The effect of N-methylation on photophysical properties of 3-aminoquinoline

Article abstract of DOI:10.1007/s10895-011-0895-2

It has been reported previously that the photophysics of 3-aminoquinoline (3AQ) is governed by a flip-flop motion of its amino group in apolar solvents and intersystem crossing in polar solvents. The nonradiative rates are governed by more than one solvent parameters, like polarity and hydrogen bonding ability of the solvent. So, 3AQ is not a well-behaved probe of any individual solvent parameter. In the present work, 3AQ has been modified synthetically; replacing an amino hydrogen atom with a methyl group and photophysical properties were studied in 22 different apolar, polar-aprotic and polar-protic solvents. It is found that Stokes' shifts and fluorescence quantum yields exceptionally low in apolar solvents as compared to those in other solvents. Such substitution causes a decrease in the extent of the effect of the hydrogen bonding ability of the solvent and that nonradiative rate become a more regular function of polarity. However, the flip-flop appears to continue to be the major nonradiative pathway, as the nonradiative rates decrease with increase in micropolarity.

Full text of DOI:10.1007/s10895-011-0895-2

J Fluoresc (2011) 21:1959–1967
DOI 10.1007/s10895-011-0895-2
ORIGINAL PAPER
The Effect of N-methylation on Photophysical Properties
of 3-Aminoquinoline
Shipra Agarwal
Received: 23 December 2010 /Accepted: 2 May 2011 /Published online: 11 May 2011
#
Springer Science+Business Media, LLC 2011
Abstract It has been reported previously that the photo-
physics of 3-aminoquinoline (3AQ) is governed by a flip–
flop motion of its amino group in apolar solvents and
intersystem crossing in polar solvents. The nonradiative
rates are governed by more than one solvent parameters,
like polarity and hydrogen bonding ability of the solvent.
So, 3AQ is not a well-behaved probe of any individual
solvent parameter. In the present work, 3AQ has been
modified synthetically; replacing an amino hydrogen atom
with a methyl group and photophysical properties were
studied in 22 different apolar, polar-aprotic and polar-protic
solvents. It is found that Stokes’ shifts and fluorescence
quantum yields exceptionally low in apolar solvents as
compared to those in other solvents. Such substitution
causes a decrease in the extent of the effect of the hydrogen
bonding ability of the solvent and that nonradiative rate
become a more regular function of polarity. However, the
flip–flop appears to continue to be the major nonradiative
pathway, as the nonradiative rates decrease with increase in
micropolarity.
Introduction
Aminoquinolines are a class of molecules with interesting
fluorescence properties [1–3]. This has prompted our group
to initiate a systematic investigation on 3-Aminoquinoline
(3AQ). The functionalization of such molecules are easy
due to the presence of amino group and can be used further
as possibility of enhancement in substrate binding, which
has great importance in sensing technology [4–6]. 3AQ in
apolar solvents, exists in non-planar configurations, in
which its -NH2 group is very flexible and do not participate
in resonance with quinoline moiety. The amino group
undergo flip–flop motions and thereby couple to the solvent
modes to efficiently deactivate the excited state in apolar
solvents. While in polar and hydrogen bonding solvents
photophysics of 3AQ is governed by intramolecular charge
transfer (ICT). Thus, its fluorescence parameter are gov-
erned by an intricate interplay of polarity and hydrogen
bonding ability [7, 8]. Such complications in its photo-
physics make 3AQ an ill-behaved fluorescent probe for
microenvironments. However, the amino group in the
compound renders it susceptible to synthetic modification
and provides an opportunity of testing the hypothesis of the
involvement of flip–flop motion and hence, the hydrogen
bonding with solvent molecules. Such modification also
opens up the possibility of the design of well-behaved
fluorescent probes, by elimination of the factors that
complicate the photophysics of 3AQ [7]. In the present
work, hydrogen atom of amino group of 3AQ has been
replaced with methyl group, as a result of this a new
compound viz., N-methylquinolin-3-amine (NMeAQ) has
been synthesized for the first time and the effect of solvents
on its photophysical properties have been investigated. In
the synthetic procedure of monomethylaion, at first ioniza-
tion of N-H bond take place and then alkylation occur
.
