Intramolecular excimer and exciplex emission of 1,4-dipyrenyl substituted cyclohexasilane

Effects of substituents in silyl groups on the absorption, fluorescence and structural properties of 1,3,6,8-tetrasilylpyrenes

1,3,6,8-Tetrasilylpyrenes and related germyl and stannyl derivatives were synthesized, and their absorption and fluorescence spectroscopic and structural properties were elucidated. The results show that the UV-vis absorption maxima of these substances in CH2Cl2 solutions shift to longer wavelengths as the size of the alkyl groups and numbers of phenyl groups on silicon increase. Fluorescence quantum yields of tetrasilylpyrenes in cyclohexane are larger than that of pyrene, and a pentamethyldisilyl derivative has an emission efficiency of 0.79. Except in the case of the SiMe2H derivative, excimer emission was not observed in concentrated solutions of these substances. The SiMe2H and SiMe3 derivatives were shown to form CT complexes with tetracyanoethylene in CH2Cl2 solutions. The calculated energy barriers for rotation of the silyl groups about the Si-C bond increase as the steric bulk of the silyl group increases. 29Si NMR chemical shifts were found to depend on the sizes of the alkyl groups and numbers of phenyl groups. Data arising from theoretical calculations suggest that the silyl groups act as electron-donating groups, and the donating ability of the groups decreases in the order SiR3 > GeR3 > SnR3.

Excimer formation in a confined space: photophysics of ladderphanes with tetraarylethylene linkers

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutene-based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5-5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited-state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter-wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion-controlled formation of the excimer.

Artificial DNAs based on alkynyl C-nucleosides as a superior scaffold for homo- and heteroexcimer emissions

DNA-like fluorescent oligomers composed of alkynyl beta-D-ribofuranosides bearing pyrene, perylene, and anthracene as a fluorophore were synthesized by solid-phase DNA synthesis. The fluorescent oligomers possess the defined number and order of the fluorophores. In these oligomers, the adjacent fluorophores efficiently interact with each other by hydrophobic interactions in their electronic ground states in a face-to-face fashion. The predominant excimer emissions were observed from not only the homooligomers (pyrene-pyrene and perylene-perylene systems) but also the heterooligomers (pyrene-perylene, pyrene-anthracene, and perylene-anthracene systems) in aqueous media.

Global and target analysis of time-resolved fluorescence spectra of Di-9H-fluoren-9-yldimethylsilane: dynamics and energetics for intramolecular excimer formation

Dynamics and energetics for intramolecular excimer formation of a diarylsilane, di-9H-fluoren-9-yldimethylsilane (DFYDMS) have been investigated by means of ps time-resolved fluorescence spectroscopy and ab initio calculation. Multiple fluorescence decay curves were globally deconvolved to generate time-resolved fluorescence spectra and decay-associated spectra (DAS), from which species-associated spectra (SAS) were obtained. It is shown in the global analysis that there are at least three excited states: Two states are the locally excited (LE) states (lambda(max) approximately 320 nm) having lifetimes of 0.70 +/- 0.04 and 1.75 +/- 0.02 ns, and another is the excimer state (lambda(max) approximately 400 nm) having a lifetime of 7.34 +/- 0.02 ns. The species which decays with 0.70 ns evolves into a species with a red-shifted spectrum, which in turn decays in 7.34 ns. The experimental and ab initio results indicate that the rise time of 0.70 ns corresponds to the conversion of the initial S(1) LE state having a near sandwich geometry to the S(1) excimer state adopting a true sandwich geometry.

Unprecedented chemiluminescence behaviour during peroxyoxalate chemiluminescence of oxalates with fluorescent or electron-donating aryloxy groups

A series of diaryl and bis(4-styrylphenyl) oxalates with electron-donating substituents or fluorescent moieties were subjected to the peroxyoxalate chemiluminescence (PO-CL) reaction, some of which were found to behave in a unprecedented manner. The reaction of bis(p-methyoxyphenyl) oxalate, as a representative example, emits light due not only to the emission from the externally added excited fluorophore, but also from the presumable excimer of p-methoxyphenol. Also, during the reaction of the bis(4-styrylphenyl) oxalates, the emission based on the fluorescence as well as the excimer of the eliminating group were observed. These experimental results suggest that such emitting species would be formed by an intra- and intermolecular electronic interaction with a high-energy intermediate, such as a dioxetanone.