Co-reporter:Kaoru Ohta, Shunrou Tokonami, Kotaro Takahashi, Yuto Tamura, Hiroko Yamada, and Keisuke Tominaga
The Journal of Physical Chemistry B November 2, 2017 Volume 121(Issue 43) pp:10157-10157
Publication Date(Web):October 12, 2017
DOI:10.1021/acs.jpcb.7b07025
To improve the power conversion efficiency of solar cells, it is important to understand the underlying relaxation mechanisms of photogenerated charge carriers in organic semiconductors. In this work, we studied the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin thin films where the diketopyrrolopyrrole unit has two n-butyl groups, abbreviated as C4-DPP-BP. We used time-resolved terahertz (THz) spectroscopy to track charge carrier dynamics with excitations at 800 and 400 nm. Compared with tetrabenzoporphyrin (BP), the extension of π-electron delocalization to the diketopyrrolopyrrole peripherals leads to an increase in absorption in the near-infrared region. Following the excitation at 800 nm, we found that the transient THz signals in C4-DPP-BP thin films decay with time constants of 0.5 and 9.1 ps, with small residual components. With excitation at 400 nm, we found that the transient THz signals decay with time constants of 0.4 and 7.5 ps. On the basis of the similarity of the decay profiles of the transient THz signals obtained with excitations at 400 and 800 nm, we considered that the decaying components are due to charge carrier recombination and/or trapping at defect sites, which do not depend on the excess energy of the photoexcitation. In contrast to BP, even without an electron acceptor, we observed the finite offset of the transient THz signals at 100 ps, demonstrating the existence of long-lived charge carriers. We also measured the photoconductivity spectra of C4-DPP-BP thin films with the excitation at both 800 and 400 nm. It was found that the spectra can be fitted by the Drude–Smith model. From these results, it was determined that the charge carriers are localized right after photoexcitation. At 0.4 ps, the product of the quantum yield of charge generation and mobility of charge carriers at 400 nm is approximately twice that obtained at 800 nm. We discuss the implications of the excess excitation energy in organic semiconductor-based devices.
Co-reporter:Masaki Okuda, Masahiro Higashi, Kaoru Ohta, Shinji Saito, Keisuke Tominaga
Chemical Physics Letters 2017 Volume 683(Volume 683) pp:
Publication Date(Web):1 September 2017
DOI:10.1016/j.cplett.2017.03.008
•Theoretical study on the frequency fluctuation of SCN− in H2O with MD simulations.•Interactions form water molecules around SCN− dominate the frequency fluctuation.•Water molecules in the hydration shell of SCN− show bulk-like hydrogen bond dynamics.•Collective reorientation of water molecules nearby SCN− is similar to that in bulk.The vibrational dynamics of SCN− in H2O are theoretically investigated by molecular dynamics simulations. Based on the vibrational solvatochromism theory, we calculate the frequency-frequency time correlation function of the SCN anti-symmetric stretching mode, which is characterized by time constants of 0.13 and 1.41 ps. We find that the frequency fluctuation is almost determined by the electrostatic interaction from the water molecules in the first-hydration shell. The collective dynamics of the water molecules in the first-hydration shell is found to be similar to that of bulk water, though the hydrogen bond between the ion and water molecule is very strong.Download high-res image (107KB)Download full-size image
Co-reporter:Partha Dutta, Jessica Afalla, Arnab Halder, Sudeshna Datta, and Keisuke Tominaga
The Journal of Physical Chemistry C 2017 Volume 121(Issue 3) pp:
Publication Date(Web):January 3, 2017
DOI:10.1021/acs.jpcc.6b10412
In this work, we have studied the temperature-dependent conductivity of graphene oxide (GO) and graphene oxide–polyaniline (GO-PANI) nanocomposites by terahertz time-domain spectroscopy (THz-TDS) from 78 to 293 K. The refractive index and absorption coefficient are related to the conductivity, and it has been found that in the THz region, the real part of the complex conductivity of GO is less than that of GO-PANI over the entire range of experimental temperatures. Both the systems exhibit an increase in these physical parameters with increasing temperature. The complex conductivity spectra of these systems in the THz region are well fitted by an analytical model that has contributions from the scattering of free electrons (Drude–Smith term) and from bound-electron oscillation (Lorentz term). The fitting analysis suggests a more ordered structure and anharmonicity for both the systems with increasing temperature and reports that GO-PANI has a greater free electron density and damping frequency of oscillation and more ordered structures than GO at all temperatures.
Co-reporter:Alvin Karlo G. Tapia
Journal of Infrared, Millimeter, and Terahertz Waves 2017 Volume 38( Issue 7) pp:885-895
Publication Date(Web):08 February 2017
DOI:10.1007/s10762-017-0363-4
The conduction properties of polyaniline (PAni) with various processing conditions were demonstrated using terahertz time-domain spectroscopy. The effects of protonation were observed by varying the doping time and annealing time. The measurements agreed with the Mott-Davis hopping model with the exponent varying inversely with the protonation conditions. Also, the density of the samples increased with compression pressure thereby increasing the inter-chain and inter-particle charge transport. Lastly, the THz response on PAni-polyethylene pellets showed dominant contribution of PAni network.
