Photophysical behavior of a potential drug candidate, trans-[2-(4-methoxystyryl)]quinoline-1-oxide tuned by environment effects

Article abstract of DOI:10.1016/j.saa.2017.06.024

Styryl and/or quinoline structural fragments, present in a large number of bioactive substances, inspired the design of various new drug candidates. In this paper, we describe the photophysical behavior of trans-[2-(4-methoxystyryl)]quinoline-1-oxide (tra

Full text of DOI:10.1016/j.saa.2017.06.024

Accepted Manuscript
Photophysical behavior of a potential drug candidate, trans-
[2-(4-methoxystyryl)]quinoline-1-oxide tuned by environment
effects
Anna Szemik-Hojniak, Irena Deperasińska, Yakov P. Nizhnik
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DOI:
Reference:
S1386-1425(17)30495-X
doi: 10.1016/j.saa.2017.06.024
SAA 15240
Spectrochimica Acta Part A: Molecular and Biomolecular
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17 June 2017
Please cite this article as: Anna Szemik-Hojniak, Irena Deperasińska, Yakov P. Nizhnik
Photophysical behavior of potential drug candidate, trans-
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[2-(4-methoxystyryl)]quinoline-1-oxide tuned by environment effects, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/
j.saa.2017.06.024
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ACCEPTED MANUSCRIPT
Photophysical behavior of a potential drug candidate, trans- [2-(4-
methoxystyryl)]quinoline-1-oxide tuned by environment effects
Anna Szemik-Hojniaka*b, Irena Deperasińskac, Yakov P. Nizhnikd
a Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 str;50-383 Wroclaw, Poland,
bUniversity of Applied Sciences, Institute of Health, 4 Zamkowa str, 58-300 Wałbrzych, Poland;
E-mail: anna.szemik@chem.uni.wroc.pl
cInstitute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
dChemistry Department, Bioo Scientific Corp., 7050 Burleson Rd., Austin, TX, 78744, USA
Abstract
Styryl and/or quinoline structural fragments, present in a large number of bioactive substances,
inspired the design of various new drug candidates. In this paper, we describe the photophysical
behavior of trans-[2-(4-methoxystyryl)]quinoline-1-oxide (trans-MSQNO) on the basis of X-ray
analysis data, theoretical calculations as well as steady state and time-resolved spectroscopy
experiments in various media. The molecule crystalizes in orthorhombic unit cell containing
eight molecules of N-oxide, space group Pbca. The N–O bond is substantially shorter in
comparison with the N-O bond in the ZnTPP unit [1.3052(11) Å vs. 1.335 (2) Å]. Variation of
emission colors from the violet (~450 nm) through blue (480 nm), green (525 nm) and yellow
(575 nm) is observed in different environments. Comparable values of lifetimes estimated both
at ambient temperature and at 77K suggest that excited state dynamics in this case is viscosity
independent. DFT and TD DFT B3LYP/6-31G (d, p) calculations performed for four different
trans-MSQNO rotamers in the gas phase, as well as nonpolar and polar media (PCM model)
suggest that an equilibrium between them can be significantly altered even by a relatively weak
interactions with the environment. It is suggested that varying intensity ratios of experimental
absorption bands in different media may be due to the dominant share of one or more rotamers
of the excited trans-MSQNO molecule. Gas phase calculations show also that the vertical ππ*,
S0 → S1, transition resulting from the HOMO→LUMO electronic configuration exhibits only a
partial CT nature. On the other hand, in polar media, a substantial increment of excited state
dipole moment of all rotamers compared to the ground state, its increase with increasing solvent
polarity and a significant red shift in the absorption and emission spectra, point to the enhanced
CT nature of the S1 excited state. Hence, the trans-MSQNO molecule may be considered a
subsequent styrylquinoline drug candidate where the CT drug-receptor interactions are of a high
importance.
*
Key words: Styryl derivatives; pKa ; TD DFT calculations; Excited states; Lifetimes.
.
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1. Introduction
Unique photophysical properties of functionalized styryl dyes with a differently substituted (-
C6H5-CH = C-) fragment are a subject of several ongoing studies [1]. These potentially bioactive
heterocycles may find application in various aspects of medicine and pharmacology [2-3].
Furthermore, substituted styryl dyes are also widely used in modern technologies such as
production of laser discs [4-5], chemical sensors [6-7], OLEDs (Organic Light-Emitting Diode)
[8-9] or organic lasers [10]. A good example of the compound that may be explored in more than
one of the abovementioned applications is trans-4-[p-[N-ethyl-N-(hydroxyethyl)aminostyryl]-N-
methylpyridinium tetraphenylborate salt, which demonstrates not only the excellent lasing
properties but also may be used in biomedical engineering as a fluorescent probe due to its
fluorescence in the orange-red region (620-690 nm) [11-12].
Styryl derivatives, which show high visible fluorescence intensity in the green part of
electromagnetic field, may find application in such biochemistry subfields as
immunofluorescence and immunohistochemistry (for example 2-[4-(dimethylamino)styryl]-1-
methylpyridinium iodide (DASPMI) or 2-[4-(dimethylamino)styryl]-1-methylquinolinium iodide
(DASQMI) [14-16].
At this point we should also mention, the trans- [4-(dimethylamino)-4`-
cyanostilbene](DCS) system, one of the most thoroughly investigated stilbens that exhibits
strong solvatochromism (large Stokes shift), relatively high quantum yield and a very short
lifetime. These features suggest its use as a probe for the solvation processes and potential
application in time-resolved spectroscopy, particularly as the probe in solvation dynamics and in
the Kerr-gated emission spectroscopy [17-19].
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Recently, we examined trans-4-[4'-(N,N-dimethylaminostyryl)]pyridine N-oxide (DPO)
that also belongs to the class of stilbens. This molecule exhibits a strong dependence of
stationary fluorescence from polarity of the solvent and demonstrates an effective charge transfer
process between the dimethylamino group, as electron donor, and the N–O group of pyridine N-
oxide moiety, as electron acceptor [20]. The DFT and TD DFT calculations have shown that in
protic environment, the trans-DPO system forms the (1:1) hydrogen bonded complexes and
undergoes the hydrogen bonded assisted charge transfer to a higher extent than in the free trans-
DPO system.
