Intramolecular Energy Migration Research Articles

C3-Symmetric Luminescent Diketone with Amido-Linkage as a Polymorphic Fluorescence Emitter.

The study introduces a novel C3-symmetric β-diketone compound, BTA-D3, and its monomeric counterpart, D, with a focus on their synthetic procedure, photophysical properties and aggregation behavior. Both compounds exhibit characteristic absorption and weak fluorescence in solution, with BTA-D3 displaying higher absorption coefficients due to its larger number of diketone units. Density Functional Theory (DFT) calculations suggest increased co-planarity of diketone groups in BTA-D3. A significant finding is the Aggregation-Induced Emission (AIE) property of BTA-D3, as its fluorescence intensity increases dramatically when exposed to specific solvent ratios. The AIE behavior is attributed to intermolecular excitonic interaction between BTA-D3 molecules in self-organized aggregates. We also studied fluorescence anisotropy of BTA-D3 and D. Despite its larger size, BTA-D3 showed reduced anisotropy values because of efficient intramolecular energy migration among three diketone units. Furthermore, BTA-D3 demonstrates unique polymorphism, yielding different emission colors and structures depending on the solvent used. A unique approach is presented for promoting the growth of self-organized aggregate structures via solvent evaporation, leading to distinct fluorescence properties. This research contributes to the understanding of C3-symmetric structural molecules and provides insights into strategies for controlling molecular alignment to achieve diverse fluorescence coloration in molecular materials.

Altering molecular photophysics by merging organic and inorganic chromophores.

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.

On the analyses of fluorescence depolarisation data in the presence of electronic energy migration. Part I: Theory and general description

A new and general procedure is described for a detailed analysis of time-resolved fluorescence depolarisation data in the presence of electronic energy migration. An isotropic ensemble of bifluorophoric molecules (D(1)-R-D(2)) has been studied to demonstrate its utility. Intramolecular donor-donor energy migration occurs between the two donor groups (D), which are covalently connected to a rigid linker group (R). These groups undergo restricted reorientational motions with respect to the R group. The analysis of depolarisation data basically involves the search for best-fit parameters which describe the local reorientational motions, the intermolecular D(1)-D(2) distance, as well as the mutual orientations of the donors. For this, the analysis is partly performed on the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process has been described by using Monte Carlo simulations and an extended Förster theory (EFT). It is found that the EFT provides the least time-consuming computational method. Since one-photon and two-photon excited fluorescence experiments can be applied for energy migration studies, a general and unified theoretical formulation is given.

Noncovalently Netted, Photoconductive Sheets with Extremely High Carrier Mobility and Conduction Anisotropy from Triphenylene-Fused Metal Trigon Conjugates

Supramolecular assembly of small molecules via noncovalent interaction is useful for bottom-up construction of well-defined macroscopic structures. This approach is attracting increasing interest due to its high potential in manufacturing novel molecular electronic and optoelectronic devices. This Article describes the synthesis and functions of a sheet-shaped assembly from novel triphenylene-fused metal trigon conjugates. These conjugates were recently designed and synthesized by a divergent method and used for the supramolecular self-assembly of sheet-like objects. In contrast to triphenylene, which absorbs photons in ultraviolet region, the triphenylene-fused metal trigon conjugate shows a strong absorption band in the visible region. The metal trigon conjugate emits green photoluminescence with significantly enhanced quantum yield and allows intramolecular energy migration, as a result of extended pi-conjugation over metal sites. It assembles via physical gelation to form noncovalent sheets that collect a wide wavelength range of photons from ultraviolet to visible regions. The noncovalent sheets allow exciton migration and are semiconducting with an extremely large intrinsic carrier mobility of 3.3 cm(2) V(-1) s(-1). They are highly photoconductive, produce photocurrent with a quick response to light irradiation, and are capable of repetitive on-off switching. Moreover, these sheets facilitate a conduction path perpendicular to the sheet plane, thus exhibiting a spatially distinctive anisotropy in conduction. The noncovalent sheet assemblies with these unique characteristics are important for molecular optoelectronic devices based on solution-processed soft materials.

Intermolecular sequential energy transfer in thin films of a white emitting copolymer

The authors investigated the energy transfer between the chromophors of a white emitting polyspirobifluorene copolymer containing a blue emitting backbone, a green, and a red emitting dye. They demonstrate that the green dye is mainly excited by blue→green Förster resonant energy transfer (FRET), with a Förster radius R0bg=35Å. The red dye is instead excited by direct blue→red FRET (R0br=21Å) and by sequential blue→green→red FRET (R0gr=31Å). Finally, the authors show that the FRET processes, fundamental to have white emission, are of intermolecular nature, while intramolecular energy migration and transfer are negligible.

Quantum signatures in the dynamics of two dipole-dipole interacting soft dimers

The quantum covariances of physically transparent pairs of observables relative to two dimers hosted in a solid matrix are exactly investigated in the temporal domain. Both dimers possess fermionic and bosonic degrees of freedom and are dipolarly coupled. We find out and describe clear signatures traceable back to the presence and persistence of quantum coherence in the time evolution of the system. Manifestations of a competition between intramolecular and intermolecular energy migration mechanisms are brought to light. The experimental relevance of our results is briefly commented.

The influence of bridging ligand electronic structure on the photophysical properties of noble metal diimine and triimine light harvesting systems

This manuscript discusses the photophysical behavior of transition metal complexes of Ru(II) and Os(II) employed in development of light harvesting arrays of chromophores. Particular emphasis is placed on the relationship between the photophysical behavior of complexes having metal-to-ligand charge transfer (MLCT) excited states and the electronic characteristics of bridging ligands used in preparing oligometallic complexes. Examples are presented that discuss intramolecular energy migration in complexes having two distinct MLCT chromophores with bridging ligands that only very weakly couple the two chromophores. In addition, systems having bridging ligands with localized triplet excited states lower in energy than the MLCT state of the metal center to which they are attached are discussed. These systems very often have excited states localized on the bridging ligand with excited state lifetimes on the order of tens of microseconds. Finally, systems having Fe(II) metal centers, with very low energy MLCT states, are discussed. In complexes also containing bridging ligands with low energy triplet states, energy partitioning between the Fe center MLCT state (or Fe localized ligand field states) and the ligand triplet state is observed; the two states relax to the ground state via parallel pathways, but the Fe(II) center does not serve as an absolute excitation energy sink.

Morphology‐dependent luminescence properties of poly(benzyl ether) dendrimers

AbstractPoly(benzyl ether) dendrimers with o‐, m‐, and p‐isomers of dialkoxybenzene at their focal points [o‐, m‐, and p‐(Gn)2Ar], having generation numbers (n) of 0–3, were synthesized. 1H NMR pulse relaxation times (T1) of the exterior MeO groups of o‐ and m‐(Gn)2Ar (n = 0–3) all remained in the range of 0.92–1.43 s. In sharp contrast, an exceptionally short T1 value (0.23 s) was observed for p‐(G3)2Ar. Although their absorption spectral profiles were slightly different from one another, an essential difference was observed for their fluorescence properties. When the generation number was increased, the fluorescence efficiency of o‐(Gn)2Ar increased, but that of p‐(Gn)2Ar decreased, whereas m‐(Gn)2Ar exhibited a relatively small change in the fluorescence efficiency. Fluorescence depolarization studies showed a highly efficient intramolecular energy migration in p‐(G3)2Ar as compared with o‐(G3)2Ar and m‐(G3)2Ar. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3524–3530, 2003

Energy migration in poly(N‐vinyl carbazole) and its copolymers with methyl methacrylate: Fluorescence polarization, quenching, and molecular dynamics

AbstractFluorescence polarization and quenching measurements were used to examine intramolecular energy migration for poly(N‐vinyl carbazole) and copolymers of N‐vinyl carbazole with methyl methacrylate. Quenching measurements of the carbazole fluorescence by CCl4 were performed in dilute solution in toluene, and fluorescence anisotropy, r, was measured for the chains dispersed in a solid matrix of poly(methyl methacrylate) (PMMA). The results suggested that the chains with a high carbazole content, that is, a high content of excimer trapping sites, do not show the highest values of the singlet energy‐migration rate. Isotropies, r−1, of the samples in vitrified PMMA corroborated such conclusions. Molecular dynamics simulations on isotactic and syndiotactic trichromophoric copolymer fragments were used to obtain parameters related to the energy‐transfer process as a function of the methyl methacrylate content. The parameters from the simulations supported the interpretation of the experiments. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1615–1626, 2003

New fluorescent bichromophoric probes for studying mechanisms of donor-donor energy migration (DDEM)

Three fluorescent probes were synthesized for studying the excitation energy migration between two identical fluorophores. Each probe has two identical fluorescent groups (dansyl, 7-nitrobenz-2-oxa-1,3-diazole-4-yl, or fluoresceinyl) linked by the rigid bis-(8-aminooctyl)amide of 4,4'-biphenyldicarbonic acid or flexible dotriacontanedioic acid spacer, which enables the intramolecular energy migration through the distance of 3.2-3.5 nm.

