Photophysical study of 2-(4′-N,N-dimethylaminophenyl)oxazolo[4,5-b] pyridine in different solvents and at various pH

Article abstract of DOI:10.1039/c2pp25039j

The spectral characteristics of 2-(4′-N,N-dimethylaminophenyl) oxazolo[4,5-b]pyridine (DMAPOP) have been investigated in solvents of different polarity and hydrogen bonding capacity. Unlike its imidazole analogue, DMAPOP emits single emission in both apro

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PAPER
Photophysical study of 2-(4′-N,N-dimethylaminophenyl)oxazolo[4,5-b]-
pyridine in different solvents and at various pH†
Anasuya Mishra and G. Krishnamoorthy*
Received 24th February 2012, Accepted 6th May 2012
DOI: 10.1039/c2pp25039j
The spectral characteristics of 2-(4′-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine (DMAPOP) have
been investigated in solvents of different polarity and hydrogen bonding capacity. Unlike its imidazole
analogue, DMAPOP emits single emission in both aprotic and protic solvents and the hydrogen-bond
induced TICT emission is not observed in any protic solvents. The solvent effect on both absorption and
the emission spectral data are analyzed by multiple parametric regression analysis. The prototropic studies
reveal that two kinds of monocations are formed by protonation of pyridine nitrogen (MC1) and the
dimethylamino nitrogen (MC3) in both ground and excited states. However three kinds of dications are
formed by protonation of pyridine and oxazole nitrogens (DC1), pyridine and dimethylamino nitrogens
(DC2), and dimethylamino and oxazole nitrogens (DC3).
molecule including DEAPOP. But their studies are restricted to
1. Introduction
nonpolar and polar aprotic solvents.14 Mac et al. also studied the
Oxazoles are an important class of molecules of the azole family
and are known for their biological activity. For example, oxazole
derivatives are used as anti-inflammatories, analgesics, anti-
pyretics and antagonists.1–4 Owing to their wide applications as
fluorescent sensors, fluorescent brighteners, and laser diodes, the
study on the optical characteristics of oxazoles has gained
interest.5–8
prototropic equilibria of DEAPOP and found two absorption
bands due to formation of two kinds of monocations. On the
other hand they reported that only one fluorescence spectrum
corresponds to one of the monocations.
Thus, we have synthesized and studied the spectral character-
istics of 2-(4′-N,N-dimethylaminophenyl)oxazolo[4,5-b]pyridine
(DMAPOP) in solvents of different polarity and hydrogen-
bonding capacity and at different pH in aqueous media. The
main objectives of the studies are to investigate (i) the effect of
protic solvents on the fluorescence characteristics of DMAPOP
and (ii) to understand the excited state prototropic equilibria of
DMAPOP.
We have recently studied the fluorescence characteristics of
2-(4′-N,N-dimethylamino)imidazo[4,5-b]pyridine
(DMAPIP,
Chart 1) and found that DMAPIP exhibits single emission in
aprotic solvents and dual emission in polar protic solvents
(except water).9 The dual emission in protic solvent is due to for-
mation of a twisted intramolecular charge transfer (TICT) state.
In water the TICT emission was quenched completely due to an
increase in the nonradiative rate by greater stabilization of the
TICT state. Our studies suggested that the hydrogen bonding of
the protic solvents with the pyridine nitrogen plays a crucial role
in the formation of TICT state in DMAPIP.9–11 Further, we have
shown that DMAPIP can be used as a probe to investigate micro-
heterogeneous systems.12,13 Therefore it is interesting to study
the effect of oxygen substitution on the spectral characteristics
and such a study may be useful in understanding the protic
solvent-induced TICT emission observed in DMAPIP. Recently
Mac et al. studied the fluorescence properties of several oxazolo-
[4,5-b]pyridines including 2-(4′-N,N-diethylaminophenyl)oxazolo-
[4,5-b]pyridine (DEAPOP, Chart 1), and found that the dyes
possess charge transfer character in the ground and the excited
states. However, they did not observe dual emission from any
2. Materials and methods
DMAPOP was synthesized by refluxing equimolar mixture of
2-amino-3-hydroxypyridine and 4-(N,N-dimethylamino)benzoic
acid in POCl3 for 8 hours by following the procedure reported
for similar compounds.15 The reaction mixture was cooled to
room temperature and poured into ice cold water. It was neutral-
ized by concentrated sodium hydroxide solution and the precipi-
tate was collected. The compound was purified by column
chromatography. The compound was confirmed by 1H NMR
spectroscopy and mass spectrometry as follows. 1H NMR
(400 MHz, CD3Cl, ppm) δ 8.46 (d, J = 5 Hz, 1H); 8.14 (d, J = 8
Hz, 2H); 7.86 (d, J = 8 Hz, 1H); 7.16 (dd, (J = 5.2, 8) Hz, 1H);
6.75 (d, J = 9.2 Hz, 2H); 3.07 (s, 6H). From HRMS the mole-
cular mass (M + 1) was found to be m/z: 240.1211.