Heterocyclic N-oxides, including styryl derivatives, often exhibit CT interactions that can
be important for understanding the drug-receptor mechanism [21-24]. Recently, we also
investigated CT behavior of the hybrid trans-MSQNO-ZnTPP complex, i.e. of trans- 2-(4-
methoxystyryl)quinoline-1-oxide (trans-MSQNO) and the ZnTPP unit.
In the hybrid, however, only weak CT interactions were observed. Hence, in search for
possible stronger interactions of this type, we decided to investigate the excited state behavior of
a free ligand, the unbounded trans-MSQNO molecule.
In this study, we report the X-ray structure of … along with the experimental results of
steady state and time-resolved spectroscopy measurementsin different media, further
complemented by the results of the DFT and TD DFT B3LYP/6-31G (d, p) calculations.
2. Material and methods
2.1. Synthesis
Synthesis of trans 2-(4-methoxystyryl)quinoline-1-oxide was performed on the basis of the
method, described elsewhere [21]: 1.6 g (0.01 mol) of 2-methylquinoline-1-oxide and 1.9 g (1.7
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mL, 0.014 mol) of 4-methoxybenzaldehyde were placed into 5 mL of 10% CH3OK in methanol.
The mixture was refluxed 3 hours under nitrogen and then the solvent was evaporated in
vacuum. The product was recrystallized from ethanol two times. Methanol and ethanol for
synthesis and recrystallizations were of analytical grade (POCH, Sigma Aldrich).Yield: 1.94 g
(70%), Yellow powder (m.p. 142.5-143oC. Anal. calc. for C18H15O2N (%): C 77.96; O 11.54; N
5.05; H 5.45. Found (%): C 77.45; O 11.45; N 5.04; H 6.11.
2.2. X-ray experiment
The crystal used for the measurements was cut from a larger crystal and had approximate
dimensions 0.30 x 0.12 x 0.06 mm. The data were collected on an Agilent Technologies
SuperNova Dual Source diffractometer using a -focus Cu K radiation source ( = 1.5418Å)
with collimating mirror monochromators. A total of 2635 frames of data were collected using -
scans with a scan range of 1 and a counting time of 5 seconds per frame with a detector offset of
+/- 40.8 and 20 seconds per frame with a detector offset of +/- 108.3. The data were collected
at 100 K using an Oxford Cryostream low temperature device. Details of crystal data, data
collection and structure refinement are listed in Table 1. Data collection, unit cell refinement and
data reduction were performed using Agilent Technologies CrysAlisPro V 1.171.37.31 [25]. The
structure was solved by direct methods using SuperFlip [26] and refined by full-matrix least-
2
squares on F with anisotropic displacement parameters for the non-H atoms using SHELXL-
2013[27]. Structure analysis was aided by use of the programs PLATON98 [28] and WinGX
[29]. The hydrogen atoms were calculated in ideal positions with isotropic displacement
parameters set to 1.2xUeq of the attached atom (1.5xUeq for methyl hydrogen atoms).
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2
2 2
2
2
The function, w(|F | - |F | ) , was minimized, where w= 1/[(δ(F )) + (0.0535*P) +
o
c
o
2
2
2
(0.9764*P)] and P = (|F | + 2|F | )/3. R (F ) refined to 0.0964, with R(F) equal to 0.0349 and
o
c
w
a goodness of fit, S, = 1.03. The data were checked for secondary extinction effects but no
correction was necessary. Neutral atom scattering factors and values used to calculate the linear
absorption coefficient are from the International Tables for X-ray Crystallography (1992) [30].
All figures were generated using SHELXTL/PC [31].
2.3. Electronic UV-VIS absorption spectra
Electronic absorption spectra in solution were recorded on a CARY-50 UV-VIS (Varian)
spectrometer at the concentration of 10-5 M of MSQNO. Solvents were of spectroscopic grade
and used fresh as purchased (Merck, Uvasol)].
2.4. Steady State Emission Spectroscopy
Emission spectra both in solution and in the solid state were recorded on FSL920 combined
fluorescence lifetime and steady state spectrofluorimeter (Edinburgh Instruments Ltd) using as
excitation source Xe900, 450W steady state xenon lamp (ozone free) with computer controlled
excitation shutter and with spectral bandwidth of ≤ 5 nm for both excitation and emission
spectra. The spectrofluorimeter was equipped with a temperature controlled the EHC-716 cell
holder. Emission at 77K was performed in a small EPR Dewar inserted into the controller of
FSL920 set up (Edinburgh Instruments Ltd) while for the solid sample the front face geometry
type holder was employed. Luminescence was detected using a red sensitive (185-850 nm)
single photon counting photomultiplier tube (R928-Hamamatsu) in a Peltier cooled housing.
Spectra were corrected for detector response and excitation source. Solvents were of
spectroscopic grade and used fresh as purchased (Merck, Uvasol)]. The concentration of the
studied solutions was about 10-5 M.
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2.5. Time-resolved spectroscopy
Fluorescence decay curves were recorded using nanosecond Time-Correlated Single Photon
Counting (TCSPC) option of FSL920 setup (Edinburgh Instruments Ltd) with LED as the
excitation source and MiniRed photomultiplier. The excitation was done at 375 nm. Data
acquisition ensured Plug-in PC Card Model TCC900 with Maximum Count Rate 3MHz, Time
Channels per Curve up to 4096, and Minimum Time per Channel 610fs.
2.6. Theoretical Calculations.
Theoretical calculations of electronic spectra for trans-MSQNO were performed at TD DFT
B3LYP/6-31G(d, p) level of theory with the Gaussian 09 [32] package of programs.
3. Results and Discussion
3.1. Crystal structure
Slow cooling down of the solution of trans 2-(4-methoxystyryl)quinoline-1-oxide in acetone
produced yellow plate crystals, suitable for structure determination by single-crystal X-ray
spectroscopy. The studied crystal is characterized by an orthorhombic unit cell containing eight
molecules of N-oxide, space group Pbca (see Table 1S in SI for other details). The X-ray
structure of the trans-MSQNO is illustrated in Fig.1.
Figure 1
The data obtained show that both the bond lengths and the valence angles in N-oxide
moiety essentially resemble those found in the previously studied by us ZnTPP complex with
this ligand [24] (see:Table 2S in SI for selected geometrical parameters). Nevertheless, two
major structural differences between the complexes ought to be noted. First of all, the N–O bond
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is remarkably shorter in the free N-oxide than in the … (add name) complex, [1.3052(11)] Å vs.
[1.335(2)] Å, respectively. This finding is in line with the data for the zinc complexes exploring
oxygen coordinating ligands. There, a strong, short and purely covalent bond in the free ligand
moiety changes its character into a donor-acceptor nature in the complex, resulting in a weaker
and slightly elongated metal-ligand bond [24, 33-34]. Secondly, the free N-oxide molecule is
more significantly distorted than the N-oxide moiety in the ZnTPP complex where dihedral
angle of quinoline ring versus that in the phenyl ring is as 41.58o vs. 20.50o (see Fig. 1S in SI
3.2. Steady state spectroscopic studies
3.2.1. Absorption and fluorescence spectra
The absorption spectra of trans-MSQNO recorded in various media and exemplified by
the spectra in n-hexane, n-PrOH and in water at pH 1, are displayed in Fig.2a. All these spectra
exhibit two well separated bands with the maxima of the first band centered at about 380 nm in
aprotic media and at 365 nm - 399 nm in protic media. In the fluorescence spectra in aprotic
solvents, the maxima peak in the 470-485 nm spectral region while in an acidified water solution
(pH 1), a broad, strongly red-shifted band assigned to cation-like fluorescence,appears at about
525 nm.
Figure 2
Various positions of absorption bands in figure 2 encouraged us to analyze the optical
properties of MSQNO within three different environmental conditions, i.e. in aprotic solvents, in
alcohols and in buffer water solutions. The most important results of this analysis are gathered in
Table 1.
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Table 1
As seen in Fig.2a, the intensity ratio of the first to the second absorption band varies
depending on the environment. In weakly apolar n-hexane, the first absorption band represents a
weaker transition than the second one, with the intensity ratio 1:2. In polar and protic n-propanol,
both transition probabilities are almost equal (1:1), whereas in highly polar, acidified water at pH
1, the intensity ratio of these two bands relative to n-hexane is reversed (2:1). One possible
reason may be a formation of various ground state rotamers and the dominating character of one
or some of them in different media what further will be discussed in chapter 3.3 (Theoretical
Calculations).
In aprotic solvents, the lowest energy absorption band shows only a slight red shift with
increasing polarity of the surrounding medium (Table 1) while in alcohols, a hypsochromic shift
is observed. In n-propanol (ε = 20.3), ethanol (ε = 24.5) and methanol (ε = 32.7), the absorption
maxima peak at 369 nm, 376 and 378 nm, respectively. In the bulk aqueous buffers, in the pH
range 1-10, two dominating forms, neutral and protonated, may been distinguished (Fig.3). It is
worth mentioning that the neutral form (Absmax = 365 nm) predominates in the broad pH range
(3–10) and equilibrium between both forms takes place only at pH 2 (see line 2 in Fig.3). In
water at pH = 1, the acidic form absorbs at ca. 400 nm.
Figure 3
Room temperature emission of trans-MSQNO can be easily recorded in solution. Several
examples of such fluorescence spectra in various solvents are displayed in Fig.2b and relevant
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photophysical data are collected in Table 1. At room temperature, an increase in polarity of
aprotic solvent leads to the red-shift in the fluorescence maxima and a slight enhancement in the
fluorescence quantum yield. In weakly polar n-hexane ( = 1.88), the maximum located around
470 nm results in a relatively small Stokes shift (4948 cm-1) and a small estimated quantum yield
(0.98x10-3). In polar acetone ( = 21.0) however, the red shift of the fluorescence maximum (485
nm) is slightly larger (5356 cm-1) and the quantum yield rises about twice (1.80 x10-3). These
results allow us to hypothesize that excited state dipole moment of trans-MSQNO may increase
in polar media and the CT nature of the emitting S1 state may be enhanced. This issue will be
further discussed in the next chapter (see chapter 3.3- Theoretical calculations).
Emission sensitivity of trans-MSQNO in higher temperatures is presented in Fig 2S(a) in
SI). Upon increasing temperature, the fluorescence maximum does not change its position while
the band intensity gradually decreases, indicating an effective nonradiative decay operating at
higher temperatures.
On the other hand, at low temperature conditions [Fig.2S(b) in SI], in the frozen n-hexane
glass, where viscosity may substantially increase and the solvent dielectric relaxation is severely
hindered, emission may occur before the relaxation is achieved. Lowering of the temperature to
77 K (λFmax = 450 nm), induces about 20 nm blue shift in the fluorescence maximum relative to
its room temperature position (470 nm). This may mean that despite the fact that the molecule
itself emits from its equilibrium geometry in the excited state, it is not relaxed in the solvent
[35].
At room temperature in alcohols, the fluorescence maxima, similarly to absorption, are
also blue shifted. In comparison with an aprotic n-hexane (470 nm), the peak position in n-
propanol and methanol shifts to 451 and 455 nm, respectively (Table 1). The quantum yield is
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fairly low, i.e., comparatively lower than those in aprotic solvents and clearly undergoes
PrOH
MeOH
= 1.60x10-4, φF
reduction with increase in alcohol polarity (φF
= 1.32x10-4). This
difference is consistent with the hypothesis of intermolecular hydrogen bond formation between
the excited fluorophore and protic solvents and is related to the enhancement of the non-radiative
processes [36].
