General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Conformational changes of intramolecular charge-transfer (CT) species were studied by means of time-resolved fluorescence spectroscopy. From a time-resolved fluorescence spectrum the parameters which describe a kinetic, compartmental model as well as the fluorescence spectral-shape parameters were estimated. The systems (Chart 1) differed in donor and acceptor strengths and possessed two different kinds of bridges: a flexible trimethylene chain and a semirigid piperidine ring. In moderately polar solvents (6 > 3.5) only fluorescence originating from an extended dipolar species (extended charge transfer (ECT) state) was found, possessing a dipole moment of about 27 D. In low dielectric media the systems with a strong cyanonaphthalene acceptor showed "harpooning": the ECT species was converted to a folded dipolar species (contact charge transfer (CCT) state), similar to a tight polar exciplex, on a time scale of less than 1 ns (flexible bridge) or 7 ns (semirigid bridge). No indications were found for an ECT in the systems with a weak naphthalene acceptor in nonpolar solvents. Semirigidly bridged systems displayed only local fluorescence, whereas in the flexibly bridged systems C T fluorescence originating from two types of CCT was found instead. IntroductionThe electronic structure of exciplexes between species with electron donor (D) and electron acceptor (A) properties can be described by taking into account a large contribution of a configuration, D+A-, in which one electron has been transferred from D to A (e.g., ref 1). In many cases fluorescence can be observed from such species (D+A--DA + hu). A characteristic feature of this charge-transfer (CT) emission is its high sensitivity to the polarity of the solvent: as a result of the large dipole moment the energy of the exciplex is lower in more polar media, which leads to a progressive red shift of the emission with increasing polarity.It has long been thought that a prerequisite for exciplex emission is "direct contact", spatial overlap between molecular orbitals of D and A. It is indeed likely that, in nonpolar media, where the electrostatic attraction between the oppositely charged fragments is large, exciplexes exist as contact ion pairs. As Weller pointed o...
Abstract. The solvent dependence of the radiative-rate constant of both inter-and intramolecular donor-acceptor systems reveals the occurrence of significant intensity borrowing from one or more local transitions. This phenomenon appears to be especially pronounced in systems of the type D -~u -A , where donor and acceptor interact via through-bond interaction over a relay of three u bonds. Within the context of a three-state model, involving the no-bond state (DA), the charge transfer state (DfA-) and a single locally excited state [(DA)*], a simplified method is given to analyse the electronic coupling elements between these states.
Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The fluorescent behavior of a series of donor-bridge-acceptor systems was studied in nonpolar solvents of different viscosity as a function of temperature. The D/A units in these systems are held apart in the ground state by a saturated hydrocarbon bridge, which was either a flexible trimethylene chain or a semirigid piperidine ring. Photoexcitation of the semirigidly bridged systems containing a "strong" 4-cyanonaphthalene acceptor leads to long-range electron transfer forming an initial extended-charge-transfer (ECT) species which subsequently transforms into a folded dipolar species (compact-charge-transfer (CCT) state, similar to a tight polar exciplex) due to the Coulomb attraction ("harpooning mechanism"). From fluorescence decay rates of the ECT and CCT species folding rates were determined at different temperatures, which were analyzed in terms of activation energies needed for the conformational change. The activation energies for the piperidine bridged systems were typically 4-6 kcal/mol, consistent with the barrier for a chair to boat inversion of the piperidine ring (1 1 -12 kcal/mol) being lowered considerably by the concomitant gain in Coulombic energy.No clear viscosity effect was found. In the flexibly bridged system with a "weak" naphthalene acceptor long-range electron transfer appears not to occur at least not in the low-polarity solvents employed and instead a conformational change precedes charge transfer. However, the flexibly bridged DA systems with a "strong" acceptor also appear to follow the "harpooning" mechanism. In this case low activation energies (2-4 kcal/ mol) were found for the folding process, which were attributed to solvent viscosity only because the steric barrier imposed by the trimethylene chain is completely compensated by the gain in Coulombic energy accompanying the ECT -CCT folding process. The effect of viscosity on the activation energies found for the harpooning mechanism is discussed within the framework of Kramers' theory.
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