Wichard Beenken - Academia.edu (original) (raw)
Papers by Wichard Beenken
e-Journal of Surface Science and Nanotechnology, 2010
We have measured the bulk and surface electronic structure of several ionic liquids of alkyl-imid... more We have measured the bulk and surface electronic structure of several ionic liquids of alkyl-imidazolium cations with different alkyl chains (EMIm, BMIm, HMIm, OMIm) and bis(trifluoromethylsulfonyl)imide anions (Tf 2 N) by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS He I and He II) as well as metastable induced electron spectroscopy (MIES). The results are compared with the densities of states (DOS) calculated by density functional theory (DFT). By XPS we found the stoichiometry of the respective ILs reproduced, and different carbon atom positions reflected by the splitting of the C(1s) -from the trifluor-methyl groups (CF 3 ) of the anion with highest binding energy to the alkyl chains of the cation with the lowest one. Furthermore with increasing alkyl chain length the peak related to the alkyl group appears at slightly lower binding energy. The more bulk-sensitive XPS spectra reveal only minor differences in the valence band structure for the studied ionic liquids, whereas the more surface-sensitive methods UPS and especially MIES display distinct changes in the peak-intensities for varying the alkyl chain length. This is a strong indication for either a non-stoichiometric composition of the upmost molecular layer of the IL-surface and/or a reorientation of the cations, probably turning the alkyl chains to the surface.
Biophysical Chemistry, 2002
The Journal of Physical Chemistry B, 2004
In conjugated polymers the concept of spectroscopic units belonging to different spatial segments... more In conjugated polymers the concept of spectroscopic units belonging to different spatial segments of the chain, which are responsible for the spectroscopic properties of the polymer, has been used to explain the spectral heterogeneity and the excitation migration by (Förster type) hopping transfer. In the present work we study the possible mechanism of segmentation of polythiophene into spectroscopic units by using quantumchemical methods (ZINDO). We found that static geometric defects such as kinks or torsions do not result in a significant localization of the excited states to a certain segment. Hence, we propose that a dynamic localization of excitation due to the interaction between the nuclear and electronic degrees of freedom is responsible for the formation of the spectroscopic units.
The Journal of Physical Chemistry B, 2007
The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(t... more The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated as a function of temperature between 100 and 620 K. We used a combination of photoelectron spectroscopies (XPS and UPS), metastable induced electron spectroscopy (MIES), and high-resolution electron energy loss spectroscopy (HREELS). The valence band and HREELS spectra are interpreted on the basis of density functional theory (DFT) calculations. At room temperature, the most pronounced structures in the HREELS, UPS, and MIES spectra are related to the CF 3 group in the anion. Spectral changes observed at 100 K are interpreted as a change of the molecular orientation at the outermost surface, when the temperature is lowered. At elevated temperatures, early volatilization, starting at 350 K, is observed under reduced pressure. † Part of the special issue "Physical Chemistry of Ionic Liquids".
The Journal of Physical Chemistry A, 2006
Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermole... more Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and threedimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles et al. AdV. Mater. 2005, 17, 2070. First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.
Physical Review Letters, 2006
In isolated conjugated polymers two explanations are in discussion for the redshift of the emissi... more In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale -exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.
Physical Chemistry Chemical Physics, 2011
We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temp... more We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temperature ionic liquids composed of bis(trifluoromethylsulfonyl)imide anions ([Tf(2)N](-)) and either 1-ethyl-3-methyl-imidazolium ([EMIm](+)) or 1-octyl-3-methyl-imidazolium cations ([OMIm](+)). [T. Ikari, A. Keppler, M. Reinmöller, W. J. D. Beenken, S. Krischok, M. Marschewski, W. Maus-Friedrichs, O. Höfft and F. Endres, e-J. Surf. Sci. Nanotechnol., 2010, 8, 241.] In the present work we analyze these spectra by means of partial density of states (pDOS) as calculated from a single ion pair of the respective ionic liquid using density functional theory (DFT). Subsequently we reconstruct the XPS and UPS spectra by considering photoemission cross sections and analyze the MIES spectra by pDOS, which provides us decisive hints to the ionic liquid surface structure.
physica status solidi (b), 2008
PACS 71.15. Mb, 71.20.Rv Stimulated by the recent experimental report of optical properties of ch... more PACS 71.15. Mb, 71.20.Rv Stimulated by the recent experimental report of optical properties of charged oligofluorenes [S. Fratiloiu et al., J. Phys. Chem. B 110, 5984 -5993 (2006)], we have used timedependent density functional theory (TD-DFT) to study the electronic structure of neutral and charged ter-fluorene with and without keto-defect. We have characterized the excited states by site-and space-based representations of the density matrix. For the defectless neutral species we found typical excitons, while the first excited states of the corresponding charged species represent long-range oscillations of the electron or hole along the whole oligomer. For the neutral ketonized ter-fluorene we found trapping of the excited electron. Similarly, the unpaired electron of the anion ground state is trapped. The excited state properties of the cation, however, are insensitive to the keto-defect.
The Journal of Chemical Physics, 2005
We have studied biphenyl by time-dependent density-functional theory. In particular, we have anal... more We have studied biphenyl by time-dependent density-functional theory. In particular, we have analyzed the dependence of singlet excitation energies and transition dipoles on the torsional angle between the phenyl groups. The torsional spectrum has been computed quantum mechanically as well as semiclassically in order to understand how this influences the broadening of absorption and luminescence spectra. Our results are in best agreement with supersonic jet spectroscopy data, but also fit astonishingly well to spectra of biphenyl in condensed phase. Furthermore, we compare the torsional and vibrational relaxation and discuss qualitatively the general consequences for poly-para-phenylenes and related conjugated polymers as poly-thiophenes, considering, in particular, how side chains and solvents may affect the optical spectra.
The Journal of Chemical Physics, 2004
We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, wh... more We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-yl-benzo͓1,2,5͔thiadiazole ͑TBT͒ units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical ͑ZINDO͒ method we can assign the characteristic bands of the ''camel-back'' absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the -conjugated electron system. Additional ''dark'' charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly͑fluorene-co-benzothiadiazole͒ ͑F8BT͒, which contains benzo͓1,2,5͔thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same.
The Journal of Chemical Physics, 2006
Conformational disorder of conjugated polymers is an important issue to be understood and quantif... more Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.
The Journal of Chemical Physics, 2004
In conjugated polymers the optical excitation energy transfer is usually described as Förster-typ... more In conjugated polymers the optical excitation energy transfer is usually described as Förster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers.
Zeitschrift für Physikalische Chemie, 2006
The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3methylimidazo... more The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated with ultraviolet and X-ray photoelectron spectroscopy as well as metastable induced electron spectroscopy. The results have been compared with density functional theory calculations. The good agreement between the experimental and theoretical data provides detailed insight into the origin of the observed spectral features. In particular, we found that a simple composition of the spectra of the isolated ions does not suffice to fit to the experimental results, but interionic interactions have to be considered.
FEBS Letters, 2000
Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple b... more Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple bacteria has revealed circular aggregates of high rotational symmetry (C8, C9 and C16, respectively). Quantum-chemical calculations indicate that in particular the waterwheel-like arrangements of pigments should show characteristic structure-sensitive spectroscopic behavior in the near infrared absorption region. Laser-spectroscopic data obtained with non-crystallized, isolated LH2 of Rhodospirillum molischianum are in line with a highly symmetric (C8) circular aggregate, but deviations have been found for LH2 of Rhodobacter sphaeroides and Rhodopseudomonas acidophila. For both the latter, C-shaped incomplete circular aggregates (as seen only recently in electron micrographs of crystallized LH1-reaction center complexes) may be a suitable preliminary model.
Chemical Physics Letters, 1997
The So-S t absorption band substructure of pinacyanol dissolved in ethanol has been investigated ... more The So-S t absorption band substructure of pinacyanol dissolved in ethanol has been investigated with nonlinear polarisation spectroscopy in the frequency domain (NLPF). The evaluation of the experimental results has been done in the frame of global analysis for a set of four NLPF spectra at two test wavelengths. Four bands with maxima located at 520, 556, 565 and 607 nm have been identified. The energy relaxation after excitation in the 565 nm band is slower (T~ = 13 ps) than in the red-most band (Tj = 7 ps). This behavior, inverse to usual excess-energy relaxation of organic molecules, suggests that these subbands belong to different isomers of pinacyanol.