Co-reporter:Naoki Yamamoto, Kaoru Ohta, Atsuo Tamura, and Keisuke Tominaga
The Journal of Physical Chemistry B 2016 Volume 120(Issue 21) pp:4743-4755
Publication Date(Web):May 9, 2016
DOI:10.1021/acs.jpcb.6b01491
We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ∼20 ps and ∼100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water–protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron-scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.
Co-reporter:Feng Zhang, Houng-Wei Wang, Keisuke Tominaga, Michitoshi Hayashi, Sunglin Lee, and Takashi Nishino
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 22) pp:4671-4676
Publication Date(Web):November 1, 2016
DOI:10.1021/acs.jpclett.6b02213
The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. Poly(l-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.
Co-reporter:Naoki Yamamoto, Tomoyo Andachi, Atsuo Tamura, and Keisuke Tominaga
The Journal of Physical Chemistry B 2015 Volume 119(Issue 29) pp:9359-9368
Publication Date(Web):December 4, 2014
DOI:10.1021/jp5099766
We have studied temperature and hydration dependent low-frequency spectra of lipid bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphoryl-3′-rac-glycerol (DMPG) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by terahertz time-domain spectroscopy (THz-TDS). We measured X-ray diffraction patterns and mid-infrared spectra of these lipid bilayers and found that the lipid bilayers have two different types of phases, i.e., the gel phase and the crystalline phase, depending on the preparation methods of the samples. In both phases, a few distinct bands were observed in the THz region. For DMPG, the peak wavenumbers of the absorption bands did not change upon hydration, while the bandwidth in the crystalline phase was smaller than that in the gel phase. We performed spectral analyses for the complex dielectric spectra for DMPG and DMPC with a model function, mainly to determine the peak wavenumbers of the absorption bands. In contrast to the case of the DMPG bilayers, the peak wavenumber of the absorption band of the DMPC bilayer shifts upon hydration. In the hydrated DMPC bilayer, it was suggested fast reorienting water molecules exist with a relaxation time of sub-picoseconds. It is suggested that the THz absorption patterns reflect the lipid packing pattern in the bilayers. The temperature dependence of the absorption band was analyzed by an empirical equation, and the anharmonicity of the vibrational potential of the low-frequency mode was quantitatively evaluated.
Co-reporter:Yusuke Iima, Daiki Kuzuhara, Zhao-Li Xue, Seiji Akimoto, Hiroko Yamada and Keisuke Tominaga
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 26) pp:13129-13135
Publication Date(Web):10 Apr 2014
DOI:10.1039/C4CP00301B
We have investigated the photophysical properties of alkyl-substituted triphyrin(2.1.1) (ATp) and benzotriphyrin(2.1.1) (BTp) by steady-state and time-resolved fluorescence spectroscopy. We focused on the effect of NH proton tautomerization, planarity of the macrocycles, and substituents on these properties. The fluorescence quantum yields (Φy) of ATp did not depend on solvent viscosity, whereas those of BTp increased with solvent viscosity, reaching a maximum value of 0.17 in paraffin. Interestingly, analyzing Φy showed that the non-radiative rate constant of BTp decreased sharply as the solvent viscosity increased. These results suggest that the substituted phenyl groups play a crucial role in suppressing molecular distortion, thus leading to decreased non-radiative relaxation in triphyrin(2.1.1). The hydrogen bond formed in the inner cavity potentially contributes to the suppression of the structural distortion, whereas the pyrrole rings in the macrocycle are close, as in porphycene.
Co-reporter:Alvin Karlo G. Tapia, Keisuke Tominaga
Chemical Physics Letters 2014 Volume 598() pp:39-42
Publication Date(Web):8 April 2014
DOI:10.1016/j.cplett.2014.02.055
•We measured the frequency- and temperature-dependence of conductivity of PAni.•We have shown the Mott–Davis behavior for PAni and correlated-barrier hopping model.•Varying %PAni suggests tuning of conductivity for applications like sensing and coating.The temperature-dependent conductivity of polyaniline emeraldine salt (PAni-ES) was studied by terahertz (THz) time-domain spectroscopy from 80 to 290 K to investigate conduction properties in the THz region. The absorption coefficient and index of refraction increase with temperature. This reflects an increasing conductivity, which indicates a thermally assisted hopping transition. The frequency-dependent behavior of the conductivity is described by using the Mott–Davis model. The model fitting parameter, S, decreases with increasing temperature, indicating a possible correlated barrier hopping mechanism. Lastly, the activation energy at THz frequencies decreases with increasing frequency, suggesting intraband transitions.