Direct observation of intra-molecular energy migration in σ-conjugated polymer by femto-second laser flash photolysis

A direct observation of energy migration processes in σ-conjugated polymers has been firstly done to a very large extent by ultra-fast transient absorption spectroscopy in near infrared (IR) region. Especially, following new results were obtained: (1) The very broad transient absorption due to excited states in various conjugated segments immediately after 266-nm excitation; (2) The shift from the very broad absorption toward the absorption with the maximum at 0.95 eV; (3) Decrease in energetic dispersion of excited states with observation time; (4) Some excited states remain in shorter conjugated segments than the radiative segments associated with the absorption maximum at 0.95 eV.

Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: contributions from intra-ligand charge transfer states.

The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are presented. The complexes ([(bpy)(2)Ru](2)bpb)(4+) and [(CO)(3)ReCl(bpb)] (bpy = 2,2'-bipyridine, bpb = 1,6-bis-(4-(2,2'-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT states localized on the metal complexes and pyrene-localized (3)(pi-pi) states. Absorption spectroscopy of both complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered (1)(pi-pi) transitions, a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex strongly suggest the presence of a pyrene(pi) to bpy(pi) intraligand charge transfer (ILCT) transition. Emission spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the bpb ligand. The (3)MLCT emission of related bipyridyl compounds lacking the pyrene is observed at higher energy than for the pyrene-containing complexes, ([(bpy)(2)Ru](2)bpb)(4+) and [(CO(3)ReCl(bpb)]. The Ru(II) complex emits at room temperature with a remarkably long lifetime (130 micros in degassed DMSO). This emission is also strongly sensitive to oxygen and is almost entirely quenched in an aerated solution. In addition, excited-state absorption spectra exhibit features not consistent with (3)MLCT or (3)(pi-pi) states of the parent chromophores. The combined characteristics suggest the emission arises from either (3)(pi-pi) or (3)ILCT states or a state with mixed parentage.

Photophysics of Carbazole-Containing Systems. 3 Fluorescence of Carbazole-Containing Oligoethers in Dilute Solution.

Fluorescence techniques, including quenching and anisotropy measurements, have been used to study the photophysical behavior of the carbazole-based oligoethers poly{9-(2,3-epoxypropyl)carbazole}, PEPCz, and poly{1,2-epoxy-6-(9-carbazolyl)-4-oxahexane}, PECzOH, and their model compounds, in dilute fluid solutions in THF. These compounds show no evidence of excimer formation under these conditions and their fluorescences exhibit first-order decay behavior regardless of the degree of polymerization of the oligomer. Both steady-state and time-resolved anisotropy measurements upon dilute, glassy solutions show that intramolecular energy migration occurs in the oligomers. Fluorescence quenching experiments, using both dimethylterephthalate and oxygen as quenchers, produced evidence which supports the supposition that in the absence of excimer trapping sites, intramolecular energy migration will be encouraged in dilute fluid solutions of the oligoethers. Unfortunately, the fluorescence quenching data did not permit evaluation of the extents of energy migration in the various polymers or the relative efficiencies with which migration occurs in the two different types of oligoether. This limitation would appear to be a general feature of fluorescence quenching measurements, despite their frequent adoption, in the past, in studies of energy migration in polymers.

Donor-Donor Energy Migration for Determining Intramolecular Distances in Proteins: I. Application of a Model to the Latent Plasminogen Activator Inhibitor-1 (PAI-1)

A new fluorescence spectroscopic method is presented for determining intramolecular and intermolecular distances in proteins and protein complexes, respectively. The method circumvents the general problem of achieving specific labeling with two different chromophoric molecules, as needed for the conventional donor-acceptor transfer experiments. For this, mutant forms of proteins that contain one or two unique cysteine residues can be constructed for specific labeling with one or two identical fluorescent probes, so-called donors ( d). Fluorescence depolarization experiments on double-labeled Cys mutant monitor both reorientational motions of the d molecules, as well as the rate of intramolecular energy migration. In this report a model that accounts for these contributions to the fluorescence anisotropy is presented and experimentally tested. Mutants of a protease inhibitor, plasminogen activator inhibitor type-1 (PAI-1), containing one or two cysteine residues, were labeled with sulfhydryl specific derivatives of 4,4-difluoro-4-borata-3a-azonia-4a-aza- s-indacence (BODIPY). From the rate of energy migration, the intramolecular distance between the d groups was calculated by using the Förster mechanism and by accounting for the influence of local anisotropic orientation of the d molecules. The calculated intramolecular distances were compared with those obtained from the crystal structure of PAI-1 in its latent form. To test the stability of parameters extracted from experiments, synthetic data were generated and reanalyzed.

