Begoña Milián-Medina

Stimulated Emission Properties of Sterically Modified Distyrylbenzene-Based H-Aggregate Single Crystals.

J-aggregation has been shown to be beneficial for light amplification in single crystals of π-conjugated organic molecules. In the case of H-aggregation, the criteria for such processes are still under debate. It has also been shown that H-aggregate arrangements with considerable π-π overlap are detrimental for light amplification. We show here that a proper alignment of the molecules in the crystal lattice, which minimizes π-π overlap between adjacent molecules, gives rise to (random) stimulated emission from cofacial arrangements similar to that of the herringbone aggregates.

Stimulated Resonance Raman Scattering and Laser Oscillation in Highly Emissive Distyrylbenzene-Based Molecular Crystals

Three-in-one: A novel distyrylbenzene-based material forms J-type aggregates in single crystals with highly polarized and bright red emission, giving rise to optical gain narrowing, for which different mechanisms (amplified spontaneous emission, laser emission and stimulated resonance Raman scattering) are observed. These are correlated with the favorable intrinsic and macroscopic properties of the crystal, in particular to the orientation of the molecules to the crystal surface.

Computational engineering of low bandgap copolymers

We present a conceptual approach to low bandgap copolymers, in which we clarify the physical parameters which control the optical bandgap, develop a fundamental understanding of bandgap tuning, unify the terminology, and outline the minimum requirements for accurate prediction of polymer bandgaps from those of finite length oligomers via extrapolation. We then test the predictive power of several popular hybrid and long-range corrected (LC) DFT functionals when applied to this task by careful comparison to experimental studies of homo- and co-oligomer series. These tests identify offset-corrected M06HF, with 100% HF exchange, as a useful alternative to the poor performance of tested hybrid and LC functionals with lower fractions of HF exchange (B3LYP, CAM-B3LYP, optimally-tuned LC-BLYP, BHLYP), which all significantly overestimate changes in bandgap as a function of system size.

π-Conjugation: π-Conjugation

In the current overview, the concept of π‐conjugation in organic compounds is reexamined. Starting from valence bond and molecular orbital (MO) theory, the principal elements of conjugation are worked out starting from a simple Hückel picture; this includes the definition of the conjugated path, bond length alternation, as well as MO localization effects, and requires the distinction between formal and effective conjugation. The latter will be needed to understand the electronic and optical properties of polyconjugated molecules, highlighting the importance of a correct quantum chemical description of ground and excited state properties. Finally, implications of conjugation on interacting systems are shortly discussed.

Excited-state switching by per-fluorination of para-oligophenylenes.

Fluorination has become a versatile route to tune the electronic and optical properties of organic conjugated materials. Herein we report a new phenomenon, excited-state switching by per-fluorination of para-oligophenylenes, placing a low intensity 1(1)B(2) state below the 1(1)B(1) state, giving rise to large Stokes shifts. The switching is attributed to the specific impact of fluorine on the delocalized and localized frontier orbitals as elucidated by quantum-chemical calculations. The sterical demands of the fluorine atom additionally diminish efficient conjugation along the chain, leading to hypsochromic shifts with respect to the unsubstituted counterparts and to a weak chain length dependence of the absorption and unstructured emission spectra and enhanced internal conversion.

Naphthalenediimide Polymers with Finely Tuned In-Chain π-Conjugation: Electronic Structure, Film Microstructure, and Charge Transport Properties

Naphthalenediimide-based random copolymers (PNDI-TVTx) with different π-conjugated dithienylvinylene (TVT) versus π-nonconjugated dithienylethane (TET) unit ratios (x = 100→0%) are investigated. The PNDI-TVTx-transistor electron/hole mobilities are affected differently, a result rationalized by molecular orbital topologies and energies, with hole mobility vanishing but electron mobility decreasing only by ≈2.5 times when going from x = 100% to 40%.

Fluoro-functionalization of vinylene units in a polyarylenevinylene for polymer solar cells

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The solar cells with the fluorinated polymer show better photovoltaic performances than those recorded with the non-fluorinated material. Comparative spectroscopic and computational studies, together with morphological, electrical and optical characterization of thin films, have been carried out to shed light on the reasons for the improvement of performances as induced by the double bond fluorination. Our study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.

Effective conjugation in conjugated polymers with strongly twisted backbones: a case study on fluorinated MEHPPV

Conjugated polymers with strongly twisted backbones, such as MEHPPV with fluorinated vinylene units (F-MEHPPV), demand a redefinition of the all-important ‘effective conjugation length’ ECL, which we extract here by a facile graphical method. In MEHPPV (being essentially planar), the ECL coincides with the ‘maximum conducive chain length’ MCC and extends over about n ≈ 9 repetition units (RU). In F-MEHPPV, the MCC is similarly long with n ≈ 8, but the ECL localizes on just one RU. The strong twist in F-MEHPPV persists in the excited state, broadening the emission spectrum and quenching the fluorescence with reduced radiative and enhanced non-radiative rates.

“Though It Be but Little, It Is Fierce”: Excited State Engineering of Conjugated Organic Materials by Fluorination

Fluorination is frequently used to significantly change the properties of conjugated organic materials due to fluorines exceptional properties; well-known is its impact on electronic structure, but it also impacts the geometry despite fluorines small size. Less known, the changes in the electronic and geometrical properties may provoke drastic changes of the excited state properties like batho- and hypsochromic shifts of absorption and emission bands (inter alia leading to excited state switching), hypo- and hyperchromic effects, spectral broadening, and changes of the nonradiative deactivation pathways. The state of the art on these issues is summarized in the current Perspective to stimulate further discussions on this intriguing subject.

Excited state absorption spectra of dissolved and aggregated distyrylbenzene: A TD-DFT state and vibronic analysis

A time-dependent density functional theory study is performed to reveal the excited state absorption (ESA) features of distyrylbenzene (DSB), a prototype π-conjugated organic oligomer. Starting with a didactic insight to ESA based on simple molecular orbital and configuration considerations, the performance of various density functional theory functionals is tested to reveal the full vibronic ESA features of DSB at short and long probe delay times.