Competition between Conformational Relaxation and Intramolecular Electron Transfer within Phenothiazine−Pyrene Dyads

Abstract

The competition between conformational dynamics and electron transfer within a series of phenothiazine-(phenyl) n -pyrene (n ) 0, 1) electron donor-acceptor dyads of potential use in organic light emitting diodes was examined using femtosecond transient absorption spectroscopy. The molecular structures of these dyads permit only torsional motions around the single bonds joining each aromatic subunit. The redox properties of these molecules are nearly independent of the phenyl bridging group, whereas spectroelectrochemistry shows that the UV/vis absorption spectra of the oxidized and reduced species vary with the bridge. Each molecule exhibits dual fluorescence emission which provides evidence for conformational heterogeneity. Emission from a locally excited state originates from a minority conformation, in which electron transfer is negligible, whereas emission because of ion pair recombination results from the majority conformation which undergoes electron transfer. The electron-transfer reactions proceed with time constants <25 ps except in the dyad with the longest donor-acceptor distance in nonpolar solution, where the free energy of the charge separation reaction is positive. If electron transfer is sufficiently fast, conformational relaxation within the ion pair state product occurs on a 100-400 ps time scale, whereas if electron transfer is slow, conformation relaxation with the locally excited state centered on phenothiazine occurs. In two of the dyads in nonpolar solvents, wherein the free energy for charge separation is estimated to be very small, strong mixing between the ion pair state and the locally excited state of phenothiazine is found. The results show that competitive conformational relaxations can have a strong influence on the charge separation dynamics of donor-bridge-acceptor molecules with single bond linkages. In turn, these conformational dynamics will undoubtedly have an important influence on the photophysics of these molecules in the solid-state environment characteristic of light-emitting diodes. † Part of the special issue "Edward Schlag Festschrift".

Aggregation-induced delayed fluorescence from phenothiazine-containing donor–acceptor molecules for high-efficiency non-doped organic light-emitting diodes

“…A similar AIE behavior was also observed for PTZ-BP in THF/water mixtures (Figure 3b), whereas the only structural difference between the two compounds is that PTZ-XT contains an oxygen bridge, which Aggregation-induced delayed fluorescence N Aizawa et al restricts the internal rotations in the acceptor unit. Hence, the observed AIE behaviors of PTZ-XT and PTZ-BP are primarily attributed to aggregation-induced restriction of conformational changes in the phenothiazine donor unit [42][43][44] and the rotation around the single bond between the donor and acceptor units.…”

Photoinduced Intramolecular Electron Transfer and Triplet Energy Transfer in a Steroid‐Linked Norbornadiene–Carbazole Dyad

“…[2,5,7,8] It is generally accepted that in donor ± {saturated hydrocarbon bridge} ± acceptor systems electron transfer and triplet energy transfer can proceed by throughbond and/or through-space mechanisms, depending on the nature of the bridge.…”

“…In this case, the electron transfer and energy transfer processes have been regarded as a through-space interaction. [19±22] The factors that govern intramolecular energy-and electron-transfer processes in rigid donor ± bridge ± acceptor systems, for instance distance and orientation between the donor and acceptor, and the electronic properties of the intervening spacer have been extensively examined by Miller and Closs, [23,24] Morrison, [11±13] Paddon-Row and Verhoevem, [6,17,25] Schuster, [26,27] Zimmerman, [28] Gust and Moore, [2,5] Wasielewski, [1,8,10,29] and many others. [3, 7, 9, 14±16] We have utilized the androstene skeleton as the bridge, mounted an antenna chromophore and a norbornadiene (or quadricyclane) group on the steroidal framework, and investigated intramolecular triplet energy transfer and electron transfer within these systems.…”

Intramolecular triplet energy transfer in benzophenone-{crown ether}-naphthalene system

“…[1][2][3][4][5][6] The principal aims of these studies have been to develop a better understanding for photoharvesting systems in organisms, [1][2][3][4][5][6] and to provide a base for designing molecular photonic devices and synthetic solar energy conversion systems. 2,5,7,8 It is generally accepted that in donor-{saturated hydrocarbon bridge}-acceptor systems triplet energy transfer can proceed via through-bond and/or through-space mechanisms, depending on the nature of the bridge. [1][2][3][4][5][6][7][8][9][10] For rigid bridge-linked donor-acceptor molecules, the through-bond mechanism appears to be favored.…”

“…2,5,7,8 It is generally accepted that in donor-{saturated hydrocarbon bridge}-acceptor systems triplet energy transfer can proceed via through-bond and/or through-space mechanisms, depending on the nature of the bridge. [1][2][3][4][5][6][7][8][9][10] For rigid bridge-linked donor-acceptor molecules, the through-bond mechanism appears to be favored. [1][2][3][4][5][6][7][8][9] Steroid, [11][12][13][14][15][16] fused norbornenyl 6,17 and many other groups 18 have been used to constitute the bridge.…”