Controlling photophysics of styrylnaphthalimides through TICT, fluorescence and: E, Z -photoisomerization interplay

Article abstract of DOI:10.1039/c6cp07255k

The photophysical properties of naphthalimide dyes NI1-3 with electron releasing 4-methoxy- (NI1), 3,4-dimethoxystyryl- (NI2) and dimethylaminostyryl (NI3) groups are examined in a variety of protic and aprotic solvents. All compounds demonstrate positive solvatochromism in the steady-state absorption and fluorescence spectra. The analysis of the dependence of the Stokes shift on the polarity of the solvent using the Lippert-Mataga equation allowed us to determine the change in the dipole moment upon excitation. The obtained data correspond to the formation of highly polar charge transfer states. Based on the transient absorption spectra and time-resolved fluorescence measurements, the presence of two different emissive states was definitely proved. The primarily formed planar Local Excited (LE) state dominates in non-polar solvents like cyclohexane and toluene where it relaxes mostly through fluorescence and E,Z-isomerisation pathways. In polar solvents, an alternative relaxation channel emerges that consists of twisting around single bond between styryl and naphthalimide fragments, which leads to the formation of a Twisted Intramolecular Charge Transfer (TICT) state. The factors affecting the fluorescence of TICT states are discussed. The observed spectral effects are rationalized using quantum-chemical calculations, X-ray data and NMR spectroscopy.

Full text of DOI:10.1039/c6cp07255k

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Chem. Phys., 2016, DOI: 10.1039/C6CP07255K.
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Physical Chemistry Chemical Physics
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Controlling Photophysics of Styrylnaphthalimides through TICT,
Fluorescence and E,Z-Photoisomerization Interplay
Pavel A. Panchenko,a,b,* Antonina N. Arkhipova,a Olga A. Fedorova,a,b Yuri V. Fedorov,a
Marina A. Zakharko,a Dmitry E. Arkhipov,a Gediminas Jonusauskasc
aA. N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of
Sciences (INEOS RAS), 119991, Vavilova str. 28, Moscow, Russia,
Tel.: +7 499 135 80 98, Fax: +7 499 135 50 85, Eꢀmail: pavel@ineos.ac.ru
bD. Mendeleev University of Chemical Technology of Russia,
125047, Miusskaya sqr. 9, Moscow, Russia,
cLaboratoire Ondes et Matière d’Aquitaine (LOMA), UMR CNRS 5798,
Bordeaux University, 33405, 351 Cours de la Libération, Talence, France
Abstract
Photophysical properties of naphthalimide dyes NI1–3 with electron releasing 4ꢀ
methoxyꢀ (NI1), 3,4ꢀdimethoxystyrylꢀ (NI2) and dimethylaminostyryl (NI3) groups are
examined in a variety of protic and aprotic solvents. All compounds demonstrate positive
solvatochromism in the steadyꢀstate absorption and fluorescence spectra. The analysis of the
dependence of the Stokes shift on the polarity of the solvent using LippertꢀMataga equation
allowed to determine the change in the dipole moment upon excitation. The obtained data
correspond to the formation of highly polar charge transfer states. Based on the transient
absorption spectra and timeꢀresolved fluorescence measurements, the presence of two different
emissive states was definitely proved. The primarily formed planar Local Excited (LE) state
dominates in nonꢀpolar solvents like cyclohexane and toluene where it relaxes mostly through
fluorescence and E,Zꢀisomerisation pathways. In polar solvents, an alternative relaxation channel
emerges that consists in twisting around single bond between styryl and naphthalimide fragment,
which leads to formation of Twisted Intramolecular Charge Transfer (TICT) state. The factors
affecting the fluorescence of TICT states are discussed. The observed spectral effects are
rationalized using quantumꢀchemical calculations, Xꢀray data and NMR spectroscopy.
Keywords
Naphthalimide, Styryl dyes, Twisted Intramolecular Charge Transfer, Transient Absorption
Spectroscopy, Solvatochromism, LippertꢀMataga Equation, E,ZꢀPhotoisomerization.

