Biomedical subjects

Toshitada Yoshihara

Publications and source records attributed to Toshitada Yoshihara.

8 recordsLinked to original sources

PubMed2006

Environment-sensitive fluorophore emitting in protic environments.

The unusual fluorescence properties of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (e.g., fluorescence quantum yield Phi(f) < 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (>450 nm) in protic solvent (e.g., Phi(f) = 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent "off-on") sensor. [structure: see text]

Chemical Phenomena↗

PubMed2006

Intramolecular charge transfer and dielectric solvent relaxation in n-propyl cyanide. N-phenylpyrrole and 4-dimethylamino-4'-cyanostilbene.

Fast intramolecular charge transfer (ICT) accompanied by dual fluorescence from a locally excited (LE) and an ICT state taking place with N-phenylpyrrole (PP) in the solvent n-propyl cyanide (PrCN) is investigated as a function of temperature between 25 and -112 degrees C. The LE and ICT fluorescence decays from -45 to -70 degrees C can be adequately fitted with two exponentials, in accordance with a two state (LE + ICT) reaction mechanism, similar to what has been observed with PP in the more polar and less viscous alkyl cyanides acetonitrile (MeCN) and ethyl cyanide (EtCN). At lower temperatures, triple-exponential fits are required for the LE and ICT decays. The ICT emission band maximum of the time-resolved fluorescence spectra of PP in PrCN at -100 degrees C displays a spectral shift from 29 230 cm-1 at t = 0 to 27 780 cm-1 at infinite time, which equilibration process is attributed to dielectric solvent relaxation. From the time dependence of this shift, in global analysis with that of the band integrals BI(LE) and BI(ICT) of the time-resolved LE and ICT fluorescence bands, the decay times 119 and 456 ps are obtained. Dielectric relaxation times of 20 and 138 ps are determined from the double-exponential spectral solvation response function C(t) of the probe molecule 4-dimethylamino-4'-cyanostilbene in PrCN at -100 degrees C. It is concluded from the similarity of the times 119 ps (PP) and 138 ps (DCS) that the deviation from double-exponential character for the fluorescence decays of PP in PrCN below -70 degrees C is due to the interference of dielectric solvent relaxation with the ICT reaction. This fact complicates the kinetic analysis of the LE and ICT fluorescence decays. The kinetic analysis for PP in PrCN is hence restricted to temperatures between -70 and -45 degrees C. From this analysis, the forward and backward ICT activation energies Ea (12 kJ/mol) and Ed (17 kJ/mol) are obtained, giving an ICT stabilization enthalpy -DeltaH of 5 kJ/mol. A comparison of the reaction barriers for PP in the three alkyl cyanides PrCN, EtCN, and MeCN (J. Phys. Chem. A 2005, 109, 1497) shows that Ea becomes smaller with increasing solvent polarity (from 12 to 6 kJ/mol), whereas Ed remains effectively constant. Both observations are indicative of a late transition state for the LE --> ICT reaction. The significance of the Leffler-Hammond postulate in this connection is discussed: not primarily the energy of the LE, ICT, and transition states but rather the extent of charge transfer in these states determines whether an early or a late transition state is present.

Electrochemistry↗

PubMed2006

Extreme fluorescence sensitivity of some aniline derivatives to aqueous and nonaqueous environments: mechanistic study and its implication as a fluorescent probe.

Effects of solvent water on the photophysical properties of a series of meta- and para-substituted anilines have been investigated by means of time-resolved fluorescence, transient absorption, and photoacoustic measurements. Some aniline derivatives exhibit extremely short fluorescence lifetime (tau(f)) and small quantum yield (Phi(f)) in water (e.g., tau(f) = 45 ps and Phi(f) = 0.0019 for m-cyanoaniline (m-ANCN) in H(2)O), which is in marked contrast with their much larger values in nonaqueous solvents (tau(f) = 7.3 ns and Phi(f) = 0.14 for m-ANCN in acetonitrile). Photoacoustic and transient absorption measurements show that the remarkable fluorescence quenching of m-ANCN in water is attributed almost exclusively to fast internal conversion. The lifetime measurements of m-ANCN in H(2)O/acetonitrile binary solvent mixtures reveal that the quenching is related to variation of hydrogen-bonding interactions between the amino group and water molecules and the conformational change of the amino group upon electronic excitation. Similar fluorescence quenching due to solvent water is also found for N-alkylated m-ANCNs. The drastic differences in the fluorescence intensity and lifetime of m-ANCNs under hydrophobic and hydrophilic environments and also the large solvent polarity dependence of the fluorescence band position suggest the possibility that they can be utilized as fluorescent probes for investigating the microenvironment of biological systems. In suspensions of human serum albumin (HSA) in water, remarkable enhancement of the fluorescence intensity and lifetime is observed for m-ANCN and its N-alkylated derivatives, demonstrating that m-ANCNs can be a candidate for novel fluorescent probe with small molecular size.

Journal Article↗

PubMed2005

Synthesis and complexing properties of [2.n](2,6)pyridinocrownophanes.