Co-reporter:Tomoyo Andachi;Naoki Yamamoto;Atsuo Tamura
Journal of Infrared, Millimeter, and Terahertz Waves 2014 Volume 35( Issue 1) pp:147-157
Publication Date(Web):2014 January
DOI:10.1007/s10762-013-0003-6
We have investigated the low-frequency spectra of a phospholipid bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by terahertz time-domain spectroscopy (THz-TDS). We focused on the temperature and hydration dependence of the low-frequency spectra of a gel-phase sample. The spectra of the dehydrated and hydrated samples showed shoulder bands at 45 and 30 cm-1, respectively. In contrast to the dehydrated sample, in the hydrated sample spectra the slope of the temperature change of the absorption coefficient increased sharply around 240 K. This result suggests that water molecules affect the change in the low-frequency dynamics. We obtained the absorption coefficient difference spectra for different hydration levels to clarify the mechanism of the spectral change.
Co-reporter:Feng Zhang, Daisuke Adachi, Sriappareddy Tamalampudi, Akihiko Kondo, and Keisuke Tominaga
Energy & Fuels 2013 Volume 27(Issue 10) pp:5957-5961
Publication Date(Web):September 5, 2013
DOI:10.1021/ef4012998
Fourier-transform infrared spectroscopy with the attenuated total reflection method provides a fast, reliable technique to monitor the conversion of oil to fatty acid methyl esters (FAME) during biodiesel production. In this work, we employed a linear calibration method to monitor the generation of FAME in the transesterification of soybean oil and methanol, catalyzed by immobilized Candida antarctica lipase B (CALB). The interaction of the immobilized CALB catalyst with the substrates and products improved its catalytic performance. The rate law changed from zero-order in the first run to quasi-first-order in the subsequent three reaction runs. The quasi-first-order rate law was characterized by a phenomenological compressed exponential function.
Co-reporter:Sayuri Yamaguchi, Motohiro Banno, Kaoru Ohta, and Keisuke Tominaga
The Journal of Physical Chemistry A 2013 Volume 117(Issue 29) pp:6323-6331
Publication Date(Web):May 23, 2013
DOI:10.1021/jp401461k
A study on the vibrational dynamics of the NH stretching mode of pyrrole–base complexes in carbon tetrachloride, using subpicosecond infrared pump–probe (PP) spectroscopy, is reported. The time evolution of the PP signal of the NH stretching mode for all the complexes was frequency-dependent; the signal decay time increased with the frequency. This frequency dependence was thought to originate from the relationship between vibrational energy relaxation (VER) and spectral diffusion. For hydrogen-bonded systems, spectral diffusion corresponds to the reorganization of the solvent environment. Qualitative analysis of the frequency dependence of the PP signal decay time indicated that a simple energy gap law could not be applied to all the pyrrole–base complexes. This conclusion was supported by spectral simulation of the PP signal using the modified Smoluchowski equation to clarify the frequency dependence of the VER and the spectral diffusion.
Co-reporter:Naoki Yamamoto, Ohki Kambara, Kohji Yamamoto, Atsuo Tamura, Shinji Saito and Keisuke Tominaga
Soft Matter 2012 vol. 8(Issue 6) pp:1997-2006
Publication Date(Web):23 Dec 2011
DOI:10.1039/C1SM06433A
We have investigated low-frequency spectra of poly-L-glutamic acid (polyE) in the powder state by terahertz time-domain spectroscopy (THz-TDS). Samples with three different secondary structures (α-helix, β-sheet, and random-coil) and different chain lengths were prepared to investigate the dependence of the THz spectra on temperature and hydration. The temperature dependence of the THz absorption spectra clearly shows that polyE, regardless of its secondary structure, undergoes dynamical transition between 190 and 240 K. We have estimated the apparent activation energy and transition temperature by phenomenological spectral analysis. We also have estimated the effective dipole moment of the amino acid residue from the real part of the dielectric permittivity at zero frequency. Both results show that the transition temperature is lower when the secondary structure undergoes a transition from a random-coil structure to an α-helix or β-sheet structure. Furthermore, both hydrating water molecules and peptide hydrogen bonds contribute to induce anharmonicity in the low-frequency vibrational motions. Meanwhile, hydration, not peptide hydrogen bonds, is crucial for the dynamical transition to occur because the onset of anharmonicity was observed only when the polypeptide is hydrated. An apparent intermolecular vibrational mode in the β-sheet structure, which suggests a highly ordered structure in the sample, did not exhibit anharmonicity at the tested temperatures and humidity levels. This result suggests that short-range or inter-strand hydrogen bonds of the α-helix or low-ordered β-sheet structures gave rise to the lower transition temperatures and the smaller effective activation energies compared with those of the random-coil structure.
