Dual-Emissive Difluoroboron Naphthyl-Phenyl β-Diketonate Polylactide Materials: Effects of Heavy Atom Placement and Polymer Molecular Weight

Abstract

Luminescent materials are important for imaging and sensing. Aromatic difluoroboron β-diketonate complexes (BF₂bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. A series of BF₂bdks with naphthyl and phenyl groups was synthesized, and photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Polymer molecular weight and dye attachment site along with bromide heavy atom placement were varied to tune optical properties of dye-PLA materials. Systems without heavy atoms have long phosphorescence lifetimes, which is useful for lifetime-based oxygen sensing. Bromine substitution on the naphthyl ring resulted in intense, clearly distinguishable fluorescence and phosphorescence peaks important for ratiometric oxygen sensing and imaging.

Scheme 1. Representative Synthesis of β-Diketonate Ligand, Boron Initiator, and Polymer Shown for bnmOH (3), BF₂bnmOH (7), and BF₂bnmPLA (11)

Figure 1

GPC trace (A) and kinetics plots (B, C) for lactide polymerization with initiator BF₂n(Br)bmOH 6 (130 °C, 6:lactide:Sn(Oct)₂ = 1:50:1/50). Aliquots were taken at 10, 30, 60, 80, 105, and 190 min as indicated.

Figure 2

Absorption spectra for boron initiators in dilute CH₂Cl₂ solutions (Abs < 0.1).

Figure 3

Boron dye initiators 5–8 in CH₂Cl₂ showing emission colors.

Figure 4

Molecular orbital diagrams for compounds 5–8 showing the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) in CH₂Cl₂.

Figure 5

Comparison of the emission spectra of dye–polymer conjugates 9–12 of different molecular weights.

Figure 6

Images showing emission color changes for polymers 9–12 of different molecular weights under air with UV lamp excitation.

Figure 7

Normalized delayed emission spectra (i.e., phosphorescence + delayed fluorescence) for polymer films 9–12 at room temperature under nitrogen (λ_ex = 369 nm). Phosphorescence maxima range from 538 to 568 nm (∼30 nm range).

Figure 8

Total emission spectra for polymers 9–12 under air and under a nitrogen atmosphere.

Figure 9

(A) Total emission spectra showing relative fluorescence and phosphorescence intensities for films of BF₂n(Br)bmPLA 10a–d of different molecular weights under N₂. (B) Images showing emission (excitation by UV lamp): top, under ambient conditions; bottom, under N₂.