Martin Brinkmann

Controllable Processes for Generating Large Single Crystals of Poly(3‐hexylthiophene)

Sowing the seeds: A simple strategy based on self-seeding allows large single crystals of long regioregular poly(3-hexylthiophene) chains to be grown from solution. When appropriately crystallized, materials differing in their degrees of regioregularity and molecular weights formed monoclinic formu2005II crystals with interdigitated hexyl side groups (see picture).

Segregated versus mixed interchain stacking in highly oriented films of naphthalene diimide bithiophene copolymers.

Highly oriented films of an electron accepting polymer semiconductor, poly{[N,N-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5-(2,2-bithiophene)} (PNDI2OD-T2), are obtained by two different methods, namely directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on friction transferred poly(tetrafluoroethylene) (PTFE) substrates. Two distinct polymorphs with unprecedented intrachain resolution are identified by high-resolution transmission electron microscopy (HR-TEM). Form I is obtained by DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T = 300 °C and cooling at 0.5 K/min. In form I, both electron diffraction and HR-TEM indicate a segregated stacking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a ∼c/2 shift between successive π-stacked chains leads to mixed π-overlaps of T2 and NDI. Form I can be transformed into form II by annealing at T > 250 °C. The different π-stacking of NDI and T2 in the two polymorphs have characteristic signatures in the UV-vis spectra, especially in the charge transfer band around 750 nm which is also observed in spin-coated films.

Orienting Semi‐Conducting π‐Conjugated Polymers

The present review focuses on the recent progress made in thin film orientation of semi-conducting polymers with particular emphasis on methods using epitaxy and shear forces. The main results reported in this review deal with regioregular poly(3-alkylthiophene)s and poly(dialkylfluorenes). Correlations existing between processing conditions, macromolecular parameters and the resulting structures formed in thin films are underlined. It is shown that epitaxial orientation of semi-conducting polymers can generate a large palette of semi-crystalline and nanostructured morphologies by a subtle choice of the orienting substrates and growth conditions.

Control of intrachain charge transfer in model systems for block copolymer photovoltaic materials.

We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor-acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor-acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.

Effects of solvent on the morphology and crystalline structure of lithium phthalocyanine thin films and powders

Abstract The effect of acetone vapours on the phase transformation x → α was studied by means of electron microscopy, X-ray diffractometry, electron spin resonance (ESR) and optical spectroscopy on thin films and powders of lithium phthalocyanine (PcLi). The exposure of amorphous films of PcLi, deposited at a substrate temperature of 15 °C onto glass slides, to solvent vapours of acetone at room temperature, induces important changes in the morphology and crystalline structure of the films. Exposure to acetone vapours under low pressure (200 mbar)leads to an x-like structure with low crystallinity, uniform coverage and no preferential ordering. Longer exposure in a saturated acetone vapour atmosphere results in an important phase transformation to the α form which leads to crystallites up to several micrometres large lying in the plane of the substrate and forming a discontinuous coverage. In the case of x powders treated in liquid acetone, a phase transformation to the α form of PcLi is observed. This x → α phasechange induced by the acetone vapour treatment is interpreted in terms of crystal size effects and compared with the phase transformation observed in thin films deposited at various substrate temperatures in the range 25–225 °C.

Polymorphism in powders and thin films of lithium phthalocyanine. An X-Ray, optical and electron spin resonance study☆

Abstract The polymorphism of lithium phthalocyanine (PcLi) was studied for powders as well as for thin films deposited on glass substrate. Powders were yielded by two electrochemical synthesis routes. While the powders obtained by electrochemical oxidation of PcLi 2 in acetonitrile and acetone lead to an x structure, the synthesis route proposed by M.A. Petit leads apparently to the α form of PcLi. Their very different sensitivity to oxygen, observed using electron spin resonance (ESR) spectroscopy, is explained by means of the crystallographic structure of both polymorphi. Thin films were prepared by vacuum deposition and phase transitions as well as preferential ordering of the crystallites are studied as a function of increasing substrate temperature. While films deposited at low substrate temperatures, below 150°C, lead to an x-like form with the molecular columns lying in the plane of the substrate, the higher substrate temperatures favour the growth of the α form and a loss in preferential ordering. Accordingly, changes in the sensitivity to oxygen are followed as a function of substrate temperature.