HPLC grade solvents were used for solvent study and milli-
pore water was used for the preparation of aqueous solutions.
Aqueous solutions in the pH range 3–11 were prepared by
Department of Chemistry, Indian Institute of Technology, Guwahati,
Guwahati 781039, India. E-mail: gkrishna@iitg.ernet.in
†Dedicated to Prof. S. K. Dogra on the occasion of his 70th birthday.
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Chart 1 Structures of DMAPOP and related molecules.
ab
max
mixing appropriate amounts of dilute solutions of NaOH and
H3PO4. Hammett’s acidity scale (H0) was used in preparing sol-
utions of pH < 1.0.16 The pH of different solutions were
measured using Jenway (model No 3510) pH meter. The pH
meter was calibrated by using three different standard buffer sol-
utions (pH 4, pH 7 and pH 10) within a range of 0.02 pH units
before any measurement. The solvents were transparent in the
spectral region of interest and were used as received. A Varian
Cary 100 UV-visible spectrophotometer and a Jobin Yvon Spex
Fluoromax 4 instrument were used to collect absorption and flu-
orescence spectra, respectively. The fluorescence quantum yields
were measured at λexc 350 nm with respect to quinine sulfate in
1 N sulfuric acid (Φf = 0.55).17 For fluorescence measurements
the absorbance of the solutions is kept at ∼0.1 at the absorption
maxima. Lifetime was measured with an Edinburgh life spec II
instrument, where the fluorescence lifetimes were determined
from time-resolved intensity decay by the method of time-corre-
lated single-photon counting using a picosecond 375 nm laser
diode as light source. The data were analyzed by the reconvolu-
tion method using the software provided by Edinburgh
instruments.
Table 1 Longest wavelength absorption maxima (λ
fluorescence emission maxima (λ
,
nm),
ab
max
, nm), Stokes shift (νSS, cm−1),
fluorescence quantum yield (Φf), life time (τ, ns), radiative rate constant
(kr, s−1) and nonradiative rate constant (knr, s−1) of DMAPOP in
different solvents
kr
knr
ab
max
em
max
Solvents
Hexane
λ
λ
ν
ˉSS
Φf
τ
(108) (108)
341, 358 364, 384, 3284 0.96 1.2 8.1
(sh) 407 (sh)
Cyclohexane 343, 360 366, 386, 3248 0.94 1.2 7.8
0.4
0.5
(sh)
354
352
362
358
365
364
366
362
364
373
376
365
408 (sh)
404
397
424
418
429
427
425
421
420
438
442
448
Dioxane
Ether
DMF
Acetonitrile
Methanol
Ethanol
1-Propanol
2-Propanol
Butanol
Glycol
3496 0.99 1.3 7.6
3220 0.88 1.3 7.0
4039 0.88 1.4 6.4
4010 0.86 1.5 5.6
4087 0.60 1.1 5.2
4053 0.83 1.5 5.6
3793 0.94 1.5 6.1
3871 0.99 1.5 6.4
3663 0.94 1.5 6.2
3979 0.51 1.0 4.9
3971 0.90 1.5 6.2
5076
0.01
1.0
0.9
0.9
3.6
1.1
0.4
0.1
0.4
4.7
0.7
Glycerol
Water
As determination of the concentration ratio [acidic form]/
[basic form] is difficult due to the complicated nature of the pro-
totropic equilibria, calculating the accurate pKa value for the pro-
totropic equilibria of DMAPOP is not feasible. However, the
pKa value for the neutral-monocation equilibrium was estimated
from the plot of absorbance against pH. Absorbance at 342 nm
where the absorbance of monocation(s) is least is used to esti-
mate the pKa value for the neutral-monocation equilibrium.
increase in the charge transfer nature. Therefore it is clear that
the substitution of electronegative oxygen increases the charge
flow from the dimethylamino group to oxazolopyridine ring.
Further the absorption spectrum of DMAPIP in water was blue
shifted with respect to that in methanol owing to the greater
hydrogen bonding of water over methanol with the dimethyla-
mino nitrogen. But no such hypsochromic effect was observed
in the absorption spectral maxima of DMAPOP, substantiating
the increase in charge flow from the dimethylamino group to the
oxazalopyridine ring.
3. Results and discussion
Multiple linear regression analysis by using Abraham et al.’s
solvatochromic approach is performed to identify the different
modes of solvation determining the absorption and emission
energies.18 The equations obtained by solvatochromic multipara-
metric method are
3.1. Solvent effect on absorption and fluorescence spectra
The absorption and the fluorescence spectral data of DMAPOP
are compiled in Table 1. Fig. 1 depicts the fluorescence spectra
in different solvents. The fluorescence spectrum is structured in
nonpolar solvents. Upon increasing the polarity and the hydro-
gen bonding capacity of the solvent the fluorescence is blurred
with a bathochromic shift. The bathochromic shift is more pro-
nounced in DMAPOP than the corresponding imidazole deriva-
tive.9 For example, from cyclohexane to methanol, a 22 nm shift
in the absorption spectra and a 43 nm shift in the fluorescence
spectra are observed for DMAPOP (Table 1). On the other hand
the bathochromic shifts are only 14 nm and 35 nm respectively
in the absorption and fluorescence spectra of DMAPIP.9 In polar
media, a red shift is observed for the electronic transition of the
type π–π* due to greater stabilization of the ππ* state by the
EðAÞ ¼ 29158 ꢀ 1131π* ꢀ 524α ꢀ 867β
EðFÞ ¼ 25970 ꢀ 2192π* ꢀ 1029α ꢀ 498β
where E(A) and E(F) are absorption and fluorescence band
maxima in cm−1, respectively. Good correlations are found for
E(A) and E(F) with Taft et al.’s π*, α and β values, where π*
denotes the dielectric effects of solvents, and α and β are the
hydrogen-bond donor ability and the hydrogen-bond acceptor
ability of the solvents, respectively.19 The regression value
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Fig. 1 Normalized fluorescence spectra (λex = 350 nm) of DMAPOP in (a) hexane, (b) cyclohexane, (c) ether, (d) dioxane, (e) acetonitrile, (f) 2-pro-
panol, (g) butanol, (h) DMF, (i) ethanol, ( j) 1-propanol, (k) DMSO, (l) methanol, (m) ethylene glycol, (n) glycerol, (o) water.
Fig. 2 Plot of ν versus ET(30) parameters.
ˉflu
obtained for absorption and fluorescence data were 0.97 and
The Lippert–Mataga plot21 was constructed (Fig. 3) by using
the relation
0.99 respectively. E0 values obtained are also in good agreement
with the experimental values in nonpolar solvent. Negative
values of π*, α and β indicate all the three parameters contribute
to the stabilization of both the ground state and the excited state
of the molecule. A good linear variation is also obtained
between the fluorescence maxima and the ET(30) values20
(Fig. 2).
o
vˉSS ¼ ½2ðμe ꢀ μgÞ=hca3ꢁΔf þ νSS
where ν is the Stokes shift, the superscript “o” indicates the
ˉSS
absence of solvent, μg and μe are dipole moments in the ground
state and excited state respectively, a is the Onsager cavity
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Fig. 3 Lippert–Mataga plot for DMAPOP.
Fig. 4 Plot of variation of fluorescence quantum yield of DMAPOP as a function of solvent polarity.
radius. The orientation polarizability Δf is defined as
Δf ¼ ½ðε ꢀ 1Þ=ð2ε þ 1Þꢁ ꢀ ½ðn2 ꢀ 1Þ=ð2n2 þ 1Þꢁ
calculation and the slope of Lippert–Mataga plot the value of μe
is calculated. The μe value thus obtained is 10.6 D. The value is
a little higher than 10.4 D reported for DEAPOP.14
Fluorescence quantum yields of DMAPOP measured in differ-
ent solvents are presented in Table 1 and the variation of
quantum yield with solvent polarity parameter ET(30) is shown
in Fig. 4. It is clear from the plot that fluorescence quantum
yield of DMAPOP decreases with increasing polarity of the
where ε and n are solvent dielectric constant and refractive
index, respectively. The geometrical optimization of DMAPOP
was done by DFT method using Gaussian 03 software to calcu-
late the μg.22 Using 5.9 D, the μg value obtained from the DFT
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solvents and the decrease is less prominent in nonpolar and polar
aprotic solvents. But in the case of polar protic solvents the
change is much more prominent with a steep slope. The signifi-
cant decrease of quantum yield observed upon increasing the
protic nature of the medium suggested the differential contri-
bution of charge transfer and hydrogen bonding interactions.23
Owing to greater viscosity of the solvent DMAPOP has higher
quantum yield in glycerol.
