Intramolecular Energy Transfer Process Research Articles

A Triphenylamine Dye Model for the Study of Intramolecular Energy Transfer and Charge Transfer in Dye‐Sensitized Solar Cells

AbstractA novel dye (2TPA‐R), containing two triphenylamine (TPA) units connected by a vinyl group and rhodanine‐3‐acetic acid as the electron acceptor, is designed and synthesized successfully to reveal the working principles of organic dye in dye‐sensitized solar cells (DSSCs). 2TPA and TPA‐R, which consist of two TPA units connected by vinyl and a TPA unit linked with rhodanine‐3‐acetic acid, respectively, are also synthesized as references to study the intramolecular energy transfer (EnT) and charge transfer (ICT) processes of 2TPA‐R in CH2Cl2 solution and on a TiO2 surface. The results suggest that the intramolecular EnT and ICT processes show a positive effect on the performance of DSSCs. However, the flexible structure and less‐adsorbed amount of dye on TiO2 may make it difficult to improve the efficiency of DSSCs. This study on intramolecular EnT and ICT processes acts as a guide for the design and synthesis of efficient organic dyes in the future.

Mechanisms of non-radiative energy transfer involving lanthanide ions revisited

In this work, the theoretical background of the Coulomb direct and exchange interactions in non-radiative energy transfer involving lanthanide ions is critically reviewed. Emphasis is given to the shielding effect produced by the filled 5s and 5p sub-shells of the lanthanide ion and the appropriate use of the so-called forced electric dipole intensity parameters of 4f–4f transitions, two aspects that have been often overlooked in the literature. An effective exchange hamiltonian, based on Mülliken-type approximations, and an expression for the resonant or quasi-resonant energy mismatch spectral overlap factor are proposed. Both cases of ion-to-ion and intramolecular energy transfer processes are discussed. Numerical estimates are compared with previous calculations, showing that order of magnitude differences appear. The analysis indicates that the donor–acceptor distance dependence of transfer rates by the exchange mechanism goes beyond the single exponential behavior adopted from Dexter’s approach.

Photophysical Properties of Lanthanide Hybrids Covalently Bonded To Functionalized MCM-41 by Modified Aromatic Carboxylic Acids

MCM-41 mesoporous silica has been functionalized with aromatic carboxylic acids salicylic acid (Sal) and 2-hydroxyl-3-methylbenzoic acid (HMBA) through co-condensation approach of tetraethoxysilane (TEOS) in the presence of the cetyltrimethylammonium bromide (CTAB) surfactant as a template. Organic ligands salicylic acid or 2-hydroxyl-3-methylbenzoic acid grafted to the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) was used as the precursor for the preparation of an organic-inorganic hybrid materials. Novel organic-inorganic mesoporous luminescent hybrid containing Ln(3+) (Tb(3+), Eu(3+)) complexes covalently attached to the functionalized ordered mesoporous MCM-41, which were designated as Ln-Sal-MCM-41 and Ln-HMBA-MCM-41, respectively, were obtained by sol-gel process. The luminescence properties of these resulting materials were characterized in detail, and the results reveal that luminescent mesoporous materials have high surface area, uniformity in the mesopore structure and good crystallinity. Moreover, the mesoporous material covalently bonded Tb(3+) complex (Tb-Sal-MCM-41 and Tb-HMBA-MCM-41) exhibit the stronger characteristic emission of Tb(3+) and longer lifetime than the corresponding Eu-containing materials Eu-Sal-MCM-41 and Eu-HMBA-MCM-41 due to the triplet state energy of modified organic ligands Sal-TEPIC and HMBA-TEPIC match with the emissive energy level of Tb(3+) very well. In addition, the luminescence lifetime and emission quantum efficiency of (5)D(0) Eu(3+) excited state also indicates the efficient intramolecular energy transfer process in Tb-SAL-MCM-41 and Tb-HMBA-MCM-41.