EPR study of the x,α and β structures of lithium phthalocyanine

The magnetic properties of the lithium phthalocyanine radical are investigated for the x,α and β crystal structures via electron paramagnetic resonance (EPR). Different magnetic behaviours (susceptibility, line shape) are evidenced in PcLi powders and interpreted in terms of the molecular packing. A Curie–Weiss susceptibility is observed for the α-phase with a mean-field temperature, TCW of ca. 4.5±1 K, whereas a two-component susceptibility must be considered for the x-phase (powders grown in acetonitrile) involving a Curie–Weiss contribution (TCWca. –3±1 K) and a thermally activated contribution with an activation energy Ea of ca. 0.04 eV. For the PcLi β structure two temperature domains can be distinguished: for T>50 K a Curie–Weiss susceptibility with a mean-field temperatureTCW of ca. –60±5 K is observed while for T<20 K a Curie–Weiss-like susceptibility withTCWca. –7±1 K dominates. The overall magnetic properties of the PcLi polymorphs are discussed in terms of McConnell’s mechanism for ferromagnetism and antiferromagnetism in solid free radicals. The magnetic properties of β-PcLi needles (magnetic susceptibility, anisotropy of spin diffusion) have been investigated. Adsorbed oxygen is shown to alter strongly the line shape, the linewidth and the g-factor anisotropy in β-PcLi crystallites and powders at temperatures below 55 K.

The dynamics of the internal phonons tris(quinolin-8-olato) aluminum(III) in crystalline β-phase

A new approach to the analysis of the internal phonons of tris(quinolin-8-olato) aluminum(III) is presented, which enlightens the role played by the ligands in determining the vibrational properties of the organometallic compound and evidences the importance of the contributions arising from the coupling terms among the three quinolinato fragments. An accurate exam of the normal modes of the meridianal isomer evidences the role of the interactions among the fragments in the vibrational dynamics of the ground state. Due to the special attention paid to the quinolinato fragments, a preliminary investigation on the vibrational properties of 8-hydroxyquinoline, taken as a model fragment, was also performed. The vibrational properties of the polymorph species β of the organometallic molecule were obtained refining the calculated frequencies, the dipole moment derivative matrix, and the polarizability derivative tensor derived by the hybrid density functional B3LYP/6-31G* comparing with the frequencies and inte...

Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto‐Electronic Properties

PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.

Optical spectroscopy of unsolvated and solvated crystalline Alq3

Abstract Three novel unsolvated crystalline phases of tris-(8-hydroxyquinoline)-aluminum(III) (Alq 3 ), namely α-, β- and γ-Alq 3 , have been synthesized and their crystalline structure determined from X-ray diffraction data on powders and single crystals. Both α- and β-Alq 3 crystals are triclinic, space group P-1, but differ for the molecular packings. In the γ-Alq 3 phase, Alq 3 molecules lie about a 3-position of the trigonal P-3 space group. A solvated Alq 3 (C 6 H 5 Cl) 1/2 phase (monoclinic, space group P2 1 / n ) was also isolated. The optical and vibrational properties of these newly isolated crystal phases of Alq 3 have been investigated by absorption, photoluminescence excitation, photoluminescence and Raman spectroscopy. The effect of the different molecular packing on the emission properties has been elucidated, the nature of the photoexcitations clarified, and the vibrational fingerprints of the α- and β-crystalline forms highlighted. The origin of the amorphous nature of the vacuum sublimed thin films is discussed on the basis of the accessibility of many different π–π links between homo- and hetero-chiral Alq 3 molecules.