Controlling Aggregation and Excited-State Intramolecular Proton Transfer in BODIPYs by Incorporation of 2-(2-Hydroxyphenyl)quinazoline and Variation of Substituents

Abstract

A series of BODIPY-based AIEgens (QB1−QB5 and Bis-QB) containing 2-(2-hydroxyphenyl)quinazoline have been synthesized and thoroughly characterized. Photophysical properties of these compounds in solution and the aggregated state have been meticulously investigated and fine-tuned via structural modifications. These display green emission (∼530 nm) in solution and bright red emission (600−655 nm) in the aggregated/solid state with increased quantum yield. Crystal structure analyses and spectral studies revealed efficient J-type aggregation in these derivatives. Significant impact of 2-(2hydroxyphenyl)quinazoline toward modulating intermolecular interactions and facilitating J-type stacking between BODIPY units has also been established. Moreover, the essential role of excited-state intramolecular proton transfer (ESIPT) in inducing emission in the aggregated state and tuning of ESIPT emission by variation of substituents have been supported by various studies.

BODIPY-Based Multichromophoric Tripodal System as a Multifunctional Material

“…The emission intensity enhanced gradually with increasing DE fractions up to f DE 80%, and subsequently, it increased dramatically at f DE ≥ 80 (up to an f de of 99%) . Abrupt enhancement of the emission intensity at f DE 90 and 99% clearly suggested efficient aggregate formation .…”

“…Notably, increasing the DE fraction up to f de 80% led to gradual aggregate formation and small emission enhancement. On the other hand, at high DE fractions ( f DE of 90 and 99%), efficient aggregate formation occurred with dramatic emission enhancement . Thus, it is concluded that this novel BODIPY clearly exhibits good emission enhancement due to aggregate formation and shows AIE features.…”

“…Aggregation features have also been followed by absorption spectral studies in the same solvent system (Figure S11, SI). The absorption band showed a noticeable red shift (∼4 nm) at f DE 90 and 99%, which also supported the aggregate formation . Solid-state absorption spectra showed a red shift (∼20 nm) for the absorption maxima relative to solution [∼495 nm (acetonitrile) to ∼515 nm (solid)] and, hence, supported aggregation (Figure S12, SI) .…”

“…The absorption band showed a noticeable red shift (∼4 nm) at f DE 90 and 99%, which also supported the aggregate formation . Solid-state absorption spectra showed a red shift (∼20 nm) for the absorption maxima relative to solution [∼495 nm (acetonitrile) to ∼515 nm (solid)] and, hence, supported aggregation (Figure S12, SI) . Further, estimation of the particle size (DLS studies) also suggested the aggregation behavior (Figure S13, SI).…”

“…In addition, some spherical aggregates are also observable in the FESEM images. The occurrence of two different shapes for the aggregates for tri-BDP may be related to the existence of more than one orientation creating these forms . Further, to support the high emission intensity of the aggregated state, comparative fluorescence lifetime studies have been performed in acetonitrile ( f de ; 0%) and in the aggregated state ( f de ; 99%) (Figure b).…”