Spectroscopic study of the benchmark Mn+-H2 complex

We have recorded the rotationally resolved infrared spectrum of the weakly bound Mn+-H2 complex in the H-H stretch region (4022-4078 cm(-1)) by monitoring Mn+ photodissociation products. The band center of Mn+-H2, the H-H stretch transition, is shifted by -111.8 cm(-1) from the transition of the free H2 molecule. The spectroscopic data suggest that the Mn+-H2 complex consists of a slightly perturbed H2 molecule attached to the Mn+ ion in a T-shaped configuration with a vibrationally averaged intermolecular separation of 2.73 A. Together with the measured Mn+...H2 binding energy of 7.9 kJ/mol (Weis, P.; et al. J. Phys. Chem. A 1997, 101, 2809.), the spectroscopic parameters establish Mn+-H2 as the most thoroughly characterized transition-metal cation-dihydrogen complex and a benchmark for calibrating quantum chemical calculations on noncovalent systems involving open d-shell configurations. Such systems are of possible importance for hydrogen storage applications

Modulating electron injection from an organic dye to a titania nanoparticle with a photochromic energy transfer acceptor

We have prepared titania nanoparticles with an organic dye sensitiser and diarylethene molecular switch attached to the surface. Spectroscopic investigations show that the dye sensitiser's electron injection efficiency is reduced when the diarylethene is switched from its colourless, ring-open isomer to its coloured, ring-closed isomer, due to the introduction of a competing energy transfer pathway

Properties of the B+-H2 and B+-D2 complexes: a theoretical and spectroscopic study

The rotationally resolved infrared spectrum of the B+-D2 ion-neutral complex is recorded in the D-D stretch vibration region (2805–2875  cm−1) by detecting B+ photofragments. Analysis of the spectrum confirms a T-shaped equilibrium geometry for the B+-D2 complex with a vibrationally averaged intermolecular bond length of 2.247 Å, around 0.02 Å shorter than for the previously characterised B+-H2 complex [V. Dryza, B. L. J. Poad, and E. J. Bieske, J. Am. Chem. Soc. 130, 12986 (2008)10.1021/ja8018302]. The D-D stretch band centre occurs at 2839.76 ± 0.10 cm−1, representing a −153.8  cm−1 shift from the Q1(0) transition of the free D2 molecule. A new three dimensional ab initio potential energy surface for the B++H2 interaction is calculated using the coupled cluster RCCSD(T) method and is used in variational calculations for the rovibrational energies of B+-H2 and B+-D2. The calculations predict dissociation energies of 1254  cm−1 for B+-H2 with respect to the B++H2 (j = 0) limit, and 1313  cm−1 for B+-D2 with respect to the B++D2 (j = 0) limit. The theoretical approach reproduces the rotational and centrifugal constants of the B+-H2 and B+-D2 complexes to within 3%, and the magnitude of the contraction of the intermolecular bond accompanying excitation of the H2 or D2 sub-unit, but underestimates the H-H and D-D vibrational band shifts by 7%–8%. Combining the theoretical and experimental results allows a new, more accurate estimation for the B+-H2 band origin (3939.64 ± 0.10  cm−1)

Isomerisation of an intramolecular hydrogen-bonded photoswitch:Protonated azobis(2-imidazole)

Photoisomerisation of protonated azobis(2-imidazole), an intramolecular hydrogen-bonded azoheteroarene photoswitch molecule, is investigated in the gas phase using tandem ion mobility mass spectrometry. The E and Z isomers exhibit distinct spectral responses, with E-Z photoisomerisation occurring over the 360-520 nm range (peak at 460 nm), and Z-E photoisomerisation taking place over the 320-420 nm range (peak at 390 nm). A minor photodissociation channel involving loss of N2 is observed for the E-isomer with a maximum efficiency at 390 nm, blue-shifted by ≈70 nm relative to the wavelength for maximum photoisomerisation response. Loss of N2 is also the predominant collision-induced dissociation channel. Electronic structure calculations suggest that E-isomer photoisomerisation involves S1(ππ∗) excitation, whereas the Z-isomer photoisomerisation involves S2(ππ∗) excitation. Conversion between the E and Z isomers through collisional excitation, which is calculated to occur through both inversion and torsion pathways, is investigated experimentally by colliding the molecular ions with nitrogen buffer gas over a range of electric fields. This study demonstrates the versatility of tandem ion mobility mass spectrometry for exploring the isomerisation of molecular photoswitches initiated by either light or collisions

Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid

Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E → Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest ππ* transition. For the E isomer of the carboxylate deprotomer, a one-way photoinitiated proton transfer generates the phenoxide deprotomer through a mechanism postulated to involve an excited-state enol–keto tautomerism followed by a series of ground-state rearrangements including a second proton transfer. This mechanism is supported by experiments in which the relevant intermediate keto isomer is prepared and spectroscopically probed and through master equation modeling of possible ground-state isomerization processes. The Z isomer of the carboxylate deprotomer shows a weak Z → E photoisomerization response that occurs in competition with photodestruction (presumably electron detachment), demonstrating that the E and Z isomers undergo different processes in their excited states. The study highlights the utility of isomer-selective spectroscopy for characterizing the photochemistry of isolated anions possessing multiple deprotonation sites