Thermal redistribution of energy from an excited [formula omitted] term to a nearby chemically reactive 3dd level in [Rh(III)(NN) 3](PF 6) 3 complexes

Below ∼ 150 K [Rh(NN) 3](PF 6) 3 complexes display 3gpπ ∗ phosphorescences characteristic of the NN diimine ligand [NN = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy)]. Above ∼ 150 K and below the melting point of glycerol, a temperature-dependent first-order photoreaction occurs. Temperature dependencies of these photochemical reactions conform to the Arrhenius equation with activation energies in the 2–3 × 10 3 cm −1 range. These results are interpreted in terms of thermal redistribution from an excited 3ππ ∗ term to a nearby chemcially reactive 3dd manifold. The relationship of the Arrhenius activation energies to the electronic states of the parent complexes and the paths of intramolecular energy migration are discussed.

Alternating copolymers consisting of arylalkyl methacrylates. I. Fluorescence properties of poly(arylalkyl methacrylate‐alt‐styrene) in organic solution

AbstractAlternating and random copolymers of 9‐phenanthrylmethyl methacrylate or 2‐(9‐carbazolyl)ethyl methacrylate with styrene were synthesized and their fluorescence properties were examined. There was no noticeable difference in the spectral features of the alternating and random copolymers in tetrahydrofuran (THF), demonstrating that this type of polymers have no quenching sites in the polymer chains. The fluorescence quenching studies indicated that the alternating copolymers permitted singlet‐state energy migration as efficiently as the corresponding random copolymers but less efficiently than the random copolymers with higher chromophore contents. These results strongly suggest that to be chromophores close to each other is most important for facilitation of an intramolecular energy migration. © 1995 John Wiley & Sons, Inc.

Intramolecular excimers and energy migration in polyesters with 2,6-naphthalene dicarboxylic acid units separated by spacers of 1–4 ethylene oxide units

The fluorescence has been measured in dilute solution and in glassy poly(methyl methacrylate) for polyesters of 2,6-naphthalene dicarboxylate and HO(CH 2 CH 2 O) m H, m = 1−4. The model compounds in which the glycol connects two molecules of 2-naphthoate were also studied. Intramolecular energy migration is implied by the anisotropy of the fluorescence in rigid media. The results are consistent with Förster radii of about 12 Å in the model compounds and 14 Å in the polymers, as found previously for a similar series based on HO(CH 2 ) m OH instead of HO(CH 2 CH 2 O) m H.

Intramolecular triplet energy migration in poly(2-vinylnaphthalene) and poly(naphthylalkyl methacrylate)s in solid solution: Effects of side chain length

The effect of side chain length on intramolecular triplet energy migration of naphthalene containing polymers was investigated by a delayed fluorescence (DF) spectroscopy for the solid solution of the polymers. The degree of triplet energy migration depends strongly on whether the chromophores are directly attached to the main chain or not.

Intramolecular energy migration in polyesters from 2,6-naphthalene dicarboxylic acid: polarization of fluorescence in the polymers and in bichromophoric model compounds

The fluorescence has been measured in dilute solution and in glassy poly(methyl methacrylate) for polyesters in which 2,6-naphthalene dicarboxylate is the rigid unit, and (CH 2) m ( m=2–6) is the flexible spacer. Bichromophoric compounds in which the same flexible spacer connects two molecules of 2-naphthoate were also studied. The anisotropy in the rigid medium demonstrates the existence of intramolecular energy migration, which becomes more important as m decreases from 6 to 2. The Förster radius is ∼ 12 A ̊ in the bichromophoric compounds and 14 Å in the polyesters.

Excimer fluorescence as a molecular probe of polymer blend miscibility: 8. Polymeric and glassy solvent host matrices

The effects of solubility parameter differences between the guest polymer and the host matrix and of guest molecular weight on the thermodynamic miscibility and chain configuration of poly(2-vinylnaphthalene) (P2VN) dispersed in polymeric or glassy solvent hosts are investigated using excimer fluorescence. For P2VN blends with a series of poly(alkyl methacrylates) in which the difference in the guest and host solubility parameters is minimized, the small increase in the excimer to monomer fluorescence ratio, I D I M , with increasing P2VN molecular weight can be rationalized by a one-dimensional energy migration model proposed by Fitzgibbon and Frank. The results indicate the possibility of small-scale phase separation or local coil contraction for P2VN with molecular weight greater than 21 000. As the thermodynamic interaction between the guest and polymeric host is altered towards immiscibility, I D I M can be related to the extent of intermolecular aggregation in the blend. To study the efficiency of intramolecular energy migration independent of intermolecular association and free of casting solvent effects, we chose a host system consisting of a series of monomeric glassy solvents. The rate of energy transfer is investigated by transient fluorescence and treated by a one-dimensional model developed by Fredrickson and Frank. The results are in excellent agreement with independently measured photostationary-state fluorescence ratios.