Evidence For Excimer Formation Research Articles

Effect of Diffusion on Photo-Induced Excited-State Energy Transfer between Fluorescent Semiconducting Molecules: Tris-(8-hydroxyquinoline) Aluminum and 6,13-Bis (Triisopropylsilylethynyl) Pentacene

In the present work, the effect of diffusion on photo-induced excitation energy transfer between fluorescent organic semiconducting molecules tris-(8-hydroxyquinoline) aluminum (AlQ3, n-type donor) and 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS-P, p-type acceptor) at a concentration range of 10–4 to 10–6 M in chloroform solution was studied by steady-state and time-domain fluorescence measurements. The donor–donor interaction strength is significantly weaker than the donor–acceptor strength (the ratio of donor–donor to donor–acceptor interaction strengths is 1.55 × 103) in chloroform solution. Considerable overlap between donor emission and acceptor absorption and high interaction parameters favors direct Förster energy transfer and exciplex (D*A) formation. Excitation energy migration does not take place between donor molecules. However, material diffusion appears to influence the donor decay dynamics by forming charge-transfer exciplex complexes and modulating the excitation energy transfer rate from the metal-to-ligand charge-transfer (MLCT) state of the donor to vibronic states of the acceptor at a low acceptor concentration. At high acceptor concentrations, energy transfer from the excited donor to acceptor occurs through the exchange mechanism along with Förster long-range dipole–dipole interactions, and the corresponding critical transfer distance is found to be ∼42 Å. A blue-shift in AlQ3 emission, a red-shift in the 0–0 vibronic peak of TIPS-P, and quenching of exciplex decay following Stern–Volmer quenching are observed, with the increase in acceptor concentration. No evidence of excimer formation is observed in acceptor molecules.

A Photoluminescent, Segmented Oligo-Polyphenylenevinylene Copolymer with Hydrogen-Bonding Pendant Chains

The syntheses and electronic spectroscopy of E,E-2,5-bis[2-hydroxyethoxy]-1,4-bis[2-(3,4,5-trimethoxyphenyl-1,2-ethenediyl]benzene, 3, and alternating block copolymer analogue alt-poly[1,8-octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-(E)-ethenediyl-2,5-(2-hydroxyethoxy)-1,4-phenylene-1,2-(E)-ethenediyl-3,5-dimethoxy-1,4-phenylene], 2, are described. Polymer 2 has UV−vis absorbances at 320 nm (shoulder) and 389 nm (maximum), with fluorescence emission at 449 nm (maximum) and 480 nm (shoulder) upon excitation at various wavelengths. Model compound 3 shows virtually identical spectral behavior. Neat solid films of polymer 2 show UV−vis absorbance in the 400−450-nm region (broad) and fluorescence at about 490 nm. The 40-nm shift from solution to neat film emission for 2 is attributed to hydrogen-bond-assisted interactions between chromophores in the solid state. The fluorescence quantum yields in chloroform at room temperature (365-nm excitation) for 2 and 3 are 0.36 and 0.74, respectively, relative to quinine sulfate standards. There is no evidence of excimer formation, although crystallographic studies of model system 3 show hydrogen-bonded chains involving the hydroxyethoxy pendant chain. The model system also has a highly planar crystal structure, consistent with its high emission quantum yield; the planarity may be favored by interactions between the pendant hydroxyethoxy groups and the ethenediyl groups of the chromophore.

Excited States of Bromine-Substituted Distyrylbenzenes: Models for Conjugated Polymer Emission

Model 1,4-bis(styryl)benzene derivatives 1a,b related to poly(2-bromo-5-methoxy-1,4-phenylenevinylene) were synthesized to probe the effect of bromination on excited-state behavior of conjugated phenylenevinylene analogues. They showed solution absorption maxima at about 385 nm, without evidence of excimer formation up to 100 μM. Their fluorescence spectra showed overlapping bands at 435 and 460 nm, with quantum yields in tetrahydrofuran of 0.16 and 0.21 for 1a and 1b, respectively. Excited-state transients were monitored by time-resolved laser flash spectroscopy at room temperature. The triplet states were characterized by absorption maxima at about 520−530 nm with lifetimes of about 0.6 μs, much longer than would be observed for prompt fluorescence states. The quantum yields of singlet to triplet-state intersystem crossing were determined to be 0.45 and 0.28 for 1a and 1b, respectively. The acceleration of triplet transient decay rates under increased oxygen pressure according to the Stern−Volmer law fu...

