Novel electronic transitions within excited doublets of dimers

We consider the electronic transition between the split components of the excited electronic doublet of a dimer species when the monomer species have permanent dipole moments. We show that the transition moment is given in terms of the permanent dipole moments of the ground and excited states of the monomers. Extension to ionic dimers and transitions between exciton bands in molecular crystals are suggested.     

Revisiting the photophysical properties and excited singlet-state dipole moments of several coumarin derivatives

The solvent effects on the electronic absorption and fluorescence emission spectra of several coumarins derivatives, containing amino, N,N-dimethyl-amino, N,N-diethyl-amino, hydroxyl, methyl, carboxyl, or halogen substituents at the positions 7, 4, or 3, were investigated in eight solvents with various polarities. The first excited singlet-state dipole moments of these coumarins were determined by various solvatochromic methods, using the theoretical ground-state dipole moments which were calculated by the AM1 method. The first excited singlet-state dipole moment values were obtained by the Bakhshiev, Kawski-Chamma-Viallet, Lippert-Mataga, and Reichardt-Dimroth equations, and were compared to the ground-state dipole moments. In all cases, the dipole moments were found to be higher in the excited singlet-state than in the ground state because of the different electron densities in both states. The red-shifts of the absorption and fluorescence emission bands, observed for most compounds upon increasing the solvent polarity, indicated that the electronic transitions were of π-π* nature.     

Determination of the ground- and excited-state dipole moments of 4’-(hexyloxy)-4-biphenylcarbonitrile and 4-isothiocyanatophenyl 4-pentylbicyclo [2,2,2] octane-1-carboxylate nematic liquid crystals and their mixtures

The dipole moments of the ground and excited states of 4′-(hexyloxy)-4-biphenylcarbonitrile and 4-isothiocyanatophenyl 4-pentylbicyclo [2.2.2] octane-1-carboxylate nematic liquid crystals and their mixtures prepared in chloroform and dichloromethane were studied at room temperature. The dipole moments of the ground states of the all samples were calculated according to the Guggenheim–Smith method. The dipole moments of their excited states were determined with the help of the Lippert equation by measuring the absorption and fluorescence spectra, solvent polarity and refractive index values. It was determined that dipole moments of the excited states were higher than those of the ground states. Moreover, the dipole moments of the ground and excited states of two nematic liquid crystals were also estimated by using molecular mechanic method (Gaussian09 program (DFT/B3LYP 6-31G(dp)). The results obtained are interpreted in detail.     

Experimental and theoretical investigation of the spectral and luminescent properties of some acridine compounds

Complex (experimental and quantum-chemical) investigation of the spectral and luminescent properties of acridine, 9-aminoacridine, 2,7-dimethyl-9-ditolylaminoacridine, and their protonated forms was performed. The electronic absorption and fluorescence spectra of the acridine dyes were studied at room temperature in ethanolic solutions at different pH values and in other solvents of different chemical nature and polarity. The energies of the excited states, the deactivation rate constants for the excited states, and the dipole moments are presented, which were obtained by calculations using the method of intermediate neglect of the differential overlap with special spectroscopic parameterization.     

The study of dipole moment of some substituted acetic acids in an electronically excited state

The electronic absorption spectra of eight substituted acetic acids have been measured at room temperature in several solvents. The ground state dipole moments are evaluated experimentally for these molecules. These ground state values are used in conjunction with the spectral results to evaluate their first electronically excited state dipole moments. For all the molecules investigated here the dipole moments in the excited state are higher than their ground state values.     

AB initio calculations of the dipole moments in low-lying electronic states of the ccn radical

The structures and dipole moments of the four low-lying electronic states (X²Π, A²Δ, B²Σ⁻ and C²Σ⁺) of the linear CCN radical are investigated by ab initio calculations at SDCI/DZP and TZP levels. For all the electronically excited states, the dipole moments are calculated to be ≈ 3.0 D. However, a significantly smaller dipole moment, ≈ 0.6 D, is predicted for the ground state. This result is consistent with the recent experiment by Suzuki, Saito and Hirota, where the MODR signals are observed for the A state CCN but not for the X state. Electronic correlation is important in determining both equilibrium bond lengths and dipole moments.     

