Combined Spectroelectrochemical and Theoretical Study of Electron-Rich Dendritic 2,5-Diaminothiophene Derivatives: <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′‑Tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine <b>11</b> is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene <b>4</b>_<b>1</b> as well as to the corresponding high-molar third dendrimer generation <b>8</b> containing the same thiophene-2,5-diamine core. The dendritic compound <b>11</b> can be reversibly oxidized in three separated steps to yield the corresponding stable monocation <b>11</b>^<b>•+</b>, dication <b>11</b>^<b>2+</b>, and tetracation <b>11</b>^<b>4+</b>. A well resolved ESR spectrum of the corresponding cation radical <b>11</b>^<b>•+</b> with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to <b>4</b>_<b>1</b>^<b>•+</b>. The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds <b>4</b>_<b>1</b>^<b>•+</b>, <b>11</b>^<b>•+</b>, and <b>8</b>^<b>•+</b> was confirmed. The computed single occupied molecular orbital (SOMO) for trication <b>11</b>^<b>•3+</b> is completely different compared to the SOMO of the corresponding monocation <b>11</b>^<b>•+</b>, and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation <b>11</b>^<b>4+</b>. The estimated theoretical limit energy of the lowest optical transition S₀ → S₁ is 2.90 eV, and it can be achieved for the 3D dendrimer generation

System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra

In this work, we examine vibrational coherence in a molecular monomer, where time evolution of a nuclear wavepacket gives rise to oscillating diagonal- and off-diagonal peaks in two-dimensional electronic spectra. We find that the peaks oscillate out-of-phase, resulting in a cancellation in the corresponding pump–probe spectra. Our results confirm the unique disposition of two-dimensional electronic spectroscopy (2D-ES) for the study of coherences. The oscillation pattern is in excellent agreement with the diagrammatic analysis of the third-order nonlinear response. We show how 2D-ES can be used to distinguish between ground- and excited-state wavepackets. On the basis of our results, we discuss coherences in coupled molecular aggregates involving both electronic and nuclear degrees of freedom. We conclude that a general distinguishing criterion based on the experimental data alone cannot be devised

Stable Radical Trianions from Reversibly Formed Sigma-Dimers of Selenadiazoloquinolones Studied by In Situ EPR/UV–vis Spectroelectrochemistry and Quantum Chemical Calculations

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>h</i>]­quinolines and 8-substituted 9-oxo-6,9-dihydro­[1,2,5]­selenadiazolo­[3,4-<i>f</i>]­quinolines with R₇, R₈ = H, COOC₂H₅, COOCH₃, COOH, COCH₃, and CN has been studied by in situ EPR and EPR/UV–vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from <i>N</i>-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV–vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations