We developed a simple methodology for the preparation of stable meso-(nitrile oxide)-substituted BODI-PYs, which were characterized by spectroscopic methods and X-ray crystallography. These compounds were used for the preparation of isoxazoline− or isoxazolyl−BODIPYs by 1,3dipolar cycloaddition reaction with dipolarophiles. Several BODIPYs possess molecular rotor behavior, including viscosity-dependent fluorescence. Transient absorption spectroscopy and time-resolved fluorescence are indicative of a 3 orders of magnitude difference in the excited-state lifetime for dichloromethane and glycerol solutions.
A series of 1,7-dipyrene-aza-BODIPY and 1,7-dipyrene-3,5-diferrocene-aza-BODIPY derivatives 5a–e with pyrene ligands covalently attached to the β-pyrrolic positions of the boron-azadipyrromethene (aza-BODIPY) core have been prepared and characterized by NMR, UV–vis, and steady-state fluorescence spectroscopy; high-resolution mass spectrometry; and X-ray crystallography. The redox processes of these donor–acceptor aza-BODIPY systems were investigated by electrochemistry (cyclic voltammetry and differential pulse voltammetry methods) and spectroelectrochemistry. The potential of the closely spaced 1,7-dipyrene fragments to promote formation of noncovalent π-complexes with nanocarbon materials (C60, C70, (6,5)-single-walled carbon nanotube, and graphene) was explored. UV–vis, steady-state fluorescent, and time-resolved transient absorption spectroscopy data indicated that the interaction between the new pyrene-aza-BODIPYs and C60 or C70 fullerenes in solution is weak, and time-resolved transient absorption spectroscopy provided no evidence of photoinduced electron transfer. X-ray crystallography on a binary solid of aza-BODIPYs 5b and C60 was indicative of a pyrene–pyrene rather than pyrene–C60 interaction motif, whereas fullerenes were found to form close contacts with the electron-rich B,O-chelated part of aza-BODIPY 5b. In contrast, pyrene–pyrene and pyrene–C60 but not aza-BODIPY–C60 interactions were observed in the crystal structure of aza-BODIPY 5d and C60. Density functional theory (DFT) and time-dependent DFT calculations were used to support conclusions based on experimental data and are suggestive of rather weak interaction energies between 1,7-dipyrene-aza-BODIPYs and nanocarbon materials. Direct comparison with an analogous control compound lacking the pyrene ligands demonstrated that the pyrene substituents were ineffective at promoting and directing complex formation with nanocarbon materials. A common measurement of complex formation, emission loss in the presence of a nanocarbon acceptor, was demonstrated to have an alternative explanation in these systems, and the general effectiveness of using pyrene ligands to build noncovalent complexes was drawn into question.
As eries of covalent ferrocene-BODIPY-fullerene triads with the ferrocene groups conjugated to the BODIPY p-systema nd the fullerene acceptorl inked at the boron hub by ac ommon catecholpyrrolidine bridge were prepared and characterizedb y1 Da nd 2D NMR, UV/Vis, steady-state fluorescence spectroscopy, high-resolution mass spectrometry, and, for one of the derivatives, X-ray crystallography.R edox processes of the new compounds were investigated by electrochemical (CV and DPV) methods and spectroelectrochemistry.D FT calculations indicate that the HOMOin all triads was delocalized between ferrocene and BODIPY p-system, the LUMO was always fullerene-centered, and the catecholcentered occupied orbitalw as close in energy to the HOMO. TDDFT calculations were indicative of the low-energy,l ow-in-tensity charge-transfer bands originated from the ferrocene-BODIPY core to fullerene excitation, which explainedt he similarity of the UV/Vis spectra of the ferrocene-BODIPY dyadsa nd ferrocene-BODIPY-fullerene triads. Photophysical properties of the new triadsa sw ell as reference BODIPYfullerenea nd ferrocene-BODIPY dyads were investigated by pump-probe spectroscopy in the UV/Vis and NIR spectralr egions following selective excitation of the BODIPY-based antenna.I nitial charget ransfer from the ferrocenet ot he BODIPY core was shown to outcompetes ub-100 fs deactivation of the excited state mediated by the catecholb ridge. However, no subsequente lectron transfer to the fullerene acceptorw as observed. The initial charge separated state relaxes by recombination with at ime constanto f1 50-380 ps.[a] Dr.We attempted to obtain single crystals of triads 3a-d.I nt he majority of cases, crystals were too small for X-ray crystallographic measurements, with typical reflection datal imited to Scheme1.Synthetic pathway for preparation of the ferrocene-BODIPY-fullerene triads 3a-d.
The excited-state dynamics and energetics of a series of BODIPY-derived chromophores bound to a catechol at the boron position were investigated with a combination of static and time-resolved spectroscopy, electrochemistry, and density functional theory calculations. Compared with the difluoro-BODIPY-derived parent compounds, the addition of the catechol at the boron reduced the excited-state lifetime by three orders of magnitude. Deactivation of the excited state proceeded through an intermediate charge-transfer state accessed from the initial optically excited π* state in <1 ps. Despite differences in the structures of the BODIPY derivatives and absorption maxima that spanned the visible portion of the spectrum, all compounds exhibited the same, rapid, excited-state deactivation mechanism, suggesting the generality of the observed dynamics within this class of compounds.
Monodispered ZnO nanocrystals (NCs) were found to quench the fluorescence of two donor−π system−acceptor (D−π−A) dyes; (E)-2-cyano-3-[5- [4-(diethylamino)phenyl]thiophen-2-yl]-2-propenoic acid, 1, and its furan analogue, 2. Parameters based on single crystal X-ray crystallography and DFT calculations confirmed planar structures for both dyes and delocalization of the donor nitrogen electrons into the πsystem. Both dyes exhibited a quasi-reversible one-electron oxidation with E o values of 0.39 and 0.35 V for 1 and 2, respectively, versus the ferrocene/ferrocenium redox couple, and spectroelectrochemical measurements revealed the absorption spectra of the oxidized products. In stark contrast to earlier reports of related dyes that did not bear a strong donor substituent, the ZnO nanocrystal fluorescence quenching efficiency was nearly quantitative. Adsorption isotherms revealed equilibrium binding constants 2.5(0.5) × 10 5 and 8(1) × 10 5 M −1 for 1 and 2, respectively, and large values for the maximum number of dyes per nanocrystal. Ultrafast fast pump−probe measurements of 1 and 2 in CH 2 Cl 2 revealed formation of singlet excited states that decayed with lifetimes of 1660(30) and 1600(100) ps, respectively. Addition of an equimolar amount of ZnO NCs caused the singlet excited state of each dye to disappear with concurrent formation of the spectral signatures for the corresponding oxidized products, thus allowing the assignment of the process to an excited state electron transfer to the ZnO NCs. Electron transfer lifetimes for 1 ranged from 14.8(4) to 18.2(6) ps as the ZnO NC diameter decreased from 5.0 to 3.2 nm, while for 2 the lifetimes ranged from 11.1(3) to 9.5(3) ps for a similar change in ZnO NC diameters. The weak dependence of the excited state electron transfer lifetimes on the diameter of the NCs is consistent with a reaction dominated by changes in their sizedependent density of states.
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