Real-space collective excitation dynamics in molecular aggregates is studied using a model where ... more Real-space collective excitation dynamics in molecular aggregates is studied using a model where the electronic system is described via exciton theory with surface hopping. The nuclear dynamics are included using the Langevin equation where temperature and zero-point motions are entered via the fluctuation-dissipation theorem. Dynamic processes like exciton relaxation, localization, polaron formation and diffusion of self-trapped excitons, which commonly require different theories, are simultaneously described with our approach. Numerical simulations of small linear aggregates are performed. Contrary to the common view we show that exciton relaxation can temporarily increase exciton delocalization. The results are discussed based on the photosynthetic light-harvesting pigment-protein complexes.
Chemical Physics, 2006
Electronic structure of two conjugated polymers, PPV and its derivative DIOXA-PPV, are studied by... more Electronic structure of two conjugated polymers, PPV and its derivative DIOXA-PPV, are studied by quantum-chemistry methods. Good agreement between the calculations and available experimental data validates the applied methods and enables us to draw conclusions about properties of excitons and polarons in these polymers. In particular, simultaneous use of different correlation plots in atomic site representation and transition densities in real space representation reveals electronic structure and spatial localization of the elementary excitations. The properties of the DIOXA-PPV are discussed in context of possible applications in molecular electronics devices.
Biophysical Journal, 2002
Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q y band) of... more Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q y band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q y band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach reveals that one emitting spectral form, characterized by a 2.3(Ϯ0.8)-fold larger dipole strength than monomeric Chl a, dominates the fluorescence spectrum of LHC-II. Considering available structural and spectroscopic data, these results can be consistently explained assuming the existence of an excitonically coupled dimer located at Chl-bindings sites a2 and b2 (referring to the original notation of W. Kü hlbrandt, D. N. Wang, and Y. Fujiyoshi, Nature, 1994, 367:614 -621), which must not necessarily correspond to Chls a and b). This fluorescent dimer, terminating the excitation energy-transfer chain of the LHC-II monomeric subunit, is discussed with respect to its relevance for intra-and inter-antenna excitation energy transfer.
Biochemistry, 2002
Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II... more Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II and CP29 have been investigated by complementary nonlinear laser spectroscopic techniques. Nonlinear absorption of fs pulses indicates an increase in absorption cross section in the red wing of the Q y -band of LHC II as compared to monomeric chlorophyll (Chl) a. Nonlinear polarization spectroscopy in the frequency domain (NLPF) exploiting also the intensity dependence of the NLPF signal reveals that in LHC II a species emitting at 682 nm is characterized by a two-fold dipole strength as compared to monomeric Chl a. NLPF experiments further indicate strong excitonic coupling between Chls a and b in CP29. Moreover, the lowest Q y -transition in CP29 (in contrast to LHC II) is assigned to a nonexcitonically coupled Chl a. Stepwise two-photon excitation of Chls a and b with fs pulses in the (red) Q y -region results in a weak "blue" fluorescence in LHC II and CP29. The dependence of the spectral shape of the blue fluorescence on excitation wavelength is consistent with the existence of strongly excitonically coupled Chls a and b in LHC II. The results bear important implications for the refinement of structural models and intra-as well as inter-antenna energy-transfer mechanisms.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1997
The unique high spectral resolution resulting from application of nonlinear polarisation spectros... more The unique high spectral resolution resulting from application of nonlinear polarisation spectroscopy in the frequency Ž . domain at 77 K directly reveals 6 subbands at 649, 657, 667, 672, 675 and 679 nm in the chlorophyll a r b Q -region of y Ž . trimeric light-harvesting complex II. A shortening from about 90 to 35 fs of the corresponding dephasing times towards the 'blue' edge of the Q -band was observed. q 1997 Elsevier Science B.V. y
e-Journal of Surface Science and Nanotechnology, 2010
