New and efficient high Stokes shift fluorescent compounds: Unsymmetrically substituted 1,2-bis-(5-phenyloxazol-2-yl)benzenes via microwave-assisted nucleophilic substitution of fluorine

Article abstract of DOI:10.1016/j.tetlet.2011.07.100

Two new unsymmetric derivatives of 1,2-bis-(5-phenyloxazol-2-yl)benzene (ortho-POPOP) were synthesized via microwave-assisted nucleophilic substitution of fluorine which appears to be significantly more efficient compared with conventional thermal activat

Full text of DOI:10.1016/j.tetlet.2011.07.100

Tetrahedron Letters 52 (2011) 5086–5089
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New and efficient high Stokes shift fluorescent compounds:
unsymmetrically substituted 1,2-bis-(5-phenyloxazol-2-yl)benzenes
via microwave-assisted nucleophilic substitution of fluorine
Rodion Yu. Iliashenko a, Nikolay Yu. Gorobets b, Andrey O. Doroshenko a,
⇑
a Kharkov V.N. Karazin National University, 4 Svobody Sqr., Kharkov 61022, Ukraine
b SSI ‘Institute for Single Crystals’ of the National Academy of Science of Ukraine, Lenin Ave. 60, Kharkov 61001, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new unsymmetric derivatives of 1,2-bis-(5-phenyloxazol-2-yl)benzene (ortho-POPOP) were synthe-
sized via microwave-assisted nucleophilic substitution of fluorine which appears to be significantly more
efficient compared with conventional thermal activation. The compounds synthesized are characterized
by high fluorescence Stokes shifts (6000–11,000 cmꢀ1) in solvents of various polarity, intermediate-to-
high fluorescence quantum yields and lifetimes in the range of several nanoseconds.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 June 2011
Revised 12 July 2011
Accepted 22 July 2011
Available online 29 July 2011
Keywords:
1,2-Bis-(5-phenyloxazol-2-yl)benzene
(ortho-POPOP)
High Stokes shift fluorescence
Nucleophilic substitution of fluorine
Microwave-assisted synthesis
Fluorescent organic compounds possessing high Stokes shift
values1 are prospective candidates for their practical application
in various fields of science and technology, where high concentra-
tions or long optical paths are required,2 for example, in scintilla-
tion techniques,3 sunlight collection, and conversion of its energy
into electricity,4 electroluminescent light sources (OLEDs),5 fluo-
rescent chemosensors,6 various biological applications, etc. Several
physico-chemical mechanisms can be applied to increase the fluo-
rescence Stokes shift of organic compounds,7 however not all of
them lead to emissions with high quantum yields.8 The most pop-
ular and most studied is the excited state proton transfer reaction,9
however it is usually connected with high radiationless excitation
energy losses.10 To our understanding, excited state conforma-
tional transformations resulting in the formation of more planar
molecular structures11 have several advantages in producing high
Stokes shifted fluorescence emission12 over the alternative twist-
ing mechanisms,13 which also induce radiationless excited state
deactivation in most of the known cases.14
of the title series are characterized by essential non-planarity caused
bythestericrepulsionof twobulkyheterocyclicmoietiesintroduced
into the ortho-positions of the central benzene ring.18 To minimize
the steric hindrance the heterocycles move out of the plane of the
central phenylene forming, with the latter, dihedral angles in the
range 30–80°. The resulting disruption of intramolecular conjuga-
tion shifts the electronic absorption spectra of ortho-POPOPs
towards the shorter-wavelength region with respect to the absorp-
tion of their planar para-isomers. In contrast, the fluorescence emis-
sion of ortho-POPOPs is observed at even longer wavelengths
compared to the para-analogs, reflecting restoration of the conjuga-
tion in the lowest singlet excited state in the flattened molecule.19 In
most cases the excited state planarization of the ortho-POPOP mole-
cules is not accompanied by essential radiationless deactivation and
the observed fluorescence quantum yields remain reasonably
high.20
Further increase of the Stokes shift values is possible by combi-
nation of several photophysical mechanisms in one molecule: for
example, excited state planarization and solvatochromic effects.