Laser-induced fluorescence excitation spectra of 1,4-Di(1-naphthyl)propane and 1-buthylnaphthalene in a supersonic jet

Laser-induced fluorescence excitation spectra of the bichromophoric molecule, 1,4-Di(1-naphthyl)propane (N4N(1,1′)), and of 1-buthylnaphthalene (1-BuN ) in a supersonic jet are reported. Comparison between the two spectra suggests that the two naphthalene moieties in the bichromophoric molecule do not interact strongly. Exciton splitting or evidence for excimer formation was not observed. However, the aliphatic substitution alone, cannot account for the significant red shift of the origin in the spectrum of the bichromophoric molecule.

Photophysics and evidence of excimer formation, linear bipyridines in solution and solid films

The photophysical properties of 1,4-bis(4′-pyridylethynyl)-2,5-di( n-hexyloxy)benzene and 1,4-bis(4′-pyridylethynyl)benzene in both solution as well as in solid films were investigated. Both compounds exhibit concentration-dependent fluorescence in solution. The fluorescent bands red shift in solution at high concentration and in solid films and the emission is ascribed to the formation of excimers.

Polarized photoluminescence from solid films of nematic and chiral-nematic poly(p-phenylene)s

Thermotropic nematic and chiral-nematic poly(p-phenylene)s were prepared into well-aligned films between fused-silica substrates in which conjugated backbones were uniaxially and helically oriented. With unpolarized photoexcitation at 350 nm, a nematic film produced a degree of linear polarization of 9 near the emission peak at 410 nm with no evidence of excimer formation. With the same photoexcitation of a chiral-nematic film, the degree of circular polarization was found to vary from −1.3 in the 390–430 nm spectral region to between +0.3 and +0.9 beyond the edge of the selective reflection band. The crossover behavior unique to light emission from the selective reflection region remains inexplicable with existing theories on light propagation through periodically structured films.

Multichromophoric cyclodextrins. 1. Synthesis of O-naphthoyl-.beta.-cyclodextrins and investigation of excimer formation and energy hopping

The synthesis and photophysical properties of /3-cyclodextrins bearing seven 2-naphthoyloxy chromophores in specific positions, either on the primary face or the secondary face, or 14 2-naphthoyloxy chromophores, seven on each face, are reported. These multichromophoric cyclodextrins are good models for the study of excitation energy migration among chromophores in well-defined positions. The investigation was performed in dichloromethane and in a mixture of ethanol and methanol that can form a glass at low temperature. The absorption spectra show that the interactions between chromophores in the ground state are weak, whereas the fluorescence spectra reveal the existence of excimers at room temperature but not at low temperature in a rigid glass. Further evidence of excimer formation is provided by the fluorescence decays. Since excimers act as energy traps, the energy hopping process was studied in a rigid glass at low temperature by steady-state and time-resolved fluorescence depolarization techniques. The steady-state anisotropy is found to be one seventh of the theoretical limiting anisotropy 0.4, which means that excitation energy hops between chromophores with essentially randomly oriented transition moments at a rate much higher than the chromophore intrinsic decay rate. Energy hopping is indeed very fast as shown by the fluorescence anisotropy decay which is at least as fast as the apparatus time resolution (a few tens of picoseconds).

Delayed Fluorescence from Upper Excited Singlet States Sn (n > 1) of the Aromatic Hydrocarbons 1,2‐benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane

AbstractDelayed fluorescence (DF.) spectra of 1,2‐benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane were measured at −80° up to the wavenumber corresponding to the energy of two triplets. With all four compounds a weak DF. Sn → S0 from the highest state Sn accessible by triplet‐triplet annihilation (TTA) was found. Lifetimes of Sn calculated (a) from the ratio of the DF's Sn → S0 and S1 → S0 and (b) from the difference in line‐width of the 0,0‐transitions Sn,0 ← S0,0 and S1,0 ← S0,0 agree reasonably well. This indicates that population of the highest accessible excited singlet state is the dominating primary process in the excited singlet channel of TTA. There is no evidence for excimer formation being the first step in TTA. DF. spectra extend up to the energy of two triplets. With pyrene the intensity distribution in the hot‐band region of the DF. S2 → S0 suggests that in TTA the same vibrational selection rules are valid as in one‐photon absorption S2 ← S0 and that vibrational relaxation within the S2 manifold is slow compared with internal conversion S2 ↝ S1. The experimental technique is described in detail and experimental difficulties arising from impurities and photoproduct formation are discussed.