Co-reporter:Kaoru Ohta, Jumpei Tayama and Keisuke Tominaga
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 30) pp:10455-10465
Publication Date(Web):28 Mar 2012
DOI:10.1039/C2CP40244K
In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN− and N3− in several polar solvents. We used an infrared (IR) pump–probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN− and N3−, we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3–6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN− and N3− is discussed in terms of the difference between protic and aprotic solvents.
Co-reporter:Motohiro Banno, Kaoru Ohta and Keisuke Tominaga
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 18) pp:6359-6366
Publication Date(Web):01 Feb 2012
DOI:10.1039/C2CP23647H
Solute–solvent interactions between acetate and D2O were investigated by vibrational spectroscopic methods. The vibrational dynamics of the COO asymmetric stretching mode in D2O was observed by time-resolved infrared (IR) pump–probe spectroscopy. The pump–probe signal contained both decay and oscillatory components. The time dependence of the decay component could be explained by a double exponential function with time constants of 200 fs and 2.6 ps, which are the same for both the COO asymmetric and symmetric stretching modes. The Fourier spectrum of the oscillatory component contained a band around 80 cm−1, which suggests that the COO asymmetric stretching mode couples to a low-frequency vibrational mode with a wavenumber of 80 cm−1. Based on quantum chemistry calculations, we propose that a bridged complex comprising an acetate ion and one D2O molecule, in which the two oxygen atoms in the acetate anion form hydrogen bonds with the two deuterium atoms in D2O, is the most stable structure. The 80 cm−1 low-frequency mode was assigned to the asymmetric stretching vibration of the hydrogen bond in the bridged complex.
Co-reporter:Sayuri Yamaguchi, Keisuke Tominaga and Shinji Saito
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 32) pp:14742-14749
Publication Date(Web):11 Jul 2011
DOI:10.1039/C1CP20912D
The low-frequency modes of the benzoic acid (BA) dimer and its analogues in carbon tetrachloride (CCl4) have been investigated by terahertz time-domain spectroscopy. The solute spectrum is obtained by subtracting the solvent contribution from that of the solution. The difference spectrum of BA in CCl4 has a broad band with a peak at 68 cm−1. To assign the observed band, the spectrum is compared with spectra of other aromatic molecules, such as benzene and phenol in addition to p-methyl BA and deuterated BA species (BA-dOH and BA-d5) in CCl4. The band at 68 cm−1 is assigned to the cogwheel mode of the BA dimer. Density functional theory calculations also support this assignment. Finally, spectral lineshape analysis based on the multimode Brownian oscillator model is applied to the THz spectra for all the samples.
Co-reporter:Yusuke Iima, Daiki Kuzuhara, Zhao-Li Xue, Hidemitsu Uno, Seiji Akimoto, Hiroko Yamada, Keisuke Tominaga
Chemical Physics Letters 2011 Volume 513(1–3) pp:67-71
Publication Date(Web):6 September 2011
DOI:10.1016/j.cplett.2011.07.075
Abstract
Excited state dynamics of benzotriphyrin(2.1.1) is investigated by time-resolved fluorescence techniques. The fluorescence quantum yield is found to be 0.005, and the lifetime of the excited state of triphyrin(2.1.1) is found to be 140 ps. Fluorescence anisotropy clearly shows that intramolecular proton transfer does not occur within the excited state lifetime. The short lifetime of the excited state is due to fast internal conversion (IC) from the S1 state, and to the nonplanarity of the macrocycle enhancing IC decay pathways.
Co-reporter:Ohki Kambara, Keisuke Tominaga, Jun-ichi Nishizawa, Tetsuo Sasaki, Houng-Wei Wang, Michitoshi Hayashi
Chemical Physics Letters 2010 Volume 498(1–3) pp:86-89
Publication Date(Web):30 September 2010
DOI:10.1016/j.cplett.2010.08.047
The vibrational modes of 2-furoic acid, a model hydrogen-bonding molecule, are assigned in the terahertz frequency region by terahertz transmittance spectroscopy of normal and deuterated crystalline forms of 2-furoic acid at 5 K and by density functional theory calculations incorporating periodic crystal structures.Graphical abstractWe demonstrated a rigorous procedure of assigning vibrational modes in the THz frequency region.Research highlights► Vibrational modes of 2-furoic acid, a model hydrogen-bonding molecule, in the THz frequency region. ► Normal and deuterated crystalline forms at 5 K. ► Density functional theory calculations incorporating periodic crystal structures.