Asymmetrization of the electronic density distribution by intro-
duction of highly electron-donating and/or electron-withdrawing
substituents into the ortho-POPOP molecule leads to a significant
rise in its excited state dipole moment, increased sensitivity to sol-
vent polarity, and thus enlarges the fluorescence Stokes shift in po-
lar media.19a
Derivativesof1,2-bis-(5-phenyloxazol-2-yl)benzene15 areortho-
analogs of the well-known scintillation luminophore POPOP [1,
4-bis-(5-phenyloxazol-2-yl)benzene]. They belong to the class of
efficient fluorescent organic compounds with abnormally high
Stokes shifts.16 In contrast to their planar para-isomers,17 molecules
⇑
Corresponding author. Tel.: +38 057 707 5335; fax: +38 057 707 5130.
E-mail addresses: andreydoroshenko@ukr.net, andrey.o.doroshenko@univer.
The key steps in the synthesis of the unsymmetrically substi-
tuted ortho-analogs of POPOP (Scheme 1) are the acylation of
x-
kharkov.ua (A.O. Doroshenko).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.100

R. Yu. Iliashenko et al. / Tetrahedron Letters 52 (2011) 5086–5089
5087
R1
Table 1
N
R1
R2
N
O
Synthesis of electron-donor substituted derivatives of ortho-POPOP by nucleophilic
substitution of fluorine under conventional and microwave conditions
a, b
O
O
a, b, c
O
O
O
O
COCl
Compound
Activation
Reaction duration (h)
Yield (%)
O
N
Thermal25
8
2
100
3
58
82
64
73
R2
2
R1
NH2
Microwave26
Thermal27
NH2
3
Microwave28
Scheme 1. Typical method for the synthesis of unsymmetrically substituted ortho-
analogs of POPOP. Reagents and conditions: (a) (chloro)anhydrides in acetone/
benzene, aqueous
x-amino acetophenone hydrochlorides, Na2CO3, room temper-
ature, 0.5 h; (b) concentrated H2SO4, 60 °C, 3 h; (c) SOCl2, 0.5 h reflux.
Table 2
Spectral properties of the investigated unsymmetrically substituted ortho-analogs of
POPOP
amino acetophenones using carboxylic acid anhydrides or acyl
chlorides, steps (a), and two oxazole ring-closures in acidic media,
steps (b). In many cases, substituents of different electronic nature,
R1 and R2, introduced into the intermediates can affect the cycliza-
tion decreasing significantly the yields of the final products. The
interests of our group include the development of new and effi-
cient methods for the synthesis of novel derivatives of ortho-PO-
POP with improved fluorescence properties.
The aim of this Letter is to report a novel approach for the syn-
thesis of unsymmetrically substituted ortho-analogs of POPOP,
which increases the variety within this series of high Stokes shift
fluorescent dyes. This work is based on the nucleophilic substitu-
tion of a halogen atom21 which is easily introduced into the
ortho-POPOP molecule by a traditional method as shown in
Scheme 1. Successful examples of such reactions for the synthesis
of dialkylamino-substituted fluorescent compounds can be found
in the literature.22 Recently, we reported the synthesis and X-ray
crystal structure investigation of fluoro-substituted ortho-POPOP
1.23 Fluorine could be replaced by an appropriate nucleophilic
group by extended heating in a sealed vessel (Scheme 2).
In the case of relatively weak nucleophilic agents such as sec-
ondary amines, the reaction performed under conventional heating
proceeds extremely slowly and was accompanied by the formation
of significant amounts of colored impurities. This led us to test
microwave irradiation as an alternative method,24 which proved
to be more convenient and rapid (Table 1).
Compounds 2 and 3 obtained by these different experimental
methods were chemically identical, however, the purity of those
synthesized via thermal activation was significantly worse. The
appearance of noticeable amounts of by-products using microwave
activation was practically not observed. Compounds prepared in
this manner needed only chromatographic separation from the
traces of the starting compound 1 remaining.