.
Keywords Quinoline Fluorescent probe Time resolved
.
.
fluorescence Intramolecular charge transfer Empirical
micropolarity
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-011-0895-2) contains supplementary material,
which is available to authorized users.
S. Agarwal (*)
Department of Chemistry,
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, India
e-mail: ashipra@chem.iitb.ac.in

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J Fluoresc (2011) 21:1959–1967
which requires an inert atmosphere because highly reactive
N-anions are generated during the reaction (Scheme 1) [9].
The fluorescence properties of this newly synthesized
molecule have been studied in 22 organic solvents (apolar,
polar-aprotic and polar-protic) using ground-state absorp-
tion and steady-state and time-resolved fluorescence meas-
urements, in order to understand the effect of N-
methylation on the excited state processes. In NMeAQ,
very large Stokes’ shifts are observed which are very
sensitive to the solvent polarities and micropolarities. Due
to these interesting properties, such compound can have
applications in studying the solvatochromic behaviours and
polarities of different environments and also in measuring
the solvent relaxation times using dynamic Stokes’ shift
method.
over magnesium sulfate, and concentrated by rotary
evaporation to give crude product. Crude product was
dissolved in 5 mL ethylacetate and decolorized with Norit
SA3 (100 mesh) activated charcoal reflux. The solution was
cooled to room temperature and filtered through celite pad
using vacuum filtration. Finally filtrate was concentrated
using rotary evaporator and at low temperature (8–10 °C)
needle like crystals appeared after 1 day. N-methylquinolin-
3-amine is obtained in 32% yield (0.49 g). Rf, 0.8, (DCM:
1
MeOH::8:2, v/v), UV (MeOH): 1max 368 nm; H NMR
(CD3OD, 400 MHz, δ (ppm)): 2.78 (s, 3H); 4.88 (s, -NH);
7.4 (t, 1H, J=6.4); 7.45–7.6 (m, 2H); 7.78–7.92 (m, 2H);
8.02 (d, 1H, J=8.2); 13C NMR (CD3OD, 100 MHz, δ
(ppm)): 29.6 (NHCH3), 124.8 (6-C), 125.1 (8-C), 126.1 (7-
C), 127.3 (8-C), 129.1 (4-C), 129.7 (5-C), 137.5 (3-C),
141.5 (10-C), 143.5 (2-C); MS (EI) m/z: 158.08 (M+);
Anal. Cacld for C10H10N2 C, 75.92; H, 6.37; N, 17.71;
found: C, 75.91; H, 6.39; N, 17.68 (Figure S1 a, b, c in
supporting information).
Experimental
Synthesis of N-methylquinolin-3-amine
Photophysical Studies
To a 250-mL, three-necked, round-bottomed flask equipped
with a magnetic stirring bar, an argon inlet, a 100-mL
pressure-equalizing addition funnel, and a rubber septum,
flushed with argon, 2.28 g (0.02 mol) of a 35% dispersion
of potassium hydride (KH) in mineral oil was added. The
mineral oil was removed by introducing 15 mL of hexane
with a syringe and stirred gently, followed by removal with
a syringe after potassium hydride settled down. To this
flask 75 mL of dry tetrahydrofuran (THF) was added,
followed by gradual addition of a solution of 1.44 g
(0.01 mol) of 3-aminoquinoline in 23 mL of dry THF, using
a dropping funnel. The magnetically stirred suspension of
potassium hydride is cooled to 0 °C, and the solution of 3-
aminoquinoline was added dropwise over a 10-min period.
After complete addition, the mixture was stirred at 0 °C for
20 min, and 3.4 mL (0.052 mol) of methyl iodide was
introduced dropwise via syringe. The cooling bath was
removed, and the reaction mixture was stirred at room
temperature for 10 hr. The reaction was quenched by
cautious addition of 5 mL of water, and the resulting
mixture was transferred to a 500-mL separatory funnel
containing 35 mL of water. The layers were separated, and
the aqueous layer was extracted three times with 30 mL of
ethyl acetate. The combined organic extracts were washed
with 75 mL of saturated sodium chloride solution, dried
3-Aminoquinoline from Lancaster, UK is used as received
to synthesize the N-methylquinolin-3-amine (NMeAQ).
