Synthesis and Photophysical Properties of Borondipyrromethene Dyes Bearing Aryl Substituents at the Boron Center

Abstract

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.

Electronic Energy Transfer to the S2 Level of the Acceptor in Functionalised Boron Dipyrromethene Dyes

“…With the donor connected through the B atom, which appears to be a poor electronic conductor, the rate of EET depends markedly on the degree of spectral overlap between donor and acceptor. A few such cases have been reported [19] and the transfer mechanism involves both Coulombic and exchange interactions. Strong indications were presented to the effect that the S 2 level on the Bodipy-based acceptor played a key role in the overall process, [19] but the spectral window between S 1 and S 2 levels is modest for conventional Bodipy dyes.…”

“…A few such cases have been reported [19] and the transfer mechanism involves both Coulombic and exchange interactions. Strong indications were presented to the effect that the S 2 level on the Bodipy-based acceptor played a key role in the overall process, [19] but the spectral window between S 1 and S 2 levels is modest for conventional Bodipy dyes. Consequently, attention has turned to expanded Bodipy derivatives in which the S 1 level is pushed towards lower energy, [20] while the S 2 level covers a wider spectral range and possesses a substantially enhanced oscillator strength.…”

“…The same spectral profile is seen for GF, thereby confirming that appendages added at the boron centre exert no real influence on the absorption spectrum. [19] Fluorescence from the GF unit in GPyPeF occurs readily and has a peak maximum at 752 nm. Thus, the Stokes shift (SS = 350 cm…”

Original Design of Fluorescent Ligands by Fusing BODIPY and Melatonin Neurohormone

“…[17][18][19]27,28 Such a π extension can be performed by introducing an aryl or a hetaryl group at the 3 and/or 5 positions of the pyrrole moiety using metal catalyzed reactions, 29−32 the Knoevenagel reaction, 33,34 vicarious nucleophilic substitution (VNS), 35 or by modifying the substituents linked to the boron atom. 36 For our purpose, aryl and styryl groups were envisaged and pyrroles 3 and 4 were first synthesized. Condensation with 2-formylmelatonin according to the previous protocol finally afforded the desired boron complexes III and IV with extended π-delocalization in 35 and 37% yield, respectively (Scheme 2).…”

Design and Synthesis of Dipyrrin Complexes Bearing Unique Structures, Properties and Functions

“…15 However, these methods usually gave the desired product in low or moderate yields and laborious puri cation was often required. On the other hand, our method has signi cant advantages: no additives or no catalyst and high yield.…”