Gradient Shape-Persistent π-Conjugated Dendrimers for Light-Harvesting: Synthesis, Photophysical Properties, and Energy Funneling

A new class of pi-conjugated dendrimers G0, G1, and G2 was developed through a double-stage divergent/convergent growth approach, in which 5,5,10,10,15,15-hexahexyltruxene was employed as the node and oligo(thienylethynylene)s (OTEs) with different lengths as the branching moieties. The dendrimers were fully characterized by (1)H and (13)C NMR, elemental analysis, gel permeation chromatography, and MALDI-TOF MS. Also, by using atomic force microscopy, it was observed that dendrimer G2 laid nearly flat on the mica surface as a single molecule. Dynamic light scattering results showed that the molecule retained its relatively flat shape in solution. To our best knowledge, dendrimer G2, with a radius approaching 10 nm and a molecular weight of 27 072 Da, was the largest among reported second generation dendrimers. The energy gradient in G2 was constructed by linking OTEs of increasing effective conjugation lengths from the dendritic rim to the core. The intramolecular energy transfer process was studied using steady-state UV-vis absorption and photoluminescent spectroscopies, as well as time-resolved fluorescence spectroscopy. Our structurally extended dendrimers showed an excellent energy funneling ability (their energy transfer efficiencies were all over 95%). All results demonstrate that these dendrimers are promising candidates as light-harvesting materials for optoelectronic devices.

Lanthanide-Centered Covalently Bonded Hybrids through Sulfide Linkage: Molecular Assembly, Physical Characterization, and Photoluminescence

A series of novel photoactive lanthanide (europium, terbium, dysprosium, samarium) hybrid materials with organic parts covalently bonded to inorganic parts via sulfide linkage have been assembled by the sol-gel process. The organic parts as molecular bridge are obtained from the functionalized thiosalicylic acids by five silane crosslinking reagents, 3-chloropropyltrimethoxysilane, 3-methacryloyloxypropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 3-(triethoxysilyl)propylisocyanate. The intramolecular energy transfer process between lanthanide ions and the molecular bridges took place within these molecular-based hybrids and especially the quantum efficiency of europium hybrids were determined, suggesting that the hybrid material systems derived from different molecular bridges present different luminescence efficiencies.

Supramolecular Metal-Polypyridyl and Ru(II) Porphyrin Complexes: Photophysical, Electron Paramagnetic Resonance, and Electrochemical Studies

A series of mixed-metal supramolecular porphyrin arrays in which the geometry of the central metal-polypyridyl moiety defines the spatial arrangement of two or more Ru(II)-porphyrin units through axial coordination have been prepared by employing self-assembly based protocols, and their photophysical and electrochemical properties have been studied. The electrochemical properties of the constituent parts of these arrays depend only on their own chemical environment, regardless of the nuclearity and the overall charge of the compound; in this way species with predetermined redox patterns can be obtained via the synthetic control of the self-assembly process. Interestingly, several of these arrays are luminescent both at room and at low temperatures, and in many cases core-to-periphery or periphery-to-core intramolecular energy transfer processes take place according to the nature of the central metal template.

Photoacoustic and Fluorescence Spectroscopy of Metallomesogens Containing Lanthanide Ions

Metallomesogens Ln(bta)3L2 (Ln3+: La3+, Eu3+, and Ho3+; bta: benzoyltrifluoroacetonate; L: Schiff base) were prepared. Photoacoustic (PA) spectroscopy was used to study physicochemical properties of the liquid crystalline metal complexes. In the region of ligand absorption, PA intensity increases for Eu(bta)3L2, La(bta)3L2, and Ho(bta)3L2, in that order. It is found that the PA intensity of the ligand bears a relation to the intramolecular energy transfer process. For the first time, phase transitions of Eu(bta)3L2 from glass state to smectic A (SmA) phase and SmA phase to isotropic liquid are monitored by PA and fluorescence (FL) spectroscopy from two aspects: nonradiative and radiative transitions. The results show that PA technique may serve as a new tool for investigating the spectral properties and phase transitions of liquid crystals containing metal ions.

Intramolecular Energy Transfer in a One-Dimensional Europium Tetracyanoplatinate

The one-dimensional, polymeric compound (C15H11N3)Eu(H2O)2(NO3)(Pt(CN)4).CH3CN (1) has been shown to display an efficient donor-acceptor intramolecular energy-transfer (IET) process where ligand donors transfer energy to the Eu (III) acceptor. Single-crystal X-ray diffraction has been used to investigate the structural features of this compound in order to help understand the observed IET process. Crystallographic data: 1, monoclinic, space group P2(1)/c, a = 12.835(1) A, b = 15.239(1) A, c = 13.751(2) A, beta = 105.594(9) degrees , V = 2590.8(5) A(3), and Z = 4 (T = 290 K).

