Quenching of the Photoisomerization of Azobenzene Self-Assembled Monolayers by the Metal Substrate

In this study, we aim at investigating the role played by the metal surface as a possible dissipative channel in the photoisomerization process of azobenzene-derivative-based self-assembled monolayers (azo-SAMs). In particular we compare the cases of gold and platinum. We study the excitonic transfer <i>phenomena</i> of two azo-derivatives (both in <i>trans</i> and in <i>cis</i> conformation) chemisorbed on Au{111} and Pt{111} to the metal surfaces. The metal effects are evaluated within the local and nonlocal regimes, showing that nonlocality in the metal response plays an important role and nonlocal accounting quenching rates are one order of magnitude smaller than the corresponding local results. The couplings are stronger for Au{111} than for Pt{111}, but for both cases the energy transfer between the molecule and the metal turns out not to be able to suppress photoisomerization

Work Function Changes of Azo-Derivatives Adsorbed on a Gold Surface

By employing state-of-the-art computational techniques, we investigate two self-assembled monolayers (SAMs), constituted by azobenzene derivatives chemisorbed on a gold surface (azo-SAMs). We study the structural features and the work function change of the azo-SAMs as a function of the conformation of the molecules (<i>trans</i> or <i>cis</i>), of the unit cell sizes, and of the anchoring site (bridge, hollow, on-top). The data obtained by the theoretical calculations are compared with both experimental and computational data of literature. Concerning the work function change due to the azo-derivative photoisomerization, the results are in agreement with the experimental data, and are qualitatively robust with respect to the structure of the SAM

Exciton Transfer of Azobenzene Derivatives in Self-Assembled Monolayers

Diphenyl-diazene and its derivative bis­[(1,1′)-biphenyl-4-yl]­diazene were found to have innovative technological applications when arranged in self-assembled monolayers (SAMs). This is due to their switching capability after photoisomerization that is preserved also when they are in a close-packed assembly over the metal surface forming SAMs. One of the possible phenomena that may hinder the photoisomerization process is the intermolecular excitonic transfer. Understanding this possibility is therefore of the utmost importance. For doing so, we tackled a quantum mechanical (QM) study that begins from the exploration of the electronic excited state properties of a single molecule, to the intermolecular exciton couplings computed at different theory levels, until the excitonic diffusion dynamics, evaluated both within a frozen SAM portion and as an average along a molecular dynamics (MD) simulation

Structural Properties of Azobenzene Self-Assembled Monolayers by Atomistic Simulations

Azobenzene self-assembled monolayers (SAMs) are examples of optomechanical nanostructures capable of producing mechanical work through the well-known azobenzene photoisomerization process. Experimental studies have provided information on their structural properties, but an atomistic description of the SAMs in both the <i>cis</i> and <i>trans</i> forms is still lacking. In this work, a computational investigation of the SAM structures is conducted by classical molecular dynamics with a dedicated force. Experimental data on the SAM unit cell is used to set up SAM models of different molecular densities. The optimal structures are identified through the comparison with structural data from X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies. The resulting SAM atomistic models are validated by comparing simulated and experimental scanning tunneling microscopy images

Niobium and Tantalum Halocyanide Clusters: The Complete Family

Synthetic procedures providing straightforward access to the whole family of Nb and Ta halide clusters with terminal cyanide ligands have been developed. Corresponding [M6X12(CN)12]4– (M = Nb, Ta; X = Cl, Br) can be accessed by ligand-exchange procedures from K4Nb6X18 (X = Cl, Br) and Bu4NCN, (Et4N)2[Ta6Cl18] and Bu4NCN and from [Ta6Br12(H2O)4Br2]·4H2O and KCN in moderate to high yields (50–80%). The products were isolated as Bu4N salts. The compounds were investigated both experimentally and by quantum chemistry, revealing correlations between structural, electrochemical, electrostatic, electronic, and topological features as a function of type of metal, halide, and charge

Way to Highly Emissive Materials: Increase of Rigidity by Introduction of a Furan Moiety in Co-Oligomers

Rigid linear organic co-oligomers are prospective materials for organic optoelectronics. In this work, we explored intramolecular factors affecting the torsional rigidity and its influence on optoelectronic properties of the alternating furan/phenylene and thiophene/phenylene co-oligomers in both ground and first singlet excited states. A furan/phenylene co-oligomer exhibits almost twice as high torsional rigidity than its thiophene analogue. The effect of intramolecular O···H and S···H interactions on torsional barriers was found to be negligible as compared with the conjugation efficiency. The higher torsional rigidity of furan and thiophene co-oligomers has been proven to be reflected in the fine structure of the UV–vis absorption spectrum of the former. The increase of furan co-oligomer rigidity as compared with its thiophene analogue lowers reorganization energy for hole, electron, and exciton transfer. Remarkably the substitution of thiophene by furan lowers by almost 20 times the reorganization energy for exciton transfer. A noteworthy finding was also that in furan co-oligomer the higher rigidity was suggested to hinder “in molecule” photoluminescence quenching due to a possible conical intersection between bright state S₁ and the T₃ excited state. Therefore, tuning of torsional rigidity impacts emission and charge transport properties, being a very powerful tool on the way to high performance emissive organic semiconductors

Synthesis and Fluorescent Behaviour of 2‑Aryl-4,5-dihydro‑1<i>H</i>‑1,2,4-triazoles

A series of new 4,5-dihydro-1<i>H</i>-1,2,4-triazoles was synthesized from amidrazones and acetylenedicarboxylic acid esters in the presence of pyridine in toluene. The synthesized compounds were characterized by ¹H, ¹³C NMR, FT-IR spectral analyses and XRD data. Optical studies revealed that most of the compounds reported here exhibited emission of blue or green-yellow light upon irradiation in acetone and showed Stokes shifts in the region of 70–96 nm and quantum yields of up to 45%. The interpretation of the experimental findings was supported by state-of-the-art quantum mechanical calculations

Acid–Base Strength and Acidochromism of Some Dimethylamino–Azinium Iodides. An Integrated Experimental and Theoretical Study

The effects of pH on the spectral properties of stilbazolium salts bearing dimethylamino substituents, namely, trans isomers of the iodides of the dipolar <i>E</i>-[2-(4-dimethylamino)­styryl]-1-methylpyridinium, its branched quadrupolar analogue <i>E</i>,<i>E</i>-[2,6-di-(<i>p</i>-dimethylamino)­styryl]-1-methylpyridinium, and three analogues, chosen to investigate the effects of the stronger quinolinium acceptor, the longer butadiene π bridge, or both, were investigated through a joint experimental and computational approach. A noticeable acidochromism of the absorption spectra (interesting for applications) was observed, with the basic and protonated species giving intensely colored and transparent solutions, respectively. The acid–base equilibrium constants for the protonation of the dimethylamino group in the ground state (p<i>K</i>_a) were experimentally derived. Theoretical calculations according to the thermodynamic Born–Haber cycle provided p<i>K</i>_a values in good agreement with the experimental values. The very low fluorescence yield did not allow a direct investigation of the changes in the acid–base properties in the excited state (p<i>K</i>_a^*) by fluorimetric titrations. Their values were derived by quantum-mechanical calculations and estimated experimentally on the basis of the Förster cycle

Presence of Two Emissive Minima in the Lowest Excited State of a Push–Pull Cationic Dye Unequivocally Proved by Femtosecond Up-Conversion Spectroscopy and Vibronic Quantum-Mechanical Computations

The long-standing controversy about the presence of two different emissive minima in the lowest excited state of the cationic push–pull dye <i>o</i>-(<i>p</i>-dimethylamino-styryl)-methylpyridinium (DASPMI) was definitively proved through the observation of dual emission, evidenced by both experimental (femtosecond up-conversion measurements) and theoretical (density functional theory calculations) approaches. From the fluorescence up-conversion data of DASPMI in water, the time resolved area normalized spectra (TRANES) were calculated, showing one isoemissive point and therefore revealing the presence of two distinct emissive minima of the excited state potential energy hypersurface with lifetimes of 0.51 and 4.8 ps. These spectroscopic techniques combined with proper data analysis allowed us to discriminate the sub-picosecond emitting state from the occurrence of ultrafast solvation dynamics and to disentangle the overlapping fluorescence (very close in energy) of the two components. Vibronic computations based on TD-DFT potential energy surfaces fully confirm those results and provide deeper insights about the key factors playing a role in determining the overall result. The two emissive minima have different structural and electronic characteristics: on one hand, the locally excited (LE) minimum has a flat geometry and an electric dipole moment smaller than the ground state; on the other hand, the twisted-intramolecular-charge-transfer (TICT) minimum shows a rotation of the methylpyridinium moiety with respect to the rest of the structure, and has an electric dipole moment significantly larger than the ground state

X‑ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with <i>N</i>,<i>N</i>-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes