1. Mike Ashfold PHOTODISSOCIATION DYNAMICS OF HETEROAROMATIC MOLECULES
School of Chemistry, University of Bristol, Bristol, U.K. BS8 1TS
63rd Ohio State University
International Symposium on Molecular Spectroscopy, 2008

2. Outline of talk
Method, and illustrative examples
Role of * states in heteroaromatics
Imidazole, pyrazole, pyrrole
Phenols and other substituted benzenes
Generalisations, and future prospects

3. H (Rydberg) atom photofragment translational spectroscopy
Detector
n=30-90
“High n” tagging
(366nm)
Lyman-
(121.6nm)
Tagging (366nm)
n=2
Photolysis
e (q = 0 ) o
Rydberg tagged
H Atoms
n=1
H Atom
Molecular Beam
Lyman-
(121.6nm)
Pulsed laser photolysis.
H atoms tagged after δt ≤10 ns by Lyman-a and Rydberg laser pulses.
Record TOF spectrum of tagged H atoms.
Explore: dependence on phot laser wavelength,
recoil anisotropy.

4. What do we learn? Eint(RAH) ~0 Ephot From TOF: Ekin(H)
Given Ekin, momentum conservation  total kinetic energy release (TKER).
Ephot > D0(RA–H) partitioned into TKER or internal energy of partner fragment (Eint(RA)).
Structure in TKER spectrum attributable to population of different vibrational (rotational) states of RA fragment.
Assign Eint(RA) peaks  D0(RAH) by energy conservation.
Peak heights in Eint(RA) spectrum  product branching fractions.

5. UV photolysis of C2H2 nm
Direct observation of H + C2H(X) products; latter show 2 (bending) vibrational excitation.
Precise bond strength: D0(HCCH) = ± 8 cm-1.
Identify ~600 cm-1 energy barrier in HCCH exit channel (defined relative to H + C2H(X) asymptote).
Products exhibit quantum state dependent recoil anisotropy.
Rationalise in terms of S1  Tn intersystem crossing and subsequent dissociation.
Mordaunt et al., JCP 108, 519 (1998) nm

6. Near UV photolysis of allene and propyne
Two isomers of C3H4
Allene: four identical CH bonds,
H2CCCH2 + h  H2CCCH + H (1) D0 ~ cm-1.
Propyne: two types of CH bond, with different strengths:
H3CCCH + h  H2CCCH + H (2) D0 ~ cm-1.
H3CCCH + h  H3CCC + H (3) D0 ~ cm-1.
Both molecules can also dissociate by eliminating H2.
Much studied systems (Bersohn, Y.T. Lee, Neumark, Jackson); data interpreted as indicating selective fission of stronger (acetylenic) C–H bond in propyne.

7. Near UV photolysis of allene and propyne
Studied H/D atom loss from photolysis of allene and propyne in range <  < nm, using H2CCCH2, H3CCCH and D3CCCH precursors.
Most products formed with low TKER (i.e. C3H3 co-fragments formed with high Eint).
No product recoil anisotropy.
- Most product TKERs are only compatible with propargyl radical formation, i.e. channel (1) or (2), not (3).
- Earlier studies likely affected by secondary photolysis of primary C3H3 and C3H2 fragments.
phot = nm
phot = nm
TKERmax(3)
TKERmax(1 or 2)

8. C3H4 fragmentation channels – a summary
Qadiri et al,
(JCP 119, (2003))
Is such behaviour the norm with larger molecules?
Hot molecule mechanism!

