Small molecules having intense luminescence properties are required to promote biological and organic material applications. We prepared five types of benzamides having pyridine, pyridazine, pyrazine, and pyrimidine rings and successfully converted them into three types of the difluoroboronated complexes, Py@BAs, as novel blue fluorophores. Py@BA having a pyridine moiety (2-Py@BA) showed no fluorescence in solution, whereas Py@BAs of pyridazine and pyrazine moieties (2,3-Py@BA and 2,5-Py@BA, respectively) emitted blue fluorescence with quantum yields of ca. 0.1. Transient absorption measurements using laser flash photolysis of the Py@BAs revealed the triplet formation of 2,3- and 2,5-Py@BAs, while little transient signal was observed for 2-Py@BA. Therefore, the deactivation processes from the lowest excited singlet state of fluorescent 2,3- and 2,5-Py@BAs consist of fluorescence and intersystem crossing to the triplet state while that of the nonfluorescent Py@BA is governed almost entirely by internal conversion to the ground state. Conversely, in the solid state, 2-Py@BA emitted intense fluorescence with a fluorescence quantum yield as high as 0.66, whereas 2,3- and 2,5-Py@BAs showed fluorescence with quantum yields of ca. 0.2. The crystal structure of 2-Py@BA took a herringbone packing motif, whereas those for 2,3- and 2,5-Py@BAs were two-dimensional sheetlike. On the basis of the difference in crystal structures, the emission mechanism in the solid state was discussed.

We prepared a variety of coumarin derivatives having expanded π-electron systems along the direction crossing the C3–C4 bond of the coumarin skeleton via a photochemical cyclization process and investigated their photophysical features as a function of the number (n) of the added benzene rings based on emission and transient absorption measurements. Upon increasing n, the fluorescence quantum yields of the π-extended coumarins increased. Expanding the π-electron system on the C3–C4 bond of the coumarin skeleton was found to be efficient for increasing the fluorescence ability more than that on the C7–C8 bond. Introducing the methoxy group at the 7-position was also efficient for enhancing the fluorescence quantum yield and rate of the expanded coumarins. The non-radiative process from the fluorescence state was not substantially influenced by the expanded π-electron system. The competitive process with the fluorescence was found to be intersystem crossing to the triplet state based on the observations of the triplet–triplet absorption. The effects of the expanded π-electron systems on the fluorescence ability were investigated with the aid of TD-DFT calculations.

We have prepared three types of carbonyl compounds, benzoylethynylmethyl phenyl sulfide (2@SPh), (p-benzoyl)phenylethynylmethyl phenyl sulfide (3@SPh) and p-(benzoylethynyl)benzyl phenyl sulfide (4@SPh) with benzoyl and phenylthiylmethyl groups, which are interconnected with a C–C triple bond and a phenyl ring. Laser flash photolysis of 3@SPh and 4@SPh in acetonitrile provided the transient absorption spectra of the corresponding triplet states where no chemical reactions were recognized. Upon laser flash photolysis of 2@SPh, the absorption band due to the phenylthiyl radical (PTR) was obtained, indicating that the C–S bond cleaved in the excited state. Triplet sensitization of these carbonyl compounds using acetone and xanthone was conducted using laser photolysis techniques. The formation of triplet 3@SPh was seen in the transient absorption, whereas the PTR formation was observed for 2@SPh and 4@SPh, indicating that the triplet states were reactive for the C–S bond dissociation. The C–S bond dissociation mechanism for 4@SPh upon triplet sensitization is discussed in comparison with those for 2@SPh and 3@SPh.

We investigated the photophysical properties of difluoroboronated β-diketones (BF2DK) with chrysene and pyrene skeletons (ChB and PyB, respectively) in solution and in the solid state. Acetylchrysenes, as the key precursors to ChBs, were photochemically prepared from the corresponding (acetylphenyl)naphthylethenes by means of a modified photocyclization method. The absorption and emission spectra of the BF2DKs were obtained in chloroform and acetonitrile, and the quantum yields and lifetimes of the fluorescence were determined. Excimeric fluorescence from PyB was absent even in highly concentrated solution. Based on the Lippert–Mataga analysis of the absorption and fluorescence features, the photophysical properties of the ChBs were discussed in comparison with those of PyB. The fluorescence states of the studied BF2DKs are shown to be of a charge-transfer character. The fluorescence quantum yields decrease with increasing the solvent polarity due to the enhanced internal conversion process. The fluorescence quantum yields in the solid state of the studied BF2DKs were determined, and it was found that PyB is fluorescent, whereas the fluorescence quantum yields of the ChBs depend on the substituted position of the chrysene moiety.

