Photoinduced valence instability in the organic molecular compound tetrathiafulvalene-p-chloranil (TTF-CA)

Abstract

It was found that a local charge-transfer excitation on the donor(D)-acceptor(A) pair leads to a semimacroscopic valence change from the quasi-ionic (D+A ) to quasineutral (D A ) states in organic molecular-compound TTF-chloranil with mixed DA stacks. Photogenerated neutralionic domain walls, which are responsible for anomalous photoconductivity, show the temporal annihilation due to the one-dimensional recombination.Various phenomena associated with the valence instability in solids have been of great interest with respect to electronic phase transitions. One of the prototypical examples is the neutral-ionic transition (NIT) observed in organic charge-transfer compounds. ' Several compounds with mixed stacks of donor (D) and acceptor (A) molecules are known to show the thermally or pressureinduced NIT from the quasineutral (DOA ) to the quasiionic (D+A ) state. ' The NIT has been attributed to the competition of the cost of energy for ionizing DoA stacks with the gain of Madelung energy in the ionic D+A lattice. ' Upon the NIT, the degree of charge transfer (CT) on the molecules shows a finite jump and the DA stacks in the ionic phase undergo the dimeric lattice distortion. Since the spin state in the ionic phase is approximately described as the S= -, ' Heisenberg chain, the lattice transition is quite analogous to the spin-Peierls transition. Such an electron (spin) -lattice interaction obviously plays an important role in promoting the NIT in real systems in addition to the above-mentioned electrostatic mechanism. In this paper, we report the observation that the valence instability in the DA compounds can be also caused by photoexcitation. The photogenerated CT excitations (D A ) or resultant charged species (D and A ) in the ionic DA lattice locally modify the Coulombic interaction and switch ofl' a channel of the collective spinlattice interaction. What we have observed in this study is the phenomenon that those local photoexcitations serve as nuclei which evolve into the neutral molecular domains over a semimacroscopic region.Among a number of CT compounds located near the neutral-ionic interface, tetrathiafulvalene (TTF)-chloranil (CA) was investigated here, since this is the most prototypical compound which shows the NIT not only by applying hydrostatic pressure above P, (equal to 11 kbar) but by lowering temperature below T, At ambient pressure, the first-order NIT is observed at 81 K in the cooling run (and at 84 K in the heating run) accompanying the stack dimerization in the ionic phase. To detect a change in molecular ionicity, optical spectra for local electronic excitations have been known to be very useful. 'In Fig. 1(a) reflectance spectra are plotted for the neutral and ionic phases taken on the (001) surface at 90 and 77 K, respectively, with the light polarization normal to the stack axis (a axis) (EJ a). The reflectance bands 13 and C are attributed to intramolecular transitions due to TTF+ molecules, while the band D is due to TTF molecules. As investigated in detail previously, 4 6 spectr...

Examples of molecular switching in inorganic solids, due to temperature, light, pressure, and magnetic field

“…[14] for model). In such a case, the physical features are indeed those of a first-order transition triggered by an external stimulus, and the single-molecule excitations, occurring in sufficient number, might create the critical germs needed for nucleation and growth of the photoinduced phase [31].…”

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…The ionic-neutral, photo-generated phase transition in TTF-CA is the archetype for a PIPT, discovered by Koshihara et al [69] a quarter of a century ago. Although numerous studies were undertaken since, there are still many open questions, for instance: What is the dependence of the PIPT on external parameters such as excitation wavelength, photon flux, and environmental temperature (between T = 4 K and T NI )?…”

“…Over the decades, a large number of ultrafast pump-probe experiments have been performed on TTF-CA in order to explore the photo-induced phase transition (PIPT) that occurs in this one-dimensional charge-transfer compound, most of them confined to the femto-and picoseconds time range [35,58,[69][70][71][72][73][74][75][76][77][78][79][80]. The studies were mainly restricted to low temperatures (typically 4 K) or very close to the phase transition (T ≤ T NI ).…”

A new approach to molecular devices using SAMs, LSMCD and Cat-CVD

“…10 nm. Thus, molecular devices, such as an organic memory device, can be expected to form when the NI transition is controlled optically or by an applied electric field [18,19]. When an electric-field-dependent material is inserted into (a) of Fig.…”