N-2-Aryl-1,2,3-triazoles: A novel class of UV/blue-light-emitting fluorophores with tunable optical properties

Article abstract of DOI:10.1002/chem.201002937

The N-2-aryl-1,2,3-triazole derivatives (NATs) were developed as a new class of UV/blue-light-emitting fluorophores. Though both N-1-aryl-1,2,3- triazoles and N-2-aryl-1,2,3-triazoles gave strong photo absorption under excitation at 330 nm, only the N-2-analogous showed strong fluorescence emission in the UV/blue range with high efficiency in various solvents (quantum yield Φ around 0.3-0.5). Significant substituted group effects were observed, allowing tunable optical properties with emission (λ_max) from 350-400 nm and Stokes shift from 38-93 nm. The computational studies along with X-ray crystal structures indicated the significance of the effective conjugation between triazole ring and aryl groups on the N-2 position. The planar intramolecular charge transfer (PICT) mechanism was proposed, which was supported by solvent effect studies. Simple derivatizations gave NAT-modified lysine and strong UV/blue emitting bis-NAT (Φ=0.76, λ_max= 390), which suggested the great potential of this new class of fluorophores in biological and material science research.

Full text of DOI:10.1002/chem.201002937

FULL PAPER
DOI: 10.1002/chem.201002937
N-2-Aryl-1,2,3-triazoles: A Novel Class of UV/Blue-Light-Emitting
Fluorophores with Tunable Optical Properties
Wuming Yan,[a] Qiaoyi Wang,[a] Quan Lin,[b] Minyong Li,[c] Jeffrey L. Petersen,[a] and
Xiaodong Shi*[a]
Abstract: The N-2-aryl-1,2,3-triazole
derivatives (NATs) were developed as
a new class of UV/blue-light-emitting
fluorophores. Though both N-1-aryl-
1,2,3-triazoles and N-2-aryl-1,2,3-tria-
zoles gave strong photo absorption
under excitation at 330 nm, only the N-
2-analogous showed strong fluores-
cence emission in the UV/blue range
with high efficiency in various solvents
(quantum yield F around 0.3–0.5). Sig-
nificant substituted group effects were
observed, allowing tunable optical
properties with emission (lmax) from
350–400 nm and Stokes shift from 38–
93 nm. The computational studies
along with X-ray crystal structures indi-
cated the significance of the effective
conjugation between triazole ring and
aryl groups on the N-2 position. The
planar intramolecular charge transfer
(PICT) mechanism was proposed,
which was supported by solvent effect
studies. Simple derivatizations gave
NAT-modified lysine and strong UV/
blue emitting bis-NAT (F=0.76, lmax
=
390), which suggested the great poten-
tial of this new class of fluorophores in
biological and material science re-
search.
Keywords: charge-transfer
· fluo-
rescent probes · quantum yield ·
Stokes shift · triazoles
Introduction
report the N-2-aryl-1,2,3-triazoles (NATs) as new UV/blue-
light-emitting compounds with tunable emission and adjust-
able Stokes shift through planar intramolecular charge
transfer (PICT).
The main challenge in developing UV/blue-light-emitting
small molecules is the optical activity and photostability di-
lemma. As shown in Scheme 1A, although condensed poly-
Photoactive molecules are important in chemistry, biology,
and material research.[1] The last two decades evidenced the
great advance in this research area, where fluorescent-active
dyes have been applied to many different research fields,
such as biological target imaging,[2] effective photosensors,[3]
and novel photoactive materials.[4]
Two key features of fluorescence-emitting molecules are
quantum efficiency and the wavelengths of excitation and
emission. Although a great number of fluorophores has
been developed over the years, effective UV/blue-light-emit-
ting small molecules are rare[5] due to the relative high-
energy gap required between the interactive orbitals, which
may cause either poor photostability or low quantum effi-
ciency.[6] However, the fast growing research areas, such as
OLED display and Fçrster resonance energy transfer stud-
ies, in photoactive compounds generated increasing needs
for effective UV/blue-light-emitting molecules. Herein, we
Scheme 1.
[a] W. Yan, Q. Wang, Prof. Dr. J. L. Petersen, Prof. Dr. X. Shi
C. Eugene Bennett Department of Chemistry
West Virginia University, Morgantown, WV 26506 (USA)
Fax : (+1)304-293-4904
AHCTUNGTREGaNNUN romatic structures have an effective aromatic conjugation,
E-mail: Xiaodong.Shi@mail.wvu.edu
usually these compounds possess a low energy gap between
excited and ground states, thus, giving emission at a longer
[b] Prof. Dr. Q. Lin
State Key Lab of Supramolecular Structure and Materials
Jilin University, Changchun, Jilin, 130012 (P.R. China)
wavelength.
Alternatively,
bis-aromatic
structures
(Scheme 1B) could provide the needed larger energy gap
for potential UV/blue emitting.[7] However, the steric repul-
sions between the ortho-substituted groups cause poor con-
jugation between the two aromatic rings and therefore, sig-
nificantly decrease the photoactivity. To overcome this prob-
[c] Prof. Dr. M. Li
Department of Medicinal Chemistry, School of Pharmacy
Shandong University, Jinan, Shandong, 250012 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002937.
Chem. Eur. J. 2011, 17, 5011 – 5018
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5011

lem, extended aromatic systems have been developed to en-
hance the conjugation (Scheme 1C).[8] Blue-light emission
has been observed with this type of molecules, such as the
commercial available blue dye disodium 4,4’-bis(2-sulfostyr-
yl)biphenyl (DSBP, STD-1).[9] However, the photolabile C=
C double bonds considerably limit the potential applications
of these compounds.[10]
Our interest in developing novel blue-emitting small mol-
ecules was initiated by our recent discovery shown in
Figure 1:[11] whereas the N-1-aryl isomer 1a’ gave almost no
while maintaining effective emission in the UV/blue range.
The structures of the NATs allowed substitutions on three
different positions: C-4, C-5, and N-2. Although the C-4 and
C-5 substitutions are away from the N-2 aryl groups, they
may strongly influence conjugation at the N-2 position
through electronic effect. This concern was confirmed by
the X-ray crystal structures of compounds 1b and 1c shown
in Figure 2.
Figure 2. X-ray crystal structures of 1b and 1c.
Figure 1. Photoluminescent spectra of N-2-aryl-1,2,3-triazoles. Sample
preparation: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at l=254 nm with
5 nm slit, quantum yield of 1a F= 0.154, quantum yield of 1a’ F=
0.009.
As shown in Figure 2, the N-2-phenyl group in compound
1b showed good conjugation with the triazole ring with a di-
hedral angle of 5.58, which supported our proposed strategy
that the N-2-aryl-triazoles could provide much improved
conjugations by avoiding the ortho-steric repulsion associat-
ed with typical bis-aromatic systems. The C-4-phenyl group,
on the other hand, showed poorer conjugation (dihedral
angle=21.88) in the solid state. This steric influence was fur-
ther emphasized by the structure of compound 1c, where a
nearly perpendicular conformation was observed (dihedral
angle=81.18) at the C-4 position when the bulky cyclohex-
enyl group was placed at the C-5 position, even with the
possibility to form intramolecular hydrogen bonds (between
the phenol hydroxyl group and the triazole N-1 nitrogen
atom). Notably, a much larger dihedral angle at the N-2-
phenyl position was observed for compound 1c (19.78) com-
pared to 1b (5.58). This result indicated the potential influ-
ence of various C-4- and C-5-substituted groups on the con-
formation at the N-2 position. Although these crystal struc-
tures provided strong supports in rationalizing the photo-
emission, the N-2-phenyl-triazole 1a showed good emission
at l=362 nm when it was excited at l=254 nm. This finding
suggested that the N-2-aryl-triazoles (NATs) may adopt ef-
fective co-planar conformation between the triazole and the
ꢀ
aryl rings even with the presence of a “free” rotatable N C
bond. This unique feature suggested that NATs might be ap-
plied as new building blocks to reach effective UV/blue
emission by overcoming the challenges associated with
other small molecule systems. A series of NATs were pre-
pared by using the method shown in Scheme 2 and the opti-
cal properties of these new UV/blue dyes were investigated.
AHCTUNGTREGaNNNU ctivity of the NATs, the free s-bond rotations at the N-2
Scheme 2. Arylation of N-H 1,2,3-triazoles.
and C-4 positions would occur when the compounds were
dissolved in solution. Therefore, to evaluate how the various
substituted groups on the C-4 and C-5 positions influenced
the overall excitation and emission, compounds 1d–1g were
prepared. The absorption and emission behavior of these
compounds are summarized in Table 1.
Results and Discussions
Influence of substituted groups on the C-4 and C-5
positions: To evaluate these new fluorophores, one key con-
cern is whether the optical properties (efficiency, excitation,
and emission wavelengths) of the NATs could be adjusted
Compared with N-2-phenyl benzotriazole 1a, the C-4-N-
2-diphenyl triazoles generally gave stronger emission (1e–
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UV/Blue-Light-Emitting Fluorophores
FULL PAPER
Table 1. Fluorescence emission behavior of compounds with different C-
5 substitution.[a]
Table 2. Substituted group effect on the N-2 position.[a]
Absorption [nm]
Emission F[b] Fluorescence
(lmax intensity
[nm]
[ꢁ103][c]
G
)
Absorption [nm]
Emission F[b]
(lmax
[nm]
Fluorescence
intensity
AHCTUNGTRENNUNG
G
)
1a
1b
1c
1d
1e
1 f
1g
222 (0.127), 305 (0.222)
206 (0.128), 263 (0.250)
227 (0.257), 292 (0.268)
220 (0.146), 296 (0.276)
220 (0.118), 294 (0.263)
220 (0.111), 290 (0.253)
221 (0.128), 285 (0.265)
362
316
380, 400
348
0.15 19.6
0.02 4.7
0.06 10.8
0.09 23.0
0.35 46.3
0.45 56.6
0.30 52.4
0.23 63.1
[ꢁ103]
ACHTUNGTRENNUNG
1e 220
(0.118), 294
N
347
350
343
344
374
342
343
0.35
0.31
0.34
0.35
0.31
46.3
138
35.0
72.1
61.6
2a 224 (0.154), 309 (0.397)
2b 240 (0.056), 294 (0.212)
2c 298 (0.274)
2d 245 (0.041), 298 (0.226)
2e 225 (0.181), 336 (0.205)
2 f 228 (0.135), 250 (0.14), 317
(0.365)
2g 220.5 (0.133), 300 (0.242)
2h 238 (0.156), 276 (0.093)
2i 222 (0.482), 260 (0.115), 296
(0.127)
[a] Sample information: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 310 nm
with 2.5 nm slit. [b] Quantum yields (F) were determined based on 1.0ꢁ
10ꢀ5 molLꢀ1 quinine sulfate dihydrate in H2O (F=0.564).
347
342
342
<0.005 <0.5
<0.005 <0.5
STD- 207 (0.452), 234 (0.228), 349
(0.710)
402, 422
1
STD- 234 (0.423), 334 (0.068)
2
365
0.56 13.9
341
341
375
0.28
0.15
0.27
52.2
1.7
29.6
[a] Sample information: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 254 nm
with 5.0 nm slit. [b] Quantum yields (F) were determined based on 1.0ꢁ
10ꢀ5 molLꢀ1 quinine sulfate dihydrate (STD-2) in H2O (F=0.56). [c] Cal-
culated photoemission integration from the original spectra. All fluores-
cence measured under identical condition (see the Supporting Informa-
tion). STD-1=disodium 4,4’-bis(2-sulfostyryl)biphenyl (DSBP).
1g). All of these substrates emitted in the UV/blue-light
range with lmax around 350 nm. The photoquenching by
strong electron-withdrawing carbonyl group was observed
(1b). Notably, the highly steric hindered compound 1g gave
similar strong emission as the much less hindered compound
1 f, which suggested the small influence of the bulkiness on
the C-5 position for the light emission.
Figure 3. X-ray crystal structures of 2g and 2h.
Variation on the N-2-position: With the N-2-aryl moiety as
the fluorophore core, it is expected that the N-2-substituted
groups will surely influence the overall optical performance.
We wondered if this could be used to adjust the fluorescence
emission and make NATs tunable dyes. Compounds with
different substituted groups on the N-2-phenyl ring were
prepared to investigate the substituent group effect on the
optical properties.
As shown in Table 2, most of the N-2-substituted aromatic
compounds 2 gave good UV/blue-light emission. The overall
quantum efficiency of NATs is around 30%, which is com-
parable with commercial blue dyes (for example, DSBP:
F=0.23, measured under identical conditions). These re-
sults suggested the promising future of these compounds as
new UV/blue-light-emitting fluorophores. Similar to other
literature reports,[12] photoquenching by NO2 (2e) and by a
carbonyl group (2 f) was observed. Interestingly, a significant
decrease of emission intensity was obtained with ortho-pyri-
dine compound 2h. Considering the similar electronic effect
of para-pyridine 2g, this result was rather surprising. The
success in obtaining the X-ray crystal structures for both 2g
and 2h helped to reveal the mechanistic reason behind this
result (Figure 3).
As shown in Figure 3, the crystal structure of 2h exhibited
a larger dihedral angle between the pyridine ring and the
triazole ring, due to the electron repulsion between the
lone-pair electrons of the nitrogen atoms on the triazole and
pyridine rings. This result supports our hypothesis that effec-
tive conjugation between the triazole ring and the N-2-aryl
groups is crucial for good fluorescence emission.
The quantum yield (F), which indicates the percentage
photon emission, is based on the photons absorbed. Al-
though the quantum yield is a very important parameter
used to evaluate fluorescent-active molecules, it may not
completely represent the performance of the fluorophores
because molecules may absorb photons with different effi-
ciency (different absorption e). The fluorescence intensities
of the NATs were calculated (under the identical condi-
tions) and are shown in Table 2.
The emission intensities of the NATs are compatible or
higher than both commercial available blue dyes quinine
sulfate dihydrate and DSBP. Impressively, compound 2a
gave very intense emission, which was more than two times
Chem. Eur. J. 2011, 17, 5011 – 5018
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X. Shi et al.
stronger than the emission of the quinine sulfate dihydrate
dye; extending conjugation is known to enhance photo ab-
sorption. However, this strategy usually does not work well
for blue-emitting dyes because conjugation gives a red shift,
resulting in the emission at a longer wavelength. To our sur-
prise, this undesired red shift did not occur in the NAT sys-
tems. For compound 2i, only a slight red shift (28 nm) was
observed compared with compound 1e. Compound 2a, on
the other hand, gave almost no red shift even with largely
increased emission intensity. These unusual photoactivities
highlighted the unique properties of these novel NAT dyes
(also see computational studies in later section for the
HOMO–LUMO distributions).
showed very little wavelength shifts. In contrast, the strong
electron-donating methoxy group, gave noticeable red shifts
when placed on either the N-2 (2d) or the C-4 (3 f) position.
The CN-substituted compound 2a gave the highest emission
intensity due to the more efficient light absorption through
conjugation. Similar trends were also observed in F- and Cl-
substituted NATs. Whereas similar quantum yields were ob-
served, the Cl-substituted NATs gave higher fluorescence
emission than the F-substituted NATs, which was due to the
better resonance conjugation between the lone-pair elec-
trons of Cl and the phenyl ring. The summarized absorption
and emission data are shown in Table 3.
Table 3. Excitation and emission behavior of substituted NATs.[a]
Substituent group electronic effect: To investigate substitu-
ent electronic effects, two series of compounds were com-
pared with substituted groups either on the para position of
the N-2-phenyl or on the para position of the C-4-phenyl.
