Photorelaxation Pathways of 4-(N,N-Dimethylamino)-4′-nitrostilbene Upon S1 Excitation Revealed by Conical Intersection and Intersystem Crossing Networks

Multi-state n-electron valence state second order perturbation theory (MS-NEVPT2) was utilized to reveal the photorelaxation pathways of 4-(N,N-dimethylamino)-4′-nitrostilbene (DANS) upon S1 excitation. Within the interwoven networks of five S1/S0 and three T2/T1 conical intersections (CIs), and three S1/T2, one S1/T1 and one S0/T1 intersystem crossings (ISCs), those competing nonadiabatic decay pathways play different roles in trans-to-cis and cis-to-trans processes, respectively. After being excited to the Franck–Condon (FC) region of the S1 state, trans-S1-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, trans-S1-FC → ISC-S1/T2-trans → CI-T2/T1-trans → ISC-S0/T1-twist → trans- or cis-S0. The singlet relaxation pathway mediated by CI-S1/S0-twist-c is hindered by the prominent energy barrier on S1 surface and by the reason that CI-S1/S0-trans and CI-S1/S0-twist-t are both not energetically accessible upon S1 excitation. On the other hand, the cis-S1-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S1/S0-twist-c and CI-S1/S0-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between αC=C twisting (cis-S1-FC → CI-S1/S0-twist-c → trans- or cis-S0) and DHP formation relaxation pathways (cis-S1-FC → CI-S1/S0-DHP → DHP-S0) on the S1 surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, cis-S1-FC → ISC-S1/T1-cis → DHP-T1 → DHP-S0, however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for cis-S1-FC mediated by ISC-S1/T2-cis are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S1 → T2 → T1 or S1 → T2 → S1 processes. The present study reveals photoisomerization dynamic pathways via conical intersection and intersystem crossing networks and provides nice physical insight into experimental investigation of DANS.

Crystal structure of bis-p-anizidinegossypol with an unknown solvate

The title compound, C44H44N2O8, (systematic name: 1,1′,6,6′-tetrahydroxy-5,5′-diisopropyl-8,8′-bis{[(4-methoxyphenyl)iminiumyl]methyl}-3,3′-dimethyl-2,2′-binaphthalene-7,7′-diolate) has been obtained by the addition ofp-anizidine to gossypol dissolved in dichloromethane. In the solid state, the title compound exists in the enamine or quinoid form. The two naphthyl moieties are inclined to one another by 72.08 (5)°. The pendant phenyl rings are inclined at 22.26 (14) and 23.86 (13)° to the corresponding naphthyl rings. In the crystal, molecules are incorporated into layers through inversion-related pairs of O—H...O interactions [graph setsR22(20) andR22(10)] and translation-related O—H...O interactions [graph setC(15)]. The packing of these layers in the crystal structure gives rise to channels in the [011] direction, with hydrophobic interactions occurring between adjacent layers. The channels are 5–7 Å wide, and the void volume of each cell is 655 Å3, corresponding to 26.6% of the cell volume. Disordered guest molecules, probably solvent and water molecules, occupy these voids of the crystal; their contribution to the scattering was removed with the SQUEEZE routine [Spek (2015).Acta Cryst. C71, 9–18] ofPLATON[Spek (2009).Acta Cryst.D65, 148–155].

Optical properties of heat-treated polyparaphenylene

The optical properties of heat-treated polyparaphenylene (PPP) were investigated by means of Raman and photoluminescence (PL) spectroscopy. Special attention is given to PPP heat-treated to temperatures (THT) near the carbonizing temperature region (THT ≈ 700°C) since polymer-based carbonaceous compounds with low-THT (<1000 °C) have been found to exhibit electrochemical properties that strongly contrast both the as-prepared polymer and fully carbonized samples. The Raman spectra show that for THT in the range 650–725°C, several Raman bands near 1300 cm−1 can be correlated with both ground-state benzenoid and excited-state quinoid PPP Ag modes. An increase in quinoid character is observed with increasing THT, which is consistent with the theoretically predicted stabilization of the quinoid form in the presence of a high density of defects. The smaller energy bandgap for π – π* transitions in the quinoid conformation relative to that for the benzenoid form allows for a resonance condition to be present for laser excitation wavelengths (λexc) near the visible (∼1–2 eV). We also report a small dispersion effect in the observed quinoid breathing mode band which can be compared to dispersion effects previously reported for the case of trans-PA. The decrease in bandgap for the defect-induced quinoid form is also evidenced in the PL spectra of samples heat-treated up to 650°C, which show vibronic structure in the blue-green emission data in the energy range 2.4–3.0 eV, with well-resolved peaks separated by quinoid phonon energies of 0.165 eV. Franck–Condon analysis shows an increase in the Huang–Rhys parameter (S) with increasing THT which can be related to changes in the electron-phonon coupling of valence and conduction band states.

