The Lowest Excited Singlet States of 1-Azaadamantane and 1-Azabicyclo[2.2.2]octane: Fluorescence Excitation Spectroscopy and Density Functional Calculations

Fluorescence Characteristics of [(Benzoyloxy)methyl]anthracene Donor-Acceptor Systems

A theoretical investigation on fluorescence properties of [(benzoyloxy)methyl]anthracene derivatives containing different groups (OCH3, CH3, H, CF3, F, CN, and NO2) as substituent on the phenyl ring is reported. Electron transfer rate constants for the molecules were calculated using Marcus theory. Theoretical electron transfer rate constants agreed with experimentally observed trend of fluorescence quenching. Large electron transfer rate constants were obtained for molecules containing strongly electron withdrawing groups as the substituent on the phenyl ring. Calculations were conducted at Hartree-Fock and density functional (HF/6-31G(d) and B3LYP/6-31G(d)) levels of theory. Density functional theory predicted spurious charge transfer excited states for molecules containing NO2, CN, and CF3 as the substituent on the phenyl ring.

Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy

Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S₁(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S₁(nπ*) and S₂(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S₁(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol⁻¹. This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

Azobenzene is perhaps the archetypal light-activated molecule, widely used for photoswitching applications, but the mechanism of isomerisation remains in doubt. Here, the authors provide high-resolution excitation spectra of trans-azobenzene, identifying the structural changes accompanying photoisomerisation.

Photoelectron studies on vibronic coupling in pyrazine

Ionization pathways from the S(1) and T(1) states of pyrazine are investigated using one- and two-photon ionization of the excited state by both resonance enhanced multiphoton ionization photoelectron spectroscopy and zero electron kinetic energy pulsed field ionization techniques. For the triplet manifold, we show that two-photon ionization of T(1) is enhanced by a vibronically induced resonance for which we determine the inducing mode and the nature of the intermediate state, as well as the (3)3s(n(-1)) Rydberg state. For the singlet manifold, we identify the mode responsible for the vibronically induced intensity of a 3p Rydberg state that was previously found to greatly perturb the 1+2(') photoelectron spectrum of S(1) by a resonance at the two-photon level.

Combined experimental-theoretical study of the lower excited singlet states of paravinyl phenol, an analog of the paracoumaric acid chromophore

The low-lying excited singlet states of paravinyl phenol (pVP) are investigated experimentally and theoretically paying attention to their similarity to excited states of paracoumaric acid, the chromophore of the photoactive yellow protein (PYP). Resonance enhanced multiphoton ionization and laser induced fluorescence spectroscopic techniques are employed to obtain supersonically cooled, vibrationally resolved excitation and emission spectra related to the lowest (1)A'(V') excited state of pVP. Comprehensive analyses of the spectral structures are carried out by means of the equation-of-motion coupled cluster singles and doubles and time dependent density functional theory methods in combination with the linear vibronic coupling model and Franck-Condon calculations. The assignments of the spectral patterns are given, mostly in terms of excitations of totally symmetric modes. Weak activity of the non-totally-symmetric modes indicates low probability of photochemical processes in the Franck-Condon region of the (1)A'(V') state. The second (1)A'(V) and third (1)A" (Ryd) excited states of pVP are characterized with regard to their electronic structure, properties, and effects of geometry relaxations. The lengthening of the double bond relevant to the trans-cis isomerization of the PYP chromophore is found for the (1)A'(V) state. A possibility of photochemical processes and strong vibronic interactions in this state can be expected. The theoretical results for the (1)A"(Ryd) state predict that dissociation with respect to the O-H bond is possible.

Electron delocalization in the radical cation of 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane, a 4-nitrogen-7-electron system

The radical cation of 1,3,6,8-tetraazatricyclo [4.4.1.1(3,8)]dodecane (TTD) has been studied using magnetic resonance and optical spectroscopic methods and computational techniques. With the help of deuterated isotopomers, assignments of EPR and resonance Raman spectra could be unequivocally established. The results demonstrate that the radical cation has D(2d) symmetry, and instantaneous electron delocalization over the four equivalent nitrogen atoms occurs. This extensive delocalization in a completely saturated system is a unique feature of the TTD radical cation. The spectroscopy of TTD, in contrast to that of simpler diamines such as 1,4-diaza[2.2.2]bicyclooctane, simultaneously reveals the consequences of orbital interactions through space and through bonds. The relationship between nitrogen pyramidalization and hyperfine coupling constants in nitrogen-centered radical cations with a number of different bonding arrangements is reviewed.

Structure and photophysics of an old, new molecule: 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane

More than a century after its initial synthesis, the static and dynamic geometry of 1,3,6,8-tetraazatricyclo [4.4.1.1(3,8)]dodecane (TTD), a fully saturated cage-like molecule, is finally established. Detection and modeling of the supersonic jet fluorescence excitation and emission spectra show that the molecule undergoes interconversion between two S(4) symmetry minima. The barrier at the D(2d) symmetric conformation is only 105 cm(-1), i.e., approximately 0.3 kcal mol(-1), and is overcome along a carbon-carbon torsional mode of a(2) symmetry. The presence of an S(4) conformation is corroborated by a Raman investigation. When excited to the first excited singlet state, the 3s Rydberg state, the molecule adopts a geometry with D(2d) symmetry. The satisfactory description of the spectroscopy of TTD obtained by a combination of quantum chemical and quantum mechanical models is discussed, and the apparent conflict between the present results and nuclear magnetic resonance and X-ray diffraction experiments is solved. Because of the close analogy between a Rydberg state and the ground state of the radical cation regarding geometry and spectroscopic properties, it is concluded that the radical cation is also of D(2d) symmetry.

Computational study of radical cations of saturated compounds with sigma-type and pi-type N-N bonds

Geometrical and electronic properties have been calculated and are compared with experimental data for three saturated diaza compounds and their radical cations and dications. The molecular geometries in the different oxidation states are consistently reproduced very well using the B3PW91 and B3LYP three-parameter density functional methods, with a modest 6-31G* basis set. The performance of the pure density functionals BLYP and BPW91 is less satisfactory. The Hartree-Fock method yields excellent results in some cases but poor results in others. Ionization potentials and electron-nuclear hyperfine interactions are reproduced moderately well with B3LYP and B3PW91. Electronic excitation energies calculated with time-dependent density functional theory agree very well with experiment in most cases. For 2,7-diazatetracyclo[6.2.2.2(3,6).0(2,7)]tetradecane 2 and its radical cation and dication, the reorganization parameters for self-electron exchange were calculated and compared with experimental and earlier computed data. The calculations allow a good estimate of the different contributions to the energy barrier, i.e., the internal and solvent reorganization energies and the work term in the case of 2+/2++.