Electronically excited states of water clusters of 7-azaindole: structures, relative energies, and electronic nature of the excited states

The geometries of 1H-7-azaindole and the 1H-7-azaindole(H(2)O)(1-2) complexes and the respective 7H tautomers in their ground and two lowest electronically excited pi-pi(*) singlet states have been optimized by using the second-order approximated coupled cluster model within the resolution-of-the-identity approximation. Based on these optimized structures, adiabatic excitation spectra were computed by using the combined density functional theory/multireference configuration interaction method. Special attention was paid to comparison of the orientation of transition dipole moments and excited state permanent dipole moments, which can be determined accurately with rotationally resolved electronic Stark spectroscopy. The electronic nature of the lowest excited state is shown to change from L(b) to L(a) upon water complexation.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

Excited state characteristics of acridine dyes: acriflavine and acridine orange

The magnitude of the Stokes shift (frequency shifts in absorption and fluorescence spectra) is observed on changing the solvents and further has been used to calculate experimentally the dipole moments (ground state and excited state) of acriflavine and acridine orange dye molecules. Theoretically, dipole moments are calculated using PM 3 Model. The dipole moments of excited states, for both molecules investigated here, are higher than the corresponding values in the ground states. The increase in the dipole moment has been explained in terms of the nature of the excited state. Acriflavine dye overcomes the non-lasing behaviour of acridine orange due to quaternization of the central nitrogen atom.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

Theoretically calculated rovibronic transition spectra of KRb

Spectroscopic properties of all the electronic states of KRb dissociating into 4s(K) + 5s(Rb), 4s(K) + 5p(Rb), 4p(K) + 5s(Rb), and 4s(K) + 4d(Rb) and some higher-lying excited states are studied with ab initio calculations. Spectroscopic constants, dipole moments, and the nature of the electronic wave functions for these states are reported. Intensities for the singlet-singlet and triplet-triplet transitions are theoretically calculated from the potential energy curves and the transition dipole moments. © 1996 John Wiley & Sons, Inc.     

Excited state dipole moments and geometries of the bases of nucleic acids

CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μ_hyb^(σ), μ_ch and μ_π has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different.Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π^* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π^* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.     

Dications of bis-triarylamino-[2.2]paracyclophanes: Evaluation of excited state couplings by GMH analysis

In this paper, we present the absorption properties of a series of bis-triarylamino-[2.2]paracyclophane diradical dications. The localized pi-pi and the charge-transfer (CT) transitions of these dications are explained and analyzed by an exciton coupling model that also considers the photophysical properties of the "monomeric" triarylamine radical cations. Together with AM1-CISD-calculated transition moments, experimental transition moments and transition energies of the bis-triarylamine dications were used to calculate electronic couplings by a generalized Mulliken-Hush (GMH) approach. These couplings are a measure for interactions of the excited mixed-valence CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states mu(ab) of the dimer equals the dipole moment difference Delta of the ground and the excited bridge state of the corresponding monomer.     

On the accuracy of computed excited-state dipole moments

The dipole moments of furan and pyrrole in many electronically excited singlet states have been determined using coupled cluster theory including large one-electron basis sets. The inclusion of connected triple excitations is shown to uniformly decrease the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) excitation energies by 0.04-0.24 eV, with an average reduction of 0.08 eV. Using a basis set larger than DZP (++)D (double-zeta plus polarization augmented with atom- and molecule-centered diffuse functions) uniformly increases the computed EOM-CCSD excitation energies by 0.03-0.29 eV, with an average increase of 0.20 eV. The corresponding shifts in excited-state dipole moments are more erratic. Including connected triple excitations changes the computed dipole moments by an rms amount of 0.17 au. More importantly, using a larger basis set shifts the dipole moments by an rms amount of 0.52 au, with an increase or a decrease being equally likely. The CC dipole moments are compared to those from time-dependent density functional theory (TD-DFT) computed by Burcl, Amos, and Handy [ Chem. Phys. Lett. 2002, 355, 8]. For 29 excited states of furan and pyrrole, the predicted TD-DFT dipole moments differ from the CC results by rms amounts of 1.6 au (HCTH functional) and 1.5 au (B97-1 functional). Including the asymptotic correction to TD-DFT developed by Tozer and Handy [ J. Chem. Phys. 1998, 109, 10180; J. Comput. Chem. 1999, 20, 106] reduces the rms differences for both functionals to 1.2 au. If those Rydberg excited states with very large polarizabilities are excluded, the rms differences from the CC results for the remaining 17 excited states become 1.31 au (HCTH) and 0.88 au (B97-1). For asymptotically corrected functionals and this subset of states, the rms differences from the CC results are only 0.54 au (HCTHc) and 0.34 au (B97-1c). Thus, the Tozer-Handy asymptotic correction for TD-DFT significantly improves the predictions of excited-state dipole moments. For excited states without very large polarizabilities, good agreement is achieved between excited-state dipole moments computed by coupled cluster theory and by the asymptotically corrected B97-1c density functional.     

