at the same irradiation wavelength if possible using the same optical density at this wavelength, but the actinometer is a weak absorber. For more precision, one uses phenantroline.

15 Fe phenantroline Fe(Phen) 3 2+ ε = M -1 cm -1 Fe(Phen) 3 2+ A = ε l c Fe 2+ Example Φ reaction = 0.1 Ph 2 P PPh 2 2+ Ph 2 P Pd Pd PPh 2 Ph 2 Ph 2 P Pd P C Pd 3 (dppm) 3 (µ-c) 2+ Products + C [Host-guest complex] Stable

16 1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission, excitation spectra and electroluminescence 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15-2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region Chapter 13: General aspects of bimolecular reactions, Stern-Volmer plots, and sensors Energy transfer Molecule* + Quencher Molecule + Quencher* Electron transfer Molecule* + Quencher Molecule + + Quencher - Molecule* + Quencher Molecule - + Quencher + Atom transfer, bond cleavage, photochemistry! Molecule* + Quencher Products

17 The effect of quenching on the emission intensity o quencher More quencher Even more quencher The effect of quenching on the emission lifetime o quencher More quencher Even more quencher Image from

18 Fluorescence quenching of quinine by bromide ions In the absence of a quencher In the presence of a quencher S 1 S 1 k RS A k F k nr A k F k nr S 0 S 0 o τ F = 1 kf + k nr τ F = 1 kf + k nr + k Q Q o τ F = τ F 1 o + τ F k Q Q

19 The Stern-Volmer plots o τ F = τ F 1 o + τ F k Q Q Also o Φ F o = 1 + τ F k Q Q Φ F Bimolecular emission quenching by 2 A diffusional (collision) energy transfer process absorption spectrum of dioxygen low-energy excited state Molecule* + 2 ( 3 Σ g- ) Molecule + 2 *( 1 g )

20 Diffusion Coefficients at 25 C Tryptophan = 0.66 x 10-5 cm/s xygen = 2.5 x 10-5 cm/s Collision radius = 5Å Lifetime without quencher; τ ο = 2.70 ns = 2.7 x 10-9 s Bi-molecular collision rate constant; k q = 1.27 x M -1 s -1 Stern-Volmer Constant; K D = 32.5 = k q τ o Efficiency of quenching per collision; f q = 1 Diffusion controlled constant; k o = k q f q = 1.27 x M -1 s τ o /τ = 32.5 M -1 [Q] τ ο /τ τ = ns [ 2 ], M Example: quenching of the Pd(TPP) phosphorescence by 2 M S M «Shield» Pd Harvey & coll., Inorg. Chem. 2005, 44, Pd(TPP)

21 Phosphorescence quenching of Bis(Pd-Porph) and (Pd)TPP Compound τ P (µs) k Q (10 9 M -1. s -1 ) k SV (10 6 M -1 ) Lowest detection (ppm) (Pd) 2 DPX (Pd)TPP (Pd) 2 DPS (Pd) 2 DPS (Pd) 2 DPB Protection against 2 quenching Shield no emission quenching by 2! HH 2 C HH 2 C R R R R HH 2 C R R R CH 2 H R R R R R CH 2 H R CH 2 H R CH 2 H

22 Emission quenching by photo-induced electron transfer H Ru 2+ e- H H H 2- Mo Q = Mo 4 2- Image from: ne application of the bimolecular photo-induced electron transfer: the photovoltaic cells π π*

23 Prof. Mihai Scarlete Thin Film Research Solar panel production

24 Action spectrum Soret Zinc(porphyrin) Zn Q-region 1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission, excitation spectra and electroluminescence 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15-2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region

25 Chapter 14: Time-resolved spectroscopy Eximers and Exciplex! Molecule(S 1 )* + Molecule (S 0 ) (Molecule) 2 *

26 CH 3 + CH 3 + Cu Cu Cu Cu + P P P P P P P P Major Minor (not observable in absorption) Harvey and collaborators, Inorg. Chem. 1997, 36, hν Excitonic processes in 1D polymers * * * hν` Emission Properties hν` hν` {Ag(dmb) 2+ } n 0.1 ms 2.0 ms 4.0 ms 6.0 ms

27 It is also good for slow processes such as phosphorescence Rel. Int Wavelength (nm) 1- The Jablonski diagram (or the state diagram of diamagnetic molecules) 2- Various natures of excited states and basics in molecular orbitals 3- Vibronic coupling and the Franck-Condon term 4- Excited state distortion vs bond order 5- The radiative and non-radiative processes 6- The Kasha rule and its exceptions 7- Photo-induced emission, excitation spectra and electroluminescence 8- Polarized emission and excitation spectra (tool for assignments) 9- Delayed fluorescence 10- Excimers and exciplexes 11- Basics in spin-orbit couplings 12- The measurements of some of the photophysical parameters 13- Bimolecular reactions, Stern-Volmer plots, and sensors 14- Time-resolved spectroscopy 15-2-Photon spectroscopy, flash photolysis and transient spectra 16- Exciton coupling and delocalized exciton in extended systems 17- Photo-induced energy transfer and the antenna effect 18- Photo-induced electron transfer and the Marcus inverted region

28 Chapter 15: 2-Photon spectroscopy (intense laser) S 2 S 1 S 0 virtual state Fps-null mouse colon stained with β-catenin 1 antibody and Alexa 488 secondary. n the right is normal tissue and on the left is a tumour. S 3 S 2 S 1 S 0

29 Absorption (T 1 -T n ) T n Inter-system crossing (very fast) Probe (lamp) S 1 T 1 Pump (laser) spectrum with pump excitation -spectrum without pump excitation transient spectrum S 0 Transient absorption S 1 inter-system crossing (very fast) very fast reaction electron or energy transfer Probe (lamp) Pump (laser) T 1 Products S 0 spectrum with pump excitation -spectrum without pump excitation transient spectrum

30 Schematic drawing of a transient absorption set-up Probe lamp Pump laser spectrum with pump excitation -spectrum without pump excitation transient spectrum anosecond Flash Photolysis Laser System (Harvey Group) scilloscope Laser Fan Screen Probe lamp Computer

31 Sample P Laser ( nm) Pump excitation nm nm YAG laser nm Growth and decay of the transient species Bleach and recovery of the ground state species The decay give the lifetime of the non-luminescing transient species

32 Delta A A 0,12 0,1 0,08 0,06 0,04 0,02 65 ns 80 ns 100 ns Wavelength(nm) Xanthone 0,11 0,07 A 0,03-0, ,05 Time(ns) Charge separated states S 1 inter-system crossing (very fast) fast reaction electron transfer + electron acceptor Probe (lamp) Pump (laser) T 1 electron recombination electron donor + electron acceptor - S 0 electron donor

33 10 9 W/cm W/cm W/cm 2 Requires very powerful lasers