The purity of NMeAQ was confirmed from the comparative
study of its fluorescence excitation spectra and absorption
spectra given in supporting information (Figure S2) which
shows that excitation and absorption spectra corresponds
and there is only one species, which doesn’t has any form
(aggregates, complexes etc) in ground state. Millipore water
is used for spectroscopic studies. Other solvents of
spectroscopic grade from Spectrochem, Mumbai, India are
distilled immediately prior to use. The steady state spectra
were recorded on JASCO V570 spectrophotometer and Varian
Cary Eclipse fluorimeter. Fluorescence quantum yields (ff) of
NMeAQ in different organic solvents are calculated after
proper correction for changes in absorbance using quinine
sulphate dissolved in 0.5 M H2SO4 as the standard, (ff=0.55)
[10]. For all the fluorescence measurements, the optical
density (OD) of the solutions have kept quite low (<0.2) at
the excitation wavelength. Time-resolved fluorescence meas-
urements are performed with TCSPC fluorescence spectrom-
eter with R3809U-50 microchannel plate photomultiplier tube
(MCP-PMT, Hamamatsu) and Picoharp300 TCSPC module
using an IBH light emitting diode with 1ex=341 nm and
repetition rate of 1 MHz as the source. The full width at half
H
N
NH2
KH, THF, MeI
N
N
Scheme 1 Synthesis of methylated derivative (NMeAQ) of 3-aminoquinoline

J Fluoresc (2011) 21:1959–1967
1961
(a)
maximum of the instrument response function is 800 ps. The
emission from the samples is collected at right angle to the
direction of the excitation beam, at magic angle polarization
(54.7°), except for the anisotropy measurements. The data are
fitted to single function after deconvolution of the instrument
response function by an iterative reconvolution technique,
using the IBH DAS 6.2 data analysis software, where reduced
χ2 and weighted residuals serve as parameters for goodness
of fit [11]. Non-radiative rate constants in different solvent
have been calculated from steady state fluorescence quantum
yield (ff) and radiative lifetimes (Cf) using the relation
kNR ¼ ð1 ꢀ ff Þ=tf .
TL
MEG
H2O
DOX
DMF
425
325
350
375
400
In order to find out the solvent polarity effect on
NMeAQ, Lippert- Mataga equation has been used,
(b)
ꢀ
ꢁ
MEG
DMF
2$m2 " ꢀ 1
n2 ꢀ 1
H2O
DOX
$u ¼
ꢀ
þ constant
ð1Þ
hca3 2" þ 1 2n2 þ 1
TL
ꢀ
ꢁ
" ꢀ 1
n2 ꢀ 1
$f ¼
ꢀ
ð2Þ
2" þ 1 2n2 þ 1
where Δυ is frequency shift (in cm−1) between absorption
and emission, and Δf is polarity function. ε and n are
dielectric constants, refractive index, respectively, of the
solvents. Their values have been obtained from literature
[12–14]. Kamlet-Taft (KT) analysis has been performed on
the absorption and fluorescence maxima in different
solvents, so as to ascertain the relative contributions of
solvent polarity and hydrogen bonding effect on the spectra
of the fluorophore. In this analysis, the spectral maxima are
fitted to a weighted linear sum of the polarity and hydrogen
bonding abilities of the solvent [15, 16].
360
400
440
480
520
560
λ/nm
Fig. 1 a Absorption spectra and b Emission spectra of NMeAQ in
different solvents: toluene (TL), dioxane (DOX), dimethylformamide
(DMF), ethanediol (MEG) and water (H2O)
photophysical properties strongly change with respect to 3-
AQ as can be seen in Table 1. Table 1 lists photophysical
characteristics of NMeAQ in different organic solvents. The
maxima of absorption and emission spectra are red shifted
with increase in the polarity of the solvents in NMeAQ
compared to that in 3AQ. This is reminiscent of the
situation in 7-amino coumarin, in which N-methylation
causes a red shifted absorption and emission spectra [17,
18]. But water shows anomalous behaviour, absorption
spectra in water is remarkably blue shifted while emission
spectra is red shifted. This is due to increase in charge
transfer character in excited state.
Typical stokes shift values are 2,475 cm-1 in n-hexane
(apolar), 4,162 cm-1 in acetonitrile (polar aprotic) and
3,918 cm-1 in methanol (polar protic) in NMeAQ. The
corresponding Stokes shifts are 2,735, 3,456 and 3,941 cm-1
in 3AQ. Thus, the Stokes shifts are greater in NMeAQ
than in 3AQ in polar solvents. However this value is less in
NMeAQ in apolar solvent. In apolar solvents the emission
spectra are distinctly blue shifted than that observed in
polar solvents. This indicates that NMeAQ is more
destabilized in excited state than ground state in apolar
solvent. The appearance of the uabs and uems (Fig. 2(a)) at
higher energies in these solvents indicates an interesting
»
u ¼ u0 þ pp þ aa þ bb
ð3Þ
where υ denotes the spectral peak frequency in a particular
solvents, π* denotes the dipolarity-polarizability, α and β
denote the hydrogen-bond donating (HBD) and hydrogen-
bond accepting (HBA) ability of the solvent and ν0 denotes
the peak frequency in absence of any solvent. The value of
the coefficient p, a, b (obtained from analyzing the spectral
parameters in a series of solvents) gives an idea about the
effect of polarity and HB on the spectral properties of a
fluorophore.
Results and Discussion
Photophysical Properties
Solvent Dependence on Steady State Absorption
and Fluorescence Spectra
UV/Vis absorption and fluorescence spectra of NMeAQ in
apolar and polar solvents are depicted in Fig. 1. The

1962
J Fluoresc (2011) 21:1959–1967
Table 1 Spectral and temporal parameters for NMeAQ in organic solvents
max
abs
max
ems
Δυ (cm−1
)
ff
τf (ns) kNR × 109(S−1
)
kR × 109(S−1
)
l
(nm)
l
(nm)
Solvent
Δf
Non-polar solvents
n-Hexane
n-Hexane
n-Heptane
0
0
0
0
0
354
336
355
355
354
388
370
387
388
389
401
417
419
418
436
428
423
429
393
426
425
426
427
426
425
436
428
430
406
448
2,475
2,735
2,329
2,395
2,541
2,686
3,720
3,530
3,854
3,945
4,032
3,983
4,162
3,456
3,773
3,644
3,998
3,828
3,773
3,792
3,945
3,883
3,918
3,941
6,497
0.057
0.16
0.053
0.056
0.050
0.12
0.31
0.14
0.24
0.68
0.60
0.33
0.45
0.31
0.39
0.37
0.35
0.34
0.18
0.13
0.11
0.28
0.12
0.68
0.09
1.06
1.19
1.08
1.19
1.06
2.2
0.89
0.70
0.87
0.80
0.83
0.31
0.15
0.24
0.19
0.04
0.11
0.15
0.14
0.34
0.07
0.08
0.11
0.04
0.11
0.11
0.26
0.06
0.33
0.06
0.38
0.05
a
0.13
0.049
0.047
0.047
0.054
0.056
0.044
0.074
0.073
0.077
0.070
0.071
0.15
Cyclohexane
3-Methylpentane
Toluene
0.013 362
0.025 361
0.201 365
0.212 360
0.264 372
0.273 365
0.284 362
0.305 364
0.305 346
0.244 367
0.249 368
0.262 364
0.263 367
0.274 367
0.276 366
0.279 372
0.288 367
0.308 368
0.309 350
0.319 347
Polar-aprotic solvents 1,4-Dioxane
5.54
3.2
Ethylacetate
THF
3.21
9.31
7.76
4.7
DMSO
DMF
Acetone
Acetonitrile
Acetonitrilea
6.31
2.01
8.41
8.05
5.94
7.99
7.98
7.62
2.56
7.96
2.71
4.76
2.38
Polar-protic solvents
1-Hexanol
1-Pentanol
t-Butanol
1-Butanol
1-Propanol
iso-Propanol
Ethanediol
Ethanol
0.046
0.046
0.059
0.042
0.022
0.017
0.0427
0.035
0.044
0.14
Methanol
Methanola
Water
0.037
a data shown is for 3AQ (in hexane, acetonitrile and methanol) and taken from reference no [7].
