Luminescent properties of some imidazole and oxazole based heterocycles: Synthesis, structure and substituent effects

Article abstract of DOI:10.1016/j.dyepig.2010.07.005

A series of 2-R-6-(aryloxazol-/imidazol-2-yl)pyridine and 2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-/oxazol-2-yl)phenol derivatives were synthesized and characterized using elemental and spectroscopic analyses as well as single-crystal X-ray diff

Full text of DOI:10.1016/j.dyepig.2010.07.005

Dyes and Pigments 88 (2011) 262e273
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Luminescent properties of some imidazole and oxazole based heterocycles:
Synthesis, structure and substituent effects
,
,
a
c
d
Abiodun O. Eseola a d, Wen Li b, Wen-Hua Sun a , Min Zhang , Liwei Xiao , Joseph A.O. Woods
*
a Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
c Faculty of Chemistry and Material Science, Langfang Teachers College, Langfang 065000, China
d Department of Chemistry, Faculty of Science, University of Ibadan, Ibadan, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-R-6-(aryloxazol-/imidazol-2-yl)pyridine and 2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]
imidazol-/oxazol-2-yl)phenol derivatives were synthesized and characterized using elemental and spec-
troscopic analyses as well as single-crystal X-ray diffraction analysis. The oxazole derivatives displayed
higher photoluminescence efficiencies than their imidazole analogues. The 2-(phenanthro[9,10-d]oxa-
zole/imidazol-2-yl)pyridine derivatives displayed quantum yields approaching unity in non-polar solvents
but were quenched in polar solvents. The oxazole analogues produced reversible fluorescence photo-
switching between 400 nm and 550 nm regions whilst the imidazole analogues underwent an irreversible
photo-induced excimer formation in polar solvents. 1H NMR was used to rationalize the proposed mode
of intermolecular interactions between excimers of the 2-(phenanthro[9,10-d]imidazol-2-yl)pyridine
derivatives.
Received 26 January 2010
Received in revised form
10 July 2010
Accepted 12 July 2010
Available online 21 July 2010
Keywords:
Photoluminescence
Organic fluorescent photoswitch
Photo-aggregation
Solvent effects
Ó 2010 Elsevier Ltd. All rights reserved.
Substituent effects
X-ray structures
1. Introduction
such as 2-(2-pyridyl)benzimidazole on to polymeric backbones
[15,16]. Simple mixing of polymers with small Mr dyes has also been
Research on organic luminescent materials has been keenly
pursued because of their importance in technological applications
related to signaling, fluorescent biosensory/chemosensory materials,
molecular switches and organic light emitting diodes (OLEDs) [1e5].
Organic fluorophores, especially nitrogen-containing heterocyclic
compounds, have attracted attention owing to their high emission
efficiency [6e11]. However, as the basic structure of a fluorophore has
to be modified, the investigation of substituent effects provides
a route to optimizing potentially advantageous properties [12].
Though the naturally occurring jellyfish fluorescent proteins
are the sole examples of biological molecules with photo-switching
capability, synthetic molecules with controllable fluorescence
switching in response to photo-irradiation are still being sought
[11,13]. Although polymeric fluorophores offer durability as
device components [14], small molecules are attractive as their
structureeproperty relationships can be readily explored unlike
polymeric materials [11]. Furthermore, polymeric fluorophores are
usually fabricated by covalently grafting small size building blocks
reported to give rugged fluorescent materials [17e19]. Rational
tuning of desired characteristics is more achievable for small mole-
cule fluorophores [20].
Molecules containing planar aromatic fragments such as pyrene,
phenanthrene, anthracene, etc., are often found to exhibit robust
photoluminescence properties that may be altered by presence or
absence of intramolecular or intermolecular stacking interactions.
Switching between free and assembled forms in solutions can
be chemically [21] or solvent induced [22]. The phenomenon is also
often manifested in significantly different emission spectral peaks
positions and/or intensities obtained in solid and solution states
of such fluorophores [22], however, there is rare report of synthetic
fluorophores which undergoes solvent dependent photo-induced
stacking in solution.
Numerous aryl-substituted imidazoles have been exploited for
their emission properties in various research fields [23e32]. Most
of this efforts have been largely spent on bidentate non-bridging
imidazo[4,5-f][1,10]phenanthroline ligands and monodentate
phenyl-substituted imidazoles. The systematic study of substituent
effects on luminescent properties in this family of compounds is
scarce and investigations on fluorescent imidazoles with receptor
capabilities that involves the five-membered ring nitrogen as one of
* Corresponding author. Tel.: þ86 10 62557955; fax: þ86 10 62618239.
E-mail addresses: liwen@mail.ipc.ac.cn (W. Li), whsun@iccas.ac.cn (W.-H. Sun).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.07.005

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
263
the donors have been largely restricted to 2-(2-pyridyl)benzimid-
azole [12,15]. To the best of our knowledge, photoluminescence
research on 2-pyridyl-substituted oxazoles is not known. Recently,
we accomplished preparation of a some new bidentate 2-(imida-
zol-2-yl/oxazol-2-yl)pyridine ligands capable of utilizing the
imidazole base as one of the bidentate donors [33]. Considering the
growing interests in organic fluorophores with receptor capabilities
towards ions (metal or non-metal), this work presents a systematic
comparison of the photoluminescent properties of twelve
new fluorophores (1e12, Scheme 1a) based on imidazole/oxazole
derivatives (Scheme 1b), from an experimental point of view.
Syntheses of new compounds 3, 4, and 7e12 as well as X-ray
structures of 8, 9 and 11 are also herein reported. In efforts to
rationalize nature of soluteesolute interactions in solution, results
of 1H NMR studies of compounds 1e4 are also discussed.
and nstd are refractive indices of solvents employed for sample and
standard respectively).
2.1. Preparation of imidazoles/oxazoles
2-(6-methylpyridin-2-yl)-1H-phenanthro[9,10-d]imidazole (3)
and 2-(6-methylpyridin-2-yl)phenanthro[9,10-d]oxazole (4): Phe-
nanthrenequinone (1.00 g, 4.80 mmol) and ammonium acetate (7.40 g,
0.096 mol) were placed in a reaction flask, charged with nitrogen.
Ethanol (15 cm3), dichloromethane (15 cm3), a catalytic amount of
glacial acetic acid (0.5 cm3) and 6-methyl-2-pyridinecarboxaldehyde
(0.63 g, 0.480 mmol) were added and the ensuing mixture was
refluxed for 3 h. After cooling, the reaction mixture was neutralized
using concentrated aqueous ammonia and the volume was reduced
under reduced pressure. The organic components were extracted
using dichloromethane (50 cm3, trice) and the combined organic
extracts was concentrated and purified on silica gel column using
dichloromethane/petroleum ether (1:4) to elute the oxazole product
first and then ethyl acetate/petroleum ether (1:4) to elute the imid-
azole product. Portions containing the products were concentrated
and addition of petroleum ether gave 3 (0.61 g, 41.0%) and 4 (0.49 g,
33.2%) as off-white and white micro-crystals respectively.
2. Experimental
All starting materials were obtained commercially as reagent
grade and used without further purification. Phenanthroline-5,6-
dione was synthesized by literature procedure [34]. Compounds 1, 2,
5 and 6 were prepared according to our reported procedures [33].
Condensation reactions were carried out under nitrogen protection
by use of standard Schlenk techniques. Organic compounds were
purified on silica gel column to exclude impurities. Ethanol solvent
was refluxed over CaH2 and distilled prior to use. Elemental analyses
were performed on a Flash EA 1112 microanalyzer. NMR spectra
were recorded on a Bruker ARX-400 MHz instrument using TMS as
internal standard. IR spectra were recorded on a Nicolet 6700 FT-IR
spectrometer as KBr discs in the range of 4000e600 cmꢀ1. The
steady-state fluorescence spectra were measured on F4500 e FL
Fluorescence Spectrophotometer while fluorescence lifetimes were
obtained by time-correlated single photon counting technique
(Edinburgh Analytical Instruments F900 Fluorescence Spectrofluo-
rimeter). Thin films were prepared on quartz slides with 1 cm width
by spin coating. Fluorescence quantum yields (VF) were calculated
by comparative method using anthracene as standard [35,36].
Compound 3: Mp. 218e220 ꢁC. Selected IR peaks (KBr disc,
cmꢀ1):
n
3435s, 3072s, 1615w, 1595s, 1573s, 1462vs, 1450vs, 1235s,
796s, 752vs. 1H NMR (CDCl3, 400 MHz, TMS):
d
11.15 (br, s, 1H); 8.76
(d, J ¼ 8.0 Hz, 2H); 8.71 (d, J ¼ 8.4 Hz, 1H); 8.32 (d, J ¼ 8.0 Hz, 1H);
8.15 (d, J ¼ 8.0 Hz, 1H); 7.77 (dd, J ¼ 8.0 Hz, 1H); 7.66 (m, 4H); 7.21
(d, J ¼ 7.6 Hz, 1H); 2.68 (s, 3H). 13C NMR (CDCl3, 100 MHz, TMS):
158.06, 148.54, 147.76, 137.49, 128.57, 127.06, 125.45, 123.59, 122.48,
118.15, 24.31. Anal. Calc. for C21H15N3$H2O: C, 77.04; H, 5.23; N,
12.84. Found: C, 76.95; H, 5.18; N, 12.79.
Compound 4: Mp. 199e201 ꢁC. Selected IR peaks (KBr disc,
cmꢀ1):
n
3061s,1616s,1591s,1569vs,1459vs,1429vs,1233vs,1080vs,
798vs, 761vs. 1H NMR (CDCl3, 400 MHz, TMS):
d
8.75 (m, 3H); 8.47
(d, J ¼ 6.8 Hz, 1H); 8.28 (d, J ¼ 8.0 Hz, 1H); 7.82 (dd, J ¼ 7.6 Hz, 1H);
7.74 (m, 4H); 7.34 (d, J ¼ 7.6 Hz, 1H); 2.79 (s, 3H). 13C NMR (CDCl3,
100 MHz, TMS): 160.96,159.58,145.72,145.57,137.22,135.59,129.77,
129.11, 127.48, 127.27, 126.82, 126.36, 126.18, 124.97, 123.71, 123.40,
123.29, 121.47, 121.03, 120.33, 24.77. Anal. Calc. for C21H14N2O$H2O:
C, 76.81; H, 4.91; N, 8.53. Found: C, 77.18; H, 4.72; N, 8.54.
Z
!
ꢀ
ꢁ
IF;x
ð
n
Þd
n
2
1 ꢀ 10ꢀA
nx
nstd
std
Z
FF;x
¼
FF;std
1 ꢀ 10ꢀA
x
IF;std
ð
n
Þd
n
2-(pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (7)
and 2-(pyridin-2-yl)oxazolo[5,4-f][1,10]phenanthroline (8):
A
(IF,x
(
n
) and IF,std
(
n
) are fluorescence intensities at wavelength
n
mixture of phenanthroline-5,6-dione (5.00 g, 0.024 mol), ammo-
nium acetate (37 g, 0.480 mol), glacial acetic acid as catalyst
(1.00 cm3) and 2-pyridinecarboxaldehyde (2.86 cm3, 0.030 mol)
for sample and standard respectively; Ax and Astd are absorbance at
excitation wavelength for sample x and standard respectively; nx
Scheme 1. (a) List of fluorophores studied in this work and (b) synthetic route to imidazole/oxazole compounds.

264
A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
was treated according to procedure for 3 and 4 above except that
chloroform was used in place of dichloromethane and the crude
mixture was purified on silica gel column using ethanol/ethyl-
acetate/petroleum ether (1:10:10) as eluent. The portions that
contained pure products were concentrated and the precipitates
were filtered, washed with small amount of ethanol and dried to
afford isolated products of 7 (2.60 g, 36.4%) and 8 (0.67 g, 9.4%) as
white powders. Single crystals of 8 suitable for X-ray analysis were
grown by slow diffusion of n-hexane into its chloroform solution.
obtain isolated quantities of 11 as colourless crystals (0.22 g, 14.8%)
and 12 as whitish micro-needles (0.52 g, 12.7%). Single crystals
suitable for X-ray analysis were grown by slow evaporation of
solvent from ethanol solution of the compound.
Compound 11; Mp. >320 ꢁC. Selected IR peaks (KBr disc, cmꢀ1):
n
3481m, 3348m, 3054m, 2957vs, 2906s, 2864s, 1615m, 1542m,
1509m, 747vs. 1H NMR (400 MHz, TMS, CDCl3);
d
13.40 (s, br, 1H);
10.01 (s, br, 1H); 8.73 (d, J ¼ 8.0 Hz, 2H); 8.55 (br, 1H); 8.24 (br, 1H);
7.72 (m, 2H); 7.65 (br, 2H); 7.51 (br, 2H); 1.48 (s, 9H); 1.35 (s, 9H).
Anal. Calc. for C29H30N2O: C, 81.56; H, 7.20; N, 6.56. Found: C, 81.42;
H, 7.11; N, 6.53.
Compound 7: Mp. >300 ꢁC. Selected IR peaks (KBr disc, cmꢀ1):
3008m, 2944m, 1590s, 1562s, 1431vs, 1070m, 738vs. 1H NMR
(400 MHz, TMS, CDCl3);
11.60 (br, s, 1H); 9.17 (d, J ¼ 4.0 Hz, 2H);
n
d
Compound 12; Mp. >300 ꢁC. Selected IR peaks (KBr disc, cmꢀ1):
9.05 (d, J ¼ 8.0 Hz, 1H); 8.67 (d, J ¼ 4.4 Hz, 1H); 8.52 (d, 7.6 Hz, 1H);
8.45 (d, J ¼ 8.0 Hz, 1H); 7.92 (dd, J ¼ 8 Hz, 1H); 7.74 (dd, J ¼ 4.4,
8.0 Hz,1H); 7.68 (dd, J ¼ 4.4, 8.0 Hz,1H); 7.39 (dd, J ¼ 7.6 Hz,1H). 13C
NMR (100 MHz, CDCl3); 149.82, 149.17 148.84, 148.78, 148.05,
144.95, 144.78, 137.48,137.35, 130.34,128.85,125.79, 124.40, 123.40,
122.84, 121.26, 118.95. Anal. Calc. for C18H11N5$½H2O: C, 70.58; H,
3.95; N, 22.86%. Found: C, 70.96; H, 3.66; N, 22.93%.
n
3077m, 3000m, 2962vs, 2907s, 2866s, 1618s, 1544s, 1255s, 749vs.
1H NMR (400 MHz, TMS, CDCl3);
d
12.03 (s, 1H); 8.76 (dd, J ¼ 8.4 Hz,
2H); 8.55 (dd, J ¼ 1.4 Hz, 8.0 Hz,1H); 8.41 (d, J ¼ 1.32 Hz, 8.2 Hz,1H);
8.05 (d, J ¼ 2.4 Hz, 1H); 7.73 (m, 4H); 7.54 (d, J ¼ 2.4 Hz, 1H); 1.55
(s, 9H); 1.44 (s, 9H). Anal. Calc. for C29H29NO2: C, 82.24; H, 6.90; N,
3.31. Found: C, 82.18; H, 6.94; N, 3.40.
Compound 8: Mp. 269e271 ꢁC. Selected IR peaks (KBr disc,
2.2. X-ray measurements
cmꢀ1):
n
3053s, 2982m, 1584m, 1560m, 1456vs, 1446vs, 1059s,
741vs. 1H NMR (400 MHz, TMS, CDCl3);
d
9.26 (d, J ¼ 3.8 Hz, 2H);
X-ray single-crystal structure determination for compound 11
was carried out on Saturn724 using CrystalClear program (Rigaku
Inc., 2008), while 8 and 9 were done on Rigaku R-AXIS Rapid IP
9.03 (dd, J ¼ 1.7, 8.1 Hz, 1H); 8.91 (d, J ¼ 4.5 Hz, 1H); 8.82 (dd, J ¼ 1.7,
8.1 Hz, 1H); 8.47 (d, J ¼ 7.9 Hz, 1H); 7.97 (dd, J ¼ 7.9 Hz, 1H); 7.97
(m, 2H); 7.51 (dd, J ¼ 4.8, 7.5 Hz, 1H). 13C NMR (100 MHz, CDCl3):
162.09, 150.47, 150.02, 149.82, 145.73, 145.19, 145.07, 144.04, 137.22,
134.77, 130.86, 129.01, 125.53, 123.64, 123.38, 123.25, 122.94, 117.97.
Anal. calc. for C18H10N4O$½H2O: C, 70.35; H, 3.61; N,18.23%. Found:
C, 70.52; H, 3.65; 18.20%.
diffractometer both employing graphite-monochromated Mo-K
a
radiation and operated at ꢀ100 ꢁC. Cell parameters were obtained
by global refinement of the positions of all collected reflections.
2-(6-methylpyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenan-
throline (9) and 2-(6-methylpyridin-2-yl)oxazolo[5,4-f][1,10]
phenanthroline (10): Phenanthroline-5,6-dione (3.00 g, 0.014 mol),
ammonium acetate (22 g, 0.29 mol) and 6-methylpyridine-2-car-
boxaldehyde (2.1818 g, 1.25 equivalent) were reacted as described
for 7 and 8 above to obtain 9 (1.10 g, 24.5%) and 10 (0.37 g, 8.2%) as
light yellow and purple micro-crystals respectively. Single crystals
of 9 suitable for X-ray analysis were grown by slow diffusion of
n-hexane into its dichloromethane solution.
Compound 9: Mp. >300 ꢁC. Selected IR peaks (KBr disc, cmꢀ1):
3057s, 3016s, 1590m, 1562s, 1440s, 1068s, 739vs. 1H NMR (400 MHz,
TMS, CDCl3);
n
d
11.60 (br, s, 1H); 9.17 (d, J ¼ 4.1 Hz, 2H); 9.05
(d, J ¼ 8.2 Hz, 1H); 8.49 (d, J ¼ 7.7 Hz, 1H); 8.52 (d, 7.6 Hz, 1H); 8.30
(d, J ¼ 8.0 Hz, 1H); 7.80 (dd, J ¼ 7.7 Hz, 1H); 7.74 (dd, J ¼ 4.4, 8.0 Hz,
1H); 7.68 (dd, J ¼ 4.4, 8.0 Hz, 1H); 7.24 (d, partially overlapped with
CDCl3 residual peak, 1H); 2.66 (s, 3H). Anal. Calc. for C19H13N5: C,
72.25; H, 4.31; N, 22.17%. Found: C, 72.47; H, 4.26; N, 22.05%.
Compound 10; Mp. 266e268 ꢁC. Selected IR peaks (KBr disc,
cmꢀ1):
(400 MHz, TMS, CDCl3);
n
3016s, 1587s, 1558s, 1459vs, 1062s, 739vs. 1H NMR
9.27 (dd, J ¼ 2.0, 5.7 Hz, 2H); 9.05 (dd,
d
J ¼ 2.1, 10.8 Hz, 1H); 8.02 (dd, J ¼ 2.1, 10.9 Hz, 1H); 8.28
(d, J ¼ 10.3 Hz, 1H); 7.80 (m, 3H); 7.38 (d, J ¼ 10.3 Hz, 1H); 2.79
(s, 3H). 13C NMR (100 MHz, CDCl3): 162.50, 159.85, 150.03, 149.75,
145.24, 145.09, 144.07, 137.40, 134.87, 131.20, 129.21, 125.54, 123.65,
123.40, 123.07, 120.62, 118.09, 24.79. Anal. calc. for C19H12N4O: C,
73.07; H, 3.87; N, 17.94%. Found: C, 72.98; H, 3.85; N, 17.87%.
2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)
phenol (11) and 2,4-di-tert-butyl-6-(phenanthro[9,10-d]oxazol-
2-yl)phenol (12): Phenanthrenequinone (2.00 g, 9.61 mmol),
ammonium acetate (14.80 g, 0.192 mol) and 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (2.25 g, 9.61 mmol) were refluxed in glacial
acetic acid (30 cm3) for 3 h. After cooling, the reaction mixture was
neutralized using concentrated aqueous ammonia and the crude
product was filtered, dried and purified on silica gel column with
dichloromethane/petroleum ether (1:10). The fractions containing
pure products were combined, concentrated and allowed to stand to
Fig. 1. Ortep plot of the molecular structure of 8∙CHCl3 showing (a) offset avoidance of
stacking interaction and (b) planarity as well as separation between molecular planes.
Some labels and hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å): O(1)eC(6)
¼
1.353(3); O(1)eC(8)
¼
1.372(3); N(2)eC(6)
¼
1.307(4); N(2)e
C(7) ¼ 1.387(3); N(1)eC(1) ¼ 1.335(4); N(1)eC(5) ¼ 1.346(4); C(4)eC(5) ¼ 1.381(4).

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
265
Intensities were corrected for Lorentz and polarization effects
and empirical absorptions. The structures were solved by direct
methods and refined by full-matrix least-squares on F2. All non-
hydrogen atoms were refined anisotropically and all hydrogen
atoms were placed in calculated positions. Structure solution and
refinement were performed using the SHELX-97 package [37].
imidazole active proton, which was absent in 1H NMR spectra of
the oxazole analogues, could be seen at around 11.5 ppm for the
pyridyl molecules and at 13.4 ppm for the phenolic ligand 11.
Due to stronger deshielding, the 13C NMR spectra of the oxazoles
all gave the most downfield peaks within 163.0e160.0 ppm
while the imidazoles gave the corresponding peak between 158.0
and 148.0 ppm. The peak is assigned to the 2-imidazole carbon
between the two heteroatoms of the oxazole (e.g. C(6), Fig. 1a)
or imidazole rings (e.g. C(6), Fig. 2a). Moreover, there were more
separations of the 13C NMR peaks in the spectra of oxazoles unlike
the observed clustering of peaks obtained for the imidazoles.
Electronically, the imidazoles are expected to have higher charge
symmetry and equivalent carbon atoms. The oxazoles generally
exhibited significantly lower melting points relative to the imid-
azoles. Compounds 1, 2, 5 and 6 were obtained according to
recently published method [33].
d
d
d
3. Results and discussion
3.1. Synthesis of organic compounds
The compounds were obtained by condensation of a-diketones
with the appropriate aryl aldehydes (pyridinecarboxaldehydes or
3,5-di-tert-butyl-2-hydroxybenzaldehyde) under reflux and in
the presence of excess ammonium acetate (Scheme 1b). Unlike for
1,2-diphenyl-1,2-ethanedione, which forms only the imidazole
condensation product, phenanthrenequinone or phenanthroline-
5,6-dione yielded both imidazole and oxazole products competi-
tively. The rigidity and co-planarity of the carbonyl groups in
the fused-ring starting materials enabled close proximity of
heteroatoms towards competitive cyclization of N^N diimine and
N^O keto-imime intermediates [33]. The oxazole products could
be differentiated from the imidazole products on the basis of 1H
NMR, 13C NMR, melting point and elemental analysis. The
3.2. X-ray structures
Molecular structures of 8, 9 and 11 were confirmed by X-ray
experiments. Compound 8 crystallized in orthorhombic Pnma
space group while compounds 9 and 11 crystallized in triclinic P-1
space group all with four molecules in each unit cell. Crystallo-
graphic data and refinement details are tabulated in Table 1.
Fig. 2. Ortep plot of the molecular structure of 9 showing (a) 1-Dimentional hydrogen bonding chain, (b) stacking separation distance and lower co-planarity between pyridyl ring
and the rest of the molecule, and (c) partial face-to-face stacking mode. Some hydrogen atoms and labels have been omitted for clarity. Selected bond lengths (Å): N(1)eC(1) ¼ 1.340
(4); N(1)eC(5) ¼ 1.340(4); N(2)eC(6) ¼ 1.331(4); N(2)eC(8) ¼ 1.369(4); N(3)eC(6) ¼ 1.361(4); N(3)eC(7) ¼ 1.371(4); N(3)eH(3N) ¼ 1.0285; C(4)eC(5) ¼ 1.384(5); N(20)eC
(60) ¼ 1.317(4); N(20)eC(80) ¼ 1.375(4); N(10)eC(50) ¼ 1.337(5); N(10)eC(10) ¼ 1.339(4); N(30)eC(60) ¼ 1.364(4); N(30)eC(70) ¼ 1.380(4); C(40)eC(50) ¼ 1.379(5).

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A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
The molecular structure of compound 8 is shown in Fig. 1 with
{N(1)eC(3)eC(2)eC(1)eC(4)eC(5)} and one of the phenanthrolineyl
rings of an adjacent partner {N(5)eC(12)eC(11)eC(10)eC(13)eC(9)}
with a separation of 3.228 Å.
the selected bond lengths around heteroatoms of the pyridyl and
oxazolyl rings. The discrete adduct of chloroform was observed
and completely devoid of networking or stacking interactions.
The observation is considered to result from absence of active
protons as in imidazoles, which usually exhibits one dimension (1-
D) networks [38]. Noteworthy also is the co-planarity (0ꢁ) between
the pyridyl plane {O(1)eN(2)eC(6)eC(7)eC(8)} and the plane
occupied by the oxazole ring atoms {N(1)eC(1)eC(2)eC(3)eC(4)e
Though attempts to obtain suitable single crystals for the
phenanthrene-substituted compounds 1e4 yielded twined crystals
for the imidazoles (1 and 3) and tiny needles for the oxazoles (2 and
4), it was anticipated that the phenol compound 11 would give
a clue about intermolecular interactions for the phenanthreneyl
systems. X-ray analysis for compound 11 revealed two crystallo-
graphically independent molecules that consist of one flat molecule
with disordered tBu and oxo atoms and another bent, but non-
disordered molecule (Fig. 3a). The angle of bending over between
the phenol plane {C(6)eC(4)eC(2)eC(3)eC(5)eC(1)} and the
phenanthreneyl fragment {C(10)eC(8)eC(9)eC(16)eC(17)eC(15)}
in the tilted molecule is as large as 14.66ꢁ. Moreover, the crystal
structure revealed that the disordered, flat molecule exists in
Zwitterionic form in which the phenolic proton is bonded to the
imidazole ring to yield oxo anion on phenol ring and imidazolium
cation. The phenolic proton of the second molecule is intact (Fig. 3a
and c). In fact, the imidazole protons were quite obvious on the
difference map and were picked before fixing all protons in calcu-
lated positions in the course of structure refinement for compound
11. The notable difference (0.034 Å) in the imidazole ring bond
lengths N(3)eC(7) (1.334(2) Å) and N(4)eC(7) (1.368(3) Å)
observed for the bent molecule in contrast to the insignificant
difference (0.001 Å) for the corresponding bonds of the imidazo-
lium ring of the flat molecule {N(2)eC(36) ¼ 1.344(3) Å; N(1)eC
(36) ¼ 1.345(3) Å} supports existence of Zwitterionic structure of
C(5)} (Fig. 1a and b). This may imply an optimum
p-conjugation
between the pyridyl ring aromatic system and the rest of the
molecule. The complete offset interaction in the structure of
compound 8, which is totally devoid of stacking, is worthy of note
(Fig. 1a and b). Furthermore, the separation between the plane of
one molecule and another (3.396e3.345 Å) is larger than observed
value for the closely related imidazole analogue 9. It could be
conclude that the oxazolyl derivatives possess inherently weaker
intermolecular interaction tendencies.
Imidazole compound 9 maintains a 1-D chain involving hydrogen
bonds between imidazole proton of one molecule and the phenan-
throlineyl nitrogen atoms of the next molecule (Fig. 2a). A twist as
large as 10.92ꢁ between the pyridyl ring {C(5)eC(4)eC(3)eC(2)eC
(1)eN(1)} and the phenanthrolineyl moiety {N4eC12eC11eC10e
C9eC13eN5eC16eC17eC18eC14eC15} was observed (Fig. 2a),
which has significance on extent of overlap that is possible between
the pyridyl
Fig. 2b and c, it could also be observed that a significantly shorter
-stacking distance exists between the pyridyl ring of one molecule
p-system and imidazo-phenanthroline bulk. As shown in
p
the flat molecule. Compound 11 also revealed strong
pep inter-
actions (Fig. 3b) between the phenanthrene rings (3.321 Å), which
held the molecules into repeating tetrameric packs (Fig. 3c).
The observed face-to-face interactions, which are due to the
rigid planar nature of the compounds 8, 9 and 11, are considered to
suggest poorer intermolecular interaction tendencies in oxazoles
and stronger interactions for the imidazoles. Although the role
of solvent of crystallization and crystal packing forces may be
significant, the stacking distances suggests that a more stable face-
to-face interaction would be expected for imidazole frameworks
than for the oxazoles.
Table 1
Crystallographic data and refinement details for 8, 9 and 11.
Parameters
8.CHCl3
19H11Cl3N4O
417.67
9
11
Formula
Fw
C
C19H13N5
311.34
173(2)
0.71073
Triclinic
P-1
10.008(2)
12.402(3)
12.504(3)
88.88(3)
78.17(3)
75.05(3)
1466.7(5)
4
C29H30N2O
422.55
173(2)
0.710747
Triclinic
P-1
10.109(2)
13.670(3)
17.355(3)
101.33(3)
101.23(3)
96.15(4)
2279.9(8)
4
Temperature (K)
Wavelength (Å)
Crystal system
space group
a (Å)
173(2)
0.71073
Orthorombic
Pnma
9.814(2)
6.6845(13)
27.146(5)
90.00(0)
90.00(0)
90.00(0)
1780.8(6)
4
b (Å)
c (Å)
3.3. Absorption properties of compounds 1e12
a
b
g
(deg)
(deg)
(deg)
The absorption spectra of compounds 1e4 in cyclohexane and
dichloromethane as well as those of compounds 5e12 in EtOH are
presented in Fig. 4aed. Comparing the respective absorption bands
of the phenanthreneyl derivatives 1e4, it is noteworthy that the
oxazoles 2 and 4 absorbed at relatively blue shifted wavelengths
compared to the imidazoles 1 and 3 (Fig. 4a). This suggests more
electron delocalization in 1 and 3 than for 2 and 4. Lower electronic
conjugation by the oxazole ring should be expected on account of
non-bonding electronic orbital orientation and greater electro-
negativity of the oxo atom. However, the band shapes are generally
the same for the oxazoles as for the imidazoles. The rest of the
imidazole/oxazole pairs generally maintained the same absorption
maximums for the oxazoles as for the corresponding imidazoles
(7 versus 8, 9 versus 10 and 11 versus 12; Table 2). Furthermore, the
extended conjugation offered by presence of the phenanthrene
group is reflected by longer absorption wavelength bands in the
region 359e367 nm observed for 1e4, 11 and 12. Shorter absorp-
tion wavelengths in the range of 323e331 nm were recorded for the
diphenyl-substituted (5 and 6) and phenanthrolineyl-substituted
Volume (Å3)
Z
Calculated
1.558
1.410
1.231
density (g cmꢀ3
)
m
(mmꢀ1
)
0.532
848
0.089
648
0.074
904
F(000)
Crystal size (mm) 0.78 ꢂ 0.71 ꢂ 0.38 0.66 ꢂ 0.34 ꢂ 0.18 0.34 ꢂ 0.30 ꢂ 0.26
q
range (deg)
1.50e27.47
2.37e25.00
ꢀ11 ꢃ h ꢃ 11
ꢀ14 ꢃ k ꢃ 14
ꢀ14 ꢃ l ꢃ 14
9390
1.54e27.49
ꢀ13 ꢃ h ꢃ 13
ꢀ17 ꢃ k ꢃ 17
ꢀ22 ꢃ l ꢃ 22
28039
Limiting indicates ꢀ12 ꢃ h ꢃ 12
ꢀ8 ꢃ k ꢃ 8
ꢀ35 ꢃ l ꢃ 35
No. of rflns
collected
No. of unique rflns 2221
R(int)
3935
5143
0.0483
99.7,
10392
0.0396
99.4,
0.0168
99.9,
Completeness
q
¼ 27.47
q
¼ 25.00
q
¼ 27.49
(%) to
No. of parameters 161
Goodness- 1.130
of-fit on F2
Final R indices
q
(o)
445
1.317
652
1.144
R1 ¼ 0.0431
R1 ¼ 0.0938
wR2 ¼ 0.1550
R1 ¼ 0.1266
wR2 ¼ 0.1657
0.272, ꢀ0.272
R1 ¼ 0.0756
wR2 ¼ 0.1916
R1 ¼ 0.0875
wR2 ¼ 0.2001
0.408, ꢀ0.410
(I > 2sigma(I)) wR2 ¼ 0.1167
R indices (all data) R1 ¼ 0.0572
wR2 ¼ 0.1343
(7e10) derivatives, and indicate lower extent of
p conjugation.
The shorter wavelength absorption bands for the phenanthrolineyl
analogues 7e10 could be attributed to perturbation of peelectron
system due to difference of nitrogen as heteroatom.
Largest diff. peak
and hole (e Åꢀ3
0.541, ꢀ0.445
)

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
267
Fig. 3. Ortep plot of 11 showing (a) two crystallographically independent molecules, (b) their mode of
pep stacking interaction, and (c) tetrameric packing. Some labels and
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): O(1)eC(1) ¼ 1.355(2); N(3)eC(7) ¼ 1.334(2); N(3)eC(8) ¼ 1.383(3); N(4)eC(7) ¼ 1.368(3); N(4)eC
(9) ¼ 1.376(2); N(4)eH(4N) ¼ 0.8904; N(1)eC(37) ¼ 1.377(3); N(1)eH(1N) ¼ 0.8800; N(2)eC(36) ¼ 1.344(3); N(2)eC(38) ¼ 1.378(3); N(2)eH(2N) ¼ 0.8800; C(34)eO(2) ¼ 1.349(4);
C(30)eO(20) ¼ 1.333(3); O(2)eH(300) ¼ 0.3936; O(20)eH(30) ¼ 0.3757.
Methyl substitution did not yield any obvious difference in
absorption properties of substituted derivatives compared to
unsubstituted derivatives (e.g. 5 and 6; Table 2). On the basis of
band shapes and wavelengths, it could be concluded that the
spectroscopic properties of the series are largely dependent on
substituent attached to the 4,5-positions of the imidazole ring. On
exposure to UV radiation (355 nm), compounds 1 and 3 exhibit new
broad absorption peaks around 430 nm (Fig. 4b). This absorbing
Observed photoluminescence parameters are summarized in
Table 2. It could be noticed that compounds 1e10 gave the primary
excited state emissions in the blue region of the visible spectrum
(368e416 nm) while compounds 11 and 12 produced large Stokes
shifted secondary emissions in the green region (470e508 nm) as
a result of excited state intramolecular protoneelectron transfer
(ESIPT) process (Table 2) [38]. According to X-ray diffraction pattern,
existence of Zwitterions in solid state of 11 (Fig. 3a) suggests the ease
for achieving photo-induced ESIPT emissions in 11e12. Emission
lifetimes in the various solvents confirm fluorescent nature of the
emissions rather than phosphorescence (Table 2).
species have been attributed to
pep stacked aggregate formation
that is photo-induced. Further experiments were conceived and
executed to explore the nature of excimer formed.
One notable observation in the photoluminescence data for
the pyridyl derivatives 1e10 (Table 2) is the strong similarities in
fluorescent parameters between 6-methyl-substituted compounds
and their corresponding unsubstituted analogues in terms of
3.4. Photoluminescence properties of compounds 1e12
All emission scans were carried out at room temperature using
same concentration (5 ꢂ 10ꢀ5 mol dmꢀ3). The photoluminescence
spectra of all compounds were obtained in cyclohexane and ethanol.
fluorescence quantum yield VF, photoluminescence lifetime
s and
energy separation between ground state So and the first excited

268
A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
Fig. 4. Absorption spectra of (a) 1e4 in cyclohexane showing blue shift of the oxazoles (2 and 4) relative to the imidazoles (1 and 2); (b) 1e4 in dichloromethane showing excimer
absorption for compounds 1 and 3 after exposure to UV; (c) 5, 6, 11 and 12 in EtOH; (d) 7, 8, 9 and 10 in EtOH. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article).
state S1 (e.g. for imidazo-pyridines, Fig. 5a: 1 versus 3, 5 versus 6, 7
versus 9; e.g. for oxazo-pyridines, Fig. 5b: 2 versus 4, 8 versus 10).
Therefore, methyl substituent in the ortho-position to pyridyl
nitrogen neither produced significant effects on ground and excited
state energies nor on excited state photo-relaxation processes for
this family of molecules in solution. In the various solvents, oxazole
analogues were observed to exhibit stronger photoluminescence
efficiencies in comparison with their respective imidazole deriva-
tives (Table 2: 1 versus 2, 3 versus 4, 7 versus 8, 9 versus 10). It could
be reasoned that the presence of active imidazolyl proton would
enhance non-radiative relaxation processes via solventesolute or
soluteesolute interactions. This view is supported by the extensive
intermolecular interactions in the solid state structures of
compounds 9 and 11 (Figs. 2 and 3), which is absent in the oxazole
compound 8 (Fig. 1). An additional probable reason is the better co-
planarity in the oxazole compounds than the imidazole compound
as suggested by crystal structure of compound 8. This factor is
favorable for enhancement of fluorescence efficiency through
better electronic overlap of the aromatic system of the pyridyl ring
and the oxazo-phenanthreneyl moieties. Furthermore, dramatic
reduction of quantum yields in going from non-polar cyclohexane
systems to the more polar ethanol solutions is in agreement
with the proposed role of intermolecular interactions towards
reducing fluorescence quantum yields. According to Table 2, this
solvent effect is more pronounced for imidazoles (e.g. 1,
VF, Cyclohexane ¼ 0.69 versus VF, Ethanol ¼ 0.02; 3, VF, cyclohexane ¼ 0.97,
VF, Ethanol ¼ 0.02) than for the oxazoles (e.g. 2, VF, Cyclohexane ¼ 0.83
4,5-position of the five-membered ring gave a general order of
phenanthreneyl > 4,5-diphenyl > phenanthrolineyl (Table 2),
which it is in accordance with capacity of their molecular structures
to delocalized peelectrons. However, the quantum yield values in
ethanol maintained quite lower and similar values.
The ESIPT emissions of compounds 11 and 12 appeared to
exhibit opposite trends of the above observations. Furthermore, the
relatively lower quantum yields observed for compounds 11 and 12
could be ascribed to inherent vibrational quenching due to the two
tBu substituents (Fig. 5b red trace).
3.5. Photo-induced fluorescence switching of compounds 1e4
It was discovered that compounds 1e4 possessed some inter-
esting properties worthy of further examination. Compounds 1 and
3 were observed to undergo photo-induced excimer formation
with broad absorption band around 425 nm (Fig. 4b) and corre-
sponding excimer emission around 550 nm in ethanol (Table 2) or
500 nm in dichloromethane (Table 3, Fig. 6a). Such excimer
formation was not observed in the non-polar cyclohexane. Also
an interesting fact is that, unlike for any other compound in the
studied series, compound 3 displayed a very brilliant solid film
photoluminescence in the green region of the visible spectrum
at room temperature (Fig. 7a). Besides electronic effects, there
is probably an interesting steric effect of the methyl group on
molecular arrangements in solid state, and is probably an evidence
of different solid state molecular arrangements in 3 compared to 1.
Though the exact mechanism for forming the new solution
species via excited states is still not clear, experimental findings
suggests that, depending on solvent properties, the molecules
versus VF,
¼
0.16; 4, VF,
¼
0.95 versus
Ethanol
Cyclohexane
VF, Ethanol ¼ 0.16). For the pyridyl molecules 1e10 in cyclohexane,
substituent effects on photoluminescence efficiencies at the