To the best of our knowledge, there are no studies concerning the influence of pH on the
emissive properties of trans-MSQNO system. Therefore, similarly to absorption, the effect of
acidity and basicity on the fluorescence spectrum was measured in the bulk aqueous buffers in
the pH range 1-10. In water, both at pH 7 and 1, the fluorescence maxima are redshifted up to
478 and 525 nm, respectively (see Figure 3S in SI), with a significant increase of the Stokes
shift value (6477 and 7654 cm-1) relative to that in nonpolar aprotic solvents (~5000 cm-1, Table
1). This indicates that in the case of trans-MSQNO molecule, a relatively weak CT character of
its excited state may be enhanced in the polar environment.
Different positions of the fluorescence band maximum of trans-MSQNO in the neutral
and acidic medium (Fig.2 and Fig 3S in SI) encouraged us to test the change in the pKa value
between the ground and excited state. In order to estimate the ground state pKa, we have
employed the literature data of experimental values of (Δλ) shifts of the Qx (14.4 nm) or Qy
(16.2 nm) absorption band maxima of ZnTPP complexes formed with different N-oxides [37].
Figure 4, demonstrates the Δλ change of the Qx band position in several ZnTPP-N-oxide
complexes, versus the pKa values of the N-oxides involved in the formation of these complexes.
From this plot we obtained the pKa of the MSQNO ligand, ca.1.5. This value seems to be
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reliable, as the pKa values of quinoline or acridine N-oxides are similar in terms of the order of
magnitude (0.86 and 1.37, respectively [37]).
Figure 4
Our estimated ground state pKa (1.5) confirms that due to the polarized >N+→ O- group
present in the neutral trans-MSQNO form, this molecule is a medium strength base [37]. In the
presence of H+ ions, it undergoes a protonation at the negatively charged oxygen atom.
Upon light irradiation, the acids and bases change their acidic and basic properties in a
different way. In order to identify these changes between ground and excited state and to
estimate the pKa value of trans-MSQNO in the excited state (pKa*), we employed the Förster
cycle method [38] using the equation (1):
pKa – pKa* = 2.1 x 10 -3(ν00HA - ν00
)
A
(1)
where: ν00 and ν00 in [cm-1] is the wavenumber of 0-0 transition of the protonated and
HA
A
neutral form, respectively.
Taking into account the estimated ground state pKa (1.5), the value of 4.10 was obtained
for the pKa in the excited state (pKa*). It should be noted, however, that if there are no acidic
functional groups present in the N-oxides, the acids are considered Brönsted-Lowry bases. It
means that these pKa values are actually pKbH+, i.e. they describe acidity of the conjugated acid
(protonated N-oxide). Hence, the higher the pKbH+, the weaker the acid (protonated N-oxide). In
other words, if pK increases in the excited state, basicity also increases. Hence, one may
conclude that the basicity of trans-MSQNO increases on excitation (pKa* 4.1) about a thousand
fold in comparison with the ground state (pKa 1.5). Such enhancement in the excited state
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basicity is due to a dominant mesomeric (+M) effect of methoxy substituent over the inductive (-
I) effect [39- 40].
Excitation of the neutral form in water at pH 7 (380-390 nm) and in acidified water at
pH 1 (395-430 nm) gives a blue (478 nm) and green emission (525 nm), respectively, which
may find application in the sensorchemistry. Their corresponding excitation spectra, exemplified
at pH 1 perfectly reproduce the absorption maxima (365, 400 nm) as shown in Fig4S (SI).
Light irradiation of the solid trans-MSQNO sample at 400 nm gives a blue-green
emission at 504 nm [Fig.5S(a) in SI] while excitation at 490 nm results in a yellow
phosphorescence at 580 nm [Fig.5S(b) in SI].
It is worth noting that for the same solid sample, the phosphorescence intensity is about
100 times stronger (~105 counts) than fluorescence (~103 counts).
3.3.Theoretical Calculations
To gain insight into the nature of electronic excited states of trans-MSQNO molecule,
DFT and TD DFT B3LYP/6-31G(d, p) computations were carried out.
The results of the ground state optimization of trans-MSQNO molecule, presented in Table 2,
demonstrate that various rotamers may coexist at room temperature. Four stereoisomers, divided
in two groups, namely [trans, syn MSQNO-0x and trans, anti MSQNO-1x (with x = 0.1)], have
been taken into account in the optimization procedure. In the first group with “trans, syn”
conformation type (MSQNO-00 and MSQNO-01), the quinoline ring and the phenyl ring are
placed on the same side of the double bond of the styryl bridge. In the second one, i.e. the
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“trans, anti” conformation (MSQNO-10 and MSQNO-11), these two substituents are located on
the opposite sides of the styryl double bond [24, 41-43], as illustrated in Table 2.
Table 2
According to the calculations, the energies of the ground state optimized rotamers differ by less
than 1 kT. Hence, one should realize that the experimental quantities describe to a greater or
lesser extent the whole group of various stereoisomers. The energy calculations also suggest that
the equilibrium between particular trans-MSQNO rotamers can be significantly altered even by
relatively weak interactions with the environment. For example, in the solid state, upon given
crystallization conditions, the X-ray data reveal only the [trans-anti MSQNO-10] rotamer
(Fig.1).
The numerical values of Table 2 allow us to compare the ground state stabilization
energy, dipole moments and vertical transition energies from the ground state to the first three
lower lying excited states (S1 – S3) as well as oscillator strengths of the individual DFT/6-
31G(d,p) optimized rotamers for the isolated trans-MSQNO molecule. The corresponding
electronic configurations are gathered in Table 3 while the simulated absorption spectra of all
optimized rotamers in a wider spectral range can be found in Table 4.
One may note that the [trans, syn MSQNO-00] rotamer is most stable, while the [trans-
anti MSQNO -11] rotamer, the representative of the second group, is the most polar. There is a
small energy difference in the ππ* type of the S0 → S1 transition between all the rotamers and it
represents a relatively high value of oscillator strength. For example, for the [trans, anti
MSQNO-10] rotamer, this value is 0.676. On the other hand, the S2 (ππ *) and S3 (nπ *) excited
states are characteristic of significantly lower oscillator strength. For the same isomer they are
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0.006 and 0.000, respectively. From the frontiers of the molecular orbitals presented in Table 3, a
conclusion may be drawn that the gas phase S0→S1 transition origins from the prevailing
electronic HOMO → LUMO configuration and shows only a weak CT character.
