Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane

The theory for modeling vibronic interactions in bichromophores was introduced in sixties by Witkowski and Moffitt [J. Chem. Phys. 33, 872 (1960)] and extended by Fulton and Gouterman [J. Chem. Phys. 35, 1059 (1961)]. The present work describes extension of this vibronic model to describe bichromophores with broken vibrational symmetry such as partly deuterated molecules. Additionally, the model is extended to include inter-chromophore vibrational modes. The model can treat multiple vibrational modes by employing Lanczos diagonalization procedure of sparse matrices. The developed vibronic model is applied to simulation of vibronic spectra of flexible bichromophore diphenylmethane and compared to high-resolution experimental spectra [J. A. Stearns, N. R. Pillsbury, K. O. Douglass, C. W. Mu?ller, T. S. Zwier, and D. F. Plusquellic, J. Chem. Phys. 129, 224305 (2008)].     

Observation of ultraviolet rotational band contours of the DNA base adenine: Determination of the transition moment

We report the first observation and analysis of rotational band contours of the jet-cooled DNA base adenine for three vibronic bands at 36,062, 36,105, and 36,248 cm(-1). The lowest npi* and pipi* states have been labeled with their excited-state vibronic symmetry, and a strong pipi*-npi* vibronic coupling via an out-of-plane vibrational mode has been revealed. The rotational band contours have been recorded by resonant two-photon ionization (R2PI) and analyzed by a genetic algorithm (GA) based fit to obtain the optimum band parameters. The vibronic band at 36,062 cm(-1) shows dominant c-type character with transition dipole moment (TDM) components mu(a)2:mu(b)2:mu(c)2 = 0.09:0.17:0.74 and those at 36 105 and 36 248 cm(-1) show abc-hybrid character with predominantly in-plane TDM components. The band at 36,062 cm(-1) has been assigned as the n --> pi* transition, and the 36,105 cm(-1) band as the pi --> pi* transition by the symmetry analysis. The band at 36,248 cm(-1) provides evidence of the strong pipi*-npi* vibronic coupling via an out-of-plane vibrational mode.     

Rotationally resolved S1<-- S0 electronic spectra of fluorene, carbazole, and dibenzofuran: evidence for Herzberg-Teller coupling with the S2 state

Rotationally resolved fluorescence excitation spectra of the S1 <-- S0 origin bands and higher vibronic bands of fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF) have been observed and assigned. Analyses of these data show that replacement of the CH2 group in FLU with a NH group in CAR and an O atom in DBF produces only localized changes in structure, in the ground state. But the three molecules exhibit different changes in geometry when they are excited by light. The S1 states of the three molecules also are electronically very different. The S1 <-- S0 transition moments of CAR and DBF are parallel to the C2 symmetry axis whereas the corresponding transition moment in FLU is perpendicular to this axis. Herzberg-Teller coupling involving the S2 state also has been observed in the spectra of higher vibronic bands of CAR and DBF. Possible reasons for these behaviors are discussed.     

Gas-phase conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene

The conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC) have been studied using resonance-enhanced two-photon ionization spectroscopy in a supersonic jet expansion. It has been estimated from the spectral analysis that 90% of THDC exists in the twist-boat (TB) conformation; the chair (C) conformer constitutes the remaining 10%. Most of the vibronic activity in the spectrum of THDC is associated with the symmetric flapping of the aromatic rings of the TB conformer. The observed S₁/S₂ exciton splitting of the TB conformer is 100 cm^?1. The S₁/S₂ transition of the C conformer is found to be forbidden. The exciton splittings of the C and TB conformers were estimated by the spectral analysis of two deuterated isotopomers of THDC. The estimated exciton splittings of the C and TB conformers are 14.7 and 101.9 cm^?1, respectively. The supramolecular model of bichromophores with identical chromophores at the CIS/6-31+G(d)//HF/6-31+G(d) level of theory predicted electronic coupling energies that are very close to the experimental exciton coupling energies.     

Symmetry segregation of the vibronic levels within the S1<--S0 system of thiophosgene, Cl2CS, by optical-optical double resonance spectroscopy

The first singlet-singlet electronic system, S1<--S0, in thiophosgene has been recorded as a laser induced fluorescence (LIF) excitation and an optical-optical double resonance (OODR) spectrum under jet-cooled conditions. In the OODR process, the sum of the frequencies of the pump and probe lasers must be fixed to the energy difference between a pair of vibronic levels in the S2(v') and S0(v") states. Detection is through the fluorescence from the S2 state. The blocking of a spectrum into its four possible symmetry components is obtained by adjusting the total pump+probe energy such that it matches the energy difference between symmetry selected levels in the S2 and S0 electronic states. In this method the pump laser is used to excite a group of "hot" sequence bands that involve combinations of the nu4 and nu6 antisymmetric vibrations. The additional data that were collected by this method were used to update and refine the analyses of the spectrum. Magnetic dipole transitions are reported for the first time.     

55Mn pulse ENDOR at 34 GHz of the S0 and S2 states of the oxygen-evolving complex in photosystem II

55Mn pulse ENDOR experiments at 34 GHz (Q-band) are reported for the S0 and S2 states of the oxygen-evolving complex of photosystem II. Their numerical analysis (i) shows that in both states all four Mn ions are magnetically coupled, (ii) allows a refinement of the hyperfine interaction (HFI) parameters obtained earlier for the S2 state at X-band (Peloquin, J. M.; Campbell, K. A.; Randall, D. W.; Evanchik, M. A.; Pecoraro, V. L.; Armstrong, W. H.; Britt, R. D. J. Am. Chem. Soc. 2000, 122, 10926-10942), (iii) provides the first reliable 55Mn HFI tensors for the S0 state, and (iv) leads to the suggestion that the Mn oxidation states in S0 and S2 are Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. In addition, a Q-band EPR spectrum is reported for the S0 state, and inversion-recovery experiments at 4.5 K directly show that the electron spin-lattice relaxation for the S0 state is about 2 orders of magnitude faster than that for the S2 state.     

Phonon-driven exciton dissociation at donor-acceptor polymer heterojunctions: direct versus bridge-mediated vibronic coupling pathways

We present a molecular-level, quantum dynamical analysis of phonon-driven exciton dissociation at polymer heterojunctions, using a linear vibronic coupling model parametrized for 3 electronic states and 24 vibrational modes. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree method. In this study, which significantly extends the two-state model of Tamura et al. (Tamura, H.; Bittner, E. R.; Burghardt, I. J. Chem. Phys. 2007, 126, 021103), we focus on the role of bridge states, which can mediate the decay of the photogenerated exciton and possibly interfere with the direct transition toward an interfacial charge-separated state. Both the direct and bridge-mediated pathways are found to depend critically on the dynamical interplay of high-frequency C=C stretch modes and low-frequency ring-torsional modes. The dynamical mechanism is interpreted in terms of a hierarchical electron-phonon model, leading to the identification of generalized reaction coordinates for the nonadiabatic process. Variation of the vibronic coupling model parameters in a realistic range provides evidence that the direct exciton decay pathway is not dynamically robust, and bridge-mediated pathways can become dominant. The ultrafast, coherent dynamics is of pronounced nonequilibrium character and cannot be modeled by conventional kinetic equations. The predicted femtosecond to picosecond decay times are consistent with time-resolved spectroscopic observations.     

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 2. Molecular deuterium and iodine monochloride

Laser-induced fluorescence and action spectroscopy experiments have identified multiple conformers of the D2...ICl van der Waals complex for both ortho-D2 (o-D2) and para-D2 (p-D2). As with the analogous H2...ICl van der Waals complexes [Darr, J. P.; Crowther, A. C.; Loomis, R. A.; Ray, S. E.; McCoy, A. B. J. Phys. Chem. A 2007, 111, 13387], the C2v conformer with the deuterium molecule localized at the iodine atom end of the dihalogen is significantly more stable than the asymmetric conformer that has the deuterium positioned orthogonally to the ICl bond axis, D0' = 223.9(2.4) versus 97.3(8)-103.9(3) cm(-1) for p-D2...I(35)Cl(X, v'=0). For both conformers, complexes containing p-D2 are found to be more strongly bound than those with o-D2. The electronically excited D2...ICl(A, v') and D2...ICl(B, v') complexes are found to have equilibrium geometries that are nearly the same as those of the ground-state asymmetric structures. Calculated D2...ICl(B, v'=3) energies and probability amplitudes obtained using a simple scaled He + ICl(B, v'=3) potential provide clues to the nature of the different excited-state levels accessed.     