10. * states in heteroaromatics: background
Domcke and co-workers reported multi-reference ab initio potential energy surfaces (PESs) of the ground and first few excited states of several heteroaromatic chromophores, and identified the following characteristic features:
Strongly absorbing diabatically bound singlet * excited states.
Optically dark, but photochemically reactive, * excited states.
Conical intersections which provide a mechanism for ultrafast deactivation of excited states.
(Sobolewski and Domcke, Chem Phys, (2000))

11. UV photolysis of heteroaromatic molecules
Amino acids
histidine
tryptophan
tyrosine
imidazole
pyrrole
indole
phenol

12. General features of 1ps* states
pp* states S0
1ps* state below 1pp* state in imidazole and pyrrole.
1ps* state above the 1pp* states in phenol and indole.
Low oscillator strengths.
Rydberg at short range; dissociative in the R(X-H) coordinate at long range.
Form conical intersections with lower energy singlet states.
6.0
5.0
1ps*
Energy / eV
4.0
3.0
1.0
1.5
2.0
Energy / eV
R(X-H) / Å
1.0
1.5
2.0
4.0
3.0
6.0
5.0
pp* states S0
1ps*
R(X-H) / Å

14. TOF spectrum of H atoms from imidazole
= 238 nm
Structure!
Intensity / arb. units
TOF / ms

15. TKER spectra of H + imidazolyl productsv6
TKER / cm-1
Intensity / arb. Units
234 nm
238 nm
236 nm
232 nm
Structure attributable to progression in 6 (C–C stretch) mode of imidazolyl radical (B3LYP).
Anisotropic recoil velocity distribution:
vH   and thus . • • • •

16. NH bond strength in imidazole
Determine N–H bond strength from TKER of H + imidazolyl(v = 0) products measured at many phot: D0(H–imidazolyl) =
33240  40 cm-1

17. Dynamics of N–H bond fission in imidazole
AL Devine, MGD Nix, B. Cronin and MNRA, JCP (2006)
7.0
6.0
Energy / eV
1A"
(1ps*)
5.0
4.0
Perpendicular excitation to 1A"(1ps*) state.
Direct dissociation to H + imidazolyl products, with excitation of in-plane C–C stretch mode, n6(a1), 952 cm-1.
Energy disposal understandable by Franck-Condon arguments.
3.0
2.0
X1A′
1.0
0.0

18. Pyrazole: TKER spectra of H + 1-pyrazolyl products
λphot = nm
Two close lying 1* excited states
Prompt dissociation, perpendicular recoil anisotropy
Population of specific vibrational levels of ground (2A2) and first excited (2B1) states of pyrazolyl fragment.
cf photodetachment studies of
radical anion, Gianola et al, JPCA (2006)

19. Pyrrole – previous studies
Blank, …. YT Lee (Chem. Phys (1994))
PTS at λ = 193 nm. Reported bimodal H atom velocity distribution, plus P(ET) distributions for HCN and (trace) NH products.
Wei, …Temps (PCCP (2003))
Imaged H atoms following excitation at nm and 217 nm. Observed fast, anisotropic recoil distribution (β ~ 0.37) plus isotropic distribution of slower H atoms.

20. N–H bond fission in pyrrole
Excitation to 1ps*(1A2) state is dipole forbidden (vibronically induced).
Weak, diffuse absorption; dissociation involves loss of an H atom.
Structured TKER spectra  populated states of pyrrolyl partner (B3LYP).
Product branching depends on phot.
234 nm
Intensity / Arb. Units
244 nm
252 nm
TKER / cm-1

21. H + pyrrolyl(v) products at phot = 244 nm; angular anisotropy
Pyrrolyl fragments formed by direct dissociation, in specific vibrational states, with very little rotational excitation.
Product modes show different polarisation behaviours.
D0(H–pyrrolyl) =  40 cm-1.
0o (Parallel)
90o (Perp)
 (Right axis)

22. Rationale for observed energy disposal in pyrrole
vibronic excitation,
prompt dissociation,
excite disappearing mode:  intramolecular (V→T) energy transfer,
 pyrrolyl(v = 0) fragments.
excite ‘spectator’ modes:
vibrational adiabaticity in parent  product evolution.
Eint(pyrrolyl) rises with increasing Ephot.
<TKER> ~dE (drop in PE en route to dissociation limit).
Mode conserved from pyrrole to pyrrolyl.
Disappearing mode.
B Cronin, MGD Nix, RH Qadiri & MNRA, PCCP, (2004)
B Cronin, AL Devine, MGD Nix and MNRA, PCCP, (2006)