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Graphical Abstract
Textual abstract for the table of contents entry
Steadyꢀstate and timeꢀresolved spectral properties of styrylnaphthalimides are studied in various
solvents

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1. Introduction
Molecular fluorescence is presently regarded as a dominant methodology used
extensively in biotechnology, flow cytometry, medical diagnostics, DNA sequencing and genetic
analysis.1 The recent development of confocal fluorescent microscopy2,3 has greatly advanced
the application of organic emissive compounds as imaging agents to study the structure,
dynamics and functions of biological macromolecules. The operational light input/output
wavelengths of most current fluorescent indicators are in the range 400 – 600 nm, which often
limits their use in life sciences, because this spectral region suffers from strong interference due
to background absorbance and autoꢀfluorescence from biological environment or endogenous
chromophores in sample media. So, it has aroused much attention to develop new nearꢀinfrared
fluorescent dyes as powerful detecting or treating tools in biological systems.4–6
Many researchers have used cyanine dyes as fluorescent labels or sensors for
biomolecules in vivo, because their spectra can reach nearꢀinfrared (NIR) region.7–9 However,
polymethine cyanine dyes are not so easy to be synthesized and modified, their photoꢀstabilities
are relatively low and their Stokes shift is usually less than 25 nm, which may cause selfꢀ
quenching and measurement error by the exciting and scattered light, and then decrease the
detection sensitivity to a great extent. Therefore, NIR dyes with a larger Stokes shift are very
promising for NIR fluorescence bioassays.
1,8ꢀNaphthalimide derivatives are famous organic fluorophores which generally exhibit
high thermo and photostability and are known to act as fluorescent brighteners and dyes for
polymer fibers,10,11 laser active media,12,13 electroluminescent materials14–16 and optical memory
devices.17–19 Because of its intense fluorescence, large Stokes shifts along with the relative ease
of synthetic operations for targeted modification of the molecular structure, this type of
compounds has found application in the construction of fluorescent chemosensors for
biologically relevant cations and anions,20–22 labels or probes for proteins, cells, lysosomes and
other acidic organelles.23–25 However, the emission bands of most naphthalimides are in blue and
greenꢀyellow regions, which is a characteristic of 4ꢀaminoꢀ, 4ꢀamidoꢀ and 4ꢀalkoxyꢀsubstituted
derivatives.26–28 Although it was known that electronꢀdonating groups at the Cꢀ4,5ꢀpositions of
naphthalene ring usually increase the fluorescence quantum yield of the compounds and cause
the red shift in the spectra, the reports on 1,8ꢀnaphthalimide derivatives with emission
wavelengths longer than 600 nm are very few.29–33
In the present work we focused on the synthesis and detailed spectroscopical
investigation of naphthalimides NI1–3 (Scheme 1) with NMe2 and OMe groups attached to
styryl fragment. It is known that compounds of similar to NI1–3 structure are capable to
penetrate cell membranes, demonstrate AIE properties and has a great potential of being used as

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imaging agents in vivo,32,33 even so, their optical properties have been explored to a much lesser
extent as compared with commonly used and 4ꢀaminoꢀ and 4ꢀalkoxynaphthalimides. The
presence of styryl group in NI1–3 is supposed to extend the conjugate system of the parent
chromophore and thus, provide the longer wavelength position of fluorescence maxima. At the
same time, as a key feature of styryl dyes,34–36 one should expect a significant impact of twisted
intramolecular chargeꢀtransfer (TICT) states formation and photoisomerization around double
bonds on the photophysics of the compounds under study. The effects of molecular structure of
styryl fragment as well as solvent polarity on the steadyꢀstate and timeꢀresolved optical
characteristics is described and discussed below.

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2. Experimental Section
General procedure for the synthesis of compounds NI1–3. Synthesis of the
naphthalimide derivatives NI1–3 is shown in Scheme 1. The first step, in which commercial
starting material 4ꢀbromoꢀ1,8ꢀnaphthalic anhydride 1 was reacted with
performed conveniently in ethanol at reflux. Next, the samples of 2 were subjected to the Heck
coupling reaction with 4ꢀmethoxystyrene, 3,4ꢀdimethoxystyrene and 4ꢀN,Nꢀ
nꢀbutylamine, was
dimethylaminostyrene under catalyst Pd(OAc)2.32 Details of synthetic procedures and
identifications are shown in the Supplementary Information.
Scheme 1. Synthetic route to compounds NI1–3. Reagents, conditions and yields: i. nꢀBuNH2, EtOH, ∆, 89%; ii.
Ar–CH=CH2, (oꢀTol)3P, Pd(OAc)2, DMF, ∆, 18–38%.
Steady-state optical measurements. The absorption spectra were taken on a Varianꢀ
Cary 5G spectrophotometer. The fluorescence quantum yield measurements were performed
using a VarianꢀCary 5G spectrophotometer and a FluoroMaxꢀ3 spectrofluorimeter. Spectral
measurements were carried out in airꢀsaturated acetonitrile solutions (acetonitrile of
spectrophotometric grade, water content <0.005%, Aldrich) at 20 ± 1 ºC; the concentrations of
studied compounds were of about 0.5–2.0ꢁ10–5 M. All measured fluorescence spectra were
corrected for the nonuniformity of detector spectral sensitivity. Coumarin 481 in acetonitrile (ϕfl
= 0.08)37 was used as reference for the fluorescence quantum yield measurements. The
fluorescence quantum yields were calculated by the Eq. (1),38
2
Si(1 - 10-A )ni
0
ϕifl
=
ϕ0fl
(1)
- A
2
i
S0(1 - 10 )n0
wherein ϕifl and ϕ0fl are the fluorescence quantum yields of the studied solution and the standard
compound respectively; Ai and A0 are the absorptions of the studied solution and the standard
respectively; Si and S0 are the areas underneath the curves of the fluorescence spectra of the

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studied solution and the standard respectively; and ni and n0 are the refraction indices of the
solvents for the substance under study and the standard compound.
Time-Resolved Fluorescence Setup. A Ti:sapphire laser system emitting pulses of 0.6
mJ and 30 fs at 800 nm and 1 kHz pulse repetition rate (Femtopower Compact Pro) with homeꢀ
built optical parametric generator and frequency mixers was used to excite the samples at the
maximum of the steadyꢀstate absorption band. All excitedꢀstate lifetimes were obtained by using
depolarized excitation light. The highest pulse energies used to excite fluorescence did not
exceed 100 nJ and the average power of excitation beam was 0.1 mW at a pulse repetition rate of
1 kHz focused into a spot with a diameter of 0.1 mm in the 10 mmꢀlong fusedꢀsilica cell. The
fluorescence emitted in the forward direction was collected by reflective optics and focused with
a spherical mirror onto the input slit of a spectrograph (Chromex 250) coupled to a streak camera
(Hamamatsu 5680 equipped with a fast single sweep unit M5676, temporal resolution 2 ps).
Convolution of a rectangular streak camera slit in the sweep range of 250 ps with electronic jitter
of the streak camera trigger pulse provided a Gaussian (over four decades) temporal apparatus
function with a full width at halfꢀmaximum of 20 ps. The fluorescence kinetics were later fitted
by means of the Levenberg–Marquardt leastꢀsquares curveꢀfitting method using a solution of the
differential equation describing the evolution in time of a single excited state and neglecting
depopulation of the ground state according to Eq. (2),
dI
dt
I(t)
= Gauss(t0 , ꢀt, A) −
(2)
τ
where
which t0 is the excitation pulse arrival delay, ꢂt – the excitation pulse width, and
amplitude. The parameter is the lifetime of the excited state. The initial condition for the
equation is
(–∞) = 0. Typically, the fit shows a χ2 value (Pirson’s criteria) better than 10–4 and a
correlation coefficient > 0.999. The uncertainty of the lifetime was better than 1%. Routinely,
I(t) is the fluorescence intensity, Gauss is the Gaussian profile of the excitation pulse, in
A
– the
τ
I
R
the fluorescence accumulation time in our measurements did not exceed 90 s.
Transient Absorption Setup. The laser system and frequencyꢀconversion apparatus
employed to excite samples were the same as for timeꢀresolved fluorescence measurements.
White light continuum (360–1000 nm) pulses generated in a 5 mm methanol cell were used as a
probe. The variable delay time between excitation and probe pulses was obtained by using a
delay line with 0.1 mm resolution. The solutions were placed in a 1 mm circulating cell.
Whitelight signal and reference spectra were recorded with a twoꢀchannel fiber spectrometer
(Avantes Avaspecꢀ2048ꢀ2). A homeꢀwritten acquisition and experimentꢀcontrol program in
LabView made it possible to record transient spectra with an average error of less than 10–4