The photoluminance spectra are depicted in Figure 4.[13]
The F-, Cl-, CN-, and OMe-substituted NATs all gave
strong emission. Interestingly, a very similar trend was ob-
served with substituted groups on either the N-2-phenyl ring
(Figure 4A) or on the C-4-phenyl ring (Figure 4B). The
NATs with electron-deficient substituted groups (F, Cl, CN)
Excitation
(lmax) [nm]
Emission
ACHTNUGRTNE(NGNU lmax) [nm]
F[b]
Fluorescence
intensity
Stokes shift
[nm]
N
AHCTUNGTRENNUNG
[ꢁ103][c]
1e
2a
2b
2c
2d
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
298
312
298
292
304
278
305
277
279
280
284
318
283
287
284
280
306
281
283
285
347
350
343
344
374
343
351
343
343
373
360
401
360
364
369
341
352
343
345
376
0.34
0.31
0.34
0.35
0.31
0.39
0.51
0.40
0.25
0.33
0.39
0.32
0.34
0.34
0.34
0.35
0.45
0.41
0.36
0.35
46.3
138
49
38
45
52
70
65
46
66
64
93
76
83
77
77
85
61
46
62
62
91
35.0
72.1
61.6
35.2
99.1
45.5
79.6
96.3
82.2
111
78.6
89.8
114
134
174
77.3
99.0
101
[a] Sample information: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 310 nm
with 2.5 nm slit. [b] Quantum yields (F) were determined based on 1.0ꢁ
10ꢀ5 molLꢀ1 quinine sulfate dihydrate (STD-2) in H2O (F=0.56). [c] Cal-
culated photo emission integration from the original spectra. All fluores-
cence measured under identical conditions (see the Supporting Informa-
tion).
NATs
Tunable UV/blue-light-emitting small molecule dyes: The
large number of NATs spectra allowed us to explore the key
factors that control the optical properties of novel tunable
NAT UV/blue dyes. First, the emission enhancement by CN
and Cl groups was confirmed, where strong emission was
observed in all cases with these two groups. When both CN
Figure 4. Photoluminescent spectra of N-2-aryl-1,2,3-triazoles. Sample in-
formation: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 310 nm with 2.5 nm
slit.
5014
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UV/Blue-Light-Emitting Fluorophores
FULL PAPER
and Cl are present (compound 3l), the highest emitting in-
tensity was achieved with lmax =352 nm. Notably, though the
fluorescence enhancement was believed to be a consequence
of the extended conjugation, no significant red shift was ob-
served. This UV emission was more than three times stron-
ger than the emission of the commercial standard blue dye
DSBP, which highlighted the superior photoactivity of these
novel UV fluorophores.
The electron-donating OMe group is an effective shift-
tuning group for the NAT dyes. The red shifts were ob-
served with OMe at either the N-2 (2d) or the C-4 (3 f) po-
sition. No further red shift occurred with OMe on both posi-
tions (3j), suggesting that cancellation of the inductive ef-
fects in 3j resulted in no further red shift. This hypothesis
was further supported by the emission of compound 3g,
where a red shift was achieved (emission lmax =401 nm)
when a combination of the OMe group and the electron-
withdrawing CN group was applied on the opposite sites of
the NATs (Figure 5).
Figure 6. Photoluminescent spectra of N-2-aryl-1,2,3-triazoles. Sample in-
formation: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 310 nm with 2.5 nm
slit.
aromatic compounds, the HOMOs and LUMOs of the
NATs were very similar for all N-2-substituted triazoles with
the LUMO being located between the triazole and the N-2-
aryl ring. Examples of compounds 2d and 3g are given in
Figure 7.
Figure 7. HOMO and LUMO orbitals of compounds 2d and 3g.
The N-2-substituted NATs are much less polar compared
with the N-1 analogous due to the homogenization of the
Figure 5. Photoluminescent spectra of N-2-aryl-1,2,3-triazoles. Sample in-
formation: 1.0ꢁ10ꢀ5 molLꢀ1 in MeOH, excitation at 310 nm with 2.5 nm
slit.
electron density through extended
p conjugation. The
photoexcitation introduces a high extension of planarity to
AHCTUNGTRENNUNG
induce the homogeneous distribution of electron density
throughout the triazole and N-2-aryl rings. This phenomen-
on, which has been well documented in the literature, is re-
ferred to as PICT.[16] What makes the 1,2,3-triazole special
comparing to other aromatic structures is the highly effec-
tive conjugation between the triazole and the N-2-aryl ring,
Besides the significant influence on the fluorescence in-
tensity and the emission wavelength, the various substituents
could also affect the Stokes shifts. As shown in Table 3, the
Stokes shifts for these tested compounds range from 38–
93 nm (Figure 6). The CN-modified NATs gave narrow
emission band with smaller Stokes shifts. Whereas the OMe-
substituted NATs gave broader emission bands with larger
Stokes shifts. The ability to modify the Stokes shifts is con-
sistent with good photoactivity of the NATs, which allowed
easy adjustment for the emission wavelength and the gap
between excitation and emission.