23. N-methylpyrrole Behaviour not limited to hydrides e.g. N-methylpyrrole
phot ~240 nm, (i.e. at energies just above the S1S0 (* ) origin).
Image CH3(v=0) fragments.
Fast peak, consistent with dissociation on repulsive excited (1*) state potential.
AG Sage, MGD Nix and MNRA, Chem Phys (2008)

25. Phenol: 275  phot  246 nm n18b(b2) n16a(a2)
Phenol has a predissociated S1(1pp*) state, with lifetime ~2 ns.
H atom TOF spectrum shows resolved structure at high TKER.
Several vibrational levels of phenoxyl populated, but no v = 0 products.
All observed product levels have a″ (b1 or a2) vibrational symmetry. 1 n=
2n18b + nn16a
n18b + nn16a
n18b(b2)
n16a(a2)
nn16a
Deduced TKER
for v=0 products
TKER spectrum when exciting
at S1 – S0 origin (λphot = nm).

26. Interpretation of energy disposal observed at long phot
S1 (1*) levels predissociate by internal conversion (IC) to high vibration levels of S0 state.
Best ‘acceptor’ levels in S0 are high OH stretch overtones (vOH~14).
OH ‘torsion’ provides coupling from S0 to S2(1A”, *) - via conical intersection (CI) at large ROH.
Analysis of coupling vector in vicinity of CI shows OH mixed with skeletal mode 16a. This motion maps through into the phenoxyl(X2B1) products.

27. Phenol: phot 244 nm n16b + nn18b
Phenoxyl radicals formed in a single vibrational progression in mode n18b (447 cm-1), the C=O in-plane wag (a’ symmetry).
Progression is built on one quantum of n16b (476 cm-1), the lowest frequency b1 mode, which promotes (or is activated by) coupling at the 1pp*/1ps* CI at short ROH.
18b(b2)
n =
n16b + nn18b 8 6 4 2
origin
222 nm photolysis
Coupling mode
16b(b1)

28. Interpretation of energy disposal observed at short phot
Progression in 18b(b2) - CO wag - is too long to be attributable to OH recoil.
Calculations of the s* orbital indicate repulsive interaction with adjacent C–H bond, leading to opening of C–C–O bond angle.
Mode 16b (b1, 476 cm-1) deduced to promote (or be activated by) efficient coupling at 1* / 1* CI – that enables dissociation to H + phenoxyl(X) products.

29. More on phenols Phenol:
Realisation that different out-of-plane product modes populated in phenol dissociation via 1(S0)/1* and 1*/1* CIs (phot >244 nm and phot <244 nm) critical to reconciling measured TKER values for the H + phenoxyl products.
D0(H–phenoxyl) = ± 40 cm-1.
MNRA, B Cronin, AL Devine, RN Dixon & MGD Nix, Science, (2006);
MGD Nix, AL Devine, B Cronin, RN Dixon and MNRA, JCP (2006).
Other ‘phenols’:
p-fluorophenol, p-chlorophenol and p-bromophenol
AL Devine, MGD Nix, B Cronin and MNRA, PCCP (2007)
o-, m- and p-methylphenol (cresols)
GA King, AL Devine, MGD Nix, DE Kelly and MNRA, PCCP (submitted)
Thiophenol
AL Devine, MGD Nix, RN Dixon and MNRA, JPCA (in press)
Catechol
TAA Oliver, GA King, MGD Nix and MNRA

30. Halophenols p-FPhOH =284.768 nm Long UV wavelengths:
TKER / cm-1
Long UV wavelengths:
p-FPhOH again similar to PhOH. Populated states of FPhO all have v16a = odd only, consistent with 1*S01* dissociation pathway.
Weak H atom yield from p-ClPhOH,
No H atom yield from p-BrPhOH.
Short UV wavelengths:
p-FPhOH and p-ClPhOH (but not p-BrPhOH) show O–H bond fission. Product energy disposal consistent with 1*/1*
coupling involving 16b – as in bare phenol.