formation of TICT emission in 4-(N,N-dimethylamino)salicylic
acid in nonpolar solvent.24 Further investigations on dual fluor-
escence of DMAPIP are in progress.
3.2 Prototropic equilibria
Different possible cations that can be formed by the protonation
of the basic nitrogens in DMAPOP are presented in Scheme 1.
Upon decreasing the pH two new bands appear in the absorp-
tion spectra of DMAPOP with two quasi-isosbestic points at
324 nm and 395 nm (Fig. 6). This indicates the formation of two
kinds of monocations. The emission spectra are quite compli-
cated. Excitation at 324 nm, the isosbestic point, resulted in two-
band systems with a quasi-isoemissive point at 412 nm (Fig. 7a).
This suggests equilibrium between the neutral form and the
monocation. On the other hand the fluorescence spectrum corre-
sponding to the neutral molecule is predominantly observed
when excited at the red-side isosbestic point (λexc = 395 nm, not
shown). The intensity of the neutral fluorescence band decreases
with increase in acid concentration. However excitation at the
red absorption band (λexc = 430 nm) produces an additional
weak emission at 560 nm (Fig. 7b). With decrease in pH the
intensity of the 560 nm emission also increases at the cost of the
neutral emission. The spectral data are compiled in Table 2. This
clearly establishes that two kinds of monocations are present in
both ground and excited states. Protonation of the dimethyl-
amino nitrogen leads to a blue shift, and protonation of the ring
nitrogens leads to a red shift and the shift is more in case of pyri-
dine nitrogen than imidazole nitrogen.25–27 Therefore the blue-
shifted band at 308 nm in the absorption spectrum and the corre-
sponding fluorescence spectrum at 350 nm can be assigned to
the monocation formed by the protonation of dimethylamino
nitrogen i.e. MC3 (Scheme 1). Based on the following facts the
red-shifted absorption and the fluorescence spectra are assigned
Radiative (kr) and nonradiative (knr) rate constants are calcu-
lated from fluorescence quantum yields and lifetime values of
DMAPOP in different solvents to understand the effect of sol-
vation on the dynamics of the excited state. A typical set of kr
and knr values are tabulated (Table 1). The logarithm of (kr/knr)
of DMAPOP is plotted against the solvent polarity parameter
ET(30) (Fig. 5). Two different straight lines are obtained, one for
aprotic solvents and the other for protic solvents. In both the
cases, upon increasing the polarity the logarithm ratio of radia-
tive to nonradiative rate decreases but a steeper slope is obtained
in the case of protic solvents. It indicates that the radiative and
nonradiative rates are more sensitive toward protic solvents. It
may be that the hydrogen bonding interaction in polar protic
environments enhances the stabilization of the S1 state of
DMAPOP as a result the nonradiative relaxation rate increases.23
The striking difference between DMAPOP and its imidazole
analogue DMAPIP is the absence of longer wavelength emission
in protic solvents. The single exponential decays in protic sol-
vents further substantiates the non-dual fluorescence nature of
DMAPOP. Our earlier studies indicate that the hydrogen bonding
of the solvent with pyridine nitrogen plays a major role in the
formation of TICT state in DMAPIP.9 The present study suggests
that the ‘NH’ group of the imidazole nitrogen may also play a
crucial role in the formation of the TICT state in DMAPIP. One
possibility that comes to mind is proton transfer from the imida-
zole ‘NH’ nitrogen to the pyridine nitrogen coupled with charge
transfer. Yoon et al. proposed such a model to explain the
Fig. 5 Plot of log(kr/knr) versus ET(30) in various solvents.
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Scheme 1 Prototropic reactions of DMAPOP.
