Martin J. Paterson

Benchmarking two-photon absorption with CC3 quadratic response theory, and comparison with density-functional response theory

We present a detailed study of the effects of electron correlation on two-photon absorption calculated by coupled cluster quadratic response theory. The hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3 has been used to investigate the effects of electron correlation on the two-photon absorption cross sections of formaldehyde (CH2O), diacetylene (C4H2), and water (H2O). In particular, the effects of triple excitations on two-photon transition cross sections are determined for the first time. In addition, we present a detailed comparison of the coupled cluster results with those obtained from Hartree-Fock and density-functional response theories. We have investigated the local-density approximation, the pure Becke-Lee-Yang-Parr (BLYP) functional, the hybrid Becke-3-parameter-Lee-Yang-Parr (B3LYP), and the Coulomb-attenuated B3LYP (CAM-B3LYP) functionals. Our results show that the CAM-B3LYP functional, when used in conjuction with a one-particle basis-set containing diffuse functions, has much promise; however, care must still be exercised for diffuse Rydberg-type states.

Structure calculation of an elastic hydrogel from sonication of rigid small molecule components

Making plans for hydrogels: Calculations successfully predict the molecular structure of a robust hydrogel. Melamine and uric acid cocrystallize with water upon sonication, and using experimental data, the minimized structure (see picture) was calculated and successfully compared with powder X-ray diffraction data of the xerogel.

Time-resolved photoelectron imaging of excited state relaxation dynamics in phenol, catechol, resorcinol, and hydroquinone

Time-resolved photoelectron imaging was used to investigate the dynamical evolution of the initially prepared S(1) (ππ*) excited state of phenol (hydroxybenzene), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) following excitation at 267 nm. Our analysis was supported by ab initio calculations at the coupled-cluster and CASSCF levels of theory. In all cases, we observe rapid (<1 ps) intramolecular vibrational redistribution on the S(1) potential surface. In catechol, the overall S(1) state lifetime was observed to be 12.1 ps, which is 1-2 orders of magnitude shorter than in the other three molecules studied. This may be attributed to differences in the H atom tunnelling rate under the barrier formed by a conical intersection between the S(1) state and the close lying S(2) (πσ*) state, which is dissociative along the O-H stretching coordinate. Further evidence of this S(1)/S(2) interaction is also seen in the time-dependent anisotropy of the photoelectron angular distributions we have observed. Our data analysis was assisted by a matrix inversion method for processing photoelectron images that is significantly faster than most other previously reported approaches and is extremely quick and easy to implement.

Induced Fit Interanion Discrimination by Binding-Induced Excimer Formation

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a 2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change, giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6, which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host complexes with a range of anions.

Magnetism in metal–organic capsules

Nickel and cobalt seamed metal-organic capsules have been isolated and studied using structural, magnetic and computational approaches. Antiferromagnetic exchange in the Ni capsule results from coordination environments enforced by the capsule framework.

Unraveling Ultrafast Dynamics in Photoexcited Aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)ππ* state (1(1)ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)πσ* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)ππ* onto the (1)πσ* state at a 1(1)ππ*/(1)πσ* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)πσ*/S(0) CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)ππ* and 1(1)ππ* states, enabling the excited-state population to evolve through a rapid sequential 2(1)ππ* → 1(1)ππ* → (1)πσ* → N-H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)πσ* driven dynamics in this ubiquitous genetic building block.

Anion binding and luminescent sensing using cationic ruthenium(II) aminopyridine complexes

The synthesis of a series of ruthenium(II) based anion sensors of the type [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(L)(2)][BF(4)] (2) is reported in which ligand L represents a series of substituted pyridinylmethyl-amine derivatives. The carbazole based ligand L(3) exhibits a fluorescent intraligand charge-transfer (ILCT) state that is quenched by ligand-to-metal charge transfer (LMCT) upon coordination to ruthenium in the 1:1 complex [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(2)(L(3))] (1 c). The 1:2 complex 2 c is fluorescent, however, and acts as a fluorescent anion sensor because of the mixing of an anion-dependent charge-transfer component into the excited state. The 1:2 complexes of type 2 all exhibit interesting low symmetry (1)H NMR spectra that also are a useful handle on anion complexation. The electronic structures of L(3), 1 c and 2 c have been probed by time-dependent DFT calculations.

Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

Non-adiabatic dynamics involving 1πσ* or 1nσ* excited electronic states play a key role in the photochemistry of numerous heteroatom containing aromatic (bio-)molecules. In this contribution, we investigate more exotic phenomena involved in σ* mediated dynamics, namely: (i) the role of purely quantum mechanical behavior; and (ii) manipulating non-adiabatic photochemistry through conical intersections (CIs) with ‘vibration-specific control’. This is achieved by investigating S–CH3 bond fission via a 1nσ* potential energy surface (PES) in thioanisole (C6H5SCH3). Using a combination of time- and frequency-resolved velocity map ion imaging techniques, together with ab initio calculations, we demonstrate that excitation to the 1ππ* ← S0 origin [1ππ*(ν = 0)] results in S–CH3 bond fission on the 1nσ* PES, despite an (apparent) energetic barrier to dissociation formed by a CI between the 1ππ* and 1nσ* PESs. This process occurs by accessing ‘classically forbidden’ regions of the excited state potential energy landscape where the barrier to dissociation becomes negligible, aided by torsional motion of the S–CH3 group out of the plane of the phenyl ring. Control over these dynamics is attained by populating a single quantum of the S–CH3 stretch mode in the 1ππ* state [1ππ*(ν7a = 1)], which mirrors the nuclear motion required to promote coupling through the 1ππ*/1nσ* CI, resulting in a marked change in the electronic branching in the C6H5S radical products. This observation offers an elegant contribution towards a vision of ‘quantum control’ in photo-initiated chemical reaction dynamics.

Wavepacket dynamics study of Cr(CO)5 after formation by photodissociation: relaxation through an (E ⊕ A) ⊗ e Jahn–Teller conical intersection

In the ultrafast photodissociation of Cr(CO)6, Cr(CO)5 is formed within 50 fs. Initially in an excited state, a Jahn–Teller (E ⊗ e) conical intersection is believed to then act as an efficient decay funnel to the ground state. Here, we present a Hamiltonian representing the first three electronic states of Cr(CO)5, obtained by fitting the linear vibronic coupling model to calculations using a complete active space self-consistent field (CASSCF) wavefunction. The intersection is in fact found to involve (( E ⊕ A) ⊗ e) pseudo-Jahn–Teller coupling, with the singly degenerate state playing an important role. The results of wavepacket dynamics using the multi-configuration time-dependent Hartree (MCTDH) method show that this model is consistent with the experimental data.

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright (1)L(a) and (1)L(b) electronic states of (1)ππ∗ character and spectroscopically dark and dissociative (1)πσ∗ states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited (1)L(a) state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived (1)L(b) state or the (1)πσ∗ state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the (1)πσ∗ state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.