31. Halophenols
Relative energetic ordering of * orbitals localised on O–H and C–Y (Y = halogen) bonds switches between 4-FPhOH and 4-BrPhOH.
(TD-DFT)
Competing C–Br bond fission in 4-BrPhOH?

32. 4-Bromophenol; imaging Br atom products
Br atoms detected at all phot  287 nm.
Slow, isotropic TKER distribution at long phot
phot < 265 nm:
Isotropic distribution of fast Br atoms (consistent with dissociation via * PES).
phot < 250 nm:
Anisotropic distribution of fast Br atoms (consistent with excitation to, and dissociation on, * PES).
S1– S0 origin

33. Catechol λphot = 280.52 nm (S1–S0 origin)
D0(HOC6H4O–H) =  50 cm-1
(reflecting stability of radical)

35. Related studies: Adenine
Identify fast, mode specific fragmentation following *S0 excitation and dissociation via a 1*/1* conical intersection. MGD Nix et al, JCP 126, (2007).
Indole shows similar behaviour. MGD Nix et al, PCCP 8, 2610 (2006)

36. Heteroaromatic molecules – Conclusions and challenges
Predicted efficacy of 1ps* states in promoting X–H bond fission in heteroaromatic biomolecular prototypes confirmed.
Radical products formed in limited sub-sets of the available vibrational state density.
Identifying populated product states can provide insights into:
- forces acting during fragmentation on 1ps* PES (e.g. imidazole)
- modes lending vibronic transition probability to 1ps* PES (pyrroles)
- parent modes involved in, or activated by, population transfer at conical intersections (e.g. the (S0)/* and */* CIs in phenol).
Analogous excited state dynamics demonstrated for case of X–CH3 bond fission (NMP).
Extension to larger, even less volatile parent molecules.
Minimising unintended multiphoton sources of H atoms.
Demonstrating analogous behaviour in the condensed phase.
(Bussandri and van Willigen, J. Phys. Chem. A (2002)).

37. Acknowledgements Mike Nix, Bríd Cronin, Adam Devine
Graeme King, Tom Oliver, Alan Sage
Richard Dixon, Andrew Orr-Ewing, Keith Rosser, Colin Western,
[Karl Welge, Ludger Schnieder, Eckart Wrede (Bielefeld)].
Funding:
EPSRC portfolio partnership LASER
EU TMR Network IMAGINE,
EU IHP Network PICNIC
Leverhulme Trust

39. Near UV photolysis of allene and propyne
Both molecules can also dissociate by eliminating H2.
Isomerisation on the ground (S0) state PES is known to occur.
Previous photolysis studies give conflicting conclusions:
- Ramsay and Thistlethwaite, Can. J. Phys. 44, 1381 (1966): UV flash photolysis of allene and propyne. Same transient product absorption detected in each case, since shown to be due to propargyl radical, H2CCCH.
- Satyapal and Bersohn, JCP 95, 8004 (1991): CH3CCD nm  Detect D atoms only, by LIF.
- Seki and Okabe, JCP 96, 3345 (1992): CD3CCH/Cl nm  HCl but not DCl.
- Jackson, .., Lee, JCP 95, 7327 (1991): H2CCCH nm. Angle resolved TOF-MS measurements of molecular products. Dominant primary process identified as H atom loss and propargyl radical formation following internal conversion to S0 state.

