New type of photoluminescence properties of 2,2’:6’,2”-Terpyridine (Terpy) is described. Orthorhombic form of Terpy crystals, which is known to be inherently nonluminous in the visible region, was found to emit strong green light in the presence of traces amount of impurities. Hydroxy-terpyridine, originally included in the Terpy reagent, was found to be the main cause of the green luminescence. Tuning of the luminescent color is possible depending on the dopant species.
Photoluminescence of organic crystals is one of the major interests for chemists and crystallographer. Merit of the organic luminescent compounds is that tuning of the color change may be achieved systematically by introducing its analogue and/or by changing molecular packing motif (polymorphism).
Among the various candidates for the organic luminescent compounds proposed until now, we have interested in 2,2':6',2''-Terpyridine (Terpy, Figue 1) as a promising compound for organic luminescence. Until now, two crystal structures of Terpy have been reported as shown in
Although, Terpy shows interesting luminescent properties as shown above, the intensity of the emitting light is weak even in case of luminescent monoclinic form. In this study, improvement of the luminescence properties
of the Terpy especially for non-luminescent orthorhombic form was challenged.
All materials used in this study were obtained from commercial sources. 2,2':6',2''-Terpyridine (purity > 98.0%) was obtained from Tokyo Chemical Industry. Some unidentified impurities are included in the Terpy reagent. To identify the impurity species and to know their effects on luminescent properties of Terpy crystals, purification of the Terpy reagent was carried by column chromatography using following materials.
Stationary phase: silica gel (Wakosil C-300, Wako Pure Chemical Industries) or activated alumina (Wako Pure Chemical Industries).
Mobile phase: hexane (purity > 95.0%, Wako Pure Chemical Industries), tetrahydrofuran (THF, purity > 99.5%, Wako Pure Chemical Industries).
In this study, purification of Terpy reagent is performed and then evaluation of photoluminescence properties of the resultant crystals, total four type, were performed.
Isolation of impure species included originally in the Terpy reagent was performed by column chromatography. An example of the method is shown in
nary phase. In case of
By this two-step column chromatography, we have obtained at least two type of impurities in addition to pure Terpy. Structure of the impurities were analyzed using following method:
Fluorescence spectra (FP-6500, JASCO), gas chromatography?mass spectrometry. (GCMS-QP2010, Shimazu), nuclear magnetic resonance (NMR, AV400M, Bruker) and FT-IR spectrometry (FT/IR4200typeA, JASCO). Crystal structure of the Terpy crystals were identified by Powder X-ray diffraction (RINT 2200, CuKα, 1.5418 Å).
shows emission spectrum of Terpy reagent (blue line, correspond to
Indentification of the impurity G was performed with the combination of spectrum measurements.
1) GC-MS: mass spectroscopy measurement of the Green impurity showed maximum mass of 249 m/z which is larger by16 m/z of mass than that of pure Terpy (233 m/z), suggesting addition of one ?OH group instead of one ?H of pyridine ring.
2) NMR data are as follows suggesting existence of Terpy skeleton and existence of ?OH group at 14.42 ppm.
1H NMR (500 MHz, CDCl3):δ14.42 (s, 1H), 8.79 (d, 1H, J = 8 Hz), 8.65 (d, 1H, J = 5 Hz), 8.54 (d, 1H, J = 5 Hz), 8.49 (d, 1H, J = 8 Hz), 8.40 (d, 1H, J = 9 Hz), 7.96 (ddd, 1H, J = 7.8, 7.8, 2 Hz ), 7.82 (ddd, 1H, J = 7.8, 7.8, 2 Hz ), 7.45 (d, 1H, J = 9 Hz ), 7.37 (ddd, 1H, J = 6.3, 6.3, 1 Hz ), 7.26 (ddd, 1H J = 6, 6, 1 Hz)
13C NMR (500 MHz, CDCl3): δ158.32, 157.20, 156.22, 148.95, 146.49, 145.40, 137.85, 136.65, 135.44, 126.46, 123.66, 123.02, 122.78, 121.94, 120.27
3) FT-IR measurements showed also existence of Terpy in addition existence of -OH vibrational mode at around 3400 cm−1.
From the above results, addition of ?OH group to Terpy as shown in
We have applied the same methodology for the blue impurities, the results were complicated and was impossible to be identified. The resultant blue impurities must be non-single components.
Now we have isolated three Terpy related compounds of pure Terpy, Green emitter TerpyOH and blue impurities (still unknown structure). Usually, luminescent properties of a compound have been studied using a pure compound and great efforts have been paid for the purification. We know that, however, we should point out
undetectable amount of traces of impurity enhanced the emission in case of Terpy. This is very interesting. Thus we have actively utilize impurities as color emitter in contrast to the usual method. Combination of these substances may lead to multi-color emission.
The Terpy reagent in orthorhorhombic form crystal emits strong light by the doping effect of some impurities. One of the impurities was identified as green emitter, 2,2':6',2''-Terpyridine-3'-ol. Existence of the other blue emitter impurities was also recognized. Multi-color luminescence of the crystals was succeeded by combination of pure Terpy with the impurities.
Masaaki Yokota,Atsushi Ito,Norihito Doki, (2016) Multi-Color Photoluminescence of Terpyridine Crystals. Advances in Chemical Engineering and Science,06,87-92. doi: 10.4236/aces.2016.62011