Mechanisms of IR amplification in radical cation polarons

Evaluating the interactions between vibrational modes and electronic transitions using frontier orbital energy derivatives

Vibrations affect molecular optoelectronic properties, even at zero kelvin. Accounting for these effects using computational modelling is costly, as it requires many calculations at geometries distorted from equilibrium. Here, we propose a low-cost method for identifying vibrations most strongly coupled to the electronic structure, based on using orbital energy derivatives as a diagnostic.

Nitrile Substituents at the Conjugated Dipyridophenazine Moiety as Infrared Redox Markers in Electrochemically Reduced Heteroleptic Ru(II) Polypyridyl Complexes

Ruthenium(II) complexes [Ru(tap)2(NN)]2+ (tap = 1,4,5,8-tetraazaphenanthrene, NN = 11-cyano-dipyrido[3,2-a:2',3'-c]phenazine (11-CN-dppz) and 11,12-dicyano-dipyrido[3,2-a:2',3'-c]phenazine (11,12-CN-dppz)) feature the C≡N groups as infrared (IR)-active redox markers. They were studied by cyclic voltammetry, UV-vis, and IR spectroelectrochemistry (SEC), and density functional theory calculations to assign the four 1e- reduction waves R1-R4 observed in dichloromethane. Generally, the NN ligands are reduced first (R1). For [Ru(tap)2(11,12-CN-dppz)]2+, R1 is sufficiently separated from R2 and delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the νs,as(C≡N) modes by -18/-28 cm-1, accompanied by a 4-fold enhancement of the νs(C≡N) intensity, comparably with reference data for free 11,12-CN-dppz. The first tap-based reduction of spin-doublet [Ru(tap)2(11,12-CN-dppz)]+ to spin-triplet [Ru(tap)2(11,12-CN-dppz)] at R2 decreased ν(C≡N) by merely -2 cm-1, while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(C≡N) by -27 cm-1 resulted from the 1e- reduction of [Ru(tap)2(11-CN-dppz)]2+ at R1 (poorly resolved from R2), and the intensity enhancement was roughly 3-fold. Concomitant 1e- reductions of the tap ligands (R2 and R3) caused only minor ν(C≡N) shifts of -3 cm-1 and increased the absorbance by overall factors of 6.5 and 8, respectively.

Real-Time Tracking of Hot Carrier Injection at the Interface of FAPbBr3 Perovskite Using Femtosecond Mid-IR Spectroscopy

One of the most effective approaches to optimizing the performance of perovskite solar cells is to fully understand the ultrafast carrier dynamics at the interfaces between absorber and transporting layers at both the molecular and atomic levels. Here, the injection dynamics of hot and relaxed charge carriers at the interface between the hybrid perovskite, formamidinium lead bromide (FAPbBr3), and the organic electron acceptor, IEICO-4F, are investigated and deciphered by using femtosecond (fs) mid-infrared (IR), transient absorption (TA), and fluorescence spectroscopies. The visible femtosecond-TA measurements reveal the generation of hot carriers and their transition to free carriers in the pure FAPbBr3 film. Meanwhile, the efficient extraction of hot carriers in the mixed FAPbBr3/IEICO-4F film is clearly evidenced by the complete disappearance of their spectral signature. More specifically, the time-resolved results reveal that hot carriers are injected from FAPbBr3 to IEICO-4F within 150 fs, while the transfer time for the relaxed carriers is about 205 fs. The time-resolved mid-IR experiments also demonstrate the ultrafast formation of two peaks at 2115 and 2233 cm-1, which can be attributed to the C≡N symmetrical and asymmetrical vibrational modes of anionic IEICO-4F, thus providing crystal clear evidence for the electron transfer process between the donor and acceptor units. Moreover, photoluminescence (PL) lifetime measurements reveal an approximately 10-fold decrease in the donor lifetime in the presence of IEICO-4F, thereby confirming the efficient electron injection from the perovskite to the acceptor unit. In addition, the efficient electron injection at the FAPbBr3/IEICO-4F interface and its impact on the C≡N bond character are experimentally evidenced and align with density functional theory (DFT) calculations. This work offers new insights into the electron injection process at the FAPbBr3/IEICO-4F interface, which is crucial for developing efficient optoelectronic devices.

Unraveling Excited State Dynamics of a Single-Stranded DNA-Assembled Conjugated Polyelectrolyte

Conformational templating of conjugated polyelectrolytes with single-stranded DNAs (ssDNAs) has the prospect of tailoring excited state dynamics for specific optoelectronic applications. We use ultrafast time-resolved infrared spectroscopy to study the photophysics of a cationic polythiophene assembled with different ssDNAs, inducing distinct conformations (flexible disordered structures vs more rigid complexes with increased backbone planarity). Intrachain polarons are always produced upon selective excitation of the polymer, the extent being dependent on backbone torsional order. Polaron formation and decay were monitored through evolution of IR-active vibrational modes that interfere with mid-IR polaron electronic absorption giving rise to Fano-antiresonances. Selective UV excitation of ssDNAs revealed that stacking interactions between thiophene rings and nucleic acid bases can promote the formation of an intermolecular charge transfer complex. The findings inform designers of functional conjugated polymers by identifying that involvement of the scaffold in the photophysics needs to be considered when developing such structures for optoelectronic applications.

Multicenter interactions and ligand field effects in platinum(II) tripyrrindione radicals

The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum(II) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert-butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt(II) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand (i.e., aqua vs. isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt(II)-Pt(II) contact in the case of the aqua complex. Overall, the properties of Pt(II) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

Operando characterization plays an important role in revealing the structure-property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

Conjugated molecular chains have the potential to act as "molecular wires" that can be employed in a variety of technologies, including catalysis, molecular electronics, and quantum information technologies. Their successful application relies on a detailed understanding of the factors governing the electronic energy landscape and the dynamics of electrons in such molecules. We can gain insights into the energetics and dynamics of charges in conjugated molecules using time-resolved infrared (TRIR) detection combined with pulse radiolysis. Nitrile ν(C[triple bond, length as m-dash]N) bands can act as IR probes for charges, based on IR frequency shifts, because of their exquisite sensitivity to the degree of electron delocalization and induced electric field. Here, we show that the IR intensity and linewidth can also provide unique and complementary information on the nature of charges. Quantifications of IR intensity and linewidth in a series of nitrile-functionalized oligophenylenes reveal that the C[triple bond, length as m-dash]N vibration is coupled to the nuclear and electronic structural changes, which become more prominent when an excess charge is present. We synthesized a new series of ladder-type oligophenylenes that possess planar aromatic structures, as revealed by X-ray crystallography. Using these, we demonstrate that C[triple bond, length as m-dash]N vibrations can report charge fluctuations associated with nuclear movements, namely those driven by motions of flexible dihedral angles. This happens only when a charge has room to fluctuate in space.

Picosecond Charge Localization Dynamics in CH3NH3PbI3 Perovskite Probed by Infrared-Activated Vibrations

Hybrid metal halide perovskites exhibit well-defined semiconducting properties and efficient optoelectronic performance considering their soft crystal structure and low-energy lattice motions. The response of such a crystal lattice to light-induced charges is a fundamental question, for which experimental insight into ultrafast time scales is still sought. Here, we use infrared-activated vibrations (IRAV) of the organic components within the hybrid perovskite lattice as a sensitive probe for local structural reorganizations after photoexcitation, with femtosecond resolution. We find that the IRAV signal response shows a delayed rise of about 3 ps and subsequent decay of pronounced monomolecular character, distinguishing it from absorption associated with free carriers. We interpret our results as a two-step carrier localization process. Initially, carriers localize transiently in local energy minima formed by lattice fluctuations. A subpopulation of these can then fall into deeper trapped states over picoseconds, likely due to local reorganization of the organic molecules surrounding the carriers.