max
The spectral and temporal parameters in Table 1 are polarity function (Δf), absorption maxima (lmabasx), emission maxima (l ), Stokes shift (Δυ),
ems
Fluorescence quantum yields (ff), radiative lifetimes (τf), nonradiative rate constants (kNR), radiative rate constants (kR). Abbreviations for the
solvents are tetrahydrofuran (THF), dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF).
observation that no specific solute–solvent interaction is
expected in apolar solvents, for which I have tried to
correlate the Stokes’ shifts (i.e.,Δu ¼ uabs ꢀ uems) with the
Δf values of the solvents. It is generally implicit that the
longer-wavelength absorption and the fluorescence bands
of a molecule are due to the transitions between the same
two ground and excited electronic states [19–22] and thus,
the Stokes’ shifts can be correlated with the solvent
polarities of different solvents. Higher uabs and uems values
in aprotic solvents than those in other protic solvents of
similar polarities indicate that the S0 and S1 states of
NMeAQ are not of the same nature. Figure 2(b) shows the
Lippert plot, which is almost liner with some deviations for
almost all the solvents except apolar solvents which have
unusually small Stokes’ shift. Another interesting point to
be noted from Fig. 2(b) is in relation to the highly viscous
alcoholic solvent; t-Butanol (tBuOH) has linear correlation
with DMSO and DMF solvents and has upward deviation
than other protic solvents. This indicates intermolecular
hydrogen bonding interaction between NMeAQ and the
solvent molecules, which causes an extra stabilization for
both S0 and S1 states of the compound. All other protic
solvents are in linear and a little lower in to the corelation
observed for aprotic solvents.
Involvement of the planar or the nonplanar structure
both are expected in the ground state of the compound,
depending on the solvent polarities. It is expected that in
apolar solvents, nonplanar structure will have energies
lower than those of the planar structure even in the ground
state. Thus, in apolar solvents, since both ground and
excited states of NMeAQ are apolar with no resonance
between the NHMe group and the quinoline moiety, the
uabs and uems appear at much higher energies compared to
those in other solvents of moderate to higher polarities. For
the same reason, the Δu values in apolar solvents are
expected to be much smaller compared to those in other
solvents. Since both the ground and excited states of
NMeAQ are of ICT character in moderate to higher polarity

J Fluoresc (2011) 21:1959–1967
1963
(a)
2.9
Abs
1000
100
10
2.8
1,4-Dioxane
ACN
2.7
2.6
2.5
2.4
2.3
Hexane
MeOH
Em
10
20
30
time (ns)
40
50
0.00
0.20
Δf
0.25
0.30
0.35
Fig. 3 The fluorescence decay profiles of NMeAQ in n-hexane,
1,4-dioxane, acetonitrile and methanol
(b)
4.5
4.0
3.5
3.0
2.5
2.0
are single exponential in all apolar solvents and polar
solvents except water and ethanediol. This is reminiscent of
the situation in aminoquinoline and aminocoumarins, where
the tetrahedral amino group is considered to undergo flip–
flop motion in apolar solvents, thereby providing an
additional nonradiative pathway for depopulation of the
excited state in these solvents [24, 25]. The flip–flop
motion is proposed to be restricted in polar solvents due to
formation of the planar ICT state. The greater suppression
of the nonradiative process in protic solvents and DMSO is
rationalized by specific hydrogen bonding interactions. The
presence of one hydrogen atom in amino group in NMeAQ
leads us to offer a similar explanation for the observations
that has been obtained in 3AQ.
Fluorescence quantum yields (ff) of NMeAQ in different
solvents were estimated at room temperature and are listed
in Table 1. It is clear from the plot of ff versus the Δf
values (Fig. 4(b)) that ff values in apolar solvents are lower
in comparison to other solvents and it increases with
increasing Δf and it is maximum for DMSO i.e. a weak,
but distinct, linear variation can be obtained in solvents of
low to medium polarity as well and for these solvents, the
line has a positive slope while in case of polar protic
solvents there is also a linear variation with negative slope
for increasing polarity, this reduction in the ff values
attribute to the intermolecular hydrogen bonding effect.
