Excited-State and Photoelectrochemical Behavior of Pyrene-Linked Phenyleneethynylene Oligomer
Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA.The Journal of Physical Chemistry B (Impact Factor: 3.3). 11/2008; 112(46):14539-47. DOI: 10.1021/jp805878c
An oligophenyleneethynylene (OPE), 1,4-bis(phenyleneethynyl)-2,5-bis(hexyloxy)benzene (2), is coupled with pyrene to extend the conjugation and allow its use as a light-harvesting molecule [Py-OPE (1)]. The absorption and emission maxima of 1 are red-shifted compared to those of 2. Similar differences in the singlet and triplet excited-state properties are evident. The fluorescence yield of 2 in toluene is 0.53, which is slightly less than the value for the parent OPE (2) of 0.66. The excited singlet and triplet of 1 as characterized from transient absorption spectroscopy exhibit lifetimes of 1.07 ns and 4.0 micros, respectively, in toluene. When 1 was cast as a film on a glass electrode (OTE) and excited with a 387-nm laser pulse, we observed the formation of excitons that decayed within a few picoseconds. When 1 was cast as a film on a SnO2-modified conducting glass electrode (OTE/SnO2), a small fraction of excitons dissociated to produce a long-lived charge-separated state. The role of the SnO2 interface in promoting charge separation was inferred from the photoelectrochemical measurements. Under visible light excitation, the OTE/SnO2 electrode was capable of generating photocurrent (approximately 0.25 mA/cm2) with an incident photon conversion efficiency (IPCE) of approximately 6%.
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ABSTRACT: Two-dimensional self-organization of a series of phenyleneethynylenes was investigated, at ambient conditions, by varying the length of alkoxy chain and introducing functional groups at the terminal positions using high-resolution scanning tunneling microscopy (STM). The model phenyleneethynylene molecule, which does not possess any functional groups, self-organizes into wire like structures on surface. High-resolution STM imaging revealed that molecules are arranged in a skewed 1D fashion. The spacing between the molecular wires was successfully modulated by replacing hexyloxy (C6) chains with dodecyloxy (C12) chains. The initial step of the formation of all the molecular assemblies involves the alkyl CH···acetylenic π interactions (CH···π) leading to the organization of molecules as two types of strips. These strips further interlock to two types of 2D organizations. The hydroxyl as well as aldehyde groups present at the terminal positions of the phenyleneethynylene molecules play an important role in the interlocking process. An end-to-end assembly was observed in the case of phenyleneethynylene molecule possessing hydroxyl groups at the terminal positions, which is attributed to the intermolecular hydrogen bonding between the strips. The adsorption of molecules with two faces results in enantiomeric 2D structures and these aspects were investigated using molecular modeling studies.The Journal of Physical Chemistry C 07/2009; 113(27). DOI:10.1021/jp901884v · 4.77 Impact Factor
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ABSTRACT: Sensitization and binding of a large footprint pyrene chromophore to planar (sapphire) and colloidal metal oxide films (TiO2 and ZrO2) is investigated. The model compound combines a 1-pyrenyl-ethynylenephenylene unit with a new adamantane-tripodal linker that binds to the surface. The linker design, combining a large footprint (approximately 2 nm2) of the tripodal linker with the meta position of the COOH anchoring groups, was suggested from atomistic models, and it aims to provide improved spacing control. The pyrene chromophore unit provides a probe of sensitizer-sensitizer interactions through its propensity to form excimers, unless neighboring pyrene units are sufficiently spaced (>or=3.5 A). Absorption and fluorescence studies, and a comparison with a pyrene-rigid rod model compound, suggest that the new tripodal anchor group allows spacing control on planar surfaces. On colloidal films, the linker provides spacing control at low surface coverage but sensitizer-sensitizer interactions are still observed on colloidal films at high surface coverage. Implications for the functionalization of metal oxide films in hybrid molecule-metal oxide semiconductor material systems are discussed.Langmuir 08/2009; 25(16):9219-26. DOI:10.1021/la9007679 · 4.46 Impact Factor
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ABSTRACT: A bipyridine-based system with phenyleneethynylene at the 4,4' positions (1) and its p-methyl (2) and p-methoxy (3) substituted derivatives were synthesized via Sonogashira coupling reactions. The photophysical properties of 1-3 and their related H+ and Zn2+ adducts (1:H+-3:H+ and 1:Zn2+-3:Zn2+) were investigated, as a function of solvent polarity, by using steady-state and time-resolved spectroscopic techniques. Molecular systems 1-3 exhibit trans conformation, whereas adducts with H+ and Zn2+ are conformationally locked cis species. The unsubstituted compound 1 emits at 360 nm with low fluorescence quantum yield (phi(fl) = 0.2%) regardless of the solvent polarity. Fluorescence spectra of 2 and 3 are bathochromically shifted in polar solvents, and the p-methoxy (3) derivative possesses phi(fl) as high as 12%. Complexation of 1-3 with H+ or Zn2+ in acetonitrile causes red-shift of the lowest energy absorption bands, whereas dramatic changes of the emission properties are found as a function of the electron donating ability of the substituents on the phenyleneethynylene moiety (-CH3 or -OCH3), suggesting a charge-transfer character of the lowest electronic transition of 1-3. 1:H+, 1:Zn2+, 2:H+ and 2:Zn2+ exhibit intense fluorescence with phi(fl) up to 33% (1:Zn2+) whilst 3:H+ and 3:Zn2+ are found to be weakly emissive. The singlet radiative and non-radiative rate constants of compounds and complexes were determined, along with triplet parameters, via phosphorescence and transient absorption spectroscopy. More conclusive evidence regarding the protonation of bipyridine nitrogen atoms of compounds 1-3 were obtained through 1H NMR titration studies. These studies indicate that the conjugate molecular systems based on 2,2'-bipyridine and phenyleneethenylenes possess tunable optical properties which can be further utilized for preparing organic and inorganic luminophores with potential application in optoelectronic systems.Photochemical and Photobiological Sciences 10/2009; 8(10):1432-40. DOI:10.1039/b9pp00002j · 2.27 Impact Factor
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