Table 3
Furthermore, it can be noted that due to the intensity distribution in the simulated
absorption spectra (Table 4) the [trans, syn MSQNO-0x] isomers form a distinct group from this
of the [trans, anti MSQNO-1x] systems. This finding may suggest that different intensity ratios
observed in different solvents for experimental absorption bands at ~ 400 nm and ~ 325
nm,(chapter 3.2.1.), may be a certain indication for the dominant isomer present under the
appropriate measurement conditions.
Table 4
Aiming to recognize the influence of the environmental conditions that may affect the
chemical and physical properties of the solute and lead to the change of excited state dipole
moment along with the nature of electronic transitions, the corresponding DFT and TD DFT/
B3LYP/6-31G (d, p) calculations have been performed in a weakly polar n-hexane and a
strongly polar acetonitrile. Computations involved ground and excited state optimization of all
four stereoisomers, calculations of the ground and excited state stabilization energy, absorption
and fluorescence maxima along with the dipole moment values in both solvents and in both
electronic states. The corresponding results are collected in Table 5.
Table 5
These data clearly show that in the non-polar solvents the [trans, syn MSQNO-00] rotamer
prevails while in the polar environment it is the [trans, anti MSQNO-10] isomer. Spectral
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characteristics of both rotamers correlate well with the experimental absorption spectra, where
the I(325)/I(400) ratio in nonpolar hexane is greater than in polar solvents. Based on these
findings, we can assume that different intensity ratios of experimental absorption bands in
different media may be due to the dominant share of one or more isomers of the excited trans-
MSQNO molecule.
It should be noted that according to the excited state calculation results( Table 5), the
energy of the [trans, anti MSQNO-1x] rotamers is lower than that of the [trans, syn MSQNO-
0x] isomers. Moreover, the excited state dipole moment of all rotamers increases as compared to
the dipole moment in the ground state. Similarly, both these dipole moments increase with the
increasing solvent polarity. It is also worth noting that upon increasing polarity of the medium,
both the absorption and the emission spectra exhibit a significant redshift. Hence, despite the fact
that the HOMO → LUMO electronic configuration originates only from a partial shift of the
electron density from the methoxy group (electron donor), to quinoline-1-oxide moiety (electron
acceptor), the results point to the CT nature of the S1 excited state of MSQNO system and are
consistent with the experimental data. We can additionally suggest that although it is not so
much visible that the S1 state intrinsically changes, it is nevertheless highly probable that the
mixing with higher-lying states (with lower oscillator strength and with more CT character)
increases as a function of the solvent polarity what in consequence increases also its CT nature.
As mentioned above, the studied compound belongs to thestyryl derivatives of which
many exhibit useful medicinal properties. In this case, particularly the presence of CT
interactions in the target compound suggest that it may also be considered as a potential drug
candidate [22-23].
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Keeping in mind that in the living systems, biological activity of heterocyclic N-oxides is
first of all related to their ligation with metalloporphyrins and formation of the 1:1 complexes
[44], we simulated such complex (Fig 6S in SI) between the trans-MSQNO molecule and the
zinc(II) meso-tetraphenylporphyrin (ZnTPP) unit. In this system, the N–O group of quinoline-1-
oxide moiety is directly coordinated to the zinc ion of the ZnTPP [45-46]. In its [TD DFT/CAM-
B3LYP/6-31G(d, p)] simulated absorption spectrum (Figure 6S in SI), the red shift observed in
the Soret band and in the Q bands position between the gas phase and a weakly polar toluene
(PCM model) confirm complex formation and suggest a potential biological activity of the trans-
MSQNO molecule.
3.4. Time-Resolved Spectroscopic Studies
3.4.1. Fluorescence and Phosphorescence lifetimes
Solvent impact on behavior of organic molecules may lead to changes both in the specific
and non-specific solute – solvent interactions as well as to the changes in dynamics of the
excited state. In the former, hydrogen-bonding abilities of protic solventscan lead to an
instantaneous formation of the hydrogen bonded associates, with different photophysical
properties [43]. Steady-state spectroscopy is not an appropriate method to study the dynamics of
the excited states.Therefore, Time Correlated Single Photon Counting (TCSPC) method has
been chosen to get some insight into the excited state behavior of trans-MSQNO molecule. The
fluorescence decays were monitored using a laser diode as the excitation source with excitation
wavelength of 375 nm.
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The decay curves analysis was performed using the non – linear least square iterative
deconvolution procedure that explores the Lavenberg – Marquardt algorithm [44]. The lifetimes
were estimated from the following equation (2):
I(t) = ∑i Ai exp (-t/τi)
(2)
where: A is the amplitude of the ith component contributing to the fluorescence lifetime τi,
assuming that ΣAi = 1. Credibility of the measurements was expressed by the reduced chi-square
(χ2) value.
Room temperature fluorescence lifetimes of thetargeted molecule were estimated in
several aprotic and protic media and the results are gathered in Table 6.
Table 6
On the basis of the data in Table 6, one can conclude that in the emission maximum in aprotic
solvents (n-hexane, toluene, and ethyl acetate) the excited state of trans-MSQNO fluorophore
decays accordingly to monoexponential function. The lifetimes of the emissive state are short
(0.09-0.19 ns) and undergo reduction with increasing solvent polarity. The corresponding decay
curves are shown in Fig5.
Figure 5
The shapes of the presented curves sugegsts a fast fluorescence decay in ethyl acetate
(τ=0.09 ns) and in water at pH 1 with τ = 0.08 ns (Figure 7S).A slower decay is observed in
neutral water at pH 7 [0.11 ns, (86%), 2.71 ns (14%)] and a significant lifetime elongation occurs
in alcohols. In the latter, similarly to water, the excited molecule decays accordingly to bi-
exponential function, as for example, in propanol: τ1 = 0.21 ns (27%), τ2 = 2.02 ns (73% ).
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One should notice, however, that in alcohols, a smaller amplitude is correlated with a shorter
time component assigned to the trans-MSQNO monomer, while a longer time constants related
to higher amplitude indicate a larger contribution of hydrogen bonded species. In neutral water,
the population with a shorter lifetime, characteristic of the monomeric form of the trans-
MSQNO molecule, is significantly larger (86%). At pH 1, the trans-MSQNO molecule occurs in
the form of cation with the shortest lifetime amongst all media explored (0.08 ns, 100%).
Moreover, it is interesting to find that despite the difference in viscosity (η) between
aprotic solvents and alcohols, e.g. between dodecane (1.508 cP) and n-PrOH (1.938 cP) [49]
there is no significant difference in their lifetimes (0.19 ns and 0.21 ns, respectively).
Furthermore, in the rigid glass matrix conditions at 77K, the profiles of the decay curves in
alcohols strongly resemble those for the room temperature conditions (Fig.8S in SI). Comparing
them with the appropriate data of Table 6, one may note that the lifetimes in n-PrOH [τ1 = 0.20
ns (28%), τ2 = 2.02 ns (72%)] and MeOH [τ1 = 0.09 ns (25%); τ2 = 1.85 ns (75 %)], are even
slightly shorter than those at room temperature and the amplitudes change almost unnoticeably.
This allows the conclusion that the time constants of trans-MSQNO are viscosity independent
[50].
The fluorescence lifetime of the solid sample of trans-MSQNO has shown to be so short
that with the SPhC set-up of 100 ps resolution (exc.400 nm, em510 nm), we were unable to
estimate it. Nevertheless, probing in the range of the phosphorescence spectrum (580-610 nm),
the monoexponential decay curve was obtained and the phosphorescence lifetime on the
microsecond time scale (τ = 10.52 usec, Χ2=1.560) was estimated (not shown).
3.4.2. Radiative and non-radiative rate constants
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The estimated fluorescence lifetimes of trans-MSQNO, allowed us to calculate its radiative
kF (kF = φF / τF ) and non-radiative, knr (knr = 1- φF / τF) rate constants in different media. This
in turn, allowed us to suggest the reason of its small quantum yield (φF), mentioned earlier. The
results of the calculations with the corresponding amplitudes (A) for each lifetime are collected
in Table 3S (in SI). In case of the bi-exponential decays, the rate constants were calculated for
both lifetimes. The results show that the trans-MSQNO system is characteristic of the higher
non-radiative than the radiative rate constants. Moreover, in alcohols, and specifically for the
longer living component assigned to the hydrogen bonded species, the value of knr is four to five
orders of magnitude higher than that for the kF values. For example, in n-propanol, knr = 4.99 x
1011 [s-1] versus kF = 7.90 x 106 [s-1]. Similarly, in methanol: knr = 5.10 x 1010[s-1] and kF = 6.80 x
106 [s-1]. Hence, the smaller quantum yield of trans-MSQNO (Table 1), in protic media is due to
the strong non-radiative processes. Since the targeted molecule possesses nonbonding electrons,
which may give rise to n−π* states, an enhanced spin−orbit coupling may lead to effective
intersystem crossing and further, to reduction in the fluorescence quantum yield [51- 52].
Investigations of Felker and Zewail confirmed that spin-orbit coupling of n−π* states in
quinoline derivatives play an important role in the deactivation of the excited species [53].
Indeed, the calculated high oscillator strengths for the S0→S1 transition (theoretical
results in Table 2), indicate high efficiency of the nonradiative transitions from the excited state.
Moreover, the results of the theoretical calculations for all trans-MSQNO rotamers demonstrate
that the energy of the T3 state is close to that of the S1 state. Thus, on this basis, we are able to
suggest that the intersystem crossing transition can be an effective non-radiative deactivation
channel of excited trans-MSQNO species.
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4. Conclusions
Herein, the structure of solid trans-2-(4-methoxystyryl) quinoline-1-oxide was
investigated by the X-ray analysis and its photophysical behavior in liquid phase by steady state
and time-resolved spectroscopy in various media. The trans-MSQNO crystal belongs to Pbca
space group and orthorhombic unit cell containing eight N-oxide molecules. An unbounded
monomeric molecule is more distorted than its complex with the ZnTPP unit.The adsorption of
trans-MSQNO was strongly dependent on the pH and in the excited state about a thousand fold
basicity increase was observed. In various media, we have seen the color change of the
emissions, starting with violet, through blue, green and yellow. The lifetimes measured both at
room temperature and at 77K are very similar, so that the dynamics of trans-MSQNO is viscosity
independent
The results of DFT and TD DFT B3LYP/6-31G(d, p) calculations for two groups of
rotamers [trans, syn MSQNO-0x and trans, anti MSQNO-1x (with x = 0.1)] allow us to
conclude that equilibrium existing between them can be significantly altered even by relatively
weak interactions with the environment, e.g. in the non-polar solvents, the [trans, syn MSQNO-
00] isomer prevails while in the polar media this is the [trans, anti MSQNO-10] species.
Despite the fact that the gas phase results indicated only a partial CT nature of the ππ*
type S0→S1 transition, the extended computations both in apolar n-hexane and polar acetonitrile
(PCM model) a substantial increment of excited state dipole moment of all isomers (anincreased
value in more polar solvents) and a significant red shift in the absorption and emission spectra
what points to an enhanced CT character of the emissive S1 state in more polar
environmentsThus, it can be expected the trans-MSQNO molecule may be considered as a
subsequent styrylquinoline drug candidate where the CT drug-receptor interactions are crucial.
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On the basis of the calculation results for all trans-MSQNO rotamers showing
that the energy of their T3 state is close to that of the S1 state, a conclusion can be drawn that a
low fluorescence quantum yield is due to effective radiationless intersystem crossing path.
