Photoinduced Energy Transfer in Bichromophoric Pyrene–PPV Oligomer Systems: The Role of Flexible Donor–Acceptor Bridges (original) (raw)
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The mechanism of short-range intramolecular electronic energy transfer in bichromophoric molecules
The Journal of Physical Chemistry, 1984
A study of intramolecular energy transfer (intra-ET) in a series of bichromophoric molecules consisting of cyclic a-diketones incorporating an ortho-, meta-, or para-substituted benzene ring is reported. Most spectroscopic properties of these molecules are described by a superposition of those of their constituent chromophores. Unique for the bichromophore molecule is the fact that, depending on the molecular geometry, energy absorbed by the aromatic chromophore is transferred in part to the a-diketone and both chromophores emit their characteristic fluorescence spectra. An extensive study was made of the intramolecular electronic energy transfer process in solution as a function of temperature. The results indicate that the transfer efficiency is strongly structure dependent suggesting that a Dexter type exchange interaction is responsible for singlet-singlet intra-ET between close chromophores in a bichromophoric molecule. The thermal dependence observed in some cases is attributed to conformational factors. A general theoretical analysis of intra-ET in bichromophoric molecules provides expressions for donor fluorescence decay and for its fluorescence quantum yield in terms of the average distance between donor and acceptor moieties and the flexibility of the chains connecting donor and acceptor. Comparison with the present experimental data supports the predictions of this analysis. It is concluded that intra-ET in bichromophoric molecules is indeed governed by short-range exchange interactions.
Journal of Photochemistry and Photobiology A: Chemistry, 1996
Results are presented on the intramolecular electronic energy transfer (intra-EET) in bichromophoric molecules of the type benzene-adiketone, in which the interchromophore bridge contains methyl substituents/3 to the c~-dicarbonyl acceptor chromophore. These results show that singlet-singlet intra-EET is independent of substitution and can be explained by a model assuming Dexter-type short-range exchange interaction. For singlet-triplet and triplet-triplet transfer, there are indications that the phosphorescence yield of the acceptor is larger than that for non-substituted bichromophoric molecules. This can be explained by through-bond interaction promoting EET via a long-range superexchange mechanism, by variations in the non-radiative decay of the triplet state of the acceptor or by chemical reaction.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2010
We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N-1,2-phenylene dip -tosylamide (B) and the multichromophore (M) where A and B are connected by a spacer containing both rigid triple (acetylenic) and flexible methylene bonds. Both steady state and time correlated single photon counting measurements demonstrate that though intermolecular energy transfer, of Forster type, between the donor and acceptor moieties occurs with rate 10 8 s −1 but when these two reacting components are linked by a spacer (multichromophore, M) the observed transfer rate (∼10 11 s −1) enhances. This seemingly indicates that the imposition of the spacer by inserting a triple bond may facilitate in the propagation of electronic excitation energy through bond. The time resolved fluorescence measurements along with the theoretical predictions using Configuration interaction singles (CIS) method by using 6-31G (d,p) basis set, implemented in the Gaussian package indicate the formations of the two excited conformers of B. The experimental findings made from the steady state and time resolved fluorescence measurements demonstrate that, though two different isomeric species of the acceptor B are formed in the excited singlet states, the prevailing singlet-singlet nonradiative energy transfer route was found from the donor A to the relatively longer-lived isomeric species of B.
Dual fluorescence and intramolecular electronic energy transfer in a bichromophoric molecule
The Journal of Physical Chemistry, 1980
The bichromophoric molecule containing phenanthrene and a-diketone moieties connected by two chains of five methylene groups has been studied. Most spectroscopic properties of this molecule are described by a superposition of those of its constituent chromophores. Unique for the combined molecule is the fact that energy absorbed by the phenanthrene chromophore is transferred in part to the a-diketone and both chromophores emit their characteristic fluorescence spectra. An extensive study was made of the intramolecular energy transfer process in solution as a function of the excitation frequency and of the sample temperature. It was clearly demonstrated that energy is transferred very efficiently to the a-diketone moiety from a thermally activated state of the bichromophoric molecule. The rate of this energy transfer is comparable to the relaxation rate of the activated state. In contrast, the transfer process from the ground vibrational level of the first excited state of the phenanthrene moiety is rather slow (-lo7 d).