Co-reporter:Carlito S. Ponseca Jr.;Ohki Kambara
Journal of Infrared, Millimeter, and Terahertz Waves 2010 Volume 31( Issue 7) pp:799-809
Publication Date(Web):2010 July
DOI:10.1007/s10762-010-9636-x
The low-frequency spectra of the amino acids l-alanine and glycine and their peptides were studied using terahertz (THz) time-domain spectroscopy (TDS) at room temperature. In a previous work (Yamamoto et al., Biophys. J. 89, L22–L24 (2005)), the low-frequency spectra of amino acids (glycine and l-alanine) and their polypeptides (polyglycine and poly- l-alanine) were studied by THz-TDS, and it was found that there is a clear difference in low-frequency dynamics between the amino acids and the polypeptides. In the present study, amino acids and short peptides were chosen in order to investigate the effect of polymerization on low-frequency spectra. We focus on two physical quantities to represent the spectral features: (1) the intensity of the reduced absorption cross section (RACS), which we define from the absorption coefficient and refractive index, and (2) the exponent in the power law behavior of the RACS. We found that the two physical quantities show different dependences on peptide chain length, suggesting that the two physical quantities reflect different dynamics and interactions. The change in RACS intensity may be due to intermolecular or intrachain motion. The validity of the assumption of constant IR activity in the investigated frequency region is critical to understanding the origin of the variation in the exponent with chain length.
Co-reporter:Ohki Kambara;Atsuo Tamura;Takashi Uchino;Kohji Yamamoto
Biopolymers 2010 Volume 93( Issue 8) pp:
Publication Date(Web):
DOI:10.1002/bip.21467
Abstract
Poly-L-lysine is known to have three different secondary structures depending on solvent conditions because of its flexible nature. In previous work (Kambara et al., Phys Chem Chem Phys 2008, 10, 5042-5044), we observed two different types of structural changes in poly-L-lysine. In the present study, we investigated the low-frequency spectrum of poly-L-lysine with a β-sheet structure in the solid state by terahertz time-domain spectroscopy. On the basis of this spectroscopic analysis, we found that the low-frequency dynamics differed from those of other polypeptides. Furthermore, we performed powder X-ray diffraction measurement on poly-L-lysine, which was found to be highly amorphous compared with other polypeptides. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 735–739, 2010.
This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com
Co-reporter:Partha Dutta, Keisuke Tominaga
Journal of Molecular Liquids 2009 Volume 147(1–2) pp:45-51
Publication Date(Web):20 July 2009
DOI:10.1016/j.molliq.2008.07.018
The low-frequency spectra of nitrobenzene in alkane solvents with different chain lengths were studied by terahertz time-domain spectroscopy (THz-TDS). The peak wavenumber of the difference spectral intensity of the absorption between the solution and the solvent was shifted to a higher wavenumber with an increase in the chain length of the alkane solvents. This feature is an evidence of the presence of a characteristic frequency of the absorption in the THz region for a particular solute–solvent interacting system. We simulate our spectra using an analytical model. The amplitudes and the time constants of different processes in the low-frequency range of our study are discussed according to results of our simulation.
Co-reporter:Partha Dutta and Keisuke Tominaga
The Journal of Physical Chemistry A 2009 Volume 113(Issue 29) pp:8235-8242
Publication Date(Web):July 1, 2009
DOI:10.1021/jp900488c
We have performed measurements of the low-frequency spectra of the refractive index and the extinction coefficient of solutions of acetone in cyclohexane at concentrations ranging between 27 and 135 mM by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. We have verified Beer’s law for the concentration range of our experiment, which establishes the fact that the solute−solvent interaction has a significant contribution to the observed spectra, whereas the interaction between the solute molecules is negligible in the present experimental conditions. Combining the data reported by Vij and Hufnagel (J. Phys. Chem., 1991, 95, 6142) with the results of the present experiments, an analytical model for the time correlation function for the total dipole moment of the system was examined in order to reproduce the low-frequency spectra. The molecular origins of the components in the TCF are discussed in light of the results of our simulation.
Co-reporter:Ohki Kambara, Atsuo Tamura, Akira Naito and Keisuke Tominaga
Physical Chemistry Chemical Physics 2008 vol. 10(Issue 33) pp:5042-5044
Publication Date(Web):11 Jul 2008
DOI:10.1039/B807385F
Two structural changes of poly-L-lysine have been studied by various spectroscopic techniques; one is a structural change of a random coil sample in solution to a mixture of α-helix and β-sheet during rapid freezing in the lyophilizing process, and the other is a pressure-induced structural change from an α-helix to a β-sheet structure for a lyophilized sample.
Co-reporter:Sayuri Yamaguchi, Motohiro Banno, Kaoru Ohta, Keisuke Tominaga, Tomoyuki Hayashi
Chemical Physics Letters 2008 Volume 462(4–6) pp:238-242
Publication Date(Web):10 September 2008
DOI:10.1016/j.cplett.2008.07.108
The vibrational dynamics of the OH stretching mode of the benzoic acid dimer in nonpolar solvents have been studied by subpicosecond, frequency-resolved infrared pump–probe spectroscopy. The signal consists of biexponential decay and a quantum beat, which is due to the intermolecular mode of the hydrogen bond coupled to the OH stretching mode. The subpicosecond component is suggested to be due to the vibrational relaxation of the low-frequency modes of the dimer, whereas the picosecond component is assigned to vibrational cooling. The frequency of the Fourier spectra of a quantum beat correlates with that of the OH stretching mode. The physical origin of the correlation is discussed.The vibrational dynamics and the anharmonic coupling between the intramolecular mode and the intermolecular mode are studied in detail.