[2.n](2,6)Pyridinocrownophanes (3a-e) were efficiently prepared by intramolecular [2 + 2] photocycloaddition of vinylpyridine derivatives under irradiation using a 400-W high-pressure mercury lamp through a Pyrex filter. They were of cis-configuration with respect to the cyclobutane ring, which was proven by the specific methine proton NMR resonances at delta 3.98-4.08. From ESI-MS analysis 3a-e were found to form 1:1 complexes with Ag+ cation. In a liquid-liquid extraction, 3 showed the highest affinity toward Ag+ cation among several heavy metal nitrates. In this series, 3c possessing four ethereal oxygen atoms was found to show the highest Ag+ affinity, according to the liquid-liquid extraction and determination of stability constant with the cation. The photoreactivity of precursor vinylpyridines is discussed by the predictor gamma(r(c)) proposed by Caldwell.

Crown Ethers↗

PubMed2005

Kinetics of intramolecular charge transfer with N-phenylpyrrole in alkyl cyanides.

For the electron acceptor/donor molecule N-phenylpyrrole (PP), the fast intramolecular charge transfer (ICT) reaction accompanied by dual fluorescence from a locally excited (LE) and an ICT state is investigated in alkyl cyanide solvents as a function of temperature. After a comparison of the X-ray crystal structure of PP with calculations from the literature, absorption and fluorescence spectra of PP in a series of solvents over a wide polarity range are discussed. ICT with PP strongly depends on solvent polarity and starts to appear in solvents more polar than diethyl ether. From an analysis of the ICT/LE fluorescence quantum yield ratio Phi'(ICT)/Phi(LE), approximate data for the change in enthalpy -DeltaH of the ICT reaction of PP are obtained, ranging from 9 kJ/mol in acetonitrile (MeCN) to 4 kJ/mol in n-butyl cyanide (BuCN). From ICT and LE fluorescence decays of PP measured as a function of temperature, the forward (Ea = 9 kJ/mol in ethyl cyanide (EtCN) and 6 kJ/mol in MeCN) and backward (Ed = 16 kJ/mol in EtCN and MeCN) ICT reaction barriers are determined. From these data, -Delta H (7 kJ/mol (EtCN); 10 kJ/mol (MeCN)) is calculated, in good agreement with the results coming from Phi'(ICT)/Phi(LE). The data for Ea show that the forward ICT barrier becomes smaller with increasing solvent polarity, whereas the absence of change for Ed comes from the compensating increase of -DeltaH. Both observations are indicative of a late transition state for the LE --> ICT reaction. For PP in EtCN and MeCN, the ICT radiative rate constant k'(f)(ICT) increases with temperature. This is caused by the ICT low transition dipole moment and hence does not contain information on the molecular structure (twisted or planar) of the ICT state. The fast ICT observed with PP supports our previous conclusion, based on a comparison of PP with its planarized derivative fluorazene, that the pyrrole and phenyl moieties in the ICT state of PP are coplanar and possess substantial electronic coupling.

Acetonitriles↗

PubMed2005

Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2'-aminoacetophenone and its derivatives in solution.

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X-C6H4-COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2(AAP), NHCH3(MAAP), N(CH3)2(DMAAP), NHCOCH3(AAAP), NHCOCF3(TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n,pi*-pi,pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic)= 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S1(pi,pi*) and S2(n,pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.

Journal Article↗

PubMed2004

Fast intramolecular charge transfer with a planar rigidized electron donor/acceptor molecule.

The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45 degrees C. The ICT state of FPP has a dipole moment mu(e)(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which mu(e)(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required.

Journal Article↗

PubMed2003

Singlet excited state dipole moments of dual fluorescent N-phenylpyrroles and 4-(dimethylamino)benzonitrile from solvatochromic and thermochromic spectral shifts.

The excited state dipole moments mue(ICT) and mue(LE) of the dual fluorescent molecules N-phenylpyrrole (PP), N-(4-cyanophenyl)pyrrole (PP4C) and N-(3-cyanophenyl)pyrrole (PP3C) are determined from solvatochromic and thermochromic measurements. It is shown that the best results are obtained when the solvatochromic as well as the thermochromic analysis of the spectral shifts is made relative to 4-(dimethylamino)benzonitrile (DMABN) as the model compound. Direct thermochromic experiments with PP4C, PP3C and DMABN in diethyl ether lead to reasonable results, but unrealistically large dipole moments mue(ICT) are found for PP, PP4C, PP3C and DMABN in acetonitrile, ethyl cyanide and n-propyl cyanide. The mue(ICT) values obtained for the N-phenylpyrroles from the thermochromic analysis in these solvents relative to DMABN (17 D) do not depend on solvent polarity: 13 D for PP, 15 D for PP4C and PP3C. The spectral shifts for the LE emission of the N-phenylpyrroles and aminobenzonitriles are much smaller than those for the ICT fluorescence, resulting in relatively small values for mue(LE). With PP and N-(4-methylphenyl)pyrrole (PP4M) the problem arises that one of the two values calculated by solving the quadratic equation for mue(LE) in the solvatochromic and thermochromic analysis cannot be discarded on photophysical or molecular grounds, as is the case for the other molecules. The experimental data for mue(ICT) of PP and PP4C are compared with theoretical values calculated for coplanar (PICT) and perpendicular (TICT) conformations of the pyrrole and phenyl or cyanophenyl groups. The experimental ICT dipole moment of PP4C has a value in between the theoretical results for mue(PICT) and mue(TICT), whereas the data for PP tend to favour the TICT configuration. It appears that in the LE state of PP and PP4M a negative charge remains on the pyrrole moiety, whereas a charge reversal takes place for the LE state of PP3C and the ICT state of PP, PP4C and PP3C.

Journal Article↗