S1 and S2 excited states of gas-phase Schiff-base retinal chromophores: a time-dependent density functional theoretical investigation

In concert with the recent photoabsorption experiments of gas-phase Schiff-base retinal chromophores (Nielsen et al. Phys. Rev. Lett. 2006, 96, 018304), quantum chemical calculations using time-dependent density functional theory coupled with different functionals and under the Tamm-Dancoff approximation were made on the first two excited states (S1 and S2) of two retinal chromophores: 11-cis and all-trans protonated Schiff bases. The calculated vertical excitation energies (Tv) and oscillator strengths (f) are consistent with the experimental absorption bands. The experimentally observed phenomenon that the transition dipole moment (mu) of S2 is much smaller that of S1 was interpreted by 3D representation of transition densities. The different optical behaviors (linear and nonlinear optical responds) of the excited states were investigated by considering different strengths of external electric fields.     

Influence of geometry relaxation on the energies of the S1 and S2 states of violaxanthin, zeaxanthin, and lutein

Precise knowledge of the excitation energies of the lowest excited states S(1) and S(2) of the carotenoids violaxanthin, lutein, and zeaxanthin is a prerequisite for a fundamental understanding of their role in light harvesting and photoprotection during photosynthesis. By means of density functional theory (DFT) and time-dependent DFT (TDDFT), the electronic and structural properties of the ground and first and second excited states are studied in detail. According to our calculations, all-s-cis-zeaxanthin and s-cis-lutein conformers possess lower total ground-state energies than the corresponding s-trans conformers. Thus, only s-cis isomers are probably physiologically relevant. Furthermore, the influence of geometric relaxation on the energies of the ground state and S(1) and S(2) states has been studied in detail. It is demonstrated that the energies of these states change significantly if the carotenoid adopts the equilibrium geometry of the S(1) state. Considering these energetic effects in the interpretation of S(1) excitation energies obtained from fluorescence and transient absorption spectroscopy shifts the S(1) excitation energies about 0.2 eV to higher energy above the excitation energy of the chlorophyll a.     

Two-dimensional infrared investigation of N-acetyl tryptophan methyl amide in solution

The linear infrared and two-dimensional infrared (2D IR) spectra in the amide-I region of N-acetyl tryptophan methyl amide (NATMA) in solvents of varying polarity are reported. The two amide-I transitions have been assigned unambiguously by using 13C isotopic substitution of the carbonyl group. The amide unit at the amino end shows a lower transition frequency in CH2Cl2 and methanol, while the acetyl end has a lower transition frequency in D2O. Multiple conformers exist in CH2Cl2 and methanol, but only one conformer is evident in D2O. The 2D IR cross peaks from the intermode coupling yield off-diagonal anharmonicities 2.5 +/- 0.5, 3.25 +/- 0.5, and 3.0 +/- 0.5 cm(-1) in CH2Cl2, methanol, and D2O, respectively, which by simple matrix diagonalization yield the coupling constants 8.0 +/- 0.5, 8.0 +/- 1.0, and 5.5 +/- 1.0 cm(-1). The major conformer in CH2Cl2 corresponds to a C7 structure, in agreement with that found in the gas phase [Dian, B. C.; Longarte, A.; Mercier, S.; Evans, D. A.; Wales, D. J.; Zwier, T. S. J. Chem. Phys. 2002, 117, 10688-10702] with intramolecular hydrogen bonding between the acetyl end C=O and the amino end N-H. The backbone dihedral angles (phi, psi) are determined to be in the ranges of (-55 +/- 5 degrees , 30 +/- 5 degrees ), (120 +/- 10 degrees , -20 +/- 10 degrees ), and (+/-160 +/- 10 degrees , +/-75 +/- 10 degrees ) in CH2Cl2, methanol, and D2O, respectively.     