Intermediate state representation approach to physical properties of electronically excited molecules

Propagator methods provide a direct approach to energies and transition moments for (generalized) electronic excitations from the ground state, but they do not usually allow one to determine excited state wave functions and properties. Using a specific intermediate state representation (ISR) concept, we here show how this restriction can be overcome in the case of the algebraic-diagrammatic construction (ADC) propagator approach. In the ISR reformulation of the theory the basic ADC secular matrix is written as a representation of the Hamiltonian (or the shifted Hamiltonian) in terms of explicitly constructable states, referred to as intermediate (or ADC) states. Similar intermediate state representations can be derived for operators other than the Hamiltonian. Together with the ADC eigenvectors, the intermediate states give rise to an explicit formulation of the excited wave functions and allow one to calculate physical properties of excited states as well as transition moments for transitions between different excited states. As for the ground-state excitation energies and transition moments, the ADC excited state properties are size consistent so that the theory is suitable for applications to large systems. The established hierarchy of higher-order [ADC(n)] approximations, corresponding to systematic truncations of the IS configuration space and the perturbation-theoretical expansions of the ISR matrix elements, can readily be extended to the excited state properties. Explicit ISR matrix elements for arbitrary one-particle operators have been derived and coded at the second-order [ADC(2)] level of theory. As a first computational test of the method we have carried out ADC(2) calculations for singlet and triplet excited state dipole moments in H(2)O and HF, where comparison to full CI results can be made. The potential of the ADC(2) method is further demonstrated in an exploratory study of the excitation energies and dipole moments of the low-lying excited states of paranitroaniline. We find that four triplet states, T1-T4, and two singlet states, S1 and S2, lie (vertically) below the prominent charge transfer (CT) excitation, S3. The dipole moment of the S3 state (17.0D) is distinctly larger than that of the corresponding T3 triplet state (11.7D).     

RMS dipole moments of the valence excited states of linear polyenes

Asymmetrical vibrations induce small oscillating dipole moments in the ground states of linear all-trans polyenes. Upon vertical excitation into one of the two low-lying valence excited states the induced in-plane dipole moments are increased 3 times and more. The rms dipole moments increase with the chain length. The inducing modes are the CCC skeletal bending and the CC stretching vibrations. CH stretching vibrations are also found to contribute non-negligible polarization.     

Spectroscopic properties and potential energy curves of low-lying electronic states of RuC

The RuC molecule has been a challenging species due to the open-shell nature of Ru resulting in a large number of low-lying electronic states. We have carried out state-of-the-art calculations using the complete active space multiconfiguration self-consistent field followed by multireference configuration interaction methods that included up to 18 million configurations, in conjunction with relativistic effects. We have computed 29 low-lying electronic states of RuC with different spin multiplicities and spatial symmetries with energy separations less than 38,000 cm(-1). We find two very closely low-lying electronic states for RuC, viz., 1Sigma+ and 3Delta with the 1Sigma+ being stabilized at higher levels of theory. Our computed spectroscopic constants and dipole moments are in good agreement with experiment although we have reported more electronic states than those that have been observed experimentally. Our computations reveal a strongly bound 1Sigma+ state with a large dipole moment which is most likely the experimentally observed ground state and an energetically close 3Delta state with a smaller dipole moment. Overall our computed spectroscopic constants of the excited states with energy separations less than 18,000 cm(-1) agree quite well with those of the corresponding observed states.     

Azulene: polarized absorption in stretched polymers and magnetic circular dichroism

Linear dichroic and magnetic circular dichroic spectroscopy is used to characterize the excited states of azulene. Comparison with crystal spectra underlines the necessity of using two-parameter equations for evaluations of stretched sheet spectra despite some previous claims to the contrary. The MCD spectrum reveals strong vibronic interactions in the second transition.The results are interpreted on the basis of Pariser—Parr—Pople (PPP) type calculations including some with extensive configuration interaction (CI). Calculated signs of the first three B terms spectrum agree with experimental signs irrespective of the details of the PPP model, and even the numerical values are in an order-of-magnitude agreement. Only the calculation with extensive CI gives agreement for signs of higher B terms. It suggest the possible presence of two excited states, but the present experimental evidence for these is insufficient. One of these new states would be best described as a predominantly doubly excited state.The origin of the B terms is traced to contributions due to magnetic mixing of individual zero-order excited states and the results permit determination of the absolute sense of magnetic dipole transition moments between excited states with respect to the sense of the vector product of the electronic dipole transition moments from the ground state to these excited states. A comparison is performed with B terms calculated using finite perturbation technique and gauge-invariant orbitals.     

Cruciforms' polarized emission confirms disjoint molecular orbitals and excited states

Steady-state and time-resolved polarized spectroscopy studies reveal that electronic excitation to the third excited state of 1,4-distyryl-2,5-bis(arylethynyl)benzene cruciforms results in fluorescence emission that is shifted an angle of ca. 60°. This result is consistent with quantum chemical calculations of the lowest electronic excited states and their transition dipole moments. The shift originates from the disjointed nature of the occupied molecular orbitals being localized on the different branches of the cruciforms.