B. Wegewijs

Determination of the barrier to intramolecular exciplex formation in a jet-cooled, bichromophoric molecule

The emissive properties of a bichromophor molecule (1) are reported. This contains an anilino group as an electron donor (D) and a 1-cyanonaphthalene group as an electron acceptor (A) interconnected by a saturated hydrocarbon bridge of limited flexibility, which holds D and A far apart in the electronic ground state. The emission spectrum of 1, both in solution and in the gas phase, indicates that quantitative formation of an intramolecular exciplex between D and A occurs. This exciplex formation was studied as a function of excitation energy in molecules of 1 isolated in a supersonic free jet. A barrier of 1700 ± 200 cm−1 was found between the Franck-Condon excited conformation and the conformation of the exciplex. Although this value is significantly higher than that reported earlier for exciplex formation between chromophores connected by a simple polymethylene chain (≈ 900 cm−1) it is much lower than the barrier predicted for folding the bridge in 1 sufficiently to bring D and A in close contact. A tentative explanation of this discrepancy is given.

Long-range, photoinduced charge separation in solvent-free donor—bridge—acceptor molecules

Abstract Charge separation in isolated supermolecules, consisting of an aniline or 4-methoxy aniline donor and a cyanonaphtalene acceptor connected by a semirigid bridge, is described in terms of irreversible sequential intramolecular relaxation involving electron transfer from an electronically excited molecular state and electrostatically driven folding. Sequential folding of the charge transfer state extended the energy range of this state, which is accessible by radiationless electron transfer. The analysis of the intramolecular electron transfer dynamics led to agreement between the distance scale for charge separation inferred from energetic and structural data and accounted for the excess energy dependence of the nuclear Franck—Condon vibrational overlap factors.

A dramatic effect of the donor ionization potential on the apparent barrier to intramolecular exciplex formation in jet-cooled, bichromophoric molecules

Abstract The fluorescence of three nearly identical donor-bridge-acceptor molecules was studied under jet-cooled conditions. These compounds contain a 1-cyanonaphthalene group as an electron acceptor (A) and an anilino derivative as an electron donor (D). Their only difference is in the substituent on the para position of the anilinogroup and therefore in ionization potential (IP). D and A are interconnected by a saturated hydrocarbon bridge of limited flexibility, which holds D and A far apart in the electronic ground state. Nevertheless, an intramolecular exciplex is formed upon excitation of the cyanonaphthalene group with sufficient excess vibrational energy. The barrier to this exciplex formation appears to be much lower than the barrier predicted for folding the bridge to bring D and A in close contact and moreover, the molecule with the lowest IP has by far the lowest barrier. This is taken as conclusive evidence that long-range electron transfer is the first step in the exciplex formation and determines the height of the barrier. The subsequent folding of the bridge would then be a result of the Coulombic attraction forces between D and A.

Exciplex-type emission from folded and extended conformations of a donor-acceptor molecule with limited flexibility

Abstract The fluorescent behaviour of a donor-bridge-acceptor compound was studied in apolar solvents as a function of temperature. In the ground-state conformation donor and acceptor are held far apart by the interconnecting saturated hydrocarbon bridge. Photoexcitation at room temperature in an apolar solvent nevertheless leads to exciplex-like emission at 454 nm from a “folded” conformation. Emission spectra at low temperatures reveal a totally different fluorescence band at 380 nm that is ascribed to a CT state with a stretched conformation. With time-resolved fluorescence measurements it was shown that this state is the precursor to the folded exciplex-state, indicating that charge separation takes place in an extended conformation. At room temperature charge separation is followed by rapid Coulomb-induced interconversion to a folded conformation, thereby efficiently quenching the 380 nm fluorescence. Further confirmation of our interpretation was obtained by time-resolved microwave conductivity experiments.

Exciplex formation in jet-cooled donor—bridge—acceptor compounds incorporating bridges with three degrees of flexibility

Abstract Intramolecular exciplex formation was studied in three types of donor—bridge—acceptor systems under jet-cooled conditions. While each of these contains the same aniline/cyanonaphthalene D/A pair the saturated hydrocarbon bridges differ in flexibility and length. With a flexible trimethylene bridge at least three conformers are concluded to be present in the jet, which display different mechanisms of exciplex formation. The main conformer is probably fully extended and required an excess excitation energy Δ E ⩾ 1100 cm −1 for exciplex formation. This is thought to correspond with a mechanism in which IVR is the primary process bringing D and A in closer proximity. A broad long wavelength excitation is furthermore assigned to the presence of a fully folded conformer undergoing direct excitation into the exciplex state. In addition a partly folded conformer appears to be present from which exciplex formation can occur at negligible excess energy. It is argued that in this partly folded conformer D and A are within “harpooning range” even for Δ E = 0, implying that after excitation at the spectral origin of the acceptor chromophore electron transfer can occur followed by electrostatically driven folding to form the emissive exciplex. With two more rigid types of bridges a harpooning mechanism is also concluded to be involved in formation of the exciplex. With these bridges however, the ground-state donor—acceptor distance is much better defined and is furthermore too large to allow electron transfer at Δ E = 0. As a result exciplex formation sets in only at excess energies sufficiently high to extend the “harpooning range” to or beyond the actual ground-state distance.

Rigid donor—bridge—acceptor systems: photoinduced charge transfer in solution and in a supersonic jet

Abstract In this paper the fluorescent behaviour is studied under jet-cooled conditions as well as in solution of two donor—bridge—acceptor molecules ( 1 and 2 ) in which the donor (D) and acceptor (A) are held rigidly apart by an extended saturated hydrocarbon bridge with an effective length of three sigma-bonds. It was found that even in the isolated molecules no excess excitation energy at all is needed to induce charge separation and that charge transfer (CT) fluorescence can be observed from both these molecules. Furthermore, broad red-shifted excitation bands are observed indicating that excitation can take place directly from the ground state into the CT state, even at energies considerably lower than that required to reach any of the locally (in D or A) excited states. Comparison of the radiative rate constants for CT fluorescence in media of different polarity indicates important intensity borrowing from local transitions in media of low polarity. Nevertheless, the admixture of locally excited configurations in the CT state is too small to diminish significantly the CT nature of the emissive state even in the gas phase.

Suppression of the harpooning mechanism in donor-bridge-acceptor systems by aza substitution in the bridge

An aza substituent was introduced in the bridge of donor-bridge-accepter systems known to undergo harpooning (long-range photoinduced charge separation followed by electrostatically driven folding). Although the substitution does not reduce the conformational flexibility of the bridge, It fully suppresses the harpooning mechanism, From a comparison with a reference compound, the solvatoehromic shifts of charge transfer fluorescence and transient absorption results, it is concluded that suppression is due to the low ionization potential of the aza substituent, which leads to a strong reduction of the positive charge on the terminal donor in the excited state.