We have measured the bulk and surface electronic structure of several ionic liquids of alkyl-imid... more We have measured the bulk and surface electronic structure of several ionic liquids of alkyl-imidazolium cations with different alkyl chains (EMIm, BMIm, HMIm, OMIm) and bis(trifluoromethylsulfonyl)imide anions (Tf 2 N) by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS He I and He II) as well as metastable induced electron spectroscopy (MIES). The results are compared with the densities of states (DOS) calculated by density functional theory (DFT). By XPS we found the stoichiometry of the respective ILs reproduced, and different carbon atom positions reflected by the splitting of the C(1s) -from the trifluor-methyl groups (CF 3 ) of the anion with highest binding energy to the alkyl chains of the cation with the lowest one. Furthermore with increasing alkyl chain length the peak related to the alkyl group appears at slightly lower binding energy. The more bulk-sensitive XPS spectra reveal only minor differences in the valence band structure for the studied ionic liquids, whereas the more surface-sensitive methods UPS and especially MIES display distinct changes in the peak-intensities for varying the alkyl chain length. This is a strong indication for either a non-stoichiometric composition of the upmost molecular layer of the IL-surface and/or a reorientation of the cations, probably turning the alkyl chains to the surface.
Biophysical Chemistry, 2002
The Journal of Physical Chemistry B, 2004
In conjugated polymers the concept of spectroscopic units belonging to different spatial segments... more In conjugated polymers the concept of spectroscopic units belonging to different spatial segments of the chain, which are responsible for the spectroscopic properties of the polymer, has been used to explain the spectral heterogeneity and the excitation migration by (Förster type) hopping transfer. In the present work we study the possible mechanism of segmentation of polythiophene into spectroscopic units by using quantumchemical methods (ZINDO). We found that static geometric defects such as kinks or torsions do not result in a significant localization of the excited states to a certain segment. Hence, we propose that a dynamic localization of excitation due to the interaction between the nuclear and electronic degrees of freedom is responsible for the formation of the spectroscopic units.
The Journal of Physical Chemistry B, 2007
The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(t... more The near-surface structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated as a function of temperature between 100 and 620 K. We used a combination of photoelectron spectroscopies (XPS and UPS), metastable induced electron spectroscopy (MIES), and high-resolution electron energy loss spectroscopy (HREELS). The valence band and HREELS spectra are interpreted on the basis of density functional theory (DFT) calculations. At room temperature, the most pronounced structures in the HREELS, UPS, and MIES spectra are related to the CF 3 group in the anion. Spectral changes observed at 100 K are interpreted as a change of the molecular orientation at the outermost surface, when the temperature is lowered. At elevated temperatures, early volatilization, starting at 350 K, is observed under reduced pressure. † Part of the special issue "Physical Chemistry of Ionic Liquids".
The Journal of Physical Chemistry A, 2006
Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermole... more Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and threedimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles et al. AdV. Mater. 2005, 17, 2070. First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.
Physical Review Letters, 2006
In isolated conjugated polymers two explanations are in discussion for the redshift of the emissi... more In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale -exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.
Physical Chemistry Chemical Physics, 2011
We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temp... more We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temperature ionic liquids composed of bis(trifluoromethylsulfonyl)imide anions ([Tf(2)N](-)) and either 1-ethyl-3-methyl-imidazolium ([EMIm](+)) or 1-octyl-3-methyl-imidazolium cations ([OMIm](+)). [T. Ikari, A. Keppler, M. Reinmöller, W. J. D. Beenken, S. Krischok, M. Marschewski, W. Maus-Friedrichs, O. Höfft and F. Endres, e-J. Surf. Sci. Nanotechnol., 2010, 8, 241.] In the present work we analyze these spectra by means of partial density of states (pDOS) as calculated from a single ion pair of the respective ionic liquid using density functional theory (DFT). Subsequently we reconstruct the XPS and UPS spectra by considering photoemission cross sections and analyze the MIES spectra by pDOS, which provides us decisive hints to the ionic liquid surface structure.
physica status solidi (b), 2008
PACS 71.15. Mb, 71.20.Rv Stimulated by the recent experimental report of optical properties of ch... more PACS 71.15. Mb, 71.20.Rv Stimulated by the recent experimental report of optical properties of charged oligofluorenes [S. Fratiloiu et al., J. Phys. Chem. B 110, 5984 -5993 (2006)], we have used timedependent density functional theory (TD-DFT) to study the electronic structure of neutral and charged ter-fluorene with and without keto-defect. We have characterized the excited states by site-and space-based representations of the density matrix. For the defectless neutral species we found typical excitons, while the first excited states of the corresponding charged species represent long-range oscillations of the electron or hole along the whole oligomer. For the neutral ketonized ter-fluorene we found trapping of the excited electron. Similarly, the unpaired electron of the anion ground state is trapped. The excited state properties of the cation, however, are insensitive to the keto-defect.
The Journal of Chemical Physics, 2005
We have studied biphenyl by time-dependent density-functional theory. In particular, we have anal... more We have studied biphenyl by time-dependent density-functional theory. In particular, we have analyzed the dependence of singlet excitation energies and transition dipoles on the torsional angle between the phenyl groups. The torsional spectrum has been computed quantum mechanically as well as semiclassically in order to understand how this influences the broadening of absorption and luminescence spectra. Our results are in best agreement with supersonic jet spectroscopy data, but also fit astonishingly well to spectra of biphenyl in condensed phase. Furthermore, we compare the torsional and vibrational relaxation and discuss qualitatively the general consequences for poly-para-phenylenes and related conjugated polymers as poly-thiophenes, considering, in particular, how side chains and solvents may affect the optical spectra.
The Journal of Chemical Physics, 2004
We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, wh... more We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-yl-benzo͓1,2,5͔thiadiazole ͑TBT͒ units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical ͑ZINDO͒ method we can assign the characteristic bands of the ''camel-back'' absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the -conjugated electron system. Additional ''dark'' charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly͑fluorene-co-benzothiadiazole͒ ͑F8BT͒, which contains benzo͓1,2,5͔thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same.
The Journal of Chemical Physics, 2006
Conformational disorder of conjugated polymers is an important issue to be understood and quantif... more Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.
The Journal of Chemical Physics, 2004
In conjugated polymers the optical excitation energy transfer is usually described as Förster-typ... more In conjugated polymers the optical excitation energy transfer is usually described as Förster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers.
Zeitschrift für Physikalische Chemie, 2006
The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3methylimidazo... more The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated with ultraviolet and X-ray photoelectron spectroscopy as well as metastable induced electron spectroscopy. The results have been compared with density functional theory calculations. The good agreement between the experimental and theoretical data provides detailed insight into the origin of the observed spectral features. In particular, we found that a simple composition of the spectra of the isolated ions does not suffice to fit to the experimental results, but interionic interactions have to be considered.
FEBS Letters, 2000
Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple b... more Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple bacteria has revealed circular aggregates of high rotational symmetry (C8, C9 and C16, respectively). Quantum-chemical calculations indicate that in particular the waterwheel-like arrangements of pigments should show characteristic structure-sensitive spectroscopic behavior in the near infrared absorption region. Laser-spectroscopic data obtained with non-crystallized, isolated LH2 of Rhodospirillum molischianum are in line with a highly symmetric (C8) circular aggregate, but deviations have been found for LH2 of Rhodobacter sphaeroides and Rhodopseudomonas acidophila. For both the latter, C-shaped incomplete circular aggregates (as seen only recently in electron micrographs of crystallized LH1-reaction center complexes) may be a suitable preliminary model.
Chemical Physics Letters, 1997
The So-S t absorption band substructure of pinacyanol dissolved in ethanol has been investigated ... more The So-S t absorption band substructure of pinacyanol dissolved in ethanol has been investigated with nonlinear polarisation spectroscopy in the frequency domain (NLPF). The evaluation of the experimental results has been done in the frame of global analysis for a set of four NLPF spectra at two test wavelengths. Four bands with maxima located at 520, 556, 565 and 607 nm have been identified. The energy relaxation after excitation in the 565 nm band is slower (T~ = 13 ps) than in the red-most band (Tj = 7 ps). This behavior, inverse to usual excess-energy relaxation of organic molecules, suggests that these subbands belong to different isomers of pinacyanol.
Real-space collective excitation dynamics in molecular aggregates is studied using a model where ... more Real-space collective excitation dynamics in molecular aggregates is studied using a model where the electronic system is described via exciton theory with surface hopping. The nuclear dynamics are included using the Langevin equation where temperature and zero-point motions are entered via the fluctuation-dissipation theorem. Dynamic processes like exciton relaxation, localization, polaron formation and diffusion of self-trapped excitons, which commonly require different theories, are simultaneously described with our approach. Numerical simulations of small linear aggregates are performed. Contrary to the common view we show that exciton relaxation can temporarily increase exciton delocalization. The results are discussed based on the photosynthetic light-harvesting pigment-protein complexes.
Chemical Physics, 2006
Electronic structure of two conjugated polymers, PPV and its derivative DIOXA-PPV, are studied by... more Electronic structure of two conjugated polymers, PPV and its derivative DIOXA-PPV, are studied by quantum-chemistry methods. Good agreement between the calculations and available experimental data validates the applied methods and enables us to draw conclusions about properties of excitons and polarons in these polymers. In particular, simultaneous use of different correlation plots in atomic site representation and transition densities in real space representation reveals electronic structure and spatial localization of the elementary excitations. The properties of the DIOXA-PPV are discussed in context of possible applications in molecular electronics devices.
Biophysical Journal, 2002
Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q y band) of... more Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q y band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q y band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach reveals that one emitting spectral form, characterized by a 2.3(Ϯ0.8)-fold larger dipole strength than monomeric Chl a, dominates the fluorescence spectrum of LHC-II. Considering available structural and spectroscopic data, these results can be consistently explained assuming the existence of an excitonically coupled dimer located at Chl-bindings sites a2 and b2 (referring to the original notation of W. Kü hlbrandt, D. N. Wang, and Y. Fujiyoshi, Nature, 1994, 367:614 -621), which must not necessarily correspond to Chls a and b). This fluorescent dimer, terminating the excitation energy-transfer chain of the LHC-II monomeric subunit, is discussed with respect to its relevance for intra-and inter-antenna excitation energy transfer.
Biochemistry, 2002
Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II... more Manifestation and extent of excitonic interactions in the plant light-harvesting complexes LHC II and CP29 have been investigated by complementary nonlinear laser spectroscopic techniques. Nonlinear absorption of fs pulses indicates an increase in absorption cross section in the red wing of the Q y -band of LHC II as compared to monomeric chlorophyll (Chl) a. Nonlinear polarization spectroscopy in the frequency domain (NLPF) exploiting also the intensity dependence of the NLPF signal reveals that in LHC II a species emitting at 682 nm is characterized by a two-fold dipole strength as compared to monomeric Chl a. NLPF experiments further indicate strong excitonic coupling between Chls a and b in CP29. Moreover, the lowest Q y -transition in CP29 (in contrast to LHC II) is assigned to a nonexcitonically coupled Chl a. Stepwise two-photon excitation of Chls a and b with fs pulses in the (red) Q y -region results in a weak "blue" fluorescence in LHC II and CP29. The dependence of the spectral shape of the blue fluorescence on excitation wavelength is consistent with the existence of strongly excitonically coupled Chls a and b in LHC II. The results bear important implications for the refinement of structural models and intra-as well as inter-antenna energy-transfer mechanisms.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1997
The unique high spectral resolution resulting from application of nonlinear polarisation spectros... more The unique high spectral resolution resulting from application of nonlinear polarisation spectroscopy in the frequency Ž . domain at 77 K directly reveals 6 subbands at 649, 657, 667, 672, 675 and 679 nm in the chlorophyll a r b Q -region of y Ž . trimeric light-harvesting complex II. A shortening from about 90 to 35 fs of the corresponding dephasing times towards the 'blue' edge of the Q -band was observed. q 1997 Elsevier Science B.V. y