Photochemistry Channels of Merocyanine Encapsulated in Sol-Gel Glasses

ABSTRACTPhotophysical properties of l-Docosyl-4-(4-hydroxystyryl)pyridiniurn bromide (SB), a merocyanine dye in solution and encapsulated in sol-gel derived glass are investigated at 298 and 77 K. In solution, the absorption spectra of SB display an equilibrium between the quinolinium and benzoid forms. The equilibrium can be shifted to either quinolinium or benzoid form under an acidic or basic condition, respectively. The emission spectra of SB, on the other hand, give not only the quinolinium and benzoid forms but also the quinoid form which emits at 500 nm. The existence of excited state quinoid form of SB is also evident in the excitation spectrum while the emission at 500 nm is monitored. Both in solution and in xerogel, the quinoid form of SB is shown to be photochemically unstable as compared to the benzoid form. It is proposed that the photoexcited quinolinium form of SB is a proton dissociative species which transforms readily to become the quinoid form. The results indicate that photochemistry channels of SB are originated from the quinoid form. Moreover, the benzoid form of SB (photochemically stable) exhibits large hyperpolarizability due to its charge-transfer characteristic, and is a desired molecular form for nonlinear optical (NLO) applications. The material processing techniques for stabilizing the benzoid form of SB in optically transparent sol-gel glasses are illustrated for the first time.

Crystal and molecular structure of potassium p-nitrophenolate monohydrate: Substituent effect on geometry of the ring in p-substituted nitrobenzene derivatives

The crystal and molecular structure of potassium p-nitrophenolate monohydrate has been determined by X-ray diffraction methods giving R = 0.050. The through resonance effect between -O- and -NO2 groups resulted in a significant deformation of the ring geometry. Application of the HOSE model yielded in determination of the resonance structure contributions and offered an argument against the classical view of the through resonance effect: percentage contribution of quinoid form with full charge transfer from -O- to -NO2 was found to be the least significant of all four possible quinoid structures. Contributions of quinoid and benzenoid structures plotted against σ+ or σp constants for 14 p-systems give a reasonable linear dependence. Application of the additive scheme of substituent effect to the same series of compounds proved a strong reasonance effect from electron donating substituents and a much weaker one (and constant in the whole series) from the nitro group.

In-Situ Derivative Cyclic Voltabsorptometric Studies On Poly-3-Methylthiophene

ABSTRACTSpectroscopic behavior of poly-3-methylthiophene (P3MT) has been studied employing derivative cyclic volt-absorptometric (DCVA) techniques. In the DCVA technique, the derivative absorption signal (dA/dt) is recorded as a function of the applied potential. The dA/dt signals, the spectroscopic analog of electrochemical currents in cyclic voltammetry, are capable of monitoring the potential dependency for the absorption band effectively discriminating against nonfaradaic signals. The DCVA studies on the P3MT system show that the neutral form of P3MT, absorbing at 490 nm (at less than 0.3 V vs. Ag), changes to the radical cation form, which absorbs at 760 nm. Initially, the formation of the radical cation goes through an isosbestic point, indicating that the conversion of the neutral to radical (polaron) form is chemically reversible. However, upon increasing the electrode potential, the rate of the radical formation at 760 nm starts to decrease, with the formation of another band at about 1250 nm, attributable to a quinoid (bipolaron) form. This trend begins above about 0.6 V, shifting to a more positive voltage as the thickness of the film grows. This observation indicates that the electrochemical conversion of the neutral to radical form, followed by the quinoid form, is a slow process controlled by the diffusion of counter ions through the film. In-situ conductivity measurements as a function of applied potentials support the observed spectroscopic behavior.