Fig. 6 Absorption spectra of DMAPOP for neutral-monocation equilibrium in water.
to the monocation formed by the protonation of the pyridine
nitrogen (MC1) rather than the oxazole nitrogen (MC2). (i) Due
to higher charge transfer character of MC1 than MC2, a large
Stokes shift is expected in the fluorescence spectrum of MC1
than MC2. The observed Stokes shift (5500 cm−1) for the mono-
cation is closer to that of MC1 of DMAPIP rather than MC2 of
DMAPIP and the Stokes shift is also much higher than that
obtained for the monocation formed by the protonation of
imidazole nitrogen (1850 cm−1) in 2-(4′-N,N-dimethylamino-
phenyl)benzoxazole (Chart 2).13,27–29 (ii) The pKa value (3.35)
obtained for DMAPOP is higher than that of oxazole nitrogen
protonation (2.35) in 2-(4′-N,N-dimethylaminophenyl)benzoxa-
zole.28 This is consistent with the fact that pyridine nitrogen pro-
tonation has a higher pKa value for the neutral-monocation
equilibrium than that of imidazole nitrogen protonation,9,25,26,28
although we observe both MC1 and MC2 in the excited state.
The results of neutral-monocation equilibrium of DMAPOP
and other related molecules (Chart 2) from the literature are
summarized in Table 3. In 2-(4′-N,N-dimethylaminophenyl)-
benzimidazole, due to an increase in charge flow from the
dimethylamino nitrogen to the imidazole nitrogen, protonation
predominantly occurs at the imidazole nitrogen and a small
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Fig. 7 Emission spectra of DMAPOP by (a) λexc = 324 nm, (b) λexc = 430 at different pH (the shoulder peak at 506 nm is due to water Raman
emission).
amount of protonation occurs at the dimethylamino nitrogen.29
When the sNH group is replaced by oxygen, it reduces the
charge density on the oxazole nitrogen in 2-(4′-N,N-dimethyl-
aminophenyl)benzoxazole and protonation at the oxazole nitro-
gen decreases and that at the dimethylamino increases.28 On the
other hand, when nitrogen is substituted in benzene ring
(DMAPIP) no protonation occurs at dimethylamino nitrogen and
it occurs at the imidazole nitrogen with a small amount at the
pyridine nitrogen. In DMAPOP the pyridine nitrogen is substi-
tuted in the benzene ring and the sNH group is replaced by
oxygen, the formation of MC2 (i.e. protonation at imidazole
nitrogen) is completely terminated and the protonation at pyri-
dine nitrogen dominates over that at dimethylamino nitrogen
(Scheme 1). It appears that substitution of more electronegative
oxygen decreases the electron density on the imidazole nitro-
gens. This is substantiated by the lower pKa values for the
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ab
max
Table 2 Absorption band maxima (λ , nm), excitation band maxima
max
Table 3 pKa values of neutral-monocation of different molecules along
with the site of protonation (see Chart 2 for monocation labelling)
(λemxacx, nm) and emission band maxima (λ , nm) for different
em
prototropic species in water
Monocation
pKa formed
ab
max
exc
max
em
Species
λ
λ
λ
max
Molecule
Neutral
MC1
MC3
DC1
DC2
DC3
TC
365
428
306
366
428
308, 321
595
360
319, 332
327
447
560
350, 365
700
480
360, 375
354, 370, 384
2-(4′-N,N-Dimethylaminophenyl)-
benzimidazolea
5.60 MC2 ≫ MC3
2.35 MC3 > MC2
2-(4′-N,N-Dimethylaminophenyl)-
benzoxazoleb
DMAPIPc
5.40 MC2 > MC1
3.35 MC1 > MC3
317, 331
325
DMAPOP
a Ref. 26. b Ref. 28. c Ref. 9.
Chart 2 Structures of monocations of DMAPOP and related molecules.
Fig. 8 Absorption spectra of DMAPOP for the monocation(s)-dication(s) equilibrium in water (the insert shows the expanded spectra).
