Nathan R. Pillsbury

Rotationally resolved studies of S0 and the exciton coupled S1/S2 origin regions of diphenylmethane and the d12 isotopologue

Rotationally resolved microwave and ultraviolet spectra of jet-cooled diphenylmethane (DPM) and DPM-d(12) have been obtained in S(0), S(1), and S(2) electronic states using Fourier-transform microwave and UV laser/molecular beam spectrometers. The S(0) and S(1) states of both isotopologues have been well fit to asymmetric rotor Hamiltonians that include only Watson distortion parameters. The transition dipole moment (TDM) orientations of DPM and DPM-d(12) are perpendicular to the C(2) symmetry axes with 66(2)%:34(2)% a:c hybrid-type character, establishing the lower exciton S(1) origin as a completely delocalized, antisymmetric combination of the zero-order locally excited states of the toluene-like chromophores. In contrast, the rotational structures of the S(2) origin bands at S(1)+123 cm(-1) and S(1)+116 cm(-1), respectively, display b-type Q-branch transitions and lack the central a-type Q-branch features that characterize the S(1) origins, indicating TDM orientations parallel to the C(2)(b) symmetry axes as anticipated for the upper exciton levels. However, rotational fits were not possible in line with expectations from previous work [N. R. Pillsbury, J. A. Stearns, C. W. Muller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 114301 (2008)] where the S(2) origins were found to be largely perturbed through vibronic interactions with the S(1) symmetric, antisymmetric torsional, and butterfly levels in close proximity. Predictions from a dipole-dipole coupling model and ab initio theories are shown to be in fair agreement with the observed TDM orientations and exciton splitting. The need to include out-of-ring-plane dipole coupling terms indicates that in-plane models are not sufficient to fully account for the excitonic interactions in this bichromophore.

Conformational effects on excitonic interactions in a prototypical H-bonded bichromophore: bis(2-hydroxyphenyl)methane.

Laser-induced fluorescence, single-vibronic level fluorescence (SVLF), UV hole burning, and fluorescence dip infrared (FDIR) spectroscopy have been carried out on bis-(2-hydroxyphenyl)methane in order to characterize the ground-state and first excited-state vibronic spectroscopy of this model flexible bichromophore. These studies identified the presence of two conformational isomers. The FDIR spectra in the OH-stretch region determine that conformer A is an OH...O H-bonded conformer, while conformer B is a doubly OH...pi H-bonded conformer with C(2) symmetry. High-resolution ultraviolet spectra ( approximately 50 MHz resolution) of a series of vibronic bands of both conformers confirm and refine these assignments. The transition dipole moment (TDM) direction in conformer A is consistent with electronic excitation that is primarily localized on the donor phenol ring. A tentative assignment of the S(2) origin is made to a set of transitions approximately 400 cm(-1) above S(1). In conformer B, the TDM direction firmly establishes C(2) symmetry for the conformer in its S(1) state and establishes the electronic excitation as delocalized over the two rings, as the lower member of an excitonic pair. The S(2) state has not been clearly identified in the spectrum. Based on CIS calculations, the S(2) state is postulated to be several times weaker than S(1), making it difficult to identify, especially in the midst of overlap from vibronic bands due to conformer A. SVLF spectra show highly unusual vibronic intensity patterns, particularly in conformer B, which cannot be understood by simple harmonic Franck-Condon models, even in the presence of Duschinsky mixing. We postulate that these model flexible bichromophores have TDMs that are extraordinarily sensitive to the distance and orientation of the two aromatic rings, highlighting the need to map out the TDM surface and its dependence on the (up to) five torsional and bending coordinates in order to understand the observations.

Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: Bis-(4-hydroxyphenyl)methane

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV-UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S(0)-S(1) origins at 35,184 and 35,209 cm(-1), respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and T), ring-flapping (R and R), and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C(2) symmetric uu and dd conformers. The S(0)-S(2) origins are assigned to bands located 132 cm(-1) above the S(0)-S(1) origins of both conformers. DFL spectra from the S(2) origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S(2) origin occurs dominantly to S(1) R(1) and S(1) R(1)β(1) levels, which are located -15 and +31 cm(-1) from it. Unusual vibronic activity in the ring-breathing (ν(1)) and ring-deformation (ν(6a)) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S(2) origin is strongly mixed (60%/40%) with S(1) 6a(1) and 1(1) levels. The direct extension of this model to the T/T and R/R inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S(1)/S(2) mixing compared to the ring modes.