40. Near UV photolysis of allene and propyne
Ni, .., Jackson, JCP 110, 3320 (1999): Molecular products from 193 nm photolysis of allene and propyne detected by 118 nm photoionization + TOF-MS. Apparent differences in C3H3/C3H2 product ratios taken as evidence for direct acetylenic CH bond fission in excited state of propyne.
Sun, …, Neumark, JCP 110, 4363 (1999): As Ni et al, but used tunable VUV photoionization. Apparent differences in C3H3 fragment photoionization efficiency curves rationalised by assuming that propyne dissociates by acetylenic CH bond fission.
Chen, ..., Rosenwaks, JCP 113, 5134 (2000): nm photolysis of CD3CCH(vC-H=3) molecules. H and D atoms observed, with very similar (low) kinetic energy releases.
DeSain and Taatjes, JPC A 107, 4843 (2003): CH3CCH nm  Monitor propargyl radical by IR kinetic absorption spectroscopy, time dependence suggests it is a primary product, quantum yield ~ 0.5.

41. More on propyne photolysis
TKER spectra of H(D) atom products from propyne photolysis at nm monitoring:
D atoms from D3CCCH
H atoms from D3CCCH
H atoms from H3CCCH
are all very similar.
Conclude that, in all cases, electronically excited C3H4 molecules undergo internal conversion (IC) to high vibrational levels of the ground (S0) state, and then isomerise at a rate that is faster than their rate of unimolecular decay.

43. The 1A2(ps*) S1-state of 2,5-dimethylpyrrole
The S1←S0 spectrum shows some resolved vibrational structure, indicating a higher barrier to dissociation than in pyrrole.
The methyl torsional modes are vibronically active in S1←S0.
The band origin and dissociation energies are lower than in pyrrole.
The fragmentation process is more structured than in pyrrole.
B Cronin, MGD Nix, AL Devine, RN Dixon & MNRA, PCCP, 8, 599, 2006

44. Features of TKER spectra from 2,5-DMP photolysis
1. Modes which track with the excitation frequency
2. A persistent peak at ~ 5100 cm-1: the value of dE for 2,5-DMP.

45. Features, and adiabatic potentials for 2,5-DMP
3. Strong features are accompanied by three satellite peaks at lower KE.
Adiabatic potentials
Part of Eint for a promoting mode must be channelled into a disappearing mode to reduce the barrier and permit dissociation.
The S1 and S0 states can be coupled at the circled conical intersection by a2 vibrational modes.

46. Dynamical coupling at the S1/S0 conical intersection
The 1A2 and 1A1 states are coupled by a2 vibrational modes.
1A2
One portion of the wavepacket will be unaffected by this interaction (v(a2) = 0).
(a2)
1A1 (S0)
A second portion will evolve onto the 1A1 potential energy surface.
Coupling back to the 1A2 surface will lead to population of two quanta of some a2 mode (19 or 20), or a combination.
Part of this wavepacket will remain on the 1A1 surface and decay via the S0 continuum.

48. Phenol: excitation near S1–S0 origin (275–270 nm)
Some modes excited in phenol S1 inter-convert in the evolution to H + phenoxyl fragments. This may reflect a Duschinsky rotation of the relevant normal coordinates, either in the IC step to S0, or at the S0 to S2 conical intersection.

49. Near UV photolysis of indole
Chromophore in the aromatic amino-acid tryptophan
 = 258 nm
 = 270 nm
TKER / cm-1
Potentials from Sobolewski et al., (PCCP 4, 1093, (2002))

50. Near UV photolysis of indole
H atoms observed at all phot<283 nm (the S1S0 origin). TKER spectrum is generally unstructured.
Additional resolved peaks seen only at phot~260 nm.
If the fastest of these H atoms are formed with indolyl (v=0) fragments 
D0(N-H) =  100 cm-1
Dominant background H atom signal attributed to 1+1 REMPI, forming indole+, and subsequent 1 photon dissociation of the cation.
 = 258 nm
 = 270 nm
TKER / cm-1