Co-reporter:Satori Hirai, Motohiro Banno, Kaoru Ohta, Dipak K. Palit, Keisuke Tominaga
Chemical Physics Letters 2007 Volume 450(1–3) pp:44-48
Publication Date(Web):14 December 2007
DOI:10.1016/j.cplett.2007.10.090
The vibrational energy relaxation of 9-fluorenone and its complex with alcohols has been investigated by sub-picosecond pump–probe spectroscopy. The infrared absorption spectrum of 9-fluorenone in alcohol shows three distinct contributions from the free 9-fluorenone, its complex with one alcohol molecule, and that with two molecules. The time constant of the vibrational energy relaxation of the free 9-fluorenone is 4.7 ± 0.1 ps, and that of the 1:1 complex is 2.3 ± 0.1 ps. The acceleration of the vibrational energy relaxation due to hydrogen-bond formation is discussed in terms of the Fermi golden rule.The vibrational energy relaxation of 9-fluorenone and its complex with alcohol has been investigated by sub-picosecond pump–probe spectroscopy.
Co-reporter:Kaoru Ohta, Keisuke Tominaga
Chemical Physics 2007 Volume 341(1–3) pp:310-319
Publication Date(Web):15 November 2007
DOI:10.1016/j.chemphys.2007.07.025
Abstract
We investigated the vibrational population relaxation of the OH stretching mode of hydrogen-bonded phenol complexes in solution by ultrafast infrared spectroscopy. For a phenol–benzonitrile complex, vibrational population relaxation takes place at around 5 ps. The decay time constants of the pump–probe signals do not depend on the probe frequency. We also measured the pump–probe signals of the OH stretching vibrations for phenol–acetone, phenol–diethylether and phenol–tetrahydrofuran complexes. We found that the time scales of vibrational population relaxation for these complexes are correlated with hydrogen bond strength; i.e., stronger hydrogen bonding leads to faster vibrational population relaxation. We did not observe a clear probe frequency dependence in relaxation rates. For these complexes, rapid evolution between different conformations causes fast fluctuation in the vibrational frequency of the OH stretching modes. This fluctuation averages out the vibrational population relaxation times of different conformations. In contrast, the decay time constants of pump–probe signals for hydrogen-bonded phenol oligomers were found to depend significantly on probe frequency. The decay time constant at the blue side (3521 cm−1) was 2.1 ps, while that at the red side (3320 cm−1) was 0.7 ps. We discuss the probe frequency dependence of vibrational population relaxation times for hydrogen-bonded phenol oligomers in solution.
Co-reporter:Kaoru Ohta, Keisuke Tominaga
Chemical Physics Letters 2006 Volume 429(1–3) pp:136-140
Publication Date(Web):29 September 2006
DOI:10.1016/j.cplett.2006.08.019
We investigated vibrational population relaxation for the anti-symmetric stretching mode of the thiocyanate ion (SCN−) in various polar solvents by ultrafast infrared spectroscopy. In aprotic solvents, vibrational population relaxation of SCN− takes place about 2–2.5 times more slowly than in protic solvents. We found that the vibrational relaxation rates and linewidths of the absorption spectra in protic solvents were larger than those in aprotic solvents. We discuss possible mechanisms of vibrational population relaxation and the relationship between relaxation rate and linewidth.We investigated vibrational population relaxation for the anti-symmetric stretching mode of the thiocyanate ion in various polar solvents by ultrafast infrared spectroscopy. We found that the vibrational relaxation rates and linewidths of the absorption spectra in protic solvents were larger than those in aprotic solvents.
Co-reporter:Asako Oka, Keisuke Tominaga
Journal of Non-Crystalline Solids 2006 Volume 352(42–49) pp:4606-4609
Publication Date(Web):15 November 2006
DOI:10.1016/j.jnoncrysol.2006.03.122
Low frequency spectra of liquid solutions of non-polar solvents were measured in the frequency range of 15–80 cm−1 at room temperature by terahertz time-domain spectroscopy to investigate the solute–solvent interaction. The extinction coefficient of the solutions with polar solutes decreases as a function of frequency, whereas that of a non-polar solute increases. It is concluded that the observed spectral change of the polar solute is caused by fast reorientational dynamics due to librational motion of solutes.
