Water soluble triazolopyridiniums as tunable blue light emitters

Article abstract of DOI:10.1039/c2cc34976k

Two [1,2,3]triazolo[1,5-a]pyridinium salts have been prepared via a novel Cu(ii)-mediated oxidative cyclization reaction. Both water soluble compounds exhibit broad-range fluorescence emission with huge Stokes shifts as well as color tunability.

Full text of DOI:10.1039/c2cc34976k

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Water soluble triazolopyridiniums as tunable blue
light emitters†‡
Xin Su,a Matthew D. Liptakb and Ivan Aprahamian*a
Cite this: Chem. Commun., 2013,
49, 4160
Received 11th July 2012,
Accepted 23rd September 2012
DOI: 10.1039/c2cc34976k
www.rsc.org/chemcomm
Two [1,2,3]triazolo[1,5-a]pyridinium salts have been prepared via a we report the novel and convenient synthesis of water soluble
novel Cu(II)-mediated oxidative cyclization reaction. Both water [1,2,3]triazolo[1,5-a]pyridinium salts via Cu(II)-mediated oxidative
soluble compounds exhibit broad-range fluorescence emission cyclization, and their interesting photophysical properties compris-
with huge Stokes shifts as well as color tunability.
ing of tunable blue light emissions with ‘‘mega’’ Stokes shifts.9
The synthesis of [1,2,3]triazolo[1,5-a]pyridinium salts has been
Blue light fluorophores have attracted much attention because of previously accomplished via oxidative cyclization of appropriate
1
hydrazones using various oxidants: Pb(OAc)4,7 N-bromosuccinimide
¨
their importance in Forster resonance energy transfer (FRET)
and OLED applications.2 The development of such fluorophores (NBS),10 2,4,4,6-tetrabromocyclohexa-2,5-dienone (TBB)11 and
has been limited by the large energy gap required between the antimony(V) chloride (SbCl5).12 The use of electrochemical ring
excited- and ground-state energy levels, which can decrease closure was also reported.13 However, these reagents/methods
quantum yields and lower photostability.3 [1,2,3]Triazolo[1,5-a]- all have their own drawbacks. For example, NBS and TBB also
pyridines4 are an important class of blue light emitting bicyclic lead to bromination resulting in mixtures of products8c and
pyridines that have been widely studied and used as synthetic SbCl5 is toxic. We have recently found that Cu(II) in acetonitrile
intermediates, metal ligands and chemosensors.5 These systems (CH3CN) can be used as the oxidant for the synthesis of
usually have large Stokes shifts that impede self-quenching and triazolopyridiniums from our hydrazone-based molecular
interference from scattered light, which are important properties switches.14 This oxidation takes place under aerobic conditions
in biological applications.6 However, these fluorphores have the to yield the appropriate triazolopyridiniums in high yields
same intrinsic shortcoming as most p-conjugated fluorophores, (Scheme 1). Significantly, no reaction is observed when the
namely poor water solubility. This limitation has to be overcome solvent is switched from CH3CN to either methanol or acetone,
if such systems are to be used for bioassay or imaging applications. which can be attributed to the enhanced oxidizing ability of
One solution to this problem is the introduction of solubilizing Cu(II) in CH3CN.15
groups; this strategy, however, usually leads to synthetic complica-
Compounds 2a and 2b were characterized using NMR
tions that could be avoided if the systems are rendered to be spectroscopy (Fig. S9, S10, S13 and S14, ESI‡). The 1H NMR
intrinsically water soluble. We hypothesized that one way of spectrum of 2a shows that all the aromatic signals are shifted
accomplishing this might be the use of the positively charged downfield relative to the parent hydrazone, which is consistent
counterparts of these systems, namely [1,2,3]triazolo[1,5-a]- with the development of a positive charge in the pyridyl ring.14
pyridinium salts.7 These compounds although discovered This effect is more pronounced in 2b, where the phenyl proton
in 1952, have received very little attention, except for a few signal is shifted downfield to 8.33 ppm as opposed to 7.38 ppm
cases in which they were used as reactive intermediates.8 Herein in 1b. Moreover, the spectrum does not contain the hydrazone
a Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755,
USA. E-mail: ivan.aprahamian@dartmouth.edu; Fax: +1 603-646-3946;
Tel: +1 603-646-9666
b Department of Chemistry, University of Vermont, Burlington, Vermont 05405, USA
† This article is part of the ChemComm ‘Emerging Investigators’ 2013 themed
issue.
‡ Electronic supplementary information (ESI) available: Experimental proce-
dures, NMR spectra, UV-vis spectra, fluorescence spectra, computational methods
and crystal structral data for compound 1b. CCDC 887676. For ESI and crystal-
Scheme 1 The synthesis of [1,2,3]triazolo[1,5-a]pyridinium salts 2a and 2b.
lographic data in CIF or other electronic format see DOI: 10.1039/c2cc34976k
c
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Table 1 A summary of the photophysical properties of 2a and 2b in H2Oa
Abs. lmax (e)
Fl. lem (Fb)
Stokes shift
FWHM
2a
2b
302 (11 100)
311 (22 100)
469 (0.05)
412 (0.30)
167/11 790
101/7882
131/6008
94/5097
a
Abs. lmax and Fl. lem are reported in nm, and the extinction coefficient
e is calculated in L molÀ1 cmÀ1. Stokes shifts and FWHMs are reported
in both nm and cmÀ1
.
Æ0.02.
b
Fig. 1 Ball-stick drawing of the crystal structure of 2a. All hydrogen atoms and
À
the ClO4 counterion are omitted for clarity.
of the additional triazolopyridinium unit. The H2O solution of 2a
emits green-blue light upon exicitation at 300 nm with a quantum