CD spectra of polynuclear complexes of diimine ligands: theoretical and experimental evidence for the importance of internuclear exciton coupling

We have recently reported on dinuclear complexes Lambda,Lambda-[Co(2)L(2)Cl(2)]CoCl(4) of two novel chiral ligands (1a and 1b) which contain pyridyl-imine chelate groups (Telfer, S. G.; Sato, T.; Kuroda, R. Chem. Commun. 2003, 1064-1065). The absolute configuration of the cobalt(II) centers was unambiguously assigned by X-ray crystallography. However, the sign of the exciton couplets in their CD spectra was opposite to that expected on the basis of the stereochemistry of the metal centers. We present a rationalization of these anomalous spectra in terms of an "internuclear" exciton coupling model which takes into account the coupling of chromophores located on different metal centers. We have performed a series of semiempirical (ZINDO) calculations which provide quantitative support to this model. These findings show that the absolute configuration of the metal centers in a polynuclear complex may be incorrectly assigned on the basis of CD data if internuclear coupling effects are not taken into consideration. We summarize the CD spectral data of number of other chiral polynuclear complexes from the literature, including dinuclear complexes bridged by the 2,2'-bipyrimidine ligand, complexes of the HAT ligand, and dinuclear triple-stranded helicates. The amplitude of the CD spectra of many of these complexes is not additive with the number of chromophores. These anomalous spectra can be accounted for by taking internuclear coupling effects into consideration.     

Vibronic structure in the S1-S0 transition of jet-cooled dibenzofuran

Fluorescence excitation spectra of dibenzofuran in a supersonic jet are observed and the vibronic structure is analyzed for the S(1) (1)A(1) (pipi) and S(0) states. An observation of the rotational envelopes reveals that the band is a B-type band. However, it is shown that most of the strong vibronic bands are A-type bands. The intensity arises from vibronic coupling with the S(2) (1)B(2) state. We find a broad emission in the dispersed fluorescence spectrum for the excitation of the high vibrational levels in the S(1) state. This indicates that intramolecular vibrational redistribution (IVR) occurs efficiently in the isolated dibenzofuran molecule.     

Symmetry analysis of the vibronic States in the upper conical potential (2(3)A') of triplet

The symmetry properties of the rovibronic resonance states (Slonczewski resonances) supported by an upright conical potential are investigated. These symmetry properties lead to a useful correlation between states calculated with and without consideration of the geometrical phase, which can assist in the assignment of those states. The vibronic resonance states of triplet H3(+) (2(3)A'), which had been studied by us before, have now been assigned to spectroscopic quantum numbers.     

DNA scaffold supports long-lived vibronic coherence in an indodicarbocyanine (Cy5) dimer

Vibronic coupling between pigment molecules is believed to prolong coherences in photosynthetic pigment–protein complexes. Reproducing long-lived coherences using vibronically coupled chromophores in synthetic DNA constructs presents a biomimetic route to efficient artificial light harvesting. Here, we present two-dimensional (2D) electronic spectra of one monomeric Cy5 construct and two dimeric Cy5 constructs (0 bp and 1 bp between dyes) on a DNA scaffold and perform beating frequency analysis to interpret observed coherences. Power spectra of quantum beating signals of the dimers reveal high frequency oscillations that correspond to coherences between vibronic exciton states. Beating frequency maps confirm that these oscillations, 1270 cm⁻¹ and 1545 cm⁻¹ for the 0-bp dimer and 1100 cm⁻¹ for the 1-bp dimer, are coherences between vibronic exciton states and that these coherences persist for ∼300 fs. Our observations are well described by a vibronic exciton model, which predicts the excitonic coupling strength in the dimers and the resulting molecular exciton states. The energy spacing between those states closely corresponds to the observed beat frequencies. MD simulations indicate that the dyes in our constructs lie largely internal to the DNA base stacking region, similar to the native design of biological light harvesting complexes. Observed coherences persist on the timescale of photosynthetic energy transfer yielding further parallels to observed biological coherences, establishing DNA as an attractive scaffold for synthetic light harvesting applications.

Dyes coupled to DNA display distance-dependent vibronic couplings that prolongs quantum coherences detected with 2D spectroscopy.     

