Haruo Shizuka

Excited state pK*a values of naphthylamines: proton-induced fluorescence quenching

Abstract The dynamic bahavior of the excited singlet of naphthylamines involving proton-induced quenching in a sulfulic acid-water mixture has been studied by means of fluorometry and nanosecond time resolved spectroscopy. A significant fluorescence quenching of neutral α- and β-naphthylamines induced by protons was observed: 1 k q = 8.9 (±0.3) × 10 9 M −1 s −1 and 1 k q = 3.3 (±0.2) × 10 8 M −1 s −1 at 300 K, respectively. On the basis of kinetic analyses, the corrected values of pK * a were determined to be −1.0 (±0.2) and −0.8 (±0.1) for α- and β-amines, respectively. The isotope effect on the quenching was also examined.

The 2pπ*–3dπ interaction in aromatic silanes. Fluorescence from the 1(2pπ, 3dπ) intramolecular charge-transfer state

The 2pπ*–3dπ interaction in the excited state of aromatic silanes has been studied by means of absorption and emission spectroscopy. Broad and structureless fluorescence spectra of phenyldisilanes and naphthyldisilanes with large Stokes shifts have been observed and they have been attributed to the emissions from the intramolecular charge-transfer (c.t.) states with large dipole moments. Evidence that the c.t. emission originates from the 1(2pπ, 3dπ) state produced by the 2pπ*(aromatic ring)→ 3dπ(Si—Si bond) intramolecular charge transfer is given by the effect of steric twisting on the emission. It is shown that the fast formation (⩽ 1 ns) of the c.t. state from the locally excited state 1(π, π*)1B2(or 1Lb) of phenyldisilanes takes place, followed by rapid decay (⩽ 1 ns) of the intersystem crossing 1(2pπ, 3dπ)→3(π, π*). However, no c.t. emission has been observed for aromatic monosilanes and polycyclic aromatic disilanes (aromatic rings 3) even in fluid polar solvents. The 2pπ*→3dπ intramolecular c.t. mechanism is discussed in comparison with that of the twisted intramolecular charge-transfer (t.i.c.t.) state.

Fluorescence quenching of aromatic molecules by inorganic anions

Abstract Fluorescence quenching of aromatic molecules by inorganic anions in H 2 OEtOH mixtures at 300 K has been studied quantitatively. A linear relationship between log k q ( k q is the quenching rate constant) and Δ G (the free energy change in electron transfer) is obtained indicating that the anion-induced quenching proceeds via an electron-transfer pathway. The potential barrier Δ G ‡ in the electron-transfer process is also given as a function of Δ G according to the Horiuchi-Polanyi rule.

Laser photolysis studies of electron transfer between triplet naphthoquinones and amines

Abstract Electron transfer between triplet naphthoquinones ( 3 Nap ∗ ; 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dimethyl-1,4-naphthoquinone in the triplet state) and N , N -dimethylaniline (DMA) or triethylamine (TEA) in acetonitrile (MeCN) and a mixture of MeCN and H 2 O (4:1, v/v) was studied by laser photolysis techniques at 295 K. By analysis of the transient absorption spectrum observed after the completion of electron transfer, the absorption spectra and coefficients of the naphthoquinone anion radicals (Nap .− ) were determined on the basis of those of the DMA cation radical (DMA .+ ) in MeCN and MeCNH 2 O (4:1, v/v). The values of the rate constants ( k et ) for electron transfer between 3 Nap ∗ and DMA were found to be close to those for diffusion-controlled processes as predicted by the Rehm-Weller theory on electron transfer. Using the determined molar absorption coefficients of Nap .− , the efficiencies ( Γ rad ) for the information of Nap .− and DMA .+ were shown to be unity since the radical ion pair produced ( 3 [Nap .− + DMA .+ ]) readily dissociates into Nap .− and DMA .+ due to the triplet spin multiplicity.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Direct measurement of proton dissociation in thhe excited state of protonated 1-aminopyrene with picosecond pulses

Abstract Direct measurement of proton dissociation in the excited singlet state of protonated I-aminopyrene in a mixture of H 2 O (or D 2 O) and acetonitrile (1:1) has been carried out by means of picosecond time-resolved spectroscopy. The proton dissociation retes k 1 1 and k D+ 1 at about 300 K were determined tobe 3.7 (±0.6) * 10 9 s -1 , respectively. These rates are in very good agreement with those obtained by nanosecond time-resolved spectroscopy with fluorometry. The excited singlet state ;K * 2 value of l-aminopyrene was also determined by dynamic analyses.