Co-reporter:Pratik Sen;Subhadip Ghosh;Sudip Kumar Mondal;Kalyanasis Sahu;Durba Roy;Kankan Bhattacharyya
Chemistry – An Asian Journal 2006 Volume 1(Issue 1-2) pp:
Publication Date(Web):10 JUL 2006
DOI:10.1002/asia.200600036
The dependence of fluorescence and solvation dynamics of coumarin 480 (C480) in a dimyristoyl–phosphatidylcholine (DMPC) vesicle on excitation wavelength (λex) was studied with femtosecond fluorescence upconversion. The study revealed an ultrafast 1.5-ps component of solvation that was not detected earlier. C480 exhibits pronounced red-edge excitation shift (REES) by 10 nm in a DMPC vesicle. This is due to the microheterogeneity of the lipid vesicle. In lipids, the probe is distributed in different locations with varying static and dynamic electrostatic responses. Solvent relaxation becomes faster and the amount of dynamic Stokes shift decreases with increasing λex. For excitation at the red end (λex=430 nm), the solvation time was found to be 1.5 ps. However, for excitation at the blue end, (λex=390 nm), there are two substantially slower components of 250 and 2000 ps. It seems that for λex=390 nm, the major contribution to total emission is due to the probe (C480) molecules in the hydrophobic and restricted locations inside the lipid bilayer. Excitation at 430 nm preferentially selects the probe molecules in a highly mobile environment (water pool of the lipid).
Co-reporter:Hiroaki Maekawa, Kaoru Ohta, Keisuke Tominaga
Journal of Molecular Structure 2005 Volumes 735–736() pp:135-143
Publication Date(Web):14 February 2005
DOI:10.1016/j.molstruc.2004.10.085
We have studied vibrational dephasing of the carbodiimide (–NCN–) anti-symmetric stretching mode of di-cyclohexylcarbodiimide (DCCD) in carbon tetrachloride and N,N-dimethylformamide by infrared photon echo technique. Both time-integrated and frequency-resolved photon echo measurements have been carried out. The time-integrated signal shows a nonexponential decay with a time scale of a few picoseconds. The signal shows its peak at around the origin of the delay time, suggesting that the vibrational frequency fluctuation is in the fast modulation limit. We found unusual frequency dependence in the rise of the frequency-resolved photon echo signals. Simulations of the photon echo signal based on the conventional formalism of nonlinear infrared spectroscopy cannot reproduce the dependence. The origin of the unusual behaviors is discussed.
Co-reporter:Pratik Sen, Taku Satoh, Kankan Bhattacharyya, Keisuke Tominaga
Chemical Physics Letters 2005 Volume 411(4–6) pp:339-344
Publication Date(Web):15 August 2005
DOI:10.1016/j.cplett.2005.06.065
Abstract
Excitation wavelength dependence of solvation dynamics and fluorescence anisotropy of coumarin 480 (C480) is studied using picosecond time resolved emission spectroscopy in a dimyristoyl-phosphatidylcholine (DMPC) vesicle. In the DMPC vesicle, C480 exhibits 10 nm red edge excitation shift indicating the heterogeneous nature of the microenvironment. The dynamic Stokes shift (Δν) and solvation time of C480 in DMPC vesicle decreases with increase in the excitation wavelength (λex). The decay of the emission spectral width and fluorescence anisotropy of C480 in DMPC also depend on λex.
Co-reporter:Kaoru Ohta, Hiroaki Maekawa, Keisuke Tominaga
Chemical Physics Letters 2004 Volume 386(1–3) pp:32-37
Publication Date(Web):1 March 2004
DOI:10.1016/j.cplett.2003.12.119
Abstract
Femtosecond infrared transient grating and three pulse photon-echo experiments have been performed to investigate the vibrational dynamics of the CN stretching mode of Fe(CN)64− in H2O. From the transient grating measurements, it was found that the vibrational population relaxation in H2O occurs about six times faster than that in D2O. This reflects the strong coupling to solvent modes for the vibrational relaxation. On the other hand, the photon-echo experiments have shown that the time-scales of the spectral diffusion processes are similar in H2O and D2O.
Co-reporter:Kaoru Ohta, Jumpei Tayama and Keisuke Tominaga
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 30) pp:NaN10465-10465
Publication Date(Web):2012/03/28
DOI:10.1039/C2CP40244K
In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN− and N3− in several polar solvents. We used an infrared (IR) pump–probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN− and N3−, we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3–6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN− and N3− is discussed in terms of the difference between protic and aprotic solvents.
Co-reporter:Sayuri Yamaguchi, Keisuke Tominaga and Shinji Saito
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 32) pp:NaN14749-14749
Publication Date(Web):2011/07/11
DOI:10.1039/C1CP20912D
The low-frequency modes of the benzoic acid (BA) dimer and its analogues in carbon tetrachloride (CCl4) have been investigated by terahertz time-domain spectroscopy. The solute spectrum is obtained by subtracting the solvent contribution from that of the solution. The difference spectrum of BA in CCl4 has a broad band with a peak at 68 cm−1. To assign the observed band, the spectrum is compared with spectra of other aromatic molecules, such as benzene and phenol in addition to p-methyl BA and deuterated BA species (BA-dOH and BA-d5) in CCl4. The band at 68 cm−1 is assigned to the cogwheel mode of the BA dimer. Density functional theory calculations also support this assignment. Finally, spectral lineshape analysis based on the multimode Brownian oscillator model is applied to the THz spectra for all the samples.