Acknowledgement
The authors are grateful to acknowledge dr Vincent Lynch of University of Texas (Austin, USA)
for his kind help in the X-ray analysis. Theoretical calculations were performed at the
Interdisciplinary Center of Mathematical and Computer Modeling (ICM) of the Warsaw
University under the computational grant No. G-32-10. This research did not receive any
specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Electronic Supplementary Information (ESI)
Packing mode of trans-MSQNO molecules along the b-axis; Temperature dependence of the
fluorescence spectra of MSQNO in c-hexane; Fluorescence spectra of trans-MSQNO in aqueous
buffer solutions performed at different pH; The mirror image effect between the fluorescence
(exc 400 nm) and excitation fluorescence spectra (em 525 nm) of trans-MSQNO in aqueous
buffer solution at pH 1; Emission spectra of the solid trans-MSQNO; TD DFT/CAM-B3LYP/6-
31G(d, p) simulated absorption spectra of the complex formed between ZnTPP unit and trans-
MSQNO ligand optimized in the gas phase and toluene (PCM model), respectively; Decay
curves of trans-MSQNO in aqueous buffer solutions at different pH excited at 375 nm; Decay
curves of MSQNO ligand at 77K in: 1- n-hexane, 2- n-PrOH, 3-MeOH; exc 375 nm, em 468 nm;
Table containing crystal data and structure refinement for trans-MSQNO; Selected experimental
values of bond lengths (Å) and angles (o) of trans-MSQNO; Radiative and non-radiative rate
constants of trans-MSQNO in different media.
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References
[1] S. H. Kim, Functional Dyes, Elsevier, Oxford, 2006.
[2] S.Br. Kondo, J Cancer. 1977, 35, 595-601.
[3] D. Kanojia, M.M. Vaidya, Oral Oncol. 2006, 42, 655-667.
[4] L.Qun , B.X.Peng, Photograph. Sci. Photochem., 1994, 12, 150-165.
[5] C.C., Lee, A.T. Hu, Dyes Pigm., 2003, 59, 63-69.
[6] Q.Li, J.S.Lee., C.Ha, C.B. Park, G.Yang, W.B.Gan, Y.T.Chang, Angew. Chem. Int. Ed.,
2004, 43, 6331- 6335.
[7] Y. Li, F. Wei, Y. Lu, S.He, L. Zhao, X. Zeng, Dyes Pigm., 2013, 96, 424-429.
[8] S. H Kim, J.Z. Cui, J.Y Park, E.M. Han, S.M.Park, Dyes Pigm., 2003, 59, 245-250.
[9] B. J Jung, C.B. Yoon., H.K. Shim, L.M.Do, T. Zyung, Adv. Funct. Mater., 2001, 11,
430-434.
[10] H. Yao, M.Yamashita, K.Kimura, Langmuir, 2009, 25, 1131-1137.
[11] U. Narang, C.F. Zhao, J.D. Bhawalkar, F.V.Bright, P.N. Prasad, J. Phys. Chem., 1996,
100, 4521-4525.
[12] E N. Kurtalie, N.Nizomov, S.I. Rahimov, J. Mol. Liq., 2011, 158, 43-45.
[13] M.Shaikh, J.Mohanty, P.K.Singh, A.C.Bhasikuttan, R.N. Rajule, V.S. Satam,
22

ACCEPTED MANUSCRIPT
S.R.Bendre, V.R. Kanetkar, H.Pal, J. Phys. Chem. A, 2010, 114, 4507- 4519.
[14] B.Strehmel, W.Rettig, J. Biomed. Opt. 1996, 1, 98-109.
[15] D. Sahoo, P. Bhattacharya, and S. Chakravorti, J.Phys.Chem.B, 2013, 116, 5226 -5233.
[16] M. Sowmiya, A.K.Tiwari, S.K.Saha, J. Photochem. Photobiol. A, 2011, 218, 76- 86.
[17] W. Rettig, W. Majenz, R.Herter, J.-F. Letard and R.Lapouyade, Pure &Appl. Chem.,
1993, 65, 1699-1704; Y. Amatats, Theor. Chem. Acc., 2000, 103, 445-450.
[18] S. Arzhantsev, K.A. Zachariasse, M. Maroncelli, J. Phys. Chem. A, 2006, 110, 3454-
3470; R.A.Rijkenberg, D. Bebelaar, W. J. Buma, and J. W. Hofstraat, J. Phys. Chem. A
2002, 106, 2446-2456.
[19] S. A. Kovalenko, R. Schanz, T. A. Senyushkina and N. P. Ernsting, Phys. Chem. Chem.
Phys., 2002, 4, 703-707; N.P. Ernsting, J. Breffke, D. Yu. Vorobyev, D. A. Duncany and
I.Pfeffer, Phys. Chem. Chem. Phys., 2008, 10, 2043-2049.
[20] A. Szemik-Hojniak, I. Deperasińska, K. Oberda, Y. Erez, D. Huppert, Y.P. Nizhnik,
Phys. Chem. Chem. Phys. 2013, 15, 9914-9923.
[21] T.O.Volkova, N.N.Nemova, Molecular Mechanisms of Apoptosis of Leukemic Cells.
Moscow: Nauka, 2006, 208-211 (in Russ.)
[22] H. H.Eldaroti, S.A.Gadir, M. S.Refat, A. M. A.Adam, Spectrochimica Acta Part A:
23

ACCEPTED MANUSCRIPT
Molecular and Biomolecular Spectroscopy, 2013, 115, 309-323.
[23] A. M. A. Adam, M. Salman, T. Sharshar, M. S. Refat, Int. J. Electrochem. Sci.,2013,
8,1274-1294.
[24] A. Szemik-Hojniak , K. Oberda, I. Deperasińska, Y. P. Nizhnik , L. Jerzykiewicz,
Polyhedron, 2015, 88, 190-198.
[25] CrysAlisPro. Agilent Technologies, 2013. Agilent Technologies UK Ltd., Oxford, UK,
SuperNova CCD System, CrysAlicPro Software System, 1.171.37.31.
