FTIR Spectroelectrochemistry of F4TCNQ Reduction Products and Their Protonated Forms.

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

Method development for binding media analysis in painting cross-sections by desorption electrospray ionization mass spectrometry.

RATIONALE: Art conservation science is in need of a relatively nondestructive way of rapidly identifying the binding media within a painting cross-section and isolating binding media to specific layers within the cross-section. Knowledge of the stratigraphy of cross-sections can be helpful for removing possible unoriginal paint layers on the artistic work. METHODS: Desorption electrospray ionization mass spectrometry (DESI-MS) was used in ambient mode to study cross-sections from mock-up layered paint samples and samples from a 17th century baroque painting. The DESI spray was raster scanned perpendicular to the cross-sectional layers to maximize lateral resolution then analyzed with a triple quadrupole mass analyzer in linear ion trap mode. From these scans, isobaric mass maps were created to map the locations of masses indicative of particular binding media onto the cross-sections. RESULTS: Line paint-outs of pigments in different binding media showed specific and unique ions to distinguish between the modern acrylic media and the lipid-containing binding media. This included: OP (EO)9 surfactant in positive ESI mode for acrylic (m/z 621), and oleic (m/z 281), stearic (m/z 283), and azelaic (m/z 187) acids in negative ESI mode for oil and egg tempera. DESI-MS maps of mock-up cross-sections of layered pigmented binding media showed correlation between these ions and the layers with a spatial resolution of 100 µm. CONCLUSIONS: DESI-MS is effective in monitoring binding media within an intact painting cross-section via mass spectrometric methods. This includes distinguishing between lipid-containing and modern binding materials present in a known mock-up cross-section matrix as well as identifying lipid-binding media in a 17th century baroque era painting.

Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT.

The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Multiple charge-transfer states, defined by the fraction of electron transfer to F4TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well-understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F4TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films coprocessed with F4TCNQ, we demonstrate that this diminished conductivity is due to formation of the nonchromophoric, weak dopant HF4TCNQ-. This species is likely formed through hydrogen abstraction from the α aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

Correlation of Coexistent Charge Transfer States in F4TCNQ-Doped P3HT with Microstructure.

Understanding the interaction between organic semiconductors (OSCs) and dopants in thin films is critical for device optimization. The proclivity of a doped OSC to form free charges is predicated on the chemical and electronic interactions that occur between dopant and host. To date, doping has been assumed to occur via one of two mechanistic pathways: an integer charge transfer (ICT) between the OSC and dopant or hybridization of the frontier orbitals of both molecules to form a partial charge transfer complex (CPX). Using a combination of spectroscopies, we demonstrate that CPX and ICT states are present simultaneously in F4TCNQ-doped P3HT films and that the nature of the charge transfer interaction is strongly dependent on the local energetic environment. Our results suggest a multiphase model, where the local charge transfer mechanism is defined by the electronic driving force, governed by local microstructure in regioregular and regiorandom P3HT.