Studies of Functional Boron Dipyrromethene Derivatives

Boron dipyrromethenes (BODIPYs) are versatile functional materials for a wide range of applications. This research work aims to explore the potential of these compounds and their aza analogues as fluorescent probes for heavy metal ions, nonlinear optical materials, and building blocks of artificial photosynthetic models. This thesis describes the molecular design, synthesis, spectroscopic characterization, and photophysical properties of several series of BODIPYs and aza-BODIPYs, as well as their potential applications in these areas.;Chapter 1 gives a brief overview of BODIPYs, focusing on their synthesis, spectroscopic properties, and applications as fluorescent probes for heavy metal ions and nonlinear optical materials. It then reviews the historical development, syntheses, and spectroscopic properties of their aza analogues. The potential applications of aza-BODIPYs in biomedicine and materials science are also discussed at the end of this chapter.;In Chapter 2, a highly selective colorimetric and near-infrared fluorescent probe for Cu2+ and Hg2+ ions is reported, which is based on a distyryl BODIPY with two bis(1,2,3-triazole)amino receptors. The compound selectively binds to Cu2+ and Hg 2+ ions in CH3CN/H2O (1:1 v/v) giving remarkably blue-shifted electronic absorption and fluorescence bands as a result of inhibition of the intramolecular charge transfer (ICT) process upon binding. The color changes can be easily seen by the naked eye. The binding stoichiometry between this probe and Cu2+ ion has been determined to be 1:2 by a Job's plot of the fluorescence data with a binding constant of (6.2 +/- 0.6) x 109 M-2. The corresponding value for Hg2+ ion is about six-fold smaller.;In addition to the ICT mechanism, photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET) are another two useful mechanisms for design of fluorescent probes. Chapter 3 reports two highly selective and sensitive BODIPY-based fluorescent probes for Hg2+ ion based on PET and FRET mechanisms. Their binding properties have been investigated by using electronic absorption and steady-state fluorescence spectroscopic methods. The probe with a bis(1,2,3-triazole)amino receptor at the meso position is highly responsive toward Hg2+ ion. By introducing two rhodamine B moieties to the BODIPY core, FRET occurs from the excited BODIPY to rhodamine B in a highly effective manner upon binding to Hg2+ ion, regarding to remarkable spectral changes.;Chapter 4 presents a series of BODIPY and aza-BODIPY derivatives bearing a donor-pi-donor or push-pull structure as nonlinear optical materials. The donor-pi-donor derivatives have been prepared by connecting 4-(diphenylamino)phenylethynyl or 4-(dimethylamino)phenylethynyl moieties to the 2- and 6-positions of the pi systems through Sonogashira coupling reactions. The linear optical absorption and fluorescence properties of these compounds have been found to be solvent-dependent. Their two-photon absorption properties have also been measured in toluene by two-photon fluorescence excitation method. For the push-pull series, an electron-donating 4-(dimethylamino)phenylethynyl group and an electron-withdrawing 4-nitrophenylethynyl group have been added to the 2- and 6-positions of the BODIPY core. The spectroscopic and electrochemical properties of these compounds have been studied. Their second-order nonlinear optical properties have also been examined by electric-field-induced second-harmonic generation method in CHCl3. The values of the dot product mu·beta are in the range from 94 x 10-48 to 330 x 10 -48 esu at 1907 nm, depending on the substituents at the 3- and 5-positions.;Chapter 5 describes the synthesis and characterization of an artificial photosynthetic model in which an aza-BODIPY core is covalently linked to a monostyryl BODIPY component and a fullerene (C60) unit. The photoinduced intramolecular processes of this triad and the model compounds have been studied in detail by steady-state and time-resolved spectroscopic methods. Upon excitation at the monostyryl BODIPY moiety, excitation energy transfer occurs to the aza-BODIPY core, which is followed by an electron transfer to the C60 unit. From the femtosecond transient absorption studies, the rate constant of charge recombination has been determined to be 7.00 x 108 s-1 in benzonitrile, giving a lifetime of 1.47 ns for the charge-separated state.;As an extension, Chapter 6 presents related supramolecular systems in which an aza-BODIPY derivative bearing two or four permethylated beta-cyclodextrin moieties binds to metal-free and zinc(II) tetrasulfonated porphyrins, as well as two cationic monostyryl BODIPYs in water. The complexation of these components has been studied by various spectroscopic methods. The resulting host-guest complexes exhibit efficient energy and/or electron transfer depending on the nature of the guests. A novel mixed array of metal-free porphyrin, aza-BODIPY, and monostyryl BODIPY has also been assembled. Upon excitation at the porphyrin unit, singlet-singlet energy transfer occurs to the aza-BODIPY core, which then obtains an electron from the monostyryl BODIPY moieties. This supramolecular hetero-array thus also serves as an artificial photosynthetic model.;Chaper 7 gives the experimental details for the work described in the preceeding chapters. All the references cited herein are given in Chapter 8.;1H and 13C{1H} NMR spectra of all the new compounds are given in the Appendix.