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
269
Table 2
Absorption and photoluminescence data (1e12) in cyclohexane and ethanol.
(ns)d
VF
krꢂ108(S-1
)
knrꢂ108(S-1
)
e
Compounds
labs-max
(nm)
lEx-max
(nm)
lEm-max
(nm)d
Stokes shift
(nm)d
s
C6H12
EtOH
1
348, 366
346, 363
360
357
389
394 [555]
29
37 [198]
2.0
2.1 [4.4]
0.69
0.02
3.450
0.095 [0.045]
1.550
4.667 [2.227]
C6H12
EtOH
2
3
332, 361
342, 359
355
366
383
406
28
40
2.4
2.3
0.83
0.16
3.458
0.696
0.708
3.652
C6H12
EtOH
Film
348, 367
346, 363
360
357
390
393 [550]
518
30
36 [193]
2.0
2.0 [4.6]
0.97
0.02
4.850
0.100 [0.043]
0.150
4.900 [2.130]
C6H12
EtOH
4
5
333, 361
342, 360
352
364
383
404
31
40
eb
0.95
0.16
e
e
2.2
0.727
3.818
C6H12
EtOH
331
327
350
348
386
397
36
49
2.4
1.6
0.26
0.01
1.083
0.063
3.083
6.188
C6H12
EtOH
6
331
329
342
342
385
394
43
52
0.7
1.4
0.35
0.01
5.000
0.071
9.286
7.071
C6H12
EtOH
7
ea
368
348
389
412
21
64
ec
ec
e
e
272, 325
4.9
0.03
0.061
1.980
C6H12
EtOH
8
280, 328
272, 323
328
347
375
382
47
35
1.3
3.5
0.18
0.03
1.385
0.086
6.308
2.771
C6H12
EtOH
9
ea
337
350
368
416
31
66
ec
ec
e
e
278, 324
5.0
0.03
0.060
1.940
C6H12
EtOH
Film
10
272, 328
272, 324
329
352
376
385
443
47
33
1.4
3.7
0.16
0.03
1.143
0.081
6.000
2.622
C6H12
EtOH
Film
11
12
340, 366
336, 365
360
370
492
470
481
132
100
3.5
2.8
0.23
0.06
0.657
0.214
2.200
3.357
c
c
C6H12
EtOH
Film
345, 365
344, 363
358
357
508
508
494
150
151
e
e
e
e
4.0
<0.01
>0.025
>2.475
a
Low solubility.
Value was in same range with response factor.
Signal too weak due to low solubility.
b
c
d
e
s
represents fluorescence emission lifetimes and values in square bracket represent photo-induced excimer emission data.
VF is fluorescence quantum yield for primary emissions.
undergo some sort of intermolecular adduct formation/aggrega-
tion, which is of a rather high stability for compounds 1 and 3,
but reversible in the case of compounds 2 and 4. This proposed
aggregate formation is supported by emission wavelengths occur-
ring in the red-shifted region for photo-generated species in
solutions as for solid film of compound 3 (Table 2). Attempts to
obtain suitable single crystals for compounds 1 and 3 only yielded
twined crystals. This is thought to result from incorporation of
already assembled aggregate units along with single molecules into
growing crystal lattices. Though the “offset face-to-face” stacking
Fig. 5. Excitation and photoluminescence spectra for (a) imidazole compounds 1, 3, 5, 6, 7, 9 and 11, and (b) oxazole compounds 2, 4, 8, 10 and 12 in ethanol. Emission scans were
excited at excitation maxima.

270
A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
Table 3
and neighbouring molecules. Although compounds
were anticipated to give similar emission characteristics as for
compounds 1 and 3 in cyclohexane on account of same heteroatom
5 and 6
Photoluminescence data for compounds 1e4 in CH2Cl2 and CH3CN.
Compounds lExꢀmax (nm) lEmꢀmax (nm)a Stokes
s
(ns)b
shift (nm)a
substitution, compounds
5 and 6 should possess additional
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
1
2
3
4
367
NDc
369
ND
364
ND
401[503]
395, 775
403
405, 785
395 [500]
395, 775
399
34 [136]
6.1
1.9
2.9
2.1
inherent thermal deactivation due to vibrations of the pendant
phenyl substituents on the 4- and 5-positions of the imidazole ring.
This conclusion is supported by the obviously larger non-radiative
rates in both non-polar and polar solvents (Table 2). The methyl
substituent did not have any notable effects on kr and knr values.
The longer lifetime of emissions from photo-induced excimer
species coupled with their kr and knr values that are lower than for
the corresponding kr and knr values for the emissions of free
molecule also suggests formation of a more rigid/larger photo-
aggregation species, which may involve stacking.
NDc
34
ND
31 [136]
ND
46
ND
2.2
2.1
353
ND
405, 790
a
Values in square bracket represent photo-induced excimer emission data.
Primary emission lifetime; same values were obtained for both visible and near-
b
infrared emissions.
c
ND ¼ Not determined.
Assignment of longer wavelength emissions to stacking of planar
moieties is known for some pyreneyl [21,22] and xanthone
compounds [39]. However, to the best of our knowledge, such photo-
induced process as observed for 1 and 3 is not known. Another
interesting feature of compounds 2 and 4 is the photo-switching of
emission band between the 400 nm and 550 nm regions of the
visible spectrum. It is believed that the new, short-lived emission
bands that appear at red-shifted wavelengths resulted from photo-
induced excimer species. Fluorescence lifetimes for compounds 1e4
as further explored in acetonitrile were generally of the order of
nanoseconds (ns). The short-lived species for compounds 1 and 3
quickly reverts to the initial species after the laser source is turned
off (Fig. 6b). This ‘‘oneoff’’ photo-switching of emission may be of
potential application in molecular signaling.
patterns observed in the structures of compounds 9 and 11 (Figs. 2c
and 3c) affords a clue about possible intermolecular arrangements
leading to the species formed after photo-irradiation, possibility of
solventesolute adduct aggregation can not be dismissed.
Except for the cases of cyclohexane solutions of compounds
1e4, it was found that the rate of non-radiative events knr effec-
tively outweighed radiative rates kr in depopulating the excited
states for solutions of compounds 1e12 at room temperature. The
derived radiative/non-radiative lifetime data (Table 2) suggest the
importance of polarepolar interactions in depopulating the excited
state since the more polar ethanol solutions generally yielded
larger non-radiative rates. The exceptions found in non-polar
solvent for the phenanthreneyl ligands 1e4 could be attributed to
the more stable conjugation in the phenanthreneyl ligands, which
is probably as a result of lesser number of heteroatoms resulting in
lesser probabilities of polar interactions between the fluorophores
The results for further measurements in dichloromethane
and acetonitrile, which were recorded to further investigate the
characteristics of the new fluorophores formed after photo-exci-
tation of compounds 1e4, are presented in Table 3. Generally,
Fig. 6. 355 nm Laser flash-photolysis experiments in dichloromethane showing (a) irreversible photo-aggregation of compound 1 and (b) photo-switching behaviour of compound 2.
Fig. 7. Photoluminescence spectra of (a) compound 3 as solid thin film and (b) compounds 1e4 in acetonitrile showing the strong near-infrared emissions.

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
271
Fig. 8. Vertical stack of 1H NMR of 1 in CDCl3 at increasing exposure to UV radiation (300 nm).
trends in these data are comparable to those observed in ethanol.
3.6. 1H NMR studies
Furthermore, near-infrared emission bands, which were quite weak
in ethanol, cyclohexane and dichloromethane, were considerably
stronger in the acetonitrile spectra (Fig. 7b). Therefore, it is
concluded that there is a strong dependence of photoluminescence
characteristics on nature of condensed media for the investigated
molecules.
It is anticipated that the affected protons of compounds 1e4 in
the photo-induced intermolecular interactions should be broad-
ened and/or shifted while the unaffected protons would retain
their peak sharpness and chemical shift values. This will only occur
for interactions that do not lead to drastic alteration of chemical
Fig. 9. Vertical stack of 1H NMR of 3 in CDCl3 at increasing exposure to UV radiation (300 nm).

272
A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
Fig. 10. Full spectrum of compound 3 showing methyl signal broadening and downfield shift.
structures. In efforts to further probe the nature of interactions that
follows photo-excitation, proton chemical shifts were investigated
for compounds 1e4 in d-CHCl3 solutions at varying exposures times
to 320 nm. The possibility of photo-stacking was thus supported in
the cases of compounds 1 and 3 (Figs. 8 and 9 respectively).
Only the imidazole analogues 1 and 3 gave dynamic changes since
the time scale for reversal of photo-switching events observed in
solutions of derivatives 2 and 4 were too short to observe changes
in the regular 1H NMR experiments.
4. Conclusion
Twelve imidazole/oxazole based fluorophores,1e12, heterocycles
were prepared and properly characterized. Structural characteriza-
tions were performed for 8, 9 and 11. On the basis of substituent
factors, a comparative study of emission properties revealed that
oxazole derivatives generally yielded better photoluminescence
efficiencies compared to their imidazole analogues. Phenanthreneyl
derivative 1e4 exhibited more promising characteristics in terms of
emission efficiency and applicability for molecular signaling, etc.
One interesting observation is that compound 3 exhibited a brilliant,
green solid state photoluminescence, which may be suitable for
organic lighting materials. Due to photo-induced intermolecular
interactions in polar solvents, laser flashing of the oxazole analogues
2 and 4 produced reversible fluorescence switching between 400 nm
and 550 nm of the visible spectrum, while the imidazole derivatives 1
and 3 showed irreversible fluorescence band changes in the same
wavelength region. Pulse laser flash (355 nm) with 10 Hz irradiation
and 1H NMR experiments suggests that excimer species formed by 2
and 4 reverts to the initial molecular forms in short time scales. It is
believed that compounds 2 and 4 are promising candidates for
further investigations towards molecular sensing/signaling applica-
tions. Ligands 1 and 3 may find application in the areas of selective
sensing of certain chemical species such as inorganic or organic ions,
biochemicals, etc, that selectively hinders excimer formation or
selectively unpacks photo-induced aggregation. Based on 1H NMR
studies, ‘‘offset face-to-face’’ stacking could not be ruled out.
It is also worthy of note that the active imidazole proton signals
for 1 and 3 (dNeH ¼ 11.14 ppm for 3) also varnished during NMR
studies, which indicates the importance of the NeH functionality
in the proposed intermolecular interactions between molecules.
We could observe that the pyridyl aromatic ring protons for ligand 1
were not affected in the process since only three protons were
mainly downfield shifted as they became progressively broadened
till they finally varnished. The coupling constants for the affected
protons show they are neighbours. Other proton signals largely
maintained their original chemical shift values and sharpness.
Hence, the mode of interactions may involve the phenanthreneyl
protons of compound 1 in a similar stacking manner to that observed
in the X-ray structure of the phenanthreneyl ligand 11 (Fig. 3b).
On the other hand, similar experiments on compound 3 revealed
that five types of proton chemical shift environments underwent
changes. This suggests that the whole length of the molecule is
involved in intermolecular interaction (Fig. 9). This view is supported
by methyl signal broadening and shift from 2.68 ppm to 2.71 ppm
(Fig. 10). The reasoning is in line with the manner of interaction
observed in solid state structure of the similarly methyl-substituted
compound 9 (Fig. 2b). It is also important to mention that the 1H
NMR experiments conducted on samples already exposed to 320 nm
irradiation and subsequently kept for days in the dark (wrapped in
aluminium foil) still gave the same signals with varnished peaks
for protons involved in intermolecular interaction. We therefore
conclude that photo-induced fluorescent switching for compounds
1 and 3 is irreversible while being reversible for compounds 2 and 4.
Supplementary material
CCDC 761256, 761257 and 761258 contain the supplementary
crystallographic data for compounds 8, 9 and 11. These data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: þ44 01223 336033; e-mail: deposit@
ccdc.cam.ac.uk).

A.O. Eseola et al. / Dyes and Pigments 88 (2011) 262e273
273
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Acknowledgement
The authors thank the National Natural Science Foundation
of China for support of this research (Grant No. 20773149). A.O.E. is
grateful to the Chinese Academy of Science (CAS), The Academy of
Science for The Developing World (TWAS) for the Postgraduate
Fellowships and Redeemer’s University for granting study leave.
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