Covalently assembly and photophysical properties of novel lanthanide centered hybrid materials by functionalized thioacylureas bridge

A novel molecular precursor (abbreviated as TAM-Si) derives from thioacetaminde (TAM) modified by 3-(triethoxysilyl)-propyl isocyanate (TEPIC) though the hydrogen transfer addition reaction. Then TAM-Si behaves as functional molecular bridge which coordinates to RE 3 (Eu 3+, Tb 3+) as well as form Si O network with inorganic precursor (TEOS) after a sol–gel process (cohydrolysis and copolycondensation reaction), resulting in the covalently bonded hybrid materials (RE–TAM-Si). On the other hand, the hybrid material of TAM-Si without introduction of RE 3+ as well has been obtained. SEM pictures indicate that the TAM-Si show the sphere micromorphology with particle size of micrometer dimension while RE–TAM-Si hybrids present different nanometer particle, which suggests that lanthanide ions has influence on the microstructure of hybrid systems through its coordinated effect. The blue emission for TAM-Si hybrids and the narrow-width green and red emissions were achieved for Tb 3+ and Eu 3+ ions, respectively, indicating that the intramolecular energy transfer process take place from photoactive group to Tb 3+ and Eu 3+ ions in these hybrid microsphere systems. Especially the lifetime and quantum efficiency for europium hybrids have been determined.

Fluorescence-Enhanced Europium-Diethylenetriaminepentaacetic (DTPA)-Monoamide Complexes for the Assessment of Renal Function

Real-time, noninvasive assessment of glomerular filtration rate (GFR) is essential not only for monitoring critically ill patients at the bedside, but also for staging and monitoring patients with chronic kidney disease. In our pursuit to develop exogenous luminescent probes for dynamic optical monitoring of GFR, we have prepared and evaluated Eu(3+) complexes of several diethylenetriamine pentaacetate (DTPA)-monoamide ligands bearing molecular "antennae" to enhance metal fluorescence via intramolecular ligand-metal fluorescence resonance energy transfer process. The results show that Eu-DTPA-monoamide complex 18b, which contains a quinoxanlinyl antenna, exhibits large (ca. 2700-fold) Eu(3+) fluorescence enhancement. Indeed, complex 18b exhibits the highest fluorescent enhancement observed thus far in the DTPA-type metal complexes. The renal clearance property was assessed using the corresponding radioactive (111)In complex 18a, and the data suggest that this complex clears via a complex mechanism that includes glomerular filtration.

Covalently bonded assembly and photoluminescent properties of rare earth/silica/poly(methyl methacrylate-co-maleic anhydride) hybrid materials

Copolymer (MMA-co-MAL) of methyl methacrylate (MMA) and maleic anhydride (MAL) was prepared and grafted by 3-aminopropyltriethoxysilane (APES), which behaves as the structural precursor for functional bridge to assemble the covalently bonded systems through the coordination to rare earth ions (Eu 3+, Tb 3+) with carboxylic groups of maleic anhydride. On the other hand, 1,10-phenanthroline (phen) is engaged in a second functional ligand to sensitize the luminescence of RE 3+ (rare earth ions) by intramolecular energy transfer process. Meanwhile, the cohydrolysis and copolycondensation processes happened between triethoxysilyl of modified copolymer (MMA-co-MAL-APES) and tetraethoxysilane (TEOS) with Si–O covalent bonds, resulting in the polymer-inorganic hybrids (phen-RE-MMA MMA-co-MAL-Si) exhibiting characteristic red or green emissions of Eu or Tb ions. Especially the luminescent quantum efficiencies of europium hybrid systems are estimated and discussed in detail.

Comment on trivalent europium lifetimes in the presence of intramolecular energy transfer processes

This work emphasizes the importance of conceptually distinguishing between lifetimes in the absence (natural lifetimes) and in the presence of intramolecular energy transfer processes in lanthanide coordination compounds. It is focused on trivalent europium compounds with mixed ligands, and a case study is made on results recently reported in the literature.