ꢀ
even with the presence of theoretically free-rotatable N C s
bonds. These co-planar structures were confirmed not only
by the X-ray crystal structures, as shown in earlier discus-
sions, but also indicated by the computational studies as the
most stable conformations for these molecules. Solvent
screenings of these dyes have been performed and indicated
little changes of the optical properties of these dyes in
EtOAc, MeOH, CH2Cl2, ClCH2CH2Cl, THF, or CH3CN,[17]
which supported the proposed intramolecular charge-trans-
fer mechanism. Moreover, the computational studies also in-
dicated that the twisted non-planar conformations were the
most energetic favored structures for N-1 isomers
Computational investigations: Time-dependent density func-
tion theory (TD-DFT) calculations were also carried out
with the Gaussian 03[14] packages by using the B3LYP/6-
31G(d) model.[15] Unlike the orbitals in other simple bis-
Chem. Eur. J. 2011, 17, 5011 – 5018
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5015

X. Shi et al.
significantly improved UV emission with a quantum yield at
0.76. Notably, the emission lmax is 390 nm, still in the UV/
blue range even with the further extended conjugation. The
fluorescence intensity of 4a was more than three-times
stronger than the commercial available blue dye DSBP,
measured at identical condition (excitation at both 254 and
310 nm). The styrene groups in 4a gave the extra synthetic
handle for any further modification, such as polymerization
and metathesis coupling, making 4a a promising UV probe
for further investigation.
Figure 8. The most stable conformation of N-1-1e’ as obtained from com-
putational studies: twisted N-1-aryl ring.
(Figure 8), which is consistent with the experimental obser-
vation that poor fluorescence emission was observed with
all N-1 isomers.
These computational studies revealed the unique features
of the NATs (N-2 isomers), which allow them to be effective
fluorophores for UV/blue-light emitting. The calculated
HOMO–LUMO gaps for 1e–3o (Table 4) are around 3.60–
4.78 eV, which are in good agreement with the experimental
values of 3.90–4.48 eV from optical absorption spectra.[18]
Effective UV/blue dyes for biological systems are rare be-
cause the protein substrates may quench the emission. To
verify the biocompatibility of the NAT fluorophore, com-
pound 4b was synthesized through coupling the NAT with
protected lysine. The optical properties of this amino-acid-
modified NAT were tested. We were pleased to find that no
photoquenching was observed. Notably, compound 4b pos-
sessed selected protected acid and amine groups, which
made it easier to be coupled with biological targets through
either the C or the N terminus. Considering the unique UV/
blue emission of this NAT, it is expected that the amino-
acid-modified NAT could be a kind of good new UV fluo-
rescence probe for various chemical and biological studies.
Further investigations are currently undergoing in the
group.
Table 4. Calculated HUMO–LUMO energy gaps.[a]
Eg
[eV]
LUMO–HOMO
[eV]
Eg
[eV]
LUMO–HOMO
[eV]
1e 4.16
2a 3.97
2b 4.16
2c 4.25
2d 4.08
3a 4.46
3b 4.07
3c 4.48
3d 4.44
3e 4.43
3.95
3.60
4.06
4.06
3.94
4.29
3.94
4.78
4.77
4.21
3 f 4.37
3g 3.90
3h 4.38
3i 4.32
3j 4.37
3k 4.43
3l 4.05
3m 4.41
3n 4.38
3o 4.35
4.34
3.73
4.51
4.51
3.72
4.27
3.90
4.04
4.03
4.16
Conclusion
We herein report the N-2-aryl-triazoles (NATs) as a novel
class of effective UV/blue-light-emitting dyes. Through a
comprehensive comparison of the substituted groups on the
C-4, C-5, and N-2 positions, a general trend was revealed re-
garding how to effectively adjust the photoactivity of these
compounds. Both emission wavelengths (350–400 nm) and
Stokes shifts (38–93 nm) could be adjusted with various sub-
stituted functional groups. The computational studies and X-
ray crystal structures confirmed the effective co-planar
structures as the most stable conformation, which was
unique to the N-2 isomers. This information could initiate
the new strategies for future dye designs. Finally, further de-
rivatizations gave highly efficient UV-emitting bis-triazoles
and biocompatible amino acid NAT probes, thus, supporting
the potential use of these novel dyes in related chemical,
material, and biological applications.
[a] Optical HOMO–LUMO gaps (Eg) were derived from the onset ab-
sorption wavelength: Eg [eV] 1240/lmax
.
Extended NAT fluorescence probes: Encouraged by the
good optical activity of these NATs and the mechanistic in-
sight for tuning optical emission, we prepared compounds
4a and 4b as two new molecular probes. Considering the
UV/blue emission was achieved through the extended conju-
Experimental Section
General procedure for preparation of N-2-aryl-1,2,3-triazoles: Into a
ꢀ
round-bottom bottle (25 mL) was successively added N H triazole
gation between triazole and N-2-aryl groups, we postulated
that the bis-triazoles should behave as good fluorophores
with even stronger emission efficiency. Compound 4a was
prepared and tested. As expected, the two triazole rings pro-
vide good conjugation with the central phenyl group (dihe-
dral angle measured from X-ray crystal structure), giving
(1 mmol), potassium carbonate (345 mg, 2.5 mmol), l-proline (23 mg,
0.2 mmol), copper(I) chloride (10 mg, 0.1 mmol), and iodo-substituted
benzene (1.2 mmol) in DMSO (10 mL) under N2 protection. The reaction
mixture was stirred and heated up to 858C for 4 h, and the reaction prog-
ress was monitored by TLC. After the reaction was completed, water
(20 mL) was added to quench the reaction. The mixture was extracted
with ethyl acetate (3ꢁ30 mL). The combined organic phases were
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UV/Blue-Light-Emitting Fluorophores
FULL PAPER
washed with brine (50 mL) and dried with anhydrous Na2SO4. The sol-
vent was removed under reduced pressure to get a residue and the resi-
due was purified by flash silica gel chromatography (hexane/ethyl acetate
20:1) to give the products.
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UV absorption spectra: Absorption spectra were recorded on a Shimadzu
UV-2500 spectrophotometer in 1 cm-length quartz cell with a resolution
of 0.2 nm. All samples were measured from a 10ꢀ5 m solution of N-2-aryl-
1,2,3-triazole in MeOH with the wavelength range between 200–800 nm.
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and emission spectra were measured on a Hitachi 7000 fluorophotometer
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=
0.564) in water. Quantum yields were calculated by using the following
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the reflective index of the solvent. The subscript reference is the respec-
tive values of the standard quinine sulfate dihydrate.
IntAreferencen2
ð1Þ
F ¼ F
reference IntrefernceAn2
reference
For explicit experimental data, including spectroscopic data, see the Sup-
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Acknowledgements
We thank the NSF (CHE-0844602, CHE-0754405) for financial support.
We also thank Dr. Xiaoqiang Yu (State Key Laboratory of Crystal Mate-
rials, Shandong University) for the helpful discussion.
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X. Shi et al.
[17] Solvatotochromism studies of compound 1e, 2a, 2d, 2e have been
carried out in various solvents. Detailed experimental results are
shown in the Supporting Information, where little changes were ob-
served in different solvent systems, strongly supported the proposed
PICT mechanism. For example, for compound 2a, the quantum
yield in different solvents are observed as following: in MeOH F=
0.31; in dichloromethane F=0.37; in acetonitrile F=0.34; in ethyl
acetate F=0.37; in THF F=0.34.
[18] See detailed computational results of all compounds 1e–3o in the
Supporting Information.
Received: October 12, 2010
Revised: November 11, 2010
Published online: March 22, 2011
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Chem. Eur. J. 2011, 17, 5011 – 5018