2,4,5,7-Tetranitro-9-fluorenone (1b) reacts readily with n-butanethiol in dipolar aprotic solvents with selective substitution of nitro groups by butylsulfanyl groups in positions 2 and 7 (2, 3); the 2,5-isomer 4 was formed only as a minor product (<1%). Condensation of fluorenones 2-4 with malononitrile yielded 9-dicyanomethylene derivatives 5-7, which showed strong intramolecular charge transfer (lambda approximately 510-560 nm) and were found to sensitize the photoconductivity of carbazole-containing polymer films. Oxidation of sulfides 2-4 gave sulfoxide 8 or sulfones 9-11, which then were converted into their corresponding dicyanomethylene derivatives 12-15. All these novel acceptors showed three reversible single-electron reduction waves (cyclic voltammetry) yielding radical anion, dianion, and radical trianion; moreover, acceptors 13-15 showed also a fourth reduction wave, representing reversible tetraanion formation. Substitution of the oxygen of the carbonyl group in the fluorenones by a dicyanomethylene group increased the thermodynamic stability (K(SEM) growth) of the radical anion; K(SEM) ranged from 3 x 10(5) to 3 x 10(9) M(-1). CV measurements characterize compounds 3, 4 (EA = 1. 86-1.89 eV) as poor acceptors, 2, 6-11 (EA = 2.13-2.31 eV) as moderate acceptors, and 5, 12-15 (EA = 2.53-2.66 eV) as strong electron acceptors. Charge-transfer complex (CTC) formation between acceptors 9, 10, 13, 14, and anthracene as a donor was monitored by the appearance of additional low-energy bands in the visible region (CTC bands) of their electron absorption spectra. Increasing the EA of the acceptors from 9-fluorenones to the corresponding 9-dicyanomethylenefluorenes increases the complexation constants K(CTC) by 2.5-3 times, while sulfonyl substituents present substantial steric hindrance for complexation (as compared to the nitro group), decreasing K(CTC) values. Two CTCs for acceptors 14 and 17 with tetrathiafulvalene (TTF) were obtained, and their structures were solved by single-crystal X-ray diffractometry, giving the stoichometries 14:TTF, 2:3, and 17:TTF:PhCl, 1:1:0.5. In the former complex the packing motif is a mixed.DDAD'A. stack; in the latter complex the D and A moieties form unusually close CT pairs, which pack in a herringbone motif.

Electron acceptors of the fluorene series. Part 5.l Intramolecular charge transfer in nitro-substituted 9-(aminomethylene)fluorenes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 compared with those from Density Functional Theory calculations. The charge transfer dynamics are studied using an ultrafast visible-pump -IR-probe photoinduced absorption technique. We demonstrate that the acceptor EA is the key -but not the only -parameter that governs charge recombination rates that scale exponentially with the acceptor EA. From the time-resolved data we deduced a model that describes charge dynamics for acceptors with low and high EAs. The two opposite trends -higher acceptor EA increases the driving force for charge separation but also inevitably increases the rate of undesirable charge recombination -should be carefully counterbalanced in designing of novel polymer-fullerene bulk heterojunctions.

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Abstract.We study charge recombination rates in blends of MEH-PPV and a comprehensive set of low molecular-weight fluorine-based acceptors, using visiblepump -IR-probe photoinduced absorption technique. The positions of acceptor's HOMO-LUMO levels are engineered by addition of different chemical substituents. The results clearly show the exponential scaling of the charge recombination rate with the acceptor LUMO energy. The minimal donor-acceptor LUMO energy difference required to form a CTC state is estimated as ~0.25 eV.Ground-state charge transfer complexes (CTCs) lay in heart of many photoinduced processes in physics, chemistry, and biology. The ground-state CTCs are characterized by a partial shift of electronic density from the donor to the acceptor molecule already in the ground state. Upon optical excitation, the electron is transferred to the acceptor almost instantaneously, while back recombination occurs in the finite time. The CTC photophysical properties strongly depend on the molecular structure of the donor-acceptor pair. For instance, in polymer-fullerene blends the CTC absorption is extremely weak and hardly detectable [1]. In contrast, in blends of soluble poly-paraphenylene vinylenes (MEH-PPV) with fluorene-type acceptors, the CTC formation can be readily detected visually as a color change. In the latter type of materials, the CTC band is so intense that it could enhance the solar energy harvesting in the bandgap of both the donor and acceptor [2], similarly to push-pull copolymers [3]. Furthermore, the polymer photoxidation stability is drastically increased by the presence of the CTCs [4]. As was recognized in early studies of small-molecule CTCs [5], the acceptor electron affinity is a key parameter that determines the back electron transfer rates.Here we demonstrate that the acceptor electron affinity governs the CTC recombination rate in full accord with the Markus theory [6]. Donor-acceptor CTCs formed in blends between the conjugated polymer MEH-PPV and a family of low-molecular weight fluorene acceptors was used for their known property to form stable CTCs already in solution. To unravel recombination dynamics of the photogenerated charges, ultrafast visible-pump --IR-probe photo-induced absorption (PIA) spectroscopy [7] was applied. EPJ Web of Conferences