yield of 0.05 Æ 0.02, whereas 2b emits deep blue light with a much
higher quantum yield (0.30 Æ 0.02). The more than eight fold
enhancement in quantum yield from 2b to 2a is much more
significant than the increase in extinction coefficient. On one
hand, 2b has one additional fluorophore than 2a. On the other
hand, since the non-conjugated phenyl ring is able to quench
fluorescence via charge transfer,18 the introduction of a second
electron-withdrawing triazolopyridinium unit in 2b can help
reduce its quenching effect leading to the increase in quantum
yield. For comparison, we also studied the photophysical proper-
ties of 2a and 2b in CH3CN, and found that they are almost
identical to those in H2O (Fig. S3, ESI‡).
Careful examinations of the emission spectra of 2a and 2b
reveal several interesting photophysical properties (Table 1).
Both compounds have very broad emission profiles, spanning
over the visible range, especially 2a that spans from 370 nm to
over 600 nm. Furthermore, the full widths at half maximum
(FWHM) for both comopounds are as large as about 100 nm or
5000 cmÀ1. Both 2a and 2b exhibit mega Stokes shifts (above
100 nm, or around 100 000 cmÀ1), which is not a common
phenomenon for simple organic dyes.9 As mentioned before, this
property is important for preventing self-quenching and light
N–H proton, which is an indication that the oxidative cycliza-
tion has taken place. The crystal structure of 2a confirms the
formation of the triazolopyridinium ring (Fig. 1).§ To the best
of our knowledge this is the first reported crystal structure of a
triazolopyridinium compound. The newly formed ring system
is perfectly planar, and the distance between the N2 and N3
atoms is 1.364(2) Å, which is slightly longer than that in the
neutral triazolopyridine (1.351(4) Å).16 The C1–N1 bond length
is 1.324(2) Å, longer than in hydrazone 1a (1.305(2) Å), but still
within the CQN double bond range. The phenyl substituent at
position 2 is not coplanar with the triazolopyridinium ring, and
the dihedral angle between them is 63.40(5)1. This is in sharp
contrast with the coplanar hydrazone 1a,14d or 2-substituted
[1,2,3]triazoles,17 in which the aryl substituents are usually
coplanar or have very small dihedral angles with the triazole
cores. Since no significant intermolecular interactions were
observed in the extended structure, the large dihedral angle
can be attributed to the electronic structure in N2.
Both 2a and 2b have good solubility in water and polar
organic solvents, such as acetone, CH3CN and EtOH. The UV/Vis
spectra of 2a and 2b show absorption maxima (lmax) around
300 nm (Fig. 2), and no appreciable solvatochromism is observed
for both compounds (Fig. S1 and S2, ESI‡). It is noteworthy that
the extinction coefficient of 2b at lmax is more than double that of
2a. Considering the fact that the triazolopyridinium ring is not in
conjugation with the phenyl ring, the increase in extinction
coefficient in 2b most probably results from the contribution
´
scattering. The CIE (Commission internationale de l’eclairage)
1931 color space chromaticity diagram was used to determine the
color of the missions, and the results show that from 2a to 2b, the
color goes from green-blue (0.159, 0.232) to deep blue (0.159,
0.094), demonstrating the color tunability of the triazolopyridinium
compounds (Fig. S4, ESI‡).
We performed time-dependent density functional theory
(TDDFT) calculations on models of 2a and 2b (Fig. S5, ESI‡)
to elucidate the quantum mechanical origins of the triazolo-
pyridinium UV/Vis and fluorescence spectra. The computations
utilized the PBE density functional,19 a triple-zeta basis set,20
and the COSMO continuum solvation model as implemented
within ORCA 2.9.1.21 Based upon the TDDFT results, the electronic
transitions responsible for the prominent UV/Vis transitions near
380 nm can be described as HOMO-LUMO transitions (Fig. S6,
ESI‡).22 Consistent with the experiment, TDDFT predicts that the
extinction coefficient for 2b is approximately triple that of 2a.
Inspection of the MOs involved in these transitions provides an
explanation for the relative intensities of this band in 2a and 2b
(Fig. 3). In both cases, the HOMO is antibonding with respect
to the triazolopyridinium–phenyl bond. For 2a the LUMO is a
triazolopyridinium-based mixture of triazolopyridinium and
phenyl p-orbitals, whereas the LUMO is a phenyl-based mixture
of the same two orbitals in 2b. Consequently, there is a better
Fig. 2 The UV/Vis absorption (5.0 Â 10À5 M) and normalized emission spectra
of 2a and 2b in H2O.
c
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Advanced Computing Core (VACC) for computer hardware
resources and Frank Neese (MPI Mu¨lheim) for providing the
ORCA 2.9.1 software package.
Notes and references
§ Crystal data of 2a: C15H14ClN3O6,
M = 367.74, monoclinic,
a = 13.7923(7), b = 5.9508(3), c = 19.9335(11) Å, U = 1608.44(15) Å3,
T = 173(2) K, space group P21/c (no.14), Z = 4, 24909 reflections
measured, 2947 unique (Rint = 0.0287) which were used in all calcula-
tions. The final Ro (F2) was 0.1178 (all data).
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anion sensing, which is currently being looked into.
This work was supported by Dartmouth College (XS and IA),
the Burke Research Initiation Award (IA) and the University of
Vermont (Startup funds to MDL). We acknowledge Dr Richard
Staples (Michigan State University) for X-ray analysis, the Vermont
3865–3868.
¨
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4162 Chem. Commun., 2013, 49, 4160--4162
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