Quantum dynamics through conical intersections: combining effective modes and quadratic couplings

We present a detailed study for the short-time dynamics through conical intersections in molecular systems related to the quadratic vibronic coupling (QVC) Hamiltonian [Müller, H.; K?ppel, H.; Cederbaum, L. S. New J. Chem. 1993, 17, 7-29] and the effective-mode formalism [Cederbaum, L. S.; Gindensperger, E.; Burghardt, I. Phys. Rev. Lett. 2005, 94, 113003]. Our approach is based on splitting the nuclear degrees of freedom of the whole system into system modes and environment modes. It was found that only three-effective environmental modes together with the system's modes are needed to describe the short-time dynamics of the complex system correctly. In addition, a detailed mathematical proof is given in the appendix to demonstrate that the exact cumulants are recovered up to the second order within the cumulant expansion of the autocorrelation function. The butatriene molecule is studied as an explicit showcase example to stress the viability of our proposed scheme and to compare with other systems.     

A solid-state NMR investigation of single-source precursors for group 12 metal selenides; M[N(iPr2PSe)2]2 (M = Zn, Cd, Hg)

The first solid-state NMR investigation of dichalcogenoimidodiphosphinato complexes, M[N(R(2)PE)(2)](n), is presented. The single-source precursors for metal-selenide materials, M[N((i)Pr(2)PSe)(2)](2) (M = Zn, Cd, Hg), were studied by solid-state (31)P, (77)Se, (113)Cd, and (199)Hg NMR at 4.7, 7.0, and 11.7 T, representing the only (77)Se NMR measurements, and in the case of Cd[N((i)Pr(2)PSe)(2)](2)(113)Cd NMR measurements, to have been performed on these complexes. Residual dipolar coupling between (14)N and (31)P was observed in solid-state (31)P NMR spectra at 4.7 and 7.0 T yielding average values of R((31)P,(14)N)(eff) = 880 Hz, C(Q)((14)N) = 3.0 MHz, (1)J((31)P,(14)N)(iso) = 15 Hz, alpha = 90 degrees , beta = 26 degrees . The solid-state NMR spectra obtained were used to determine the respective phosphorus, selenium, cadmium, and mercury chemical shift tensors along with the indirect spin-spin coupling constants: (1)J((77)Se,(31)P)(iso), (1)J((111/113)Cd,(77)Se)(iso), (1)J((199)Hg,(77)Se)(iso), and (2)J((199)Hg,(31)P)(iso). Density functional theory magnetic shielding tensor calculations were performed yielding the orientations of the corresponding chemical shift tensors. For this series of complexes the phosphorus magnetic shielding tensors are essentially identical, the selenium magnetic shielding tensors are also very similar with respect to each other, and the magnetic shielding tensors of the central metals, cadmium and mercury, display near axial symmetry demonstrating an expected deviation from local S(4) symmetry.     

Doppler-free two-photon excitation spectroscopy and the Zeeman effects. Perturbations in the 14(0)1 and 1(0)(1)14(0)1 bands of the S1 <-- S0 transition of C6D6

Doppler-free two-photon excitation spectra and the Zeeman effects for the 1 band of the S1 1B2u <-- S0 1A1g transition in gaseous benzene-d6 were measured. Although the spectral lines were strongly perturbed, almost all of the lines near the band origin could be assigned. From a deperturbation analysis, the perturbation near the band origin was identified as originating from an anharmonic resonance interaction. Perturbation centered at K = 28-29 in the 14(0)1 band was analyzed, and it was identified as originating from a perpendicular Coriolis interaction. The symmetry and the assignment of the perturbing state proposed by Schubert et al. (Schubert, U.; Riedle, E.; Neusser, H. J. J. Chem. Phys. 1989, 90, 5994.) were confirmed. No perturbation originating from an interaction with a triplet state was observed in both bands. From the Zeeman spectra and the analysis, it is demonstrated that rotationally resolved levels are not mixed with a triplet state. The intersystem mixing is not likely to occur at levels of low excess energy in the S1 state of an isolated benzene. Nonradiative decay of an isolated benzene in the low vibronic levels of the S1 state will occur through the internal mixing followed by the rotational and vibrational relaxation in the S0 state.