Charge-Transfer Formation and Geometry of the Naphthalene−Trimethylamine van der Waals Complex

Characterization of Push-Pull-Type Benzo[X]quinoline Derivatives (X = g or f): Environmentally Responsive Fluorescent Dyes with Multiple Functions

Benzo[X]quinoline (X = g or f: BQX) derivatives bearing bis-trifluoromethyl and amine groups have been designed as push-pull-type fluorescent dyes. Through the synthesis of BQX derivatives from 2,7-diaminonaphthalene, linear-type (BQL) and angular-type (BQA) structural isomers were obtained. X-ray structures of single crystals from six given BQX derivatives revealed that the BQL and BQA series adopt planar- and bowl-shaped structures. In the fluorescence spectra, interestingly, the BQL series emitted in the near-infrared region over 700 nm in polar solvents. Based on the visible absorptions and base properties related to the amine moiety, the ammonia responsiveness was investigated using an ion-exchange reaction by the BQX-HCl salt. By exploiting the environmentally responsive fluorescence probe, cell imaging through confocal laser microscopy was conducted using HeLa and 3T3-L1 cells, emitting specific lipid droplets. The results indicate that BQX derivatives have multiple functions and may be applied in materials chemistry and biochemistry.

Structure and intermolecular vibrations of perylene·trans-1,2-dichloroethene, a weak charge-transfer complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

Analysis of the effects of protic, aprotic, and multi-component solvents on the fluorescence emission of naphthalene and its exciplex with triethylamine

The emission spectra of naphthalene (NP)-triethylamine (TEA) systems were measured under steady-state illumination conditions in some protic and aprotic solvent-tetrahydrofuran (THF) mixtures. The fluorescence spectrum of the NP-TEA system in THF could be separated into two component bands (band A at 329 nm (fluorescence of NP) and band B at 468 nm (emission from an intermolecular exciplex)). The intensities of bands A and B decreased with increasing solvent polarity. The intensity of band B also decreased owing to the hydrogen-bonding interaction between TEA and protic solvents, but in this case the intensity of band A increased. The decrease in the intensity of band A with increasing solvent polarity is considered to be caused by the enhanced formation of an ion-pair parallel to the formation of an exciplex with increasing solvent polarity. The decrease in the intensity of band B is considered to be caused by the enhanced formation of ion-pair both parallel to and through the formation of the exciplex. The increase in the intensity of band A and the decrease in that of band B upon the addition of protic solvents is caused by the decrease in the concentration of free TEA. Acetonitrile only has a polar effect and trichloroacetic acid only has a hydrogen-bonding (protonation) effect, while alcohols have both the effects.

Solvent-specific photolytic behavior of octachlorodibenzo-p-dioxin

The photolysis of octachlorodibenzo-p-dioxin (OCDD) was investigated in various organic solvents under lambda > 300 nm irradiation. The rates of OCDD photolysis were highly solvent-specific. OCDD was photodegraded rapidly in toluene, n-hexane, CCl4, and 1-octanol, whereas it underwent negligible photodegradation in acetonitrile, acetone, and methanol. Both OCDD photolysis and fluorescence emission did not take place in very polar solvents because polar solvent molecules efficiently quenched the excited OCDD (OCDD*). The addition of acetonitrile to an OCDD solution in toluene rapidly quenched both the fluorescence emission and the photolysis rate, which can be described by Stern-Volmer analysis. The efficient photolysis in nonpolar (or less polar) solvents seems to be mediated through a charge-transfer path where the solvent and OCDD* act as an electron donor and acceptor, respectively. However, OCDD photolysis in CCl4 seems to represent the opposite case in which the solvent is an electron acceptor and OCDD* is an electron donor. Hammett sigma constants that approximately represent the electron-donating power in structurally related aromatic solvents show a good correlation with the photolysis rates. We propose that the solvent specificity in OCDD photolysis is mainly ascribed to the difference in the electron donating (or accepting) tendency among various solvents. When triethylamine that easily donates an electron to form a charge-transfer exciplex with OCDD* was added, a marked enhancement in the photolysis rate was observed.