Photophysics and photochemistry of trans-4-nitrostilbenes and trans-2,4-dinitrostilbenes: Effect of intramolecular charge transfer

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.

Adsorption of 4-(N,N-Dimethylamino)-4'-nitrostilbene on an Amorphous Silica Glass Surface

Stilbenes are a compelling class of organic photoswitches with a high degree of tunability that sensitively depend on their environment. In this study, we investigate the adsorption properties of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS), a push-pull stilbene, on amorphous silica glass. Plane-wave density functional theory (DFT) calculations are used to understand how the trans and cis isomers of DANS interact with the amorphous surface and which are the most preferred modes of adsorption. Our calculations revealed that the O-H···O hydrogen bonds between the nitro group and hydroxyl groups of the silica surface dominate the intramolecular interaction. In addition to hydrogen bonding, O-H···π interactions with the aromatic ring and double bond play a critical role in adsorption, whereas C-H···O interactions are present, but contribute little. Therefore, both isomers of DANS favor parallel orientations such that not only the functional groups but also the aromatic parts can strongly interact with the glass surface.

Green spectrofluorimetric assay of dantrolene sodium via reduction method: application to content uniformity testing

Green analysis has turned out to be of a great value in all areas, including pharmaceutical analysis. Thus, it is extremely important to consider the environmental influence of each step in developing any analysis technique. The present work illustrates a validated, simple and green spectrofluorimetric method for the analysis of dantrolene sodium (DAN). The developed process is characterized by being of high sensitivity as well as being relatively inexpensive. The suggested technique based on the formation of a highly fluorescent product of DAN via its reduction by the aid of Zn/HCl system. The resulting fluorophore showed a powerful fluorescence at λ _em 344 nm after excitation at λ _ex 279 nm. Calibration graph revealed a great linear regression (r = 0.9998) within concentration ranging from 0.05 to 2.0 µg ml^-1. The suggested method had very low detection and quantification limits of 0.010 and 0.031 µg ml^-1, respectively. The applied technique was effectively used in the determination of DAN in its pharmaceutical preparations. The results were compared with those from using the official United States Pharmacopeia (USP) method and they were in a good agreement. Moreover, content uniformity testing of DAN in capsules was performed adopting the investigated technique with satisfying results. The greenness of the suggested technique was confirmed by the three standard assessment tools. Therefore, the developed technique can be used in the routine quality control analysis of DAN with minimum harmful impact on nature or individuals.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

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 α_C=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.

Silyl-based initiators for two-photon polymerization: from facile synthesis to quantitative structure–activity relationship analysis

Silyl-based stilbene derivatives were straightforwardly prepared and successfully applied in two photon lithography as photoinitiators.

Unusual high fluorescence of two nitro-distyrylbenzene-like compounds induced by CT processes affecting the fluorescence/intersystem-crossing competition

Fluorescence/intersystem crossing/photoisomerization competition in the relaxation of nitro-substituted distyrylbenzene-like compounds: charge transfer induced, unusually high, fluorescence yield in polar solvents.

N,N-Dihexyl-4-[2-(4-nitro-phen-yl)vin-yl]aniline

The title compound, C(26)H(36)N(2)O(2), was prepared by Horner olefination of p-dihexyl-amino-benzaldehyde and diethyl p-nitro-benzyl-phospho-nate. It crystallizes with two independent mol-ecules in the asymmetric unit. Both have similar geometries of the π-systems but the conformations of all hexyl chains are different. Whereas one hexyl chain of the first mol-ecule shows the typical all-anti conformation, the second is arranged in a gauche-anti-gauche-anti conformation with N-C-C-C, C-C-C-C, C-C-C-C and C-C-C-C torsion angles of -65.1 (4), 167.3 (3), 63.3 (4), and 179.4 (3)°. One of the hexyl chains in the other mol-ecule has an anti-anti-gauche-anti conformation [N-C-C-C, C-C-C-C, C-C-C-C and C-C-C-C torsion angles = 179.6 (3), -179.8 (3), -68.7 (5) and -178.8 (4)°], the other starts with an anti-gauche-gauche sequence. Molecules A and B are composed of five planar subunits. The angle sums around the N atoms are in the range 356 (2)-360.0 (2)°. Torsion angles between these segments do not exceed 4.9 (4)°, except for one of the alkyl chains each [molecule A = 26.2 (4)°; molecule B = -6.0 (4)°]. The high planarity of the molecules and the short aniline C-N bonds [1.385 (3) Å in molecule A and 1.378 (3) Å in molecule B] indicate a strong electronic coupling through the stilbene unit. One methylene group is disordered over two positions with an occupancy ratio of 0.72:0.28.

Fluorescence Spectroscopic Properties and Crystal Structure of a Series of Donor−Acceptor Diphenylpolyenes

A series of p-nitro-p'-alkoxy(OR)-substituted (E,E,E)-1,6-diphenyl-1,3,5-hexatrienes (1a, R = Me; 1b, R = Et; 1c, R = n-Pr; 1d, R = n-Bu) were prepared. The absorption and fluorescence spectra in solution were almost independent of the alkoxy chain length. The absorption maximum showed only a small dependence on the solvent polarity, whereas the fluorescence maximum red-shifted largely as the polarity increased. The solid-state absorption and fluorescence spectra were red-shifted relative to those in low polar solvents and were clearly dependent on the alkoxy chain length. The fluorescence maxima for the crystals of 1b and 1d were observed at 635-650 nm, which were red-shifted by 40-50 nm relative to those for 1a and 1c. The Stokes shifts were all relatively small (3000-3500 cm-1). For all four compounds, the fluorescence decay curves in the solid state were able to be analyzed by single-exponential fitting to give the lifetimes of 1.1-1.3 ns. This indicates that the emission of 1a-d is not originated from an excimer or molecular aggregates, but from only one emitting monomeric species. The fluorescence quantum yields of 1a-d were considerably high compared with the values for organic solids, which is consistent with their monomeric origin of emission. Single-crystal X-ray structure analyses of 1a, 1c, and 1d showed that the crystal packing was dependent on the alkoxy chain length. The crystals of 1a and 1c had herringbone structure, whereas that of 1d had pi-stacked structure. Strong pi-pi interaction in the crystal of 1d would be the cause of the spectral red shifts relative to those for 1a and 1c. No observation of excimer fluorescence from crystal 1d can be attributed to the limited overlap between the pi-planes of the molecules due to its "slipped-parallel" structure.

Solvatochromic Behavior of Donor−Acceptor-Polyenes: Dimethylamino-cyano-diphenylbutadiene

4-(Dimethylamino)-4'-cyano-1,4-diphenylbutadiene (DCB) and 4-(dimethylamino)-2,6-dimethyl-4'-cyano-1,4-diphenylbutadiene (DMDCB) have been characterized spectroscopically. Quantum chemical calculations were performed for comparison. Solvatochromic shifts of the fluorescence were strong and showed a linear dependence on the solvent polarity parameters, whereas shifts in the absorption spectra are very weak only correlate better with the polarizability of the solvents. Excited state dipole moments derived from fluorescence using the Onsager model are very large and similar for both compounds. It is concluded that a strongly allowed and highly dipolar pi, pi* state is the lowest excited state in polar solvents. The strong difference in absorption and fluorescence solvatochromic slopes suggests that the simple Onsager model with a point dipole approximation is not sufficient here.