Co-reporter:Motohiro Banno, Kaoru Ohta and Keisuke Tominaga
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 18) pp:NaN6366-6366
Publication Date(Web):2012/02/01
DOI:10.1039/C2CP23647H
Solute–solvent interactions between acetate and D2O were investigated by vibrational spectroscopic methods. The vibrational dynamics of the COO asymmetric stretching mode in D2O was observed by time-resolved infrared (IR) pump–probe spectroscopy. The pump–probe signal contained both decay and oscillatory components. The time dependence of the decay component could be explained by a double exponential function with time constants of 200 fs and 2.6 ps, which are the same for both the COO asymmetric and symmetric stretching modes. The Fourier spectrum of the oscillatory component contained a band around 80 cm−1, which suggests that the COO asymmetric stretching mode couples to a low-frequency vibrational mode with a wavenumber of 80 cm−1. Based on quantum chemistry calculations, we propose that a bridged complex comprising an acetate ion and one D2O molecule, in which the two oxygen atoms in the acetate anion form hydrogen bonds with the two deuterium atoms in D2O, is the most stable structure. The 80 cm−1 low-frequency mode was assigned to the asymmetric stretching vibration of the hydrogen bond in the bridged complex.
Co-reporter:Yusuke Iima, Daiki Kuzuhara, Zhao-Li Xue, Seiji Akimoto, Hiroko Yamada and Keisuke Tominaga
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 26) pp:NaN13135-13135
Publication Date(Web):2014/04/10
DOI:10.1039/C4CP00301B
We have investigated the photophysical properties of alkyl-substituted triphyrin(2.1.1) (ATp) and benzotriphyrin(2.1.1) (BTp) by steady-state and time-resolved fluorescence spectroscopy. We focused on the effect of NH proton tautomerization, planarity of the macrocycles, and substituents on these properties. The fluorescence quantum yields (Φy) of ATp did not depend on solvent viscosity, whereas those of BTp increased with solvent viscosity, reaching a maximum value of 0.17 in paraffin. Interestingly, analyzing Φy showed that the non-radiative rate constant of BTp decreased sharply as the solvent viscosity increased. These results suggest that the substituted phenyl groups play a crucial role in suppressing molecular distortion, thus leading to decreased non-radiative relaxation in triphyrin(2.1.1). The hydrogen bond formed in the inner cavity potentially contributes to the suppression of the structural distortion, whereas the pyrrole rings in the macrocycle are close, as in porphycene.
Co-reporter:Ohki Kambara, Atsuo Tamura, Akira Naito and Keisuke Tominaga
Physical Chemistry Chemical Physics 2008 - vol. 10(Issue 33) pp:NaN5044-5044
Publication Date(Web):2008/07/11
DOI:10.1039/B807385F
Two structural changes of poly-L-lysine have been studied by various spectroscopic techniques; one is a structural change of a random coil sample in solution to a mixture of α-helix and β-sheet during rapid freezing in the lyophilizing process, and the other is a pressure-induced structural change from an α-helix to a β-sheet structure for a lyophilized sample.
![3-{[(2,3-dihydroxypropoxy)(hydroxy)phosphoryl]oxy}-2-(tetradecanoyloxy)propyl tetradecanoate](http://img.cochemist.com/ccimg/61400/61361-72-6.png)
![3-{[(2,3-dihydroxypropoxy)(hydroxy)phosphoryl]oxy}-2-(tetradecanoyloxy)propyl tetradecanoate](http://img.cochemist.com/ccimg/61400/61361-72-6_b.png)
![POLY[OXY[(1R)-1-METHYL-2-OXO-1,2-ETHANEDIYL]]](http://img.cochemist.com/ccimg/27000/26917-25-9.png)
![POLY[OXY[(1R)-1-METHYL-2-OXO-1,2-ETHANEDIYL]]](http://img.cochemist.com/ccimg/27000/26917-25-9_b.png)
![Poly[oxy[(1S)-1-methyl-2-oxo-1,2-ethanediyl]]](http://img.cochemist.com/ccimg/26200/26161-42-2.png)
![Poly[oxy[(1S)-1-methyl-2-oxo-1,2-ethanediyl]]](http://img.cochemist.com/ccimg/26200/26161-42-2_b.png)
![(6R,9AR)-OCTAHYDRO-2H-PYRIDO[1,2-A]PYRAZIN-6-YLMETHANOL](http://img.cochemist.com/ccimg/13700/13699-48-4.png)
![(6R,9AR)-OCTAHYDRO-2H-PYRIDO[1,2-A]PYRAZIN-6-YLMETHANOL](http://img.cochemist.com/ccimg/13700/13699-48-4_b.png)
