Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s

The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab-initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.Comment: 12 pages, 13 Figure

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π–π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.P.T. thanks the “Conseil Régional du Limousin” for financial support and CALI (CAlcul en LImousin). Financial support from the Czech Science Foundation (P208/12/G016), the Ministry of Education, Youth and Sports of the Czech Republic (project LO1305), and the Operational Program Education for Competitiveness-European Social Fund (project CZ.1.07/2.3.00/20.0058 of the Ministry of Education, Youth and Sports of the Czech Republic) is also gratefully acknowledged. The work at IMDEA was supported by the Spanish Ministerio de Economía y Competitividad (MINECO; project CTQ2014-58801)

Probing the molecular orientation of a single conjugated polymer via nano-gap SERS

Determining the molecular orientation at the single molecule level is of key importance for a wide range of applications ranging from molecular electronic devices to biomedical applications. In this work surface-enhanced Raman scattering (SERS) was used to probe the light-emitting conjugated polymer F8-PFB at the single molecule level using nanoparticles on an extended metallic film nanogap. The directional field enhancement of the nanogap combined with density functional theory (DFT) calculations was used to determine the orientation of the molecule. This analysis revealed that the spin-coated conjugated polymer preferentially aligns its molecular chains parallel to the metallic substrate. The integration of this approach in nanofabrication and synthesis will have a profound impact on different fields ranging from molecular electronic devices to biomedical applications

Excited-state non-radiative decay in stilbenoid compounds:an ab initio quantum-chemistry study on size and substituent effects

In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-consistent field/complete-active-space second-order perturbation theory (CASSCF/CASPT2) are combined. The strategy is firstly benchmarked for ethene, styrene, and stilbene; next it is applied to DSB and representative DCS molecules. Two energy descriptors are derived from the approximated PES, the Franck-Condon energy and the energy gap at the elongated, twisted, and pyramidalized structures. These energy descriptors correlate fairly well with the non-radiative decay rates, which validates our computational strategy. Ultimately, this strategy may be applied to predict the luminescence behavior in related compounds

Inverted energy gap law for the nonradiative decay in fluorescent floppy molecules:larger fluorescence quantum yields for smaller energy gaps

A data survey on experimental fluorescence quantum yields of (multi)substituted dicyano-distyrylbenzenes in fluid solution evidences that non-radiative decay increases with the Franck-Condon energy (E-FC), being opposite to the conventional energy gap law. Quantum-chemistry indicates that this is controlled by access to the conical intersection (CI) following the Bell-Evans-Polanyi principle as a first-step approximation for this family of molecules; the variations in E-FC among the different compounds are found to be decisive, while those of E-CI are estimated to be weaker or even enhancing the effect. The current findings may have significant consequences for the design of molecules for organic solid state emitters