Six difluoroboronated β-diketones having the phenanthrene skeleton (Phe@Ar) are prepared. Based on the measurements of the fluorescence quantum yields, lifetimes and transient absorption, the photophysical features of Phe@Ar are studied in comparison with those of difluoroboronated diketones having phenyl, naphthyl and anthryl moieties. β-Diketones having 1-, 2-, 3- and 9-phenanthryl moieties (PheDKAr) were prepared as the precursor to Phe@Ar. 1-Acetylphenanthrene was synthesized by the photocyclization method as the key building block of PheDKAr having the 1-phenanthryl moiety. The counter aromatic moieties (Ar) of the prepared PheDKAr are varied with phenyl, furyl and thienyl rings (Ar = Ph, F and T, respectively) to investigate the effects of π-conjugation on the fluorescence properties. The prepared Phe@Ars are fluorescent with appreciable fluorescence quantum yields which depend on the substitution position of the phenanthrene moiety. 3-Phe@Ph having the 3-phenanthryl moiety provides the largest fluorescence quantum yield (0.81) in acetonitrile among the Phe@Ars whereas 2-Phe@Ph having the 2-phenanthryl moiety shows the smallest fluorescence quantum yield (0.07) in acetonitrile. All the Phe@Ars show fluorescence also in the solid state, and the fluorescence spectra and quantum yields were determined. Transient absorption measurement using laser flash photolysis of the Phe@Ars revealed the triplet formation. DFT and TD-DFT calculations of Phe@Ars rationalize the dependency of the fluorescence quantum yields on the substitution position of the phenanthrene skeleton in terms of difference in the oscillator strength for the HOMO–LUMO transition.

•2-Alkyl-1,3-diketone having five-membered rinds are prepared.•The alkyl diketones in the keto form tautomerize via the triplets to the enol form.•The keto form shows the absorption in the UV-C region while the enol does in the UV-A.•The enol form recovers to the original keto form via thermal processes in a few days.•The enol form disperses the absorbed photon energies via internal conversion.Keto-enol tautomerization in 2-alkyl-1,3-diketons having five-membered rings was studied using steady state and laser flash photolysis techniques in solution. The alkyl keto-diketones undergo photoinduced tautomerization mainly in the triplet state to the enol in acetonitrile. The alkyl enol-diketones, thus formed, undergo thermal tautomerization to the original keto forms in a few days. The alkyl enol-diketones show fast internal conversion from the excited singlet state to the ground state without yielding the corresponding isomeric forms (rotamer). Based on the computation for state energies of the keto, enol and isomer forms, a schematic energy diagram for the tautomerization was drawn.Download full-size image

•BF2 compleses of 1,3-diketones having the ferrocene moiety (Fc@Phs) were prepared.•Fc@Phs show visible colors and no emission in solution.•No photodecomposition occurred upon photolysis of Fc@Phs in cyclohexane.•Laser photolysis studies of Fc@Phs revealed no formation of the triplet states.•Fc@Phs efficiently convert absorbed photon energy to heat.We synthesized 1,3-diketones having the ferrocene moiety (FcDKPhs) and the corresponding difluoroboronated complexes (Fc@Phs), and investigated the photophysical and photochemical properties in solution by steady-state and laser flash photolyses. FcDKPhs and Fc@Phs had absorption bands with large absorptivities in the UV-region and ones with small absorptivities in the Vis-region, but provided neither fluorescence nor phosphorescence. FcDKPhs decomposed upon exposure to UV–vis light while Fc@Phs were stable. No transient signals obtained upon laser flash photolysis of FcDKPhs and Fc@Phs demonstrated absence of intersystem crossing to the triplet states. Fc@Phs efficiently convert absorbed photon energy to heat.

•1-Benzoyl-2-phenylacetylene (BPA) is prepared.•The triplet energy of BPA is found to be 65 kcal mol-1.•The excited state character of BPA is similar to that of benzophenone.•The absorption spectra of triplet BPA and the ketyl radical are identified.1-Benzoyl-2-phenylacetylene (BPA) having a moderate triplet energy (65 kcal mol−1) was prepared, and steady state and laser photolysis techniques were conducted to characterize the photochemical features of the triplet state and the BPA ketyl radical that was formed via H-abstraction of the triplet n,π* from alcohol and phenol. The photophysical and photochemical properties of BPA similar to those of benzophenone may encourage photochemists to use it as an n, π* triplet sensitizer scheduled for triplet-energy-tuned experiments upon laser flash photolysis in solution.

Photophysical and photochemical features of [3n]cyclophanes (3nCPs) (n = 2–6) in solution were investigated by emission and transient absorption measurements. The studied 3nCPs show excimer fluorescence without locally excited fluorescence whereas some of them emit excimer phosphorescence in rigid glass at 77 K. The probability of excimeric phosphorescence from transannular π-electron systems was shown to strictly depend on the symmetric molecular structures. A feature of intersystem crossing from an excimeric fluorescence state to the excimeric triplet state was observed. Transient absorption spectra obtained upon laser flash photolysis of 3nCP revealed formation of the triplet excimer states. Triplet sensitization of 33CP using xanthone as the sensitizer demonstrated formation of triplet 33CP via triplet energy transfer whereas from the xanthone ketyl radical formation, it was inferred that triplet xanthone undergoes H atom abstraction from 32CP, providing a benzylic 32CP radical as the counter species. Based on kinetic and spectroscopic data obtained upon laser flash photolysis, differences in photochemical reactions of triplet xanthone between 32CP and 33CP were discussed.

