Investigation of the long effective conjugation length in defect-free insulated molecular wires

Due to the “insulation” of the π-conjugated backbones, insulated molecular wires (IMWs) are expected to be applied to various optoelectronic applications and nanotechnology.[1] Recently, Kazunori et al have succeeded in the synthesis of a self-threading polythiophene with a polyrotaxane-like 3D architecture (PSTB, see Figure 1), for which an intrawire hole mobility of 0.9 cm2 V−1 s−1 has been measured.[2] Here, we aim to evaluate the extent of π-conjugation along polythiophene backbones sheathed within defect-free “insulating” layers. A comparison between the experimental Raman spectra of the self-threading oligomers (i.e. 2STB-5STB) and the corresponding PSTB polymer indicates that: (i) the ratio of relative intensities of the two most intense Raman bands (I1375/1445) increases with the elongation of the size chain but does not saturate up to the pentamer, and (ii) π-conjugation spreads over 17–18 thiophene units in the polymer. Whether the effective conjugation length of the polymer is better described by using the long oligomer extrapolation approach[3] or periodic DFT calculations of the polymer is discussed in detailed by exploiting the very recent potentialities of state-of-the-art quantum chemical simulations of vibrational properties for crystalline solids.[Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Molecular and electronic structure investigation of encapsulated polytiophenes

Insulated molecular wires (IMWs) are expected to be applied to various optoelectronic applications due to their unique photophysical, electronic, and mechanical properties which originate from the absence of -stacking.[1] Kazunori et al have succeeded in the synthesis of a self-threading polythiophene with a polyrotaxane-like 3D architecture (PSTB, see Figure 1a), for which an intrawire hole mobility of 0.9 cm2 V−1 s−1 has been measured.[2] In this study we aim to evaluate for the first time the extension of the -conjugation in encapsulated polythiophenes. A comparison between the experimental Raman spectra of the self-threading PSTB polymer with their correspondent oligomers (i.e. 2STB-5STB) suggests that the effective conjugation length in the polymer is longer than five monomer units. Whether the effective conjugation length of the polymer is better described by using the long oligomer extrapolation approach or periodic DFT calculations of the polymer is discussed in detailed by exploiting the very recent potentialities of state-of-the-art quantum chemical simulations of vibrational properties for crystalline solids.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Optical Absorption and Emission Properties of End-capped Oligothienoacenes: A Joint Theoretical and Experimental Study

The electron–vibration coupling in a family of silyl end-capped oligothienoacenes is investigated on the basis of a joint experimental and theoretical study using UV–vis absorption and emission spectroscopies and density functional theory calculations. Well-resolved vibronic progressions are found in the low-temperature absorption and emission profiles of these silyl-functionalized organic molecules. As the size of the oligomer lengthens a bathochromic shift is observed in the near-UV–vis range, indicative of the extension of the effective π-conjugation. The absorption and emission bands are practically mirror-symmetric. The combination of two normal modes with frequencies of $\sim 1500 cm^{−1}$ and $\sim 500 cm^{−1}$ determines the main vibronic progression in absorption and emission for all the series, although for larger oligomers (n = 6, 7, and 8) the presence of low-frequency normal modes ($\sim100 cm^{−1}$) is also evident. The spacing of the vibrational features is slightly larger in absorption than in emission; this agrees with the predicted shifting of the C−C stretching modes of the inner-most ring toward the high-frequency region as a result of the reversal of the single–double C−C pattern in the electronic excited-state. Our calculations indicate that the contributions of the end-capping groups to the total relaxation energy of the $S_0 \rightarrow S_1$ and $S_1 \rightarrow S_0$ transitions are almost negligible. This result suggest that the vibronic structure and to a large extent the spectral profiles of the silyl end-capped oligothienoacenes are mainly determined by their respective oligothienyl core.Chemistry and Chemical Biolog

π-Dimerization of Heptathienoacene Radical Cations

Oligothienoacenes, the fused-ring analog of pi-linked oligothiophenes, belong to the most promising candidates for organic electronic applications. This is in part due to their fully planar structure that avoids conformational disorder and allows for densely packed solid-state structures resulting in high charge carrier mobilities. In recent years, there has been a growing interest in the study of the pi-dimerization of conjugated radical cations with a dual purpose: (i) elucidation of the nature of the charge-transport phenomena in p-doped semiconducting polymers and (ii) development of supramolecular bonding ideas for applications in material science, such as actuators. However, the π-dimerization of planar conjugated radical cations in solution is scarce and usually encountered at low temperatures. In this work, we investigate the exceptional pi-dimerization capability showed by radical cations of a heptathienoacene alpha,beta-substituted with four n-decyl side groups (D4T7•+) by using a joint experimental and theoretical approach. D4T7 radical cations are found to exhibit an exceptional ability to form pi-dimer dications even at ambient temperature. Our results evidence the presence of two different transitory oxidized species formed during the course of the one-electron oxidation: (i) different conformations of the [D4T7•+]2 pi-dimer dications and (ii) the intermediate [D4T7]2•+ pi-dimer radical cations.[5] The nature and structure of these transitory species and ultimate pi-dimer dication are rigorously analyzed with the help of the DFT and TD-DFT calculations. Our study would provide valuable guidance for the further development of pi-dimer based supramolecular architectures.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Investigaciones arqueométricas en las cuevas de Ardales y Las Palomas de Teba (Comarca del Guadalteba, Málaga)

We have performed a Raman miscroscopic analysis of a series of prehistoric artifacs found inside the Ardales Cave (Ardales, Málaga, Spain), such as a number of fixed and portable fat-burning stone lamps, some lithic tools and wall paintings, by means of either a portable Raman spectrometer for in situ studies in the field or bench-top Raman spectrometers and Raman microscopes in the laboratory of Vibrational Spectroscopy at the Central Building of Scientific Facilities of the University of Málaga (SCAI, UMA).We have performed a Raman miscroscopic analysis of a series of prehistoric artifacs found inside the Ardales Cave (Ardales, Málaga, Spain), such as a number of fixed and portable fat-burning stone lamps, some lithic tools and wall paintings, by means of either a portable Raman spectrometer for in situ studies in the field or bench-top Raman spectrometers and Raman microscopes in the laboratory of Vibrational Spectroscopy at the Central Building of Scientific Facilities of the University of Málaga (SCAI, UMA).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Caracterización de sistemas basados en carbazol con transformación reversible entre birradical aislado y σ-agregados

El desarrollo de la química de moléculas orgánicas π-conjugadas que tienen una capa de valencia que no está completamente llena, también conocidas como sistemas de capa abierta, ha atraído mucho interés en estos últimos años. Estos sistemas presentan gran ventaja para aplicaciones en espintrónica, electrónica orgánica u óptica no lineal, entre otros.[1] Sin embargo, los sistemas π-conjugados con carácter birradical suelen ser poco estables a temperatura ambiente. Por tanto, el diseño y caracterización de compuestos birradicales estables presenta hoy en día un gran interés.[2] En este trabajo, presentamos un estudio experimental y teórico, basado en espectroscopias Raman y UV-Vis-NIR junto con cálculos DFT, de un compuesto bicarbazol que presenta transformaciones termocrómicas y piezocrómicas reversibles entre una especie quinoide de capa cerrada y otra especie biradical de capa abierta que forma agregados mediante enlaces de tipo-σ.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Estudio del efecto fotocrómico reversible de un sistema quinoide basado en carbazol

Hoy en día, el uso de materiales de naturaleza orgánica se presenta como un recurso ecológico y asequible para el diseño de dispositivos electrónicos. Entre ellos, los materiales sensibles a estímulos tienen además numerosas aplicaciones como interruptores moleculares, sensores moleculares o memorias moleculares.[1] En este trabajo, estudiamos un compuesto quinoide basado en carbazol (ver figura 1) con propiedades fotocrómicas (cambia de color por la acción de la luz). En estado sólido presenta un color blanco asociado a una estructura tetramérica producida por la generación de puentes σ C-C de cuatro monómeros birradicalarios.[2] No obstante, dicho agregado tetramérico puede romperse mediante alguna fuerza externa, obteniendo las unidades monoméricas independientes, adoptando un color violeta intenso el compuesto. El estudio detallado de sus propiedades fotocrómicas y termocrómicas será abordado mediante técnicas espectroscópicas (Raman, IR y UV-Vis-NIR) y cálculos DFT. FiguraUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Estudio de las propiedades inter- e intramoleculares de oligotienoacenos y polímeros heteroaromáticos

En esta Tesis doctoral se estudian polímeros derivados de tiofeno, furano y selenofeno, con el fin de estimar la longitud de conjugación efectiva, morfología en estado sólido y el efecto de la polidispersidad de estos polímeros semiconductores mediante espectroscopía Raman. También se ha realizado un estudio en profundidad de la deslocalización electrónica de especies cargadas de diferentes sistemas tiofénicos mediante experimentos de electroquímica y espectroelectroquímica UV-Vis-NI

Pi-Dimerization of Oligothienoacene Radical Cations

Oligothienoacenes, the fused-ring analog of -linked oligothiophenes, belong to the most promising candidates for organic electronic applications. This is in part due to their densely packed solid-state structures resulting in high charge carrier mobilities.1 In recent years, there has been a growing interest in the study of the -dimerization of conjugated radical cations with a dual purpose: (i) elucidation of the nature of the charge-transport phenomena in -doped semiconducting polymers and (ii) development of supramolecular bonding ideas for applications in material science.2 However, the π-dimerization of planar conjugated radical cations in solution is scarce and usually encountered at low temperatures. In this work, the exceptional -dimerization capability showed by radical cations of oligothienoacenes is investigated for the first time. To this end, we make use of a joint experimental and theoretical approach that combines different spectroscopic techniques with advanced DFT calculations. Our results evidence that the incorporation of bulky TIPS groups prevents the -dimerization while the ,-substitution with n-decyl groups in heptathienoacene or -substitution with thienyl groups in tetrathienoacene favors the -dimer formation.3 The nature, structure and stability of the different aggregate structures formed in the course of the oxidation are rigorously analyzed with the help of exhaustive DFT and TD-DFT calculations.Universidad de Málaga. Campus de Excelencia de Andalucía Tec

Multistep π dimerization of tetrakis(n-decyl)heptathienoacene radical cations: a combined experimental and theoretical study

Radical cations of a heptathienoacene a,b-substituted with four n-decyl side groups (D4T7C+) form exceptionally stable p-dimer dications already at ambient temperature (Chem. Comm. 2011, 47, 12622). This extraordinary p-dimerization process is investigated here with a focus on the ultimate[D4T7C+]2 p-dimer dication and yet-unreported transitoryspecies formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the p-dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one-electron oxidation: 1) a different transient conformation of the ultimate [D4T7C+]2 p-dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]2C+ p-dimer radical cations formed prior to the fully oxidized [D4T7]2C+ p-dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems