288

Original Research Paper

J. Jpn. Soc. Colour Mater., 93〔9〕，288–291（2020）―Special Edition

Central (Japan) Section : New colorant technologies―

Relationship between Crystal Packing and Solid-State Fluorescence Quantum Yieldin Pyrazine Monoboron Complexes
Yasuhiro Kubota＊,†, Yuki H aishima＊, Kazumasa F unabiki＊ and Masaki M atsui＊＊ Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu, Gifu 501-1193, Japan†Corresponding Author, E-mail: kubota@gifu-u.ac.jp(Received May 26, 2020; Accepted August 17, 2020 )AbstractThe relationship between crystal packing and solid-state fluorescence quantum yield (Φf) in pyrazine monoboron complexes 1～6 wasinvestigated. The maximum fluorescence wavelength (Fmax) of the BPh 2 complexes 4～6 was blue-shifted ( 524～628 nm) and the Φf values
were higher ( 0.13～0.29 ) than those of the corresponding BF 2 complexes 1～3 (Fmax: 531～672 nm, Φf: 0.04～0.13 ), probably due toinhibition of the intermolecular interactions. Despite weaker intermolecular interactions of the dimethylamino-substituted BPh 2 complex 6compared to non-substituted BF 2 complex 1, 6 (Φf = 0.13, Fmax = 628 nm) had the same Φf value as 1 (Φf = 0.13, Fmax = 542 nm) due to
red-shifted Fmax, which promoted non-radiative processes.
Key-words: Boron complex, β-iminoenolate ligand, Pyrazine, Solid-state fluorescence, Crystal packing

1．Introduction
Despite the wide-ranging applications of solid-state
fluorescent organic dyes in a variety of areas such as organic
light-emitting diodes 1 ), solid-state dye lasers 2 ), and organic
light-emitting field-effect transistors 3 ), the developmentof these dyes is still challenging. Common organic dyesthat fluoresce in dilute solution often quench or reduce thefluorescence intensity in the solid state; this phenomenon iscalled aggregation-caused quenching (ACQ) or concentrationquenching 4 ). The solid-state fluorescence properties oforganic dyes are known to be strongly dependent on themolecular packing 5 ). In many cases, the intermolecular π-πinteractions in the solid state lead to ACQ 6 ). The introductionof bulky substituent group in organic dyes should prevent theformation of intermolecular π-π interactions and is often used
for the expression of fluorescence and enhancement of thefluorescence quantum yield in the solid state 7 ).
Organoboron complexes are some of the most vitalfluorescent dyes. Due to the rigid molecular skeleton,organoboron complexes often fluoresce in solution 8 , 9 ).
Additionally, the photophysical properties such as absorption
and fluorescence wavelengths can be controlled by the typeof ligand. However, similar to the common organic dyes,
organoboron complexes exhibit the ACQ phenomenon in thesolid state. In the course of our studies on the organoboron
compounds 10-14 ), we have found that the introduction of【Figures and illustrations】Figures and illustrations published in black andwhite in the journal can be seen in color at our public website「J-STAGE」.
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© 2020 Journal of the Japan Society of Colour Materialbulky substituent groups, particularly on the boron atom, is agood strategy for enhancement of the fluorescence quantum
yield of organoboron compounds. We have also reported
pyrazine monoboron complexes featuring a β-iminoenolate
ligand that show interesting fluorescence properties insolution such as fluorescence solvatochromism 15 ). In thispaper, we reported the relationship between crystal packing
and solid-state fluorescence quantum yield in the pyrazine
monoboron complexes.

2．Experimental
The synthesis and fluorescence properties of pyrazine
monoboron complexes 1～6 in solution were reported in
our previous paper 15 ). Fluorescence spectra in the solid state
were measured with a Jasco FP-8600 spectrofluorometer.
Fluorescence quantum yields in the solid state wererecorded with a Hamamatsu Quantaurus-QY instrument.
The single crystals of 1 , 4, and 6 were obtained by theslow diffusion of hexane into chloroform solutions of thecorresponding compounds. Single-crystal X-ray diffraction
measurements were performed on a Rigaku Mercury 375R/
M CCD ( XtaLAB-mini ) diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71075 Å). The
crystal structures were solved using direct methods withSHELXS-97 or SHELXS2013 and refined by full-matrix
least-squares methods with anisotropic thermal parameters
for non-hydrogen atoms on F 2 using SHELXL- 97 orSHELXL2013. All hydrogen atoms were placed using AFIXinstructions. The Cambridge Crystallographic Data Center(CCDC) numbers for 1, 4, and 6 are 1567751, 1567752, and－20－