Solvent
ma
ka
mf
kf
D
mST
uf
sf
F-substituted ortho-POPOP, 1
Hexane
Toluene
1,4-Dioxane
1,2-DCE
EtOAc
DMF
MeCN
EtOH
30,960
30,480
30,660
30,620
30,940
30,840
30,860
30,760
323
328
326
327
323
324
324
325
23,640
23,200
23,160
23,100
23,180
22,880
22,940
22,840
423
431
432
433
432
437
436
438
7320
7280
7500
7520
7760
7960
7920
7920
0.72
0.71
0.86
0.84
0.68
0.63
0.54
0.52
4.02
3.95
4.45
4.31
4.27
4.47
4.41
5.02
OH-substituted ortho-POPOP, 2
Hexane
Toluene
1,4-Dioxane
1,2-DCE
EtOAc
DMF
MeCN
EtOH
30,440
30,640
30,460
30,520
30,700
30,900
31,020
30,600
329
326
328
328
326
323
322
327
23,080
22,520
21,980
21,880
21,840
20,620
20,960
20,580
433
444
455
457
458
485
477
486
7360
8120
8480
8640
0.62
0.59
0.57
0.64
0.55
0.59
0.65
0.23
3.70
3.41
3.93
4.08
4.17
4.47
4.86
2.42
8860
10,280
10,060
10,020
Piperidyl-substituted ortho-POPOP, 3
Hexane
Toluene
1,4-Dioxane
1,2-DCE
EtOAc
DMF
MeCN
EtOH
29,940
29,060
28,980
29,080
29,320
28,820
28,900
29,160
334
344
345
344
341
347
346
343
23,640
21,840
21,100
20,280
20,120
19,500
19,460
18,060
423
458
474
493
497
513
514
554
6300
7220
7880
8800
9200
9320
9440
11,100
0.56
0.40
0.64
0.56
0.42
0.65
0.31
0.39
1.49
1.80
2.64
3.03
2.88
3.28
2.53
1.78
m
a, ka,–Positions of the long-wavelength absorption band (cmꢀ1/nm),
tions of the fluorescence band (cmꢀ1/nm), ST–fluorescence Stokes shift (cmꢀ1),
Dm
m
f, kf,–posi-
u
f–fluorescence quantum yield with respect to quinine sulfate in 0.1 M aqueous
H2SO4,30
s
f–mean fluorescence lifetime (ns).
POPOP, its fluorescence emission decay laws were rather compli-
cated and fitted satisfactorily only by bi-exponential functions
with positive pre-exponentials. Thus, the fluorescence lifetimes
presented in Table 2 are the mean values calculated using the com-
The effect of microwave irradiation on the reaction time in the
case of the strong nucleophilic agent, OH-, application of which re-
sulted in the formation of compound 2, was less significant, than in
the case of compound 3 (a weak nucleophile). However, in this case
a 1.4 fold increase in the yield was detected.
The optical properties of the starting and newly synthesized
derivatives of ortho-POPOP were determined in aprotic solvents
of different polarity and ethanol (Table 2).29 Owing to the excited
state conformational rebuilding typical of any ortho-analog of
P
P
mon equation1:
time and
s
¼
ais2i
=
aisi, where si is the fitted decay
a
i the corresponding pre-exponential factor. Photophysi-
cal data for compound 1 were not published previously and hence
they are included in Table 2 and discussed here briefly.
High fluorescence Stokes shifts within the range of 6300–
11,100 cmꢀ1 (Fig. 1, Table 2) are typical of all the studied com-
pounds. Surprisingly, the fluorescence quantum yields of fluo-
rine-substituted molecule 1 were the highest of all the known
derivatives of ortho-POPOP with relatively high mean lifetimes in
the range of 4–5 ns. This is a reflection of the decreased radiation-
less decay efficiency caused by the presence of fluorine. The solva-
tochromism of compound 1 was not very high, indicating the
intermediate positive mesomeric effect of the fluorine atom as a
substituent in heteroaromatic conjugated molecules. Thus, F has
relatively low influence on the excited state electron density redis-
tribution and the total increase of the excited state dipole moment.
The hydroxy derivative 2 demonstrates a pronounced Stokes
shift increase with solvent polarity from 7300 to 10,000 cmꢀ1. Its
fluorescence lifetime shows a clear tendency to increase in aprotic
solvents as well and in contrast, nearly a 2-fold decrease together
N
N
N
N
Nu
Nu:
F
O
O
O
O
Δ or MW
- F-
1
HO-
HN
Nu:
2
3
Scheme 2. Nucleophilic substitution of the F atom in compound 1 resulting in the
formation of compounds 2 and 3.

5088
R. Yu. Iliashenko et al. / Tetrahedron Letters 52 (2011) 5086–5089
Even in the case of the strong nucleophile OHꢀ, we obtained an in-
crease in the yield and less by-products.