From the above data it was inferred that solvent polarity
effect on spectral shifts that the S0 and S1 states of NMeAQ
in apolar solvents are of different characteristics than in
other solvents because of this ff values in apolar solvents
are quite different than those in other solvents.
0.20
0.35
0.00
0.25
Δf
0.30
Fig. 2 a Plot of energies of absorption ðnabsÞ and emission ðnem
Þ
against solvent polarity function (Δf). b Plot of Stokes shift ðΔnÞ
against solvent polarity function (Δf) in apolar (solid triangle), aprotic
(hollow circle), and protic (solid circle) solvents. The reported data are
in the following solvent systems in the order of increasing polarity: (1)
n-heptane, (2) n-hexane, (3) cyclohexane, (4) 3MP, (5) toluene (6) 1,4-
dioxane, (7) ethyl acetate, (8) tetra hydrofuran, (9) dimethyl
formamide, (10) DMSO, (11) acetone, (12) acetonitrile, (13) 1-
t
hexanol, (14) 1-pentanol, (15) BuOH, (16) 1-BuOH, (17)1-propanol,
(18) iso-propanol, (19) ethanol, (20) ethanediol (21) methanol (22)
water. (For abbreviations see Table 1)
solvents, the uabs and uems undergo substantial red shift with
solvent polarity.
The fluorescence quantum yields, in the plot of ff versus
the ET(30) (Figure S3(b) in supporting information) and ff
versus the Δf, first increase with increasing Δf and ET(30)
and then after DMSO it decreases for higher Δf and ET(30)
values with significant downward deviation in protic
solvents. This is usual; as generally, an increased Stokes’
shift is accompanied by a decrease in fluorescence quantum
yield due to an increase in nonradiative rates, as per the
energy gap law [23]. Figure 3 shows the radiative lifetimes
in four different solvents. Notably, the fluorescence decays
In order to explore this possibility further, the depen-
dence of the radiative and nonradiative rates as a function
of solvent polarity has been examined. In this study, the
empirical micropolarity parameter, ET(30) has also been

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J Fluoresc (2011) 21:1959–1967
the more polar ICT state which is planar. The sudden
decrease of the kNR between ET(30) 31.1 and 35 can be
rationalized as a crossover from the predominantly apolar
pyramidal conformation of the amino group to the
predominantly planar one. The sudden increase, on crossing
over to protic solvents, can be explained by further less
hindered motion of the amino group as compared to 3AQ
due to the formation of hydrogen bonds. A similar
explanation can be offered for the even greater decrease
in the nonradiative rate in DMSO and tBuOH. The protic
solvents act as hydrogen bond donors as well as acceptors.
DMSO, on the other hand, acts only as a hydrogen bond
acceptor. The lone pair on the nitrogen atom involves in
hydrogen bonding with the protic solvent molecules and
reduces its participation in the ICT process, this interaction
is absent in DMSO. Thus, in this solvent, the movement of
the amino group is restricted by the hydrogen bonds
between amino nitrogen atoms and the solvent molecule,
but the lone pair is absolutely free to participate in ICT,
thereby restricting the motion of the amino group to an
even greater extent.