Photochrome-doped organic films for photonic key-pad locks and multi-state fluorescence

The spectroscopic properties of poly(methyl methacrylate) polymer films doped with two kinds of photochromic molecular switches are investigated. A green-fluorescent sulfonyl diarylethene (P1) is combined with either a non-fluorescent diarylethene (P2) or red-fluorescent spiropyran (P3). Photoswitching between the colorless and colored isomers (P1: o-BTFO4 ↔ c-BTFO4, P2: o-DTE ↔ c-DTE, P3: SP ↔ MC) enables the P1+P2 and P1+P3 films to be cycled through three distinct states. From the initial state (o-BTFO4 + o-DTE/SP), irradiation with UV light generates the second state (c-BTFO4 + c-DTE/MC), where c-BTFO4 → c-DTE/MC energy transfer is established. Irradiation with green light then generates the third state (c-BTFO4 + o-DTE/SP), where the energy transfer acceptor is no longer present. Finally, irradiation with blue light regenerates the initial state. For the P1+P2 film, only one state is fluorescent, with the irradiation inputs required to be entered in the correct order to access this state, acting as a keypad lock. For the P1+P3 film, the states emit either no fluorescence, red fluorescence, or green fluorescence, all using a common excitation wavelength. Additionally, once the fluorescence is activated with UV light, it undergoes a time-dependent color transition from red to green, due to the pairing of P-type and T-type photochromes. These multi-photochromic systems may be useful for security ink or imaging applications

Photodissociation Dynamics of N⁺₃

The photodissociation dynamics of N+3 excited from its (linear) 3Σ−g/(bent) 3A″ ground to the first excited singlet and triplet states is investigated. Three-dimensional potential energy surfaces for the 1A′, 1A″, and 3A′ electronic states, correlating with the 1Δg and 3Πu states in linear geometry, for N+3 are constructed using high-level electronic structure calculations and represented as reproducing kernels. The reference ab initio energies are calculated at the MRCI+Q/aug-cc-pVTZ level of theory. For following the photodissociation dynamics in the excited states, rotational and vibrational distributions P(v′) and P(j′) for the N2 product are determined from vertically excited ground state distributions. Due to the different shapes of the ground state 3A″ potential energy surface and the excited states, appreciable angular momentum j′ ∼ 60 is generated in diatomic fragments. The lifetimes in the excited states extend to at least 50 ps. Notably, results from sampling initial conditions from a thermal ensemble and from the Wigner distribution of the ground state wavefunction are comparable

Reversible photoswitching of isolated ionic hemiindigos with visible light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40–80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches

Ultrafast photoisomerisation of an isolated retinoid

The photoinduced excited state dynamics of gas-phase trans-retinoate (deprotonated trans-retinoic acid, trans-RA−) are studied using tandem ion mobility spectrometry coupled with laser spectroscopy, and frequency-, angle- and time-resolved photoelectron imaging. Photoexcitation of the bright S3(ππ*) ← S0 transition leads to internal conversion to the S1(ππ*) state on a ≈80 fs timescale followed by recovery of S0 and concomitant isomerisation to give the 13-cis (major) and 9-cis (minor) photoisomers on a ≈180 fs timescale. The sub-200 fs stereoselective photoisomerisation parallels that for the retinal protonated Schiff base chromophore in bacteriorhodopsin. Measurements on trans-RA− in methanol using the solution photoisomerisation action spectroscopy technique show that 13-cis-RA− is also the principal photoisomer, although the 13-cis and 9-cis photoisomers are formed with an inverted branching ratio with photon energy in methanol when compared with the gas phase, presumably due to solvent-induced modification of potential energy surfaces and inhibition of electron detachment processes. Comparison of the gas-phase time-resolved data with transient absorption spectroscopy measurements on retinoic acid in methanol suggest that photoisomerisation is roughly six times slower in solution. This work provides clear evidence that solvation significantly affects the photoisomerisation dynamics of retinoid molecules

Nonadiabatic Dynamics between Valence, Nonvalence, and Continuum Electronic States in a Heteropolycyclic Aromatic Hydrocarbon

Internal conversion between valence-localized and dipole-bound states is thought to be a ubiquitous process in polar molecular anions, yet there is limited direct evidence. Here, photodetachment action spectroscopy and time-resolved photoelectron imaging with a heteropolycyclic aromatic hydrocarbon (hetero-PAH) anion, deprotonated 1-pyrenol, is used to demonstrate a subpicosecond (τ1 = 160 ± 20 fs) valence to dipole-bound state internal conversion following excitation of the origin transition of the first valence-localized excited state. The internal conversion dynamics are evident in the photoelectron spectra and in the photoelectron angular distributions (β2 values) as the electronic character of the excited state population changes from valence to nonvalence. The dipole-bound state subsequently decays through mode-specific vibrational autodetachment with a lifetime τ2 = 11 ± 2 ps. These internal conversion and autodetachment dynamics are likely common in molecular anions but difficult to fingerprint due to the transient existence of the dipole-bound state. Potential implications of the present excited state dynamics for interstellar hetero-PAH anion formation are discussed