Effect of through bond coupling and conformation on the photophysical properties of σ-bridged systems comprising a vinylnaphthalene donor and a dicyanovinyl acceptor

Abstract

The electron transfer kinetics of the donor-bridge-acceptor (D-B-A) compounds investigated is governed by three different effects: polarity of the solvent, change of conformation (especially rotation of the single bond of the vinylnaphthalene moiety) and the p-character of the bridge (bicyclooctane, norbornane, stellane). The expected increase of the rate of electron back transfer with increasing p-character of the bridge orbitals is only clearly seen in medium and very polar solvents (methylene chloride and acetonitrile). In less polar solvent (2-methyl-THF) the temperature dependence of the charge-transfer (CT) fluorescence of the system with the bicyclooctane bridge points to the existence of two emitting CT conformers. Semiempirical MO calculations with configuration interaction reveal that 2 or even 3 (norbornane bridge) local minima exist on the energy surface of the CT state. They differ mainly in the angle of rotation of the single bond mentioned above, but also by deformation of the bridge structure. In non-polar solvent (methylcyclohexane) the fluorescence behaviour becomes even more complex because emission from the CT state(s) and from the locally excited state overlap. The analysis points to an equilibrium between different rotamers on the energy surface of the lowest locally excited state, on the one hand, and of the CT state, on the other hand. Investigations of the compound with the stellane bridge are hampered by significant photodegradation in methylcyclohexane solution.

Charge Transfer in Donor-Bridge-Acceptor Systems: Static Disorder, Dynamic Fluctuations, and Complex Kinetics

“…It is worth noting that nonmonoexponential transients attributed to the process of intramolecular CT were reported for a variety of D-B-A systems studied by different time-resolved spectroscopy techniques. In particularly, decay curves that do not follow a single exponential kinetic law were reported for "harpooning" semiflexibly bridged donor-acceptor systems, 104 σbridged triads comprising a vinylnaphthalene donor and a dicyanovinyl acceptor 105 as well as for a porphyrin-base molecular wire bridging ferrocene (a hole acceptor) and fullerene (a hole donor). 80 In addition, similar decay profiles were observed for a number of D-DNA-A assembles 78,79,81,106,107 and for a series of helical peptides connecting an electron donor (an indole group) and an acceptor moiety (pyrene attached to N-terminus).…”

“…108 Experimental decay curves for these systems are often fitted with the sum of several exponentials with different time constants or with a single exponential as long as the fitting procedure is restricted only to one particular narrow time window. For example, Kurzawa et al 105 found that the fluorescent transients used to obtain information about the formation and decay of the CT state in vinylnaphthalene donorσ-bridge-dicyanovinyl acceptor compounds can be described with the sum of three exponentials with different time decay constants. Fiebig, Barton, Zewail and their co-workers 107 reported that femtosecond transient absorption of DNA assemblies also exhibits three components.…”

Charge on the move: how electron-transfer dynamics depend on molecular conformation

“…[35][36][37] It was found, in general, that trans orientations give higher rates than the corresponding cis geometries. 39,40 Interesting behaviour can be observed from the occurrence of slowly interconverting conformers in such systems, where solvent polarity plays a major role in defining the charge-transfer properties. 38 A similar effect has been reported for donor-acceptor dyads linked via a s-bond.…”

“…38 A similar effect has been reported for donor-acceptor dyads linked via a s-bond. 39,40 Interesting behaviour can be observed from the occurrence of slowly interconverting conformers in such systems, where solvent polarity plays a major role in defining the charge-transfer properties.…”

Photoinduced Intramolecular Charge Transfer in Donor-acceptor Dyad and Donor-bridge-acceptor Triad

“…Over these distances it is energetically more favorable for electrons to tunnel through unoccupied orbitals of the intervening medium than to travel through empty space. Usually, such tunneling is described by two kinds of mechanisms [13][14][15][16][17][18][19][20]: superexchange or through bond and sequential or charge hopping, and the corresponding model description can be simplified as donor-bridgeacceptor (D-B-A) triad, in which a donor (D) group is covalently linked to an acceptor (A) group through a rigid or flexible bridging unit (B). For D-B-A triad, the rate of electron transfer can be controlled by changing the structural and electronic properties of bridge molecules such as the length or degree of conjugation or polarity of the solvent.…”