ABSTRACT
Because of their excellent optical and electronic properties, oligofluorenes and polyfluorenes have been investigated for years. Recently developed star-shaped oligomers bearing a truxene or isotruxene core are interesting two-dimensional oligomers. Since employment of a condensed ring system will be effective in further extension of π-conjugation system, we studied electronic and vibrational properties of radical cation of CITFn, star-shaped condensed oligomer with isotruxene core and fluorene unit, by means of the radiation chemical methods. Absorption spectra of radical cation of CITFn were measured in the wide spectral range, which revealed extended π-conjugation of CITFn. Furthermore, time-resolved resonance Raman spectra during pulse radiolysis revealed that the oxidation of CITFn induced structural change to enhance quinoidal character. The Raman data and theoretical calculation indicated that the rigid framework of the present star-shaped oligomer which makes the oligomer a planar structure is quite important in extension of the conjugation pathway.
ABSTRACT
Oligofluorenes attract wide attention due to their excellent fluorescent properties. For the detailed understanding of the excited state properties, ultrafast processes have to be clarified. Here, we have investigated the structural relaxation in the singlet excited state of star-shaped oligofluorenes with a truxene or isotruxene core, to which oligofluorenes (n = 1-4) were attached. The transient absorption peak showed red-shift with time upon excitation. The fluorescence decay profiles in the picosecond domain showed the fast component in addition to the component corresponding to the singlet excited state lifetime. These ultrafast phenomena can be attributed to the structural relaxation, i.e., planarization, in the singlet excited state. The planarization process was supported by the theoretical calculation based on the time-dependent density functional theory. Furthermore, dependence of two-photon absorption cross section on the core of the star-shaped oligofluorene has been elucidated.
ABSTRACT
The synthesis, photochemical and electrochemical properties, and electronic structures of a series of star-shaped ladder-type oligophenylenes Sn (n = 7, 10, 13, 16, 19, and 22), including one multibranched case S19mb, are reported and compared with the linear para-phenylene ladders Rn (n = 2-5 and 8) and the stepladder analogues SFn (n = 10, 16, and 22). The n value refers to the number of π-conjugated phenylene rings. Functionalized isotruxenes are the key synthetic building blocks, and S22 is the largest monodispersed ladder-type oligophenylene known to date. The Sn systems possess the structural rigidity of Rn and the ortho-para phenylene connectivity of SFn. Consequently, Sn represents the first class of branched chromophores with fully two-dimensional conjugation in both ground- and excited-state configurations. Evidences include the excellent linear correlations for the optical 0-0 energies or the first oxidation potentials of Sn and Rn against the reciprocal of their n values, delocalized HOMO and LUMO based on density functional theory calculations, and molecule-like fluorescence anisotropy. The resulting model of effective conjugation plane (ECP) for the two-dimensional π-conjugated systems compliments the concept of effective conjugation length (ECL) for one-dimensional oligomeric systems. Other implications of the observed structure-property relationships are also included.
ABSTRACT
The synthesis and brakelike performance of a new molecular system (1) consisting of a pentiptycene rotor and a 2-methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variable-temperature (1)H and (13)C NMR spectroscopy and DFT calculations, and the switching between the brake-on and brake-off states was conducted by a combination of photochemical and electrochemical isomerization. Because of the greater steric hindrance between the rotor and the brake units in the Z form ((Z)-1) than in the E form ((E)-1), rotation of the rotor is slowed down 500-fold at room temperature (298 K) on going from (E)-1 to (Z)-1, corresponding to the brake-off and brake-on states, respectively. The (E)-1â(Z)-1 photoisomerization in acetonitrile is efficient and reaches an (E)-1/(Z)-1 ratio of 11:89 in the photostationary state upon excitation at 290 nm, attributable to a much larger isomerization quantum efficiency for (E)-1 versus (Z)-1. An efficient (Z)-1â(E)-1 isomerization (96%) was also achieved by electrochemical treatment through the radical anionic intermediates. Consequently, the reversibility of the E-Z switching of 1 is as high as 85%. The repeated E-Z switching of 1 with alternating photochemical and electrochemical treatments is also demonstrated.
ABSTRACT
The synthesis of star-shaped ladder-type oligophenylenes Sn (n = 4, 7, 10, 13, and 16), where n is the number of pi-conjugated phenylene rings, is reported. The two-dimensional conjugation interactions in Sn result in a lower bandgap for S16 than the currently known ladder-type poly(p-phenylene)s (LPPPs).
ABSTRACT
Three multistep approaches toward facile syntheses of isotruxene (1) and isotruxenone (3) are reported. The ortho-para conjugated backbone in the precursor 4 was constructed by either Co-catalyzed [2 + 2 + 2] cyclotrimerization or the [4 + 2] Diels-Alder reactions. The regioselectivity of the triple intramolecular Friedel-Crafts acylation of 4 plays the key role in determining the overall yield. Compared to the previous one-step method, the current approaches are more efficient in terms of product yield (27-36% vs 4-18%) and purification (i.e., free of column chromatography).
ABSTRACT
The synthesis, photophysics, cyclic voltammetry, spectroelectrochemistry, and electrogenerated chemiluminescence (ECL) properties of a series of unsymmetrical star-shaped oligophenylenes IT1-IT4 are reported. The electronic couplings among the three oligofluorene arms in IT1-IT4 are strong due to the para-ortho branched isotruxene core. The ortho conjugation effect results in band splitting in the absorption spectra for both the neutral and the radical ionic form with a stronger effect for the latter. However, such an ortho conjugation effect becomes weaker as the oligofluorene arms are longer. The same fluorescence maxima displayed by IT2-IT4 suggest that the exciton coherence size (or the bound electron-hole pair) is no larger than 16 phenylene rings. The little chain-length dependence of the first reduction and oxidation potentials for IT1-IT4 suggests that the reversible electron-transfer processes of the neutral species are mainly associated with the isotruxene core. The ECL of IT1-IT4 is from the singlet excited state, but the spectra are red shifted with respect to the fluorescence spectra of dilute solutions due to the reabsorption effect. Our results also reveal that the meta conjugation interactions in the previously reported C 3-symmetrical truxene-oligofluorene analogs T1-T4 (Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.; Perepichka, I. F.; Bradley, D. D. C.; Koeberg, M. J. Am. Chem. Soc. 2004, 126, 13695-13702) are rather weak.
ABSTRACT
A room-temperature light-driven molecular brake (1), consisting of a pentiptycene rotator, a 3,5-dinitrophenyl brake, and a photoisomerizable ethenyl spacer, is reported. The rotation rates of the rotator differ by about 9 orders of magnitude between the brake-on (cis-1) and brake-off (trans-1) states.