1.0
0.5
1
References and notes
1. (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, Third edition; Springer
Science: Singapore, 2006; (b) Valeur, B. Molecular Fluorescence; Wiley-VCH
GmbH: Germany, 2002.
2. (a) Eichhorn, M. Appl. Phys. B 2009, 96, 369–377; (b) Bordeau, G.; Lartia, R.;
Teulade-Fichou, M.-P. Tetrahedron Lett. 2010, 51, 4429–4432.
3. Birks, J. B. The Theory and Practice of Scintillation Counting; Pergamon Press:
Oxford, 1967; (b) Zorn, C.; Bowen, M.; Majewski, S.; Walker, J.; Wojcik, R.;
Hulburt, C.; Moser, W. Nucl. Instr. Meth. Phys. Res. 1988, A273, 108–116.
4. (a) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729–
15735; (b) McNamara, W. R.; Snoeberger, R. C.; Li, G.; Schleicher, J. M.; Cady, C.
W.; Poyatos, M.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista,
V. S. J. Am. Chem. Soc. 2008, 130, 14329–14338.
0.0
1.0
278 294 313 333 357 385 417 455 500 556 625
Wavelength, nm
5. (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913–915; (b) Tamoto,
N.; Adachi, C.; Nagai, K. Chem. Mater. 1997, 9, 1077–1085.
2
6. de Silva, A. P.; Fox, D. B.; Huxley, A. J. M.; Moody, T. S. Coord. Chem. Rev. 2000,
205, 41–57.
7. Vollmer, F.; Rettig, W.; Birckner, E. J. Fluoresc. 1994, 4, 65–69.
8. Volchkov, V. V.; Uzhinov, B. M. High Energy Chem. 2008, 42, 153–169.
9. (a) Ormson, S. M.; Brown, R. G. Progr. React. Kinet. 1994, 19, 45–91; (b)
LeGourrierec, D.; Ormson, S. M.; Brown, R. G. Progr. React. Kinet. 1994, 19, 221–
295.
0.5
10. Vollmer, F.; Rettig, W. J. Photochem. Photobiol. A: Chem. 1996, 95, 143–155.
11. (a) Siebert, R.; Winter, A.; Schubert, U. S.; Dietzek, B.; Popp, J. J. Phys. Chem. C
2010, 114, 6841–6848; (b) Yang, C.-C.; Hsu, C.-J.; Chou, P.-T.; Cheng, H. C.; Su, Y.
O.; Leung, M.-K. J. Phys. Chem. B 2010, 114, 756–768.
12. Doroshenko, A. O. Theor. Exper. Chem. 2002, 38, 133–152.
13. (a) Grabowski, Z. R.; Rotkiewicz, K.; Semiarczuk, A.; Cowley, D. J.; Baumann, W.
Nouv. J. Chim. 1979, 3, 443–453; (b) Rettig, W. Angew. Chem., Int. Ed. 1986, 25,
971–988.
14. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 103, 3899–4031.
15. (a) Doroshenko, A. O.; Patsenker, L. D.; Baumer, V. N.; Chepeleva, L. V.;
Van’kevich, A. V.; Kirichenko, A. V.; Yarmolenko, S. N.; Shershukov, V. M.;
Mitina, V. G.; Ponomaryov, O. A. Mol. Engineering 1994, 3, 353–363; (b)
Doroshenko, A. O.; Kyrychenko, A. V.; Baumer, V. N.; Verezubova, A. A.;
Ptyagina, L. M. J. Mol. Struct. 2000, 524, 289–296; (c) Doroshenko, A. O.;
Baumer, V. N.; Verezubova, A. A.; Ptyagina, L. M. J. Mol. Struct. 2002, 609, 29–37.
16. Doroshenko, A. O.; Kirichenko, A. V.; Mitina, V. G.; Ponomaryov, O. A. J.
Photochem. Photobiol. A: Chem. 1996, 94, 15–26.
0.0
278 294 313 333 357 385 417 455 500 556 625 714
Wavelength, nm
1.0
3
0.5
0.0
17. Ambats, I.; Marsh, R. E. Acta Cryst. 1965, 19, 942–948.
18. Doroshenko, A. O.; Baumer, V. N.; Kirichenko, A. V.; Shershukov, V. M.;
Tolmachev, A. V. Chem. Heterocycl. Comp. 1997, 33, 1341–1349.
19. (a) Doroshenko, A. O. Chem. Phys. Rep. 1999, 18, 873–879; (b) Doroshenko, A. O.
Russ. J. Phys. Chem. 2000, 74, 773–777; (c) Doroshenko, A. O.; Kyrychenko, A. V.;
Waluk, J. J. Fluoresc. 2000, 10, 41–48.
20. Kirichenko, A. V.; Doroshenko, A. O.; Shershukov, V. M. Chem. Phys. Rep. 1998,
17, 1643–1651.
21. Vlasov, V. M. Russ. Chem. Rev. 2003, 72, 681–704.
22. (a) Ossowski, T.; Zarzeczan´ ska, D.; Zalewski, L.; Niedziałkowski, P.; Majewski,
R.; Szyman´ ska, A. Tetrahedron Lett. 2005, 46, 1735–1738; (b) Demchenko, S. A.;
Pivovarenko, V. G. Ukr. Bioorg. Acta 2006, 1, 11–16; (c) Yushchenko, D. A.;
Bilokin, M. D.; Pyvovarenko, O. V.; Duportail, G.; Mély, Y.; Pivovarenko, V. G.
Tetrahedron Lett. 2006, 47, 905–908.
294 313 333 357 385 417 455 500 556 625 714
Wavelength, nm
Figure 1. Electronic absorption spectra of compounds 1–3 in hexane (violet, light
violet for 3 in EtOH). Fluorescence spectra excited at 330 nm in hexane (blue) and
EtOH (red), Stokes shifts are shown by arrows of the corresponding color. Insets:
fluorescence in EtOH.
with the fluorescence quantum yield in ethanol. Probably, this is
the effect of specific interactions with protic solvent molecules,
however, excited state OH group photodissociation was not ob-
served as was the case when the alcohol solution was made basic.
Introduction of the most pronounced electron-donor group
within the examined series (compound 3) results in a definite sys-
tematic decrease in both the fluorescence quantum yield and mean
lifetime, however they still remain reasonably high. The ethanol
solution of 3 demonstrated further acceleration of radiationless de-
cay, but it was not as high as could be expected in the cases when
formation of twisted intramolecular charge transfer (TICT) states
took place.13,14 We note, that a significant Stokes shift increase
was observed for 3 on going from acetonitrile to ethanol. This is
a reflection of the role of hydrogen bonding in the photophysics
of this compound, connected with the increase in the electron-
withdrawing parameters of its H-bonded oxazole rings and inten-
sification of the intramolecular excited state donor–acceptor
interactions.
23. Ilyashenko, R.; Wera, M.; Błazejowski, J.; Doroshenko, A. Acta Cryst. 2010, E66,
o2379–o2380.
24. A monomode microwave system Emrys™ Creator EXP from Biotage (Uppsala,
Sweden) equipped with an IR temperature sensor and pressure control system
was used for MW-assisted synthesis. The absorption mode was set to ‘normal’
with the initial power at 300 W. Process vials with 5 ml of the reaction mixture
volume were used.
25. 1-[5-(4-Hydroxyphenyl)oxazol-2-yl]-2-(5-phenyloxazol-2-yl)benzene
(2).
Thermal activation: A solution of 0.3 g (7.8 ꢁ 10ꢀ4 mol) of compound 1 with
0.1 g of KOH in DMSO (15 ml) was heated over 8 h using a glycerol bath at
120 °C with periodic monitoring by fluorescence at an excitation wavelength of
330 nm (by cooling and removing microliter portions of the reaction mixture
for analysis). The reaction mixture was poured into 100 ml of cold H2O and
acidified with AcOH. The resulting precipitate was filtered, washed with H2O
and dried. Purification was done by column chromatography on silica/benzene.
Yield 0.174 g, (4.6 ꢁ 10ꢀ4 mol, 58%).
26. 1-[5-(4-Hydroxyphenyl)oxazol-2-yl]-2-(5-phenyloxazol-2-yl)benzene
(2).
Microwave activation: A solution of 0.3 g (7.8 ꢁ 10ꢀ4 mol) of compound 1
with 0.1 g of KOH in DMSO (5 ml) was heated to 120 °C for 2 h in a closed glass
vial under microwave irradiation (visual monitoring by fluorescence color).
Separation and purification was as described above. Yield 0.246 g
(6.5 ꢁ 10ꢀ4 mol, 82%). Compound 2: C24H16N2O3, white fine-crystalline solid,
mp 102–103 °C, 1H NMR (Varian Mercury VX-200, 200 MHz, DMSO-d6): 6.7 (d,
J = 7.6 Hz, 2H), 7.28–7.31 (m, 5H), 7.49–7.52 (m, 3H), 7.67 (d, J = 7.6 Hz, 2H),
7.85 (s, 1H), 8.08 (m, 2H), 9.2 (br s 1H). MS (Varian 1200 L, EI, 70 eV): 381, 380
(M+), 351, 324, 303. No molecular ions for initial compound 1 and its most
probable first fragment ions were detected in the MS of sample 2 (382, 363/
362, 356).
In conclusion, a new approach for the synthesis of unsymmetri-
cally substituted ortho-analogs of POPOP including aromatic nucle-
ophilic substitution of a fluorine atom has been elaborated.
Microwave irradiation makes the above reaction significantly more
efficient, clean and rapid, especially in the case of the relatively
weak nucleophilic reagent—the secondary cyclic amine piperidine.

R. Yu. Iliashenko et al. / Tetrahedron Letters 52 (2011) 5086–5089
5089
27. 1-{5-[4-(N-Piperidyl)phenyl]oxazol-2-yl}-2-(5-phenyloxazol-2-yl)benzene (3).
Thermal activation: A solution of 0.3 g (7.8 ꢁ 10ꢀ4 mol) of compound 1 in a
mixture of piperidine (2 ml) and DMF (0.5 ml) was heated to 200 °C in a steel
autoclave over 100 h (more than 2 weeks, up to 8 h/working day) with
monitoring of the reaction progress by periodic removal microliter samples of
the reaction medium for fluorescence analysis (excitation at 350 nm). After the
reaction was complete, the mixture was poured into 100 ml of cold H2O, and
the resulting precipitate filtered, washed with H2O and dried. Purification was
accomplished by column chromatography on silica/benzene. Yield 0.22 g
(5 ꢁ 10ꢀ4 mol, 64%).
NMR (200 MHz, DMSO-d6): 1.6 (m, 6H), 3.73 (m, 4H), 7.42 (d, J = 7.2 Hz, 2H),
7.55 (m, 3H), 7.72 (d, J = 7.2 Hz, 4H), 7.80 (s, 2H), 8.00 (m, 2H), 8.28 (d,
J = 7.5 Hz, 2H). MS (high fragmentation, only the most intense fragment ion
masses are listed): 448, 447 (M+), 428, 412, 399, 398. No molecular ions for
initial compound 1 and its most probable first fragment ions were detected in
the MS of sample 3.
29. Electronic absorption spectra were measured on
spectrophotometer and fluorescence spectra on
a
a
HITACHI U3210
HITACHI F4010
fluorescence spectrometer. Synchronous scan fluorescence spectra were
applied as simple test indicating the purity of newly synthesized
compounds. Fluorescence lifetimes were detected on sub-nanosecond
a
28. 1-{5-[4-(N-Piperidyl)phenyl]oxazol-2-yl}-2-(5-phenyloxazol-2-yl)benzene (3).
Microwave activation: A solution of 0.3 g (7.8 ꢁ 10ꢀ4 mol) of compound 1 in
piperidine (2 ml) and DMF (0.5 ml) was heated to 200 °C in a closed glass vial
under microwave irradiation (visual monitoring by fluorescence color).
Separation and purification was as described above. Yield 0.26 g
(5.7 ꢁ 10ꢀ4 mol, 73%). Compound 3: C29H25N3O2, yellow amorphous solid, 1H
a
kinetic spectrometer, consisting of an MDR-12 monochromator (LOMO,
Russia), a TimeHarp 200 TCSPC device, a PLS 340-10 picosecond LED driven
by a PDL 800-B device (PicoQuant GmbH, Germany) and a Hamamatsu H5783P
PMT (Hamamatsu, Japan).
30. Melhuish, W. H. J. Res. Nat. Bur. Stand. U.S.A. 1972, 76A, 547–560.