Boron Dipyridylmethene (DIPYR) Dyes: Shedding Light on Pyridine-Based Chromophores

Article abstract of DOI:10.1021/acs.joc.7b00786

Boron dipyrromethene (BODIPY) derivatives have found widespread utility as chromophores in fluorescent applications, but little is known about the photophysical properties of pyridine-based BODIPY analogues, dipyridylmethene dyes. Indeed, it has been repo

Full text of DOI:10.1021/acs.joc.7b00786

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Article
Boron dipyridylmethene (DIPYR) dyes:
shedding light on pyridine-based chromophores
Jessica H. Golden, John W. Facendola, Daniel Sylvinson, Cecilia
Quintana Baez, Peter I. Djurovich, and Mark E Thompson
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b00786 • Publication Date (Web): 04 Jul 2017
Downloaded from http://pubs.acs.org on July 4, 2017
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The Journal of Organic Chemistry
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Boron dipyridylmethene (DIPYR) dyes:
shedding light on pyridineꢀbased
chromophores
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Jessica H. Golden,† John W. Facendola,† Daniel Sylvinson M. R.,† Cecilia Quintana Baez,‡
Peter I. Djurovich,† and Mark E. Thompson*,†,‡
Department of Chemistry† and Mork Family Department of Chemical Engineering and Materials
Science‡, University of Southern California, Los Angeles, California, 90089, United States
FOR TABLE OF CONTENTS ONLY
ABSTRACT:
Boron dipyrromethenes (BODIPY) derivatives have found widespread utility as chromophores in
fluorescent applications, but little is known about the photophysical properties of pyridineꢀbased
BODIPY analogues, dipyridylmethene dyes. Indeed, it has been reported that boron difluoride
dipyridylmethene (DIPYR) is nonꢀemissive, and that derivatives of DIPYR have modest, if any,
luminescence. In this report, we explore this littleꢀtouched area of chemical space and investigate
the photophysical properties of three simple DIPYR dyes: boron dipyridylmethene, boron
diquinolylmethene, and boron diisoquinolylmethene. The three dyes absorb strongly in the blueꢀ
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The Journal of Organic Chemistry
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green part of the spectrum (λem = 450–520 nm, ε = 2.9–11 x 104 Mꢀ1cmꢀ1) and display green
fluorescence with high quantum yields (ΦPL = 0.2, 0.8 and 0.8, respectively). Key photophysical
properties in these systems were evaluated using a combination of TDꢀDFT and extended multiꢀ
configurational quasiꢀdegenerate second order perturbation theory (XMCQDPT2) methods and
compared to experiment, revealing that high quantum yields of the quinoline and isoquinoline
derivatives are a result of the relative reordering of S1 and T2 state energies upon benzannulation
of the parent structure. The intense absorption and high emission efficiency of the benzannulated
derivatives make these compounds an intriguing class of dyes for further derivatization.
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INTRODUCTION:
The boron dipyrromethene (BODIPY) core structure is pervasive among fluorescent dyes
employed in biological and optoelectronic applications. BODIPYs have been utilized in protein
and DNA labeling,1ꢀ5 as fluorescent switches and chemosensors,6ꢀ8 as laser dyes,9 and in
photovoltaics.10,11 Their physical and photophysical properties can be readily tuned by making
simple substitutions on the pyrroles or at the meso position, and they tend to have little
dependence on solvent polarity or pH, which makes them ideal fluorophores for in vivo
labeling.12 These features, along with a high luminance efficiency and photostability, have led to
the popularity of BODIPY compounds. Despite the wide landscape of BODIPY dyes available,
further investigation into the derivatization of BODIPYs and the development of alternative
fluorescent dyes remains an active area of research due to specific limitations presented by
BODIPYs.13 Indeed, some simple synthetic modifications of BODIPYs have proven challenging;
for example, although the class of dyes has been known since 1968, it was not until 2009 that the
parent structure was successfully isolated.14ꢀ16 Given the enormous body of work available on the
derivatization and application of BODIPYs and related fluorophores, we found it surprising that
very little research has been reported on the photophysical properties of pyridineꢀbased
BODIPYꢀlike dyes.
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In this work we aim to expand the scope of BODIPYꢀlike dyes by the replacement of the
pyrroles in BODIPY with pyridine moieties. The resultant class of boron difluoride
dipyridylmethene (DIPYR) fluorescent dyes (Figure 1) are similar in geometric structure to
anthracene and dibenz[a,j]anthracene, and superficially appear to be a mix between acenes and
BODIPYs. Considering their intriguing combination of structural features, it is therefore
somewhat surprising that only a handful of studies have appeared regarding the photophysical
properties for this class of compounds. For example, the first report on the preparation of
DIPYR in 1973 made no mention of luminescent properties,17 while a later paper describing the
photophysical properties of the related boron dipyridylamine dyes incorrectly described DIPYR
as being nonꢀemissive.18 It is likely for this reason that followꢀup research on this class of dye
molecules has been limited.19 However, dipyridylamine analogues of DIPYR dyes with nitrogen
at the meso position do display UVꢀviolet emission (λem = 362–416 nm) and have luminescent
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quantum yields (ΦPL = 0.4)18 similar to values found for the blueꢀemissive mesoꢀcyano
substituted DIPYR dyes (λ = 450 nm, ΦPL = 0.2).19 These promising photophysical properties
piqued our interest in the study of the parent DIPYR dyes with methine at the meso position.
Herein we report the synthesis, structural and photophysical properties of this promising new
family of pyridineꢀ, quinolineꢀ, and isoquinolineꢀderived fluorescent dyes. We find that
benzannulation of DIPYR leads to higher absorptivity and substantial increases in luminescent
efficiency, with quantum yields (ΦPL = 0.8) that approach values comparable to the most highly
efficient BODIPY dyes. The increase in fluorescent efficiency in the quinoline and isoquinoline
DIPYR derivatives is attributed to a decrease in the rate of intersystem crossing caused by a
change in energetic ordering of the S1 and T2 excited states.
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