[26] SuperFlip. L.Palatinus, G. Chapuis, J. Appl. Cryst., 2007, 40, 786-790.
[27] G.M. Sheldrick, SHELXL-2013. Program for the Refinement of Crystal Structures.
Acta Cryst., 2008, A64, 112.
[28] A. L.Spek, 1998. PLATON, A Multipurpose Crystallographic Tool. Utrecht University,
The Netherlands.
[29] WinGX 1.64, An Integrated System of Windows Programs for the Solution, Refinement
and Analysis of Single Crystal X-ray Diffraction Data. L.J.J.Farrugia, Appl. Cryst.
1999, 32. 837-838.
[30] International Tables for X-ray Crystallography ,1992. Vol. C, Tables 4.2.6.8 and
6.1.1.4, A. J. C. Wilson, editor, Boston: Kluwer Academic Press.
24

ACCEPTED MANUSCRIPT
[31 G.M.Sheldrick, SHELXTL/PC (Version 5.03). Siemens Analytical X-ray Instruments,
1994, Inc., Madison, Wisconsin, USA.
[32] M. J. Frisch et all,. Gaussian 09, Revision B.01; Gaussian, Inc., Wallingford CT, 2010.
[33] Z.Xu, Scientific Reports , 2013, 3, 2914-2920.
[34] C. C Broomell, F. W. Zok, and J.H. Waite, Acta Biomater, 2008, 4, 2045–2051.
[35] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer Science-
Business Media, LLC, 2006.
[36] M. H. Abraham, L. Honcharova, S. A. Rocco, W. E. Acree, Jr and K. M. De
Fina , New J. Chem., 2011, 35, 930-936.
[37] V. P. Andreev, P.S. Sobolev, D.O. Zaitsev, A.B.Il’yukhin , Russ. J. Gen. Chem,
2014, 84, 115-124.
[38] T. Förster, Z. Elektrochem. 1950, 54, 42-46.
[39] D. Pines, E.Pines, Solvent assisted photoacidity, in: Hydrogen Transfer Reactions,
Willey-VCH Verlag GmbH & Co, 2007, pp. 377- 415.
[40] N. Agmon, J.Phys.Chem.A, 2005, 109, 15-30.
[41] R.Burdujan, J. D`Angelo, D. Desmaele, F. Zouhiri, P.Tauc, J-C. Brochon, Ch.Auclair,
J-F, Mouscadet, P. Pernot, F. Tfibel, M. Enescu, M-P. Fontaine-Aupart., Phys. Chem.
Chem.Phys., 2001, 3, 3797-3804.
25

ACCEPTED MANUSCRIPT
[42] J. L. R Williams, S.K. Webster, J.A. Van Allan, J. Org. Chem., 1961, 26, 4893-4895.
[43] J. L. R Williams, J. Org. Chem., 1960, 25, 1839-1840.
[44] K.Takekawa, S.Kitamura, K.Sugihara, S.Ohta. Xenobiotica, 2001, 31, 769-82.
45] K. Oberda, I. Deperasińska, Y. Nizhnik, L. Jerzykiewicz, A. Szemik-Hojniak,
Polyhedron, 2011, 30, 2391-2399.
[46] K. Oberda, I. Deperasińska, Y.P. Nizhnik, A. Szemik-Hojniak, Polyhedron, 2013,
51, 61- 69.
[47] D. B. Spry, A. Goun, M. D. Fayer, J. Phys. Chem. A, 2007, 111, 230-237.
[48] D. V. O’Connor, D. Phillips, Time-Correlated Single Photon Counting, Academic
Press: London, 1984.
[49] G. Kavarnos, Fundamentals of photoinduced electron transfer, VCH Publishers, Inc,
New York, 1993.
[50] B.L.McClain, I.J. Finkelstein, M.D. Fayer, J. Am Chem Soc. 2004, 126, 15702-15710.
[51] W. S.Yeh, L. A. Philips, S. P. Webb, L. F. Buhse, J. H. Clark “Excited-State
Solvation Dynamics in 4-Aminophthalimide” in Ultrafast Phenomena IV, Springer
Series in Chemical Physics, 1984, 38, 359-364.
[52] Janic, I.; Kawasaki, A. Adv. Mol. Relax. Proc., 1973, 5, 185−191.
[53] Felker, R. M.; Zewail, A. H. Chem. Phys. Lett., 1983, 74, 448−453.
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Figure Captions
Figure 1. ORTEP-view of trans-MSQNO showing the atom labeling scheme. Displacement
ellipsoids are scaled to the 50% probability level.
Figure 2. Absorption and fluorescence spectra of trans-MSQNO in different media; a)
Absorption: n-hexane (black), n-PrOH (red), water at pH 1 (blue); b) Fluorescence: n-hexane
(blue, exc350 nm), n-PrOH (black, exc 369 nm), H2O at pH 1 (red, exc 375 nm), H2O at pH 7
(green, exc 365 nm).
Figure 3. Absorption spectra of trans-MSQNO in various water buffer solutions at different pH.
Figure 4. The plot of pKa values versus Δλ shift of the Qx band of complexes formed between
the ZnTPP unit and different N-oxides; y = 0.6797x - 8.0573, R2= 0.9525 [37].
Figure 5. Room temperature decay curves of trans-MSQNO, excited at 375 nm, in: ethyl acetate
(dark green, em 472 nm) compared to those in protic solvents : water at pH =7 (light blue, em
478 nm, n-PrOH (dark blue, em 469 nm) and MeOH (red, em 450 nm).
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Figures
Figure 1. ORTEP-view of trans-MSQNO showing the atom labeling scheme. Displacement
ellipsoids are scaled to the 50% probability level.
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Figure 2. Absorption and fluorescence spectra of trans-MSQNO in different media; a)
Absorption: n-hexane (black), n-PrOH (red), water at pH 1 (blue); b) Fluorescence: n-hexane
(blue, exc350 nm), n-PrOH (black, exc 369 nm), H2O at pH 1 (red, exc 375 nm), H2O at pH 7
(green, exc 365 nm).
29