Most early studies of molecular photodissociation dynamics were undertaken on condensed phase systems, and irradiation was for the most part incoherent, continuous and polychromatic. New technologies (e.g. wavelength-tunable pulsed lasers, vacuum technology, molecular beams and greatly increased computing power) led to a switch in behaviour: most recent dynamical studies of photofragmentations have been conducted in the gas phase, under isolated molecule conditions. Now the pendulum is swinging again. The availability of broadly tunable femtosecond lasers and the development of ultrafast spectroscopic techniques mean that detailed studies of molecular photodissociations are feasible in solution. The particular focus of our current work is to explore the extent to which dynamical insights provided by earlier gas phase studies inform our understanding of molecular photodissociations in a liquid. In favorable cases, we can also envisage situations where photodissociation studies in solution might offer insights into systems that defy investigation in the gas phase – e.g. thermally unstable molecules, molecules with very low vapor pressures, or ring-opening reactions, where differentiating the precursor and product can be challenging.
Much of our more recent gas-phase research has focused on the role of (n/π)σ* excited states in photoinduced bond fissions in molecules ranging in size and complexity from ammonia to adenine, with particular attention on X-Y bond fissions (where X = O, S or N and Y = H, CH3 or a halogen atom) in (hetero)aromatic systems. Our first attempt at a like-for-like comparison of the gas and solution phase photodissociation dynamics of a centered on 4-methylthiophenol (4-MePhSH) at wavelengths ~267 nm and was performed in collaboration with the group of Prof. Stephen Bradforth (Univ. Southern California). H Rydberg atom photofragment translational spectroscopy studies show prompt S–H bond fission in the gas phase, and formation of the 4-MePhS radical partner in both its ground (X) and first excited (A) electronic states, and with significant vibrational excitation. Ultrafast transient UV pump – broadband UV/visible probe studies of this same dissociation in cyclohexane and ethanol solution revealed similarly prompt dissociation (in <50 fs), radical formation in both electronic states, and additional processes (e.g. vibronic quenching of the products, radical pair recombination, etc) specific to the solution phase.
This, and related, systems were investigated further by ultrafast UV-pump – broadband IR probe methods at the ULTRA Facility at the Rutherford Appleton Laboratory. This complementary technique allows us to monitor UV-pump induced changes in the parent ground (S0) state population (via a bleach) and in the ground state 4-MePhS radical (via its time evolving absorption) in deuterated acetonitrile. Kinetic analysis returns the proportion of radicals that subsequently recombine (~40%), and vibrational relaxation timescales (~10 ps). The UV photodissociation dynamics of 4-methylthioanisole in deuterated acetonitrile solution were also investigated: changing the departing fragment from H to a Me radical results in a much increased excited state lifetime, but S–Me bond fission is still observed – as in the gas phase. O–H bond fission following excitation to the S1 (1ππ*) state of phenol in the gas phase is deduced to occur by tunneling through the substantial energy barrier under the conical intersection between the S1 and S2(1πσ*) potential energy surfaces. Here, too, UV pump transient absorption studies of phenol in non-polar solvents like cyclohexane reveal striking similar dynamics immediately post-excitation, along with signatures of solvent-induced geminate recombination at longer pump-probe delays. This body of work has been summarized in a recent PCCP Perspective article.
Transient IR spectra measured following 267 nm photoexcitation of 4-MePhSMe in CD3CN solution, showing the time evolution of (i) a parent excited state absorption, (ii) a ground state bleach feature and (iii) an absorption attributable to the 4-MePhS radical product.
In other recent work, we have used both UV pump – UV/vis and UV pump – IR probe spectroscopies to interrogate the solution-phase photodissociation dynamics of two molecules, allyl phenyl ether and phenyl acetate, for which there is as yet no equivalent gas phase data. The solution-phase excitation, subsequent radical pair recombination and hydrogen migration steps observed for these molecules are better known as the photo-Claisen and photo-Fries rearrangements, respectively, and are of synthetic utility. This investigation further extends the prior work of the group by illustrating the role of (n/π)σ* states in mediating dissociation even in cases where the departing fragment is significantly larger than H (or even a Me radical). Such studies allow us to track the progress of these rearrangements out to 2 ns, visualizing (through key spectroscopic signatures) the initial photoinduced O–C bond fission and the subsequent recombinations, and enabling quantum yield estimates for these various intermediate processes.