Conformational Isomerization and Collisional Cooling Dynamics of Bis(2-hydroxyphenyl)methane

Stimulated emission pumping-population transfer spectroscopy (SEP-PTS) has been used to directly measure the energy threshold to isomerization between the two conformational isomers of bis(2-hydroxyphenyl)methane. These conformers have been shown in the preceding paper (DOI 10.1021/jp8098686) to be an OH...O H-bonded structure (conformer A) and a doubly OH() ...pi H-bonded conformer (conformer B). Lower and upper bounds on the energy threshold for A-->B isomerization are at 1344 and 1399 cm(-1), respectively, while the corresponding bounds on the B-->A isomerization are 1413 and 1467 cm(-1). The difference between these thresholds provides a measure of the relative energies of the two minima, with DeltaE(AB) = E(A) - E(B) = 14-123 cm(-1). The transition-state structure responsible for this energy threshold has been identified using DFT B3LYP, DFT M05-2X, and MP2 calculations, all with a 6-31+G* basis set. Only the DFT M05-2X calculations correctly reproduce both the energy ordering of the two minima and the magnitude of the barrier separating them. Below the energy threshold to isomerization, we have used the extensive Franck-Condon progressions present in the SEP spectrum of conformer A to undertake an SEP-PT study of its vibrational relaxation rate, as a function of internal energy over the 0-1200 cm(-1) region. The position of SEP excitation in the expansion was systematically varied in order to change the rate and number of cooling collisions that occur between SEP excitation and probe steps and the initial temperature at which SEP occurs. From this data set, three energy regimes were identified, each with a unique value of the average energy lost per collision with helium (region 1: 13 cm(-1)/collision for E = 300-1200 cm(-1), region 2: 0.6 cm(-1)/collision for E = 200-300 cm(-1), and region 3: 7 cm(-1)/collision for E < 200 cm(-1)). In region 1, the vibrational density of states is sufficient to support efficient loss of energy via Deltav = -1 collisions, involving the lowest-frequency vibrations of the molecule (with a frequency of 26 cm(-1)). In region 2, the vibrational energy levels are sufficiently sparse that energy gaps exist, reducing the efficiency of relaxation. In region 3, a combination of the quantum nature of the helium, attractive forces, and orbiting resonances may be responsible for the increased efficiency at lowest-energy regime.

Conformational Isomerization of 5-Phenyl-1-pentene Probed by SEP-Population Transfer Spectroscopy

Stimulated emission pumping-population transfer (SEP-PT) spectroscopy is used to experimentally determine upper and lower bounds on the energy thresholds to conformational isomerization between 14 X-->Y reactant-product conformer pairs of isolated 5-phenyl-1-pentene (5PPene). This work builds directly on the spectroscopic assignments of the five observed conformers of 5PPene in the preceding paper. The observed thresholds fall into two energy ranges: near 600 cm(-1) for isomerization processes that involve only reorientation of the terminal vinyl group, and in the 1200-1374 cm(-1) range for barriers that involve hindered rotation about the alkyl chain carbon-carbon bonds. As a result, this latter threshold opens up much of the conformational phase space to exploration, with multiple isomerization pathways connecting any two of the conformational minima.

Lowest triplet (n,π*) state of 2-cyclohexen-1-one: Characterization by cavity ringdown spectroscopy and quantum-chemical calculations

The cavity ringdown (CRD) absorption spectrum of 2-cyclohexen-1-one (2CHO) was recorded over the range 401.5-410.5 nm in a room-temperature gas cell. The very weak band system (ε ≤ 0.1 M(-1) cm(-1)) in this spectral region is due to the T1(n, π*) ← S0 electronic transition. The 0(0)(0) origin band was assigned to the feature observed at 24,558.8 ± 0.3 cm(-1). We have assigned 46 vibronic transitions in a region extending from -200 to +350 cm(-1) relative to the origin band. For the majority of these transitions, we have made corresponding assignments in the spectrum of the deuterated derivative 2CHO-2,6,6-d3. From the assignments, we determined fundamental frequencies for several vibrational modes in the T1(n, π*) excited state of 2CHO, including the lowest ring-twisting (99.6 cm(-1)) and ring-bending (262.2 cm(-1)) modes. These values compare to fundamentals of 122.2 cm(-1) and 251.9 cm(-1), respectively, determined previously for the isoconfigurational S1(n, π*) excited state of 2CHO and 99 cm(-1) and 248 cm(-1), respectively, for the S0 ground state. With the aid of quantum-mechanical calculations, we have also ascertained descriptions for these two modes, thereby resolving ambiguities appearing in the previous literature. The ring-twisting mode (ν39) contains a significant contribution from O=C-C=C torsion, whereas the ring-bending mode (ν38 in the ground state) involves mainly the motion of C-5 with respect to the plane containing the other heavy atoms. The CRD spectroscopic data for the T1(n, π*) state have allowed us to benchmark several computational methods for treating excited states, including time-dependent density functional theory and an equation-of-motion coupled cluster method. In turn, the computational results provide an explanation for observed differences in the T1(n, π*) vs. S1(n, π*) ring frequencies.