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neutral-monocation equilibrium of oxazoles compared to those
of imidazoles (Table 3). Since the presence of pyridine nitrogen
also decreases the electron density on the oxazole nitrogen,
oxazole nitrogen could not be protonated in DMAPOP. This may
be due to higher electron density on diethylamino group (over
dimethylamino group), as in DEAPOP the protonation at the
diethylamino group is higher than that at pyridine nitrogen.14
Decreasing the pH of a solution below 2 causes a decrease in
the absorbance of MC1 and the absorption spectrum of MC3
also undergoes a bathochromic shift with an increase in absor-
bance (Fig. 8). At pH 0.18, a clear structured band is observed at
317 nm with a long tail on the red side. These changes are con-
sistent with a shift of the equilibrium towards the dication from
the monocation. The absence of an isosbestic point shows the
complicated nature of the equilibrium. Three distinct fluor-
escence spectra were obtained at pH 0.18 while exciting at
316 nm, 428 nm, and 620 nm (Fig. 9). In the absorption spectra
of dications, though non-zero absorbance is obtained at around
Fig. 9 Normalised emission spectra of DMAPOP for all three dications at pH 0.18: (a) λexc = 316 nm (DC3), (b) λexc = 428 nm (DC2), (c) λexc
=
620 nm (DC1).
Fig. 10 Normalised excitation spectra of DMAPOP at pH 0.18: (a) λem = 387 nm (DC3), (b) λem = 470 nm (DC2), (c) λem = 710 nm (DC1).
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595 nm no clear band is observed. However the excitation
spectra recorded clearly shows a band at 595 nm (Fig. 10). The
excitation spectra of all the three emission bands at dicationic pH
(0.18) are given in Fig. 10. The fluorescence data along with the
respective excitation spectral data are compiled in Table 2. The
same as the monocations, three types of dications are possible
(Scheme 1). A large red shift in the excitation and the emission
spectra shows that both the ring nitrogens were protonated; there-
fore emission band at 702 nm and the corresponding excitation
band at 595 nm can be assigned to DC1, which is formed from
MC1. The excitation band at 360 nm and the fluorescence band
at 480 nm are blue shifted with respect to those of MC1 and red
shifted to those of MC3 and these bands can be assigned to DC2
formed by the protonation of both pyridine and dimethylamino
nitrogens. The last excitation band at 319 nm and the emission
band at 360 nm can be assigned to DC3 and as expected it is red
shifted with respect to MC3 and blue shifted compared to MC1
and other dications. The large shift in the emission spectrum of
DC1 compared to the other dications may be due to higher
charge transfer in DC1, where both protonations occurred at the
Fig. 11 Emission spectra for dication-trication equilibrium of DMAPOP (λexc = 318 nm).
Fig. 12 Normalised excitation and emission spectra of DMAPOP for trication (in conc. H2SO4): (a) λem = 370 nm, (b) λexc = 320 nm.
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Fig. 13 Normalised emission spectra of DMAPOP for all the prototropic species.
acceptor ring, and, on the other hand, the protonation at dimethyl-
amino nitrogen (donor) will retard the charge transfer in DC2
and DC3.
Acknowledgements
The authors acknowledge department of science and technology
(DST), New Delhi for the financial support.
The fluorescence spectra of DMAPOP at pH lower than
0.18 are shown in Fig. 11. The spectral changes indicate
the shifting of equilibrium towards trication. In concentrated sul-
phuric acid at H0 −9.4 single excitation and single emission
spectra were obtained (Fig. 12). These spectra confirm the for-
mation of trication by protonation of all three nitrogens.
For comparison purposes the fluorescence spectra of all the pro-
totropic species of DMAPOP (Scheme 1) are compiled in
Fig. 13.
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4. Conclusion
The emission characteristics of DMAPOP are significantly
different from those of its imidazole analogue DMAPIP in protic
solvents. It emits single emission from the intramolecular charge
transfer state in both protic and aprotic solvents. No protic
induced dual emission is observed in DMAPOP. Further investi-
gations are required to understand the mechanism of dual emis-
sion in DMAPIP. Multiparametric regression analysis suggested
all three parameters contribute to the stabilization of both ground
and excited states. However the relative contribution of dipolar
interactions and hydrogen-bond donating ability of the solvent
enhanced in the excited state and that of the acceptor ability of
solvents is reduced in the excited state. The quantum yield
decreases in polar protic solvents due to an increase in the non-
radiative rate by hydrogen bonding interactions. DMAPOP also
forms two kinds of monocations in the ground and the excited
states. But, unlike DMAPIP, monocations are formed by protona-
tion of pyridine and dimethylamino nitrogen. Though the dica-
tion equilibrium is dominated by DC3, two other dications DC1
and DC2 also exist in aqueous media.
6 J.-P. Fouassier, D.-J. Lougnot, F. Wieder and J. Faure, Laser investigations
of some oxazole and oxadiazole derivatives, J. Photochem., 1977, 7,
17–28.
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