Ultrafast spectroscopy also allows study of photochemical processes that surely occur in the gas phase but are challenging to study – for example, because the product is simply an isomer of the original parent molecule. The photoinduced ring opening of molecules like furanone and thiophenone are examples of such processes. Here, too, UV pump – broadband IR probe studies allow us to follow the production and evolution of the ketene (thioketene) product formed via ring-opening though C–O(C–S) bond fission. Work on these and related systems is on-going, as part of a broader programme of work exploring the role of (n/π)σ* excited states play in many areas of molecular photochemistry.
Transient IR spectrum illustrating the time evolving parent C=O stretch bleach and the transient absorption of the product C=C=O stretch vibration following 267 nm photoexcitation of a solution of thiophenone in acetonitrile.
This research project is now supported by an ERC Advanced Grant which has allowed establishment of an ultrafast laser laboratory in the School of Chemistry at the University of Bristol. The laboratory is equipped with a Coherent Vitara and Legend Elite amplified Ti:S laser system (5 W, 1 kHz), two OPAs with difference and sum frequency generation, and two 128-pixel MCT array spectrometers for transient IR absorption spectroscopy. The laser systems are also being used to study ultrafast reaction dynamics in solution.
Our ultrafast dynamics studies have been supported by EPSRC (e.g. via Programme Grant EP/G00224X/1), the ERC (Advanced Grant 290966 CAPRI) and STFC (through access to the ULTRA Facility at the Central Laser Facility, Rutherford Appleton Laboratory).
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Representative publications:
UV-induced isomerization dynamics of N-methyl-2-pyridone in solution, D. Murdock, S.J. Harris, I.P. Clark, G.M. Greetham, M. Towrie, A.J. Orr-Ewing and M.N.R. Ashfold, J. Phys. Chem. A 119, 88-94, (2015).
On the participation of photo-induced N–H bond fission in aqueous adenine at 266 and 220 nm: a combined ultrafast transient electronic and vibrational absorption spectroscopy study, G.M. Roberts, H. Marroux, M.P. Grubb, M.N.R. Ashfold and A.J. Orr-Ewing, J. Phys. Chem. A, 118, 11211-25, (2014).
Tracking the Paternò-Büchi reaction in real time using transient electronic and vibrational spectroscopies, S.J. Harris, D. Murdock, M.P. Grubb, I.P. Clark and M.N.R. Ashfold, J. Phys. Chem. A, 118, 10240-5, (2014).
Transient UV pump-IR probe investigation of heterocyclic ring-opening dynamics in the solution phase: the role played by ns* states in the photoinduced reactions of thiophene and furanone, D. Murdock, S.J. Harris, J. Luke, M.P. Grubb, A.J. Orr-Ewing and M.N.R. Ashfold, Phys. Chem. Chem. Phys. 16, 21271-9, (2014).
Exploring the energy disposal immediately after bond-breaking in solution: The wavelength dependent excited state dissociation pathways of para-methylthiophenol, Y. Zhang, T.A.A. Oliver, S. Das, A. Roy, M.N.R. Ashfold and S.E. Bradforth, J. Phys. Chem. A. 117, 12125-37 (2013).
Comparing molecular photofragmentation dynamics in the gas and liquid phases, S.J. Harris, D. Murdock, Y. Zhang, T.A.A. Oliver, M.P. Grubb, A.J. Orr-Ewing, G.M. Greetham, I.P. Clark, M. Towrie, S.E. Bradforth and M.N.R. Ashfold, Phys. Chem. Chem. Phys. 15, 6567-82 (2013).