Inorganic Chemistry, 2002
Steady-state and time-resolved spectroscopic properties of bichromophoric species containing [Ru(bpy) 3 ] 2+ and pyrene (pyr) units linked together by flexible poly(ethylene glycol) chains of variable length, [Ru(bpy) 2 (bpy-pyr)]-(PF 6 ) 2 (1) and [Ru(bpy) 2 (bpy-O6-pyr)](PF 6 ) 2 (2), have been investigated in acetonitrile solvent. The complexes were designed with the aim of examining the intercomponent energy-transfer processes taking place after light absorption at the two chromophores and the influence of the distance separation between them; in the case of complex 2, the linking chain in the extended conformation is as long as 21 Å. Direct excitation of the pyrene unit (λ exc ) 410 nm) results in singlet-to-singlet energy transfer (an antenna effect) to the Ru-based component, 1 pyr f 1 MLCT, which we analyze in terms of the Förster mechanism taking place with unit efficiency. Analysis of the time-resolved pyrene fluorescence reveals that the actual center-to-center distance separation (d cc ) between the photoactive centers changes according to a Gaussian distribution, with an average d cc ) 13.6 Å (distribution width, a ) 2.8 Å) and 12 Å (a ) 10.2 Å), for 1 and 2, respectively; this is ascribed to folding of the poly(ethylene glycol) linking chain. In O 2 -free solvent at room temperature, after population of the 1 MLCT level (which takes place either because of direct excitation by using λ exc > 355 nm or via the "antenna" effect) and subsequent intersystem crossing localized at the Ru center, 1 MLCT f 3 MLCT, a triplet−triplet thermal equilibration is established which involves the physically separated centers, 3 MLCT T 3 pyr, with K eq ) 11 (the energy gap between the two levels is 480 cm -1 , as determined from luminescence data obtained at 77 K). As a consequence of this equilibrium, the 3 MLCT luminescence lifetime becomes τ Ru ∼ 9 µs both in 1 and 2, i.e., 1 order of magnitude longer than for the unsubstituted [Ru(bpy) 3 ] 2+ luminophore. In air-equilibrated solvent, diffusional quenching by O 2 effectively depletes the 3 pyr level and only the forward 3 MLCT f 3 pyr energy transfer step is observed with k en ) 4 × 10 8 and 2 × 10 8 s -1 for 1 and 2, respectively. As briefly discussed, reasons for the high rate constants observed for the various triplet−triplet steps may be traced back to the folding properties of the linking chains. (F.B.). † Dedicated to Prof. Vincenzo Balzani, in recognition of his contribution to the development of inorganic photochemistry. ‡ Istituto ISOF-CNR, Bologna. § University of Bristol. (1) Wasielewski, M. R. Chem. ReV. 1992, 92, 435.
The ultrafast energy transfer process in naphtole–nitrobenzofurazan bichromophoric molecular systems
Journal of Photochemistry and Photobiology A: Chemistry, 2007
This work presents an experimental and computational study of the intramolecular electronic energy transfer process occurring in two newly synthesized bichromophoric species: )oxy]acetate (r-Bi). In both f-Bi and r-Bi the donor chromophore is the [(4-chloro-1-naphthyl)oxy]acetate moiety, whereas the acceptor units belong to the family of the 4dialkylaminonitrobenzoxadiazoles, well-known fluorescent probes. The two bichromophores differ in the structural flexibility. In f-Bi, acceptor and donors are linked by a diethanolamine moiety, whereas in r-Bi through a (3S, 4S)3,4-dihydroxypyrrolidine ring. By means of steady-state and timeresolved UV-vis spectroscopies we carried out a detailed analysis of the photo-response of donor and acceptor chromophores as individual molecules and when covalently linked in f-Bi and r-Bi. The intramolecular energy transfer process occurs very efficiently in both the bichromophores. The rate constant and the quantum efficiency of the process are k ET = (2.86 ± 0.16) × 10 11 s −1 and Q = 0.998 in f-Bi, and k ET = (1.25 ± 0.08) × 10 11 s −1 and Q = 0.996 in r-Bi. Semiempirical calculations were utilized to identify the energy and the nature of the electronic states in the isolated chromophores. Molecular mechanics calculations have been performed to identify the most stable structures of the bichromophoric compounds. The predictions of Förster theory are consistent with the experimental results and provide a suitable way to evaluate the structural differences between the two compounds.