Nevertheless, it is important to note that two distinct
regions with different types of dependence between non-
radiative rates and polarity are identified. The considerable
deviation of the data points on either side of the lines is too
large to be entirely due to errors in the experiment. It is
more likely that they indicate different kinds of specific
solvent effects. For example, there is a negative deviation
for highly viscous solvent ethylene glycol and positive
deviation for acetone which contains C=O bond. Moreover,
the question that arises now is that whether the so-called
unusual behavior is observed in the low-polarity or the high
polarity region, as there are weak linear correlations of
opposite natures in the two regions. At this point, it is
useful to recall that in the absence of excited state reactions
like twisted intramolecular charge transfer (TICT), excited
state proton transfer (ESPT), photoisomerization etc., log
kNR can have a direct as well as inverse variation with
solvent polarity, depending on the relative polarities of the
ground and the excited states. In molecules with a more
polar excited state than the ground state, there is a Stokes
shifted emission quantified by the well-known Lippert
(a)
(b)
1E-9
1E-10
0.60
0.45
0.30
0.15
0.00
0.20
0.25
0.30
Δf
Fig. 4 A linear variation of the non-radiative rate (a) and quantum
yield (b) with the solvent polarity function (Δf). The data in apolar
(circle), aprotic (hollow square), and protic (solid triangle), The solid
line is linear fit
used with Δf, as some kind of a specific solute–solvent
interaction at the molecular level is suspected to operate
and in such cases, it is well established that ET(30) is a
more useful polarity parameter compared to measures of
bulk polarity like the dielectric constant [26]. The values of
radiative rate constants (kR) are found to be almost constant
for all the solvents used, but the non-radiative rate constants
(kNR) exhibit a rather interesting solvent dependence
(Table 1). While observing the plot of log kNR rates with
the empirical solvent polarity parameter ET(30) (Figure S3
(a)) it yields a straight line with a positive slope for the
high-polarity solvents and for low polarity the log kNR
values decreases linearly with increasing micropolarity from
apolar to polar aprotic solvents i.e. it yields straight line with
negative slope with some deviations. The log kNR in DMSO
turns out to be smaller than other solvents of similar
polarity. Thus, the polarity dependence appears to generally
decrease with increasing polarity, with the breaks in the
trends signifying the onset of a different kind of mechanism
that influences the nonradiative process. Similar to the case
of aminocoumarins, the tetrahedral amino group in 3AQ
can undergo flip–flop motion in apolar solvents. The apolar
excited state with tetrahedral amino group is destabilized
with increase in the polarity of the solvent, in comparison to
Table 2 KT analysis of absorption and emission peak frequencies and ln kNR for NMeAQ
p(π*) a(α)
−617 (55%) −494 (43%)
b(β)
−28 (2%)
−456(16%)
−100 (6%)
−659 (20%)
ν0
R
NMeAQ
3AQa
Abs
Em
Abs
Em
28,039
25,308
29,736
26,948
0.97
0.92
0.97
0.98
−2,066 (68%)
−975 (55%)
−481 (16%)
−681 (39%)
−1,559 (47%)
−1,088 (33%)
a Data taken from reference no. 7. For absorption and emission, units are in cm−1 . The figures in parentheses indicate the relative percentage. R
denotes regression coefficient.

J Fluoresc (2011) 21:1959–1967
1965
Table 3 Relevant geometrical parameters involved in photophysical
properties of NMeAQ
more than that from α. Thus the fluorescence of the
NMeAQ is found to be influenced more by the polarity and
less by the hydrogen bonding ability of the medium, than
that of 3AQ. This observation is commensurate with the
inference drawn in the previous section.
B3LYP/gas
Onsager/H2O
CIS
C2-N2-H
115.19
115.62
115.46
C2-N2-Me
H1-N2-Me
C1-C2-N2-H
C3-C2-N2-Me
121.87
121.84
125.07
115.89
115.76
116.03
Density Functional Theory Calculations
−19.86(−25.0)
12.38(22.5)
−19.04(−34.4)
12.72(17.9)
−6.42(−13.0)
17.70(21.8)
Geometrical Features and Natural Bond Orbital Analysis
Ground State (B3LYP, Onsagar) and excited state (CIS) calculations
are performed using 6-311++G** and 6-31+G* respectively.
Paranthesis contains corresponding value for 3AQ ref. no 7.
The geometry of the neutral form of NMeAQ has been
optimized independently using the Density functional
theory (DFT) (Table 3). The co-planarity of the methyl
substitute amino group with ring system play an important
role in the possibility of flip–flop motion of the methyl
substituted amino group and intramolecular charge transfer
(ICT) in the system. Hence the bond angle C2-N2-H, C2-
N2-Me, Me-C-H and the H/Me-N2-C-C dihedral angles
(dh) obtained from different level of calculation shown in
Table 3 provide an idea about the feasibility of the flip-flop
motion. It is seen that the value dihedral angle (dh) ‘C1-C2-
N2-H’ decreases in excited state from −19.86° (dh in
ground state) to −6.42° where as value of the dihedral angle
‘C3-C2-N2-Me’ increases in the excited state in NMeAQ.
However comparing the corresponding dihedral angles
shown in parentheses in Table 3 for 3AQ, it is clear that
the degree of planarity of the methyl substituted amino
group in NMeAQ in excited state is less than that of amino
group in 3AQ. This indicates some hindrance in flip-flop
motion of the substituted methyl group which slows down
the non-radiative process in apolar and polar aprotic
solvent. The theoretical calculation also supports the
occurrence of the ICT state though the planarity in the
excited state is less in compare to that in 3AQ. Experimen-
tally it has been observed that the red shifted absorption and
emission almost in all solvents. This is due to increase in
charge transfer character in excited state. This can be
interpreted via, natural bond orbital (NBO) analysis [34]
which has been performed at B3LYP/6-311++G** level of
theory and the respective results is presented in Table 4.
The perturbation E(2) charge-transfer energies corresponding
to the interaction of lone pair of N2 with antibonding (π*)
Mataga equation [27]. Moreover, in this kind of molecules
as the energy gap between the excited and the ground state
decreases in polar solvents, there is an increase in the non-
radiative rate with polarity [28, 29]. The increase in the
energy gap between their ground and excited states forms
the basis of the widely used ET(30) scale of polarity as
observed in Lucifer yellow ethylenediamine (LYen).
Kamlet-Taft Analysis of the Spectral Data
Micropolarity is not only the parameter which effect on
photophysical behavior of fluorophore. Other parameters
like hydrogen bonding donation ability (α) and hydrogen
bonding accepting ability (β) may also play important roles
when the fluorophore has both acid and basic group. In
such a situation it is convenient to approach a linear
solvation energy relationship proposed by Kamlet and Taft
[30–33]. Such a treatment has been performed earlier on
3AQ [7]. Upon performing a similar multiple regression
fitting of the data of NMeAQ in all solvents to Kamlet Taft
(KT) function, the following are observed: The relative
contributions of polarity and hydrogen bonding ability,
towards the absorption maxima of NMeAQ and 3AQ are
the same. This indicates that the ground state intramolecular
interaction in NMeAQ is more or less the same as in 3AQ.
The relative contribution of polarity to emission, however,
is increased to 68% in NMeAQ, as against 47% in 3AQ
(Table 2). Moreover, equal contributions are observed from
α and β, unlike in 3AQ where the contribution from β was
Table 4 Second order perturbation theory analysis of Fock matrix in NBO basis for NMeAQ
Donor (i)
Acceptor (j)
aE(2)/kcalmol−1
bE(j)-E(i)/a.u.
cF(i,j)/a.u.
dLP N1
LP N1
LP N2
σ* (C4–C5)
σ* (C1–C2)
π* (C2–C3)
9.36
10.36
36.95
0.88
0.84
0.30
0.082
0.084
0.097
Fi2j
a E(2) means energy of hyperconjugative interactions, b energy difference between donor and acceptor i and j NBO
hsjFjsi
Eð2Þ ¼ ꢀns
¼ ꢀns
;
"
»ꢀ"s
ΔE
s
orbitals, c F(i,j) is the Fock matrix element between i and j NBO orbitals, nσ is the population of the donor σ orbital. d LP=lone pair.

1966
J Fluoresc (2011) 21:1959–1967
orbital of C2-C3 is 8 kcal/mole more in NMeAQ than in 3AQ
[7]. This clearly indicates an increase in charge transfer
character with substitution of methyl group. The other
parameters which are obtained in NBO analysis are energy
difference (ΔEi,j) in between donor and acceptor orbital
which is 0.30 a.u. and Fock matrix element(Fi,j) which is
0.097.
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Acknowledgements This work is supported by a DBT Research
Associateship. The Department of Chemistry, IIT Bombay, is thanked
for having provided its facilities. The author thanks Prof. Anindya
Datta and students of the Ultrafast Dynamics group for their support
and encouragement.

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