Study of lanthanide liquid-crystalline complexes by Photoacoustic and luminescence spectroscopy

Lanthanide complexes Ln(bta)3L2 (Ln3+: Eu3+, Tb3+ and Ho3+; bta: benzoyltrifluoroacetonate; L: N-octadecyl-2-hydroxy-4-tetradecyloxybenzal- dimine) are synthesized. Their photoacoustic (PA) spectra are reported and interpreted. In the region of ligand absorption, PA intensity increases for Eu(bta)3L2, Tb(bta)3L2 and Ho(bta)3L2, respectively. It is found that the PA intensity of the ligand bears a relation to the intramolecular energy transfer process. By comparison with luminescence spectra, the energy transfer process and phase transition of lanthanide complexes are studied from two aspects: radiative and nonradiative processes.

Synthesis, Photophysical Characterization, and Surface Photovoltage Spectra of Windmill‐Shaped Phthalocyanine–Porphyrin Heterodimers and Heteropentamers

AbstractThe synthesis and characterization of three nonperipherally substituted, covalently‐linked phthalocyanine–porphyrin hybrid molecules is described. The optical and photophysical properties of these compounds are studied in detail. The intramolecular energy transfer process is investigated and quantified in terms of the quantum yield of the energy transfer and is found to be highly effective in these molecules. The photophysical properties of the phthalocyanine core of these three compounds are significantly modified by the presence of the porphyrin moieties. These molecules display interesting optical‐power‐limiting behavior and a surface photovoltaic effect. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Lanthanide-centered luminescent hybrid microsphere-particles obtained by sol–gel method

Luminescent hybrid micro-particles have been prepared by traditional sol–gel method. The precursor derived from 2-amino pyridine exhibited a self-organization under the coordination to RE 3+ (Eu 3+, Tb 3+) evaluated by SEM (micrometric scale) and X-ray diffraction studies (nanometric scale). The adapting traditional routes used in this paper affected the shape of the materials and can be taken as a new method to control the hydrolysis-polycondensation process. Fourier transform infrared (FTIR), diffuse reflectance ultraviolet–visible spectra (DRUVS) and 1H NMR spectra were used to confirm the modifications. The material's phosphorescent spectra and luminescence spectra were recorded. The efficient intramolecular energy transfer process between carboxylic group and lanthanide ions took place within these molecular-based hybrids. The narrow-width green and red emissions were achieved for Tb 3+ and Eu 3+ ions, respectively.

Molecular Assembly and Photophysical Properties of Quaternary Molecular Hybrid Materials with Chemical Bond

In this paper, two long chain aliphatic carboxylic acids (oleic acid [OLA] and stearic acid [STA]) are modified with cross-linking molecules (N-2-aminoethyl-3-aminopropyl-methyl-dimethoxylsiliane, (AEAPMMS, H(2) N(CH(2))(2)HN(CH(2))(3)SiCH(3)(OCH(3))(2) and 3-aminopropyl-methyl-diethoxylsiliane (APMES, H(2) N(CH(2))(3)SiCH(3)(OC(2)H(5))(2)) resulting in four new kinds of structural molecular bridge OLA (STA)-AEAPMMS (APMES). Subsequently, ternary molecular complex systems with four molecular bridges OLA (STA)-AEAPMMS (APMES) and 2,2-bipyridyl (bipy) of lanthanides (terbium and europium) or zinc ions were assembled, which resulted in four novel kinds of quaternary molecular hybrid materials (named as bipy-Ln (Zn)-OLA (STA)-AEAPMMS (APMES) with strong chemical bonds (N-Ln(Zn)-O coordination bonds and Si-O covalent bonds) after a sol-gel (cohydrolysis and copolycondensation) process of the modified molecular bridges (as structural ligand) with inorganic precursor (tetraethoxysilane, TEOS). And especially bipy behaves as functional ligand to sensitize the luminescence of terbium or europium ions through the effective intramolecular energy transfer process, which gives rise to the characteristic emission of metal ions. The design and assembly from structural and functional ligands can help achieve a candidate technology for molecular hybrids.