Inorganic Chemistry, 2003
The supramolecular systems [Ru(Pyr n bpy)(CN) 4 ] 2-(n) 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN) 4 ] 2chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet−singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n) 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyr n bpy)(CN) 4 ] 2systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5−1.3 ns (n) 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260−450 ns (n) 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excitedstate partition of 12−6% MLCT (n) 1−2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.
Ultrafast Intramolecular Electronic Energy-Transfer Dynamics In a Bichromophoric Molecule
J. Phys. Chem. …, 2004
Intramolecular electronic energy-transfer (intra-EET) dynamics has been investigated in 2-(9-anthryl)-1Himidazo [4,5-f] [1,10]-phenanthroline (AIP), a newly synthesized bichromophoric molecule, using the steadystate and time-resolved absorption and fluorescence spectroscopic techniques. In AIP, anthracene (AN) and 1H-imidazo [4,5-f] [1,10]-phenanthroline (IP) molecules are directly linked to each other through a CC σ bond and without any intervening molecular bridge. Two constituent chromophoric moieties of this bichromophoric molecule interact relatively weakly in the ground state. In the excited singlet state, however, the AN moiety transfers its excitation energy quantitatively (the efficiency of energy transfer, φ EET , is near unity) and rapidly (the rate of energy transfer, k EET , is 1.8 × 10 11 s-1 in methanol) to the unexcited IP moiety. k EET decreases linearly with increase in viscosity of the solvents, and the process is significantly retarded in rigid glass matrixes. These observations suggest that, for an efficient EET process, the molecule needs to attain a conformational geometry, which is different from that of the ground state, by undergoing a conformational relaxation process following photoexcitation. The theoretically calculated energy-transfer rate (5.1 × 10 9 s-1) due to the Förster dipole-dipole-induced resonance-interaction mechanism is about 2 orders of magnitude smaller than the experimentally determined energy-transfer rate. Hence, the Dexter throughspace exchange-interaction mechanism, which becomes predominant at shorter interchromophoric separation (R ∼ 6.3 Å in AIP) and requires specific conformation for efficient orbital overlap, should have the major contribution to the intra-EET process in AIP. Viscosity dependence of k EET suggests that we possibly measure the rate of the conformational relaxation process using the intra-EET process as the probe.
Pure and Applied Chemistry, 2000
The photophysical properties are compared of systems containing electron donoracceptor (D/A) pairs linked by saturated hydrocarbon bridges with various degrees of flexibility. It is concluded that even in fully extended conformations rapid (subnanosecond) photoinduced electron transfer can occur, thus providing a mechanism for quenching of 'local' fluorescence that is not restricted by the conformational dynamics of the bridge. Especially in solvents of low dielectric constant electrostatic forces strongly modify the conformational dynamics occurring after the initial charge separation (harpooning mechanism'). Furthermore it is shown that the extended charge separated state may undergo radiative recombination resulting in the observation of 'exciplex-like' emission. For flexibly bridged systems this allows the occurrence of multiple 'exciplex' emission from widely different conformations ranging from fully extended to fully folded. The distance across which charge separation and radiative recombination occur with significant rate can be extended by through-bond interaction (TJ3I) via the bridge, but even if the bridge structure and conformation do not allow for important TBI these rates can be quite significant for bridges with a length up to that corresponding to an extended pentamethylene chain.