Synthesis and electronic properties of C60–o-quinodimethane adducts

Several C60–o-quinodimethane adducts having various aromatic rings have been synthesized by the simple thermal reaction of C60 with dihydrocyclobutaarenes. According to MM2 calculations, the most stable conformation of the cyclohexene ring in these adducts is not the half chair, but the half boat. On the basis of variable temperature NMR results, its inversion rate was found to depend on the attached aromatic rings, mainly due to the steric effect. The absorption spectra in the visible region were substantially identical for all the adducts, exhibiting a weak and broad band around 700 nm with vibrational structures. The fluorescence spectra due to the C60 moiety of the adducts were also similar to each other and independent of the excitation wavelength. The fluorescence excitation spectra were in good agreement with the corresponding absorption spectra. No fluorescence could be observed from the attached aromatic rings even in the case of pyrene, because of the quenching by the intramolecular S1(pyrene)–So(C60) energy transfer. The similar transient absorption spectra of the C60 adducts were ascribed to the Tnâ†�T1 absorption of the C60 moiety. These results show that little intramolecular electronic interaction between the C60 and aromatic moieties takes place in the adducts.

Emitting states of benzyl, p-fluorobenzyl and p-cyanobenzylradicals

Abstract Polarization studies of the fluorescence emission and excitation spectra have been carried out by the photoselection method for the benzyl, p -fluorobenzyl and p -cyanobenzyl radicals in poly (vinyl alcohol) films at 77 K. The emitting states have been assigned to the 1 2 A 2 electronic state for the benzyl and p -fluorobenzyl radicals and the 2 2 B 2 state for the p -cyanobenzyl radical. Flu lifetimes have been determined to be 1.22±0.10, 0.36±0.04 and 0.43±0.04 μs for the three radicals, respectively.

Recovery mechanism of the reaction intermediate produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane

Abstract The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane was studied in various solvents by nanosecond laser flash photolysis. The recovery rate and mechanism depend strongly on the nature of the solvent. Unimolecular recovery of the intermediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s −1 respectively) despite the small activation energy (3.6 kcal mol −1 in 3-methylpentane). The slow unimolecular recovery rate can be ascribed to the small frequency factor (7.0 × 10 5 s −1 ), i.e. the large negative entropy change for the formation of the chelated enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecular recovery process via hydrogen-bond interactions between two intermediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen exchange between the intermediate and alcohol molecule(s) accelerates the recovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine in acetonitrile, increase the recovery rate considerably by an additional process through the enolate anion.

Internal conversion of o-aminoacetophenone in solution

The photophysical properties of o-aminoacetophenone (o-AAP) in solution have been studied by using a femtosecond laser–single photon counting system and time-resolved thermal lensing (TRTL) method. The fluorescence quantum yield (Φf) and lifetime (τf) of o-AAP depend strongly on the nature of the solvent. In nonpolar solvents, o-AAP gives very small Φf values (Φf = 2.4 × 10−4 in n-hexane) and remarkably short fluorescence lifetimes (τf = 9.4 ps in n-hexane), suggesting the presence of very fast nonradiative deactivation processes. The measurements of the quantum yield (Φisc) of intersystem crossing based on the energy transfer and TRTL methods clearly show that the fast radiationless processes in nonpolar solvents are due to internal conversion. In aprotic solvents, the rate (kic) of internal conversion for o-AAP decreases significantly with increasing solvent polarity (kic = 1.0 × 1011 s−1 in n-hexane, kic = 2.4 × 109 s−1 in acetonitrile). In protic solvents, the kic value tends to increase with an increase of hydrogen-bonding donor ability of the solvent. The internal conversion rate in aprotic solvents is scarcely affected by deuterium substitution of the NH2 group in o-AAP, while a large isotope effect is found for o-AAP in deuteriated protic solvents. It is concluded that the efficient S1 → S0 internal conversion in nonpolar aprotic solvents arises from vibronic interactions between close-lying 1(π,π*) and 1(n,π*) states (the proximity effect), and in protic solvents intermolecular hydrogen-bonding interactions with solvent molecules also facilitate the nonradiative process.