The mesolysis mechanisms for eight aromatic thioether radical anions (ArCH2SAr′•–) generated during radiolysis in 2-methyltetrahydrofuran were studied by spectroscopic measurements and DFT calculation. Seven of ArCH2SAr′•– underwent mesolysis via dissociation of the σ-bond between the benzylic carbon and sulfur atoms, forming the corresponding radical and anion with the stepwise mechanism or concerted mechanism. Conversely, no mesolysis in the benzyl β-naphthyl sulfide radical anion was found. From the Arrhenius analysis of the mesolysis with the stepwise mechanism, apparent activation energies (ΔEexp) were determined and compared with those (ΔEcal) estimated by the DFT calculations. Two types of C–S bond dissociation are possible to give the C radical and S anion (ArCH2•/Ar′S–) and the C anion and S radical (ArCH2–/Ar′S•). The dissociation energies (BDE(ArCH2•/Ar′S–) and BDE(ArCH2–/Ar′S•)) were estimated by the DFT calculations, and BDE(ArCH2•/Ar′S–) were found to be smaller than BDE(ArCH2–/Ar′S•). The formation of ArCH2•/Ar′S– was observed on the mesolysis of five ArCH2SAr′•–, while one ArCH2SAr′•– provided ArCH2–/Ar′S•. Chemical properties governing the mesolysis mechanisms of ArCH2SAr′•– are discussed.

Facile synthesis of fulminene ([6]phenacene) was achieved through the Mallory reaction of 1-(1-naphthyl)-2-(1-phenanthryl)ethene or the 9-fluorenone-sensitized photo-ring-closure of 1-(1-naphthyl)-2-(1-phenanthryl)ethane. The electronic spectral properties of fulminene were investigated for the first time using photoluminescence as well as transient absorption spectroscopy. The spectral features were compared with those of a series of lower phenacene homologs such as phenanthrene ([3]phenacene), chrysene ([4]phenacene), and picene ([5]phenacene). For the [n]phenacene series, both the fluorescence and phosphorescence bands linearly red-shifted with an increase in the number of the benzene rings (n). Trends in the energy levels of the excited singlet (ES) and the triplet (ET) states were expressed as Es = −2.6n + 89.1 (kcal mol−1) and ET = −1.8n + 66.2 (kcal mol−1), respectively. In the case of fulminene, laser flash photolysis displayed a transient spectrum with an absorption maximum (λmaxT–T) at 675 nm, which was assigned as the triplet fulminene excited state. The λmaxT–T values for the [n]phenacene series showed a linear correlation as a function of the ring number n, given by an equation, λmaxT–T = 60n + 318 (nm).

The photothermal tautomerization processes between enol and keto forms of 4-tert-butyl-4′-methoxydibenzoylmethane (trade name, Avobenzone) in acetonitrile have been studied by steady-state and laser flash photolysis. The keto form is produced upon photolysis of the enol in only acetonitrile with a quantum yield of 0.014. The molar absorptivity of the keto form was determined. Phototautomerization from the keto to the enol form was not seen. Laser flash photolysis of the keto form recognized the formation of the triplet state. In the dark, the keto form underwent thermal tautomerization to the enol with a lifetime of 5.1 h at 295 K. The enolization rate in acetonitrile was not accelerated by the presence of alcohols and/or water but increased with increasing temperature and followed the Arrhenius expression. The activation energy and the frequency factor were determined for the enolization process from the keto to the enol form. On the basis of the energy states of the tautomers and isomers as estimated by DFT calculations, a schematic energy diagram was determined for the photothermal tautomerization processes in acetonitrile.

A facile formation of picene was achieved by photosensitization of 1,2-di(1-naphthyl)ethane using 9-fluorenone as a sensitizer. This sensitized photoreaction is the first photochemical cyclization of ethylene-bridged naphthalene moieties to afford the picene skeleton. 5,8-Dibromopicene, prepared by this procedure using 1,2-di[1-(4-bromonaphthyl)]ethane as the substrate, was readily converted to novel functionalized picenes by conventional substitution and cross-coupling reactions.

We have prepared three types of carbonyl compounds, benzoylethynylmethyl phenyl sulfide (2@SPh), (p-benzoyl)phenylethynylmethyl phenyl sulfide (3@SPh) and p-(benzoylethynyl)benzyl phenyl sulfide (4@SPh) with benzoyl and phenylthiylmethyl groups, which are interconnected with a C–C triple bond and a phenyl ring. Laser flash photolysis of 3@SPh and 4@SPh in acetonitrile provided the transient absorption spectra of the corresponding triplet states where no chemical reactions were recognized. Upon laser flash photolysis of 2@SPh, the absorption band due to the phenylthiyl radical (PTR) was obtained, indicating that the C–S bond cleaved in the excited state. Triplet sensitization of these carbonyl compounds using acetone and xanthone was conducted using laser photolysis techniques. The formation of triplet 3@SPh was seen in the transient absorption, whereas the PTR formation was observed for 2@SPh and 4@SPh, indicating that the triplet states were reactive for the C–S bond dissociation. The C–S bond dissociation mechanism for 4@SPh upon triplet sensitization is discussed in comparison with those for 2@SPh and 3@SPh.