Evaluation of intramolecular energy transfer process in the lanthanide(III) bis- and tris-(TTA) complexes: Photoluminescent and triboluminescent behavior

This work reports the energy transfer mechanism process of [Eu(TTA) 2(NO 3)(TPPO) 2] ( bis-TTA complex) and [Eu(TTA) 3(TPPO) 2] ( tris-TTA complex) based on experimental and theoretical spectroscopic properties, where TTA=2-thienoyltrifluoroacetone and TPPO=triphenylphosphine oxide. These complexes were synthesized and characterized by elemental analyses, infrared spectroscopy and thermogravimetric analysis. The theoretical complexes geometry data by using Sparkle model for the calculation of lanthanide complexes (SMLC) is in agreement with the crystalline structure determined by single-crystal X-ray diffraction analysis. The emission spectra for [Gd(TTA) 3(TPPO) 2] and [Gd(TTA) 2(NO 3)(TPPO) 2] complexes are associated to T→S 0 transitions centered on coordinated TTA ligands. Experimental luminescent properties of the bis-TTA complex have been quantified through emission intensity parameters Ω λ ( λ=2 and 4), spontaneous emission rates ( A rad), luminescence lifetime ( τ), emission quantum efficiency ( η) and emission quantum yield ( q), which were compared with those for tris-TTA complex. The experimental data showed that the intensity parameter value for bis-TTA complex is twice smaller than the one for tris-TTA complex, indicating the less polarizable chemical environment in the system containing nitrate ion. A good agreement between the theoretical and experimental quantum yields for both Eu(III) complexes was obtained. The triboluminescence (TL) of the [Eu(TTA) 2(NO 3)(TPPO) 2] complexes are discussed in terms of ligand-to-metal energy transfer.

Theoretical studies toward understanding the excited state dynamics of a bichromophoric molecule

By means of the time-dependent density functional theory, the authors study the torsional dynamics of the lowest singlet electronic excited state (S1) of a bichromophoric molecule, 2-(9-anthryl)-1H-imidazo [4,5-f]-phenanthroline (AIP). The intramolecular dynamical relaxation process, the S1 potential energy surface, and the vibrationally resolved electronic absorption and fluorescence spectra are estimated. The results reveal that the strong electron-nuclear coupling leads to a dynamic structural distortion in S1 state so that the mirror-image symmetry of absorption and fluorescence spectra of AIP breaks down. The torsional motion between the donor and acceptor moieties in AIP favors the intramolecular electronic energy transfer process. The transfer rate is dominated by the relaxation time along S1 low-frequency torsional motion.

Molecular fabrication and photoluminescence of novel terbium co-polymer using 4-vinyl pyridine as the efficient second ligand

Three terbium binary complexes with pyridine carboxylic acids (nicotinic, picolinic and methyl nicotinic acid) were synthesized in the first stage. Then the above complexes were introduced into 4-vinyl pyridine–ethyl methacrylate co-polymer matrices using the former as the second ligand. The composition of the hybrids was characterized by FT-IR and the photophysical process of the luminescence was investigated by UV spectra and fluorescence spectra. The experiment results substantiate that 4-vinyl pyridine could coordinate to terbium ions directly with nitrogen atoms of the pyridine ring. Accordingly, the intramolecular energy transfer process completed within polymer hybrids and strong green emissions of Tb3+ have been obtained.

Molecular design and photo-physics of quaternary hybrid terbium centered systems with novel functional di-urea linkages of strong chemical bonds through hydrogen transfer addition

Two kinds of crosslinking reagents, 3-aminopropyl-methyl-diethoxylsiliane (H 2N(CH 2) 3SiCH 3(OC 2H 5) 2, abbreviated as APMES) and N-2-aminoethyl-3-aminopropyltriethoxylsiliane (H 2N(CH 2) 2HN(CH 2) 3Si(OC 2H 5) 3, abbreviated as AEAPES) are modified by 3-(triethoxysilyl)-propyl isocyanate (TESPIC) to afford two novel crosslinking molecular derivatives as bridge ligands. Then the ternary organic–inorganic molecular-based hybrid system with these functional bridge ligands and 1,10-phenanthroline (phen) are constructed with the two components equipped with covalent bonds. The two components of APMES (or AEAPES) and TESPIC is linked through –NHC( O)NH– groups which is applied to coordinate to Tb 3+ and further formed Si–O backbones after co-hydrolysis and co-polycondensation processes, while phen behaves as the main energy donor for the sensitization of luminescence of Tb 3+. Luminescence spectra were utilized to characterize the photo-physical properties of the obtained hybrid system and the above spectroscopic data reveal that the phen in this favorable hybrid system behaves the main energy donor for the luminescence of Tb 3+. In this way, the intra-molecular energy transfer process took place within these molecular-based hybrids and strong green and blue emissions of Tb 3+ have been achieved.