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times the optical density for all wavelengths. The temporal resolution of our setup was better
than 60 fs. Temporal chirp of the probe pulse was corrected by a computer program with respect
to a Lawrencian fit of a Kerr signal generated in a 0.2 mm glass plate used in a place of the
sample.
Determination of
yields of the forward and backward reactions of
compounds NI1–3, we preliminary calculated the absorption spectra of the corresponding
isomers and the ratios of quantum yields ꢀꢁ→ꢂ/ꢀꢂ→ꢁ using the Fisher method39 from the
absorption spectra of the ꢀisomers and the spectra of two photostationary states obtained by
E
,
Z
-photoisomerization quatum yields. To determine the quantum
E,Z
ꢀphotoisomerization (ꢀꢁ→ꢂ and ꢀꢂ→ꢁ) of
Zꢀ
E
photoirradiation at two different wavelengths. As a light source DRK 120 mercury lamp (120 W)
was used. Particular spectral lines of this lamp were picked out using glass light filters from the
standard set of samples of colored optical glasses. The changes in the absorption spectra upon
irradiation and the spectra of
S1–9).
Zꢀisomers are presented in the Supplementary Information (Fig.
The quantum yields ꢀꢁ→ꢂ and ꢀꢂ→ꢁ were calculated as follows. For the reversible
photochemical reaction40
the equations for the rate of formation of substances A and B are written in the form
ꢄ ꢆ
ꢃ ꢅ
= −ꢀꢈ→ꢉ ∙ ꢊꢋꢈꢌꢍ + ꢀꢉ→ꢈ ∙ ꢊꢋꢉꢌꢍ
= +ꢀꢈ→ꢉ ∙ ꢊꢋꢈꢌꢍ − ꢀꢉ→ꢈ ∙ ꢊꢋꢉꢌꢍ
(3)
(4)
ꢃꢇ
ꢄ ꢆ
ꢃ ꢎ
ꢃꢇ
where ꢊꢋꢈꢌꢍ and ꢊꢋꢉꢌꢍ are the amounts of radiation quanta absorbed by substances A and B within
time ꢃꢇ
ꢀꢈ→ꢉ and ꢀꢉ→ꢈ are the quantum yields of the forward and backward reaction
ꢄ ꢆ ꢄ ꢆ
;
respectively; ꢃ ꢅ and ꢃ ꢎ are the changes in the number of molecules of the corresponding
substances A and B within time ꢃꢇ
.
The amount of quanta absorbed by substances A and B is determined as follows:
ꢐꢈ
ꢊꢋꢈꢌꢍ = ꢊꢋꢏꢌꢍ
∙
∙
(5)
(6)
ꢐꢏ
ꢐꢉ
ꢐꢏ
ꢊꢋꢉꢌꢍ = ꢊꢋꢏꢌꢍ
in this case
ꢏ
ꢊꢋꢌꢍ = ꢊꢑ(1 − 10ꢒꢓ
)
ꢔ
(7)

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where ꢐꢏ is the total absorption of all components, ꢐꢈ and ꢐꢉ are the absorption by the
corresponding substance, ꢊꢋꢏꢌꢍ is the total amount of absorbed radiation quanta within time ꢃꢇ
,
and ꢊꢑ is the incident radiation intensity.
The quantum yields ꢀꢈ→ꢉ and ꢀꢉ→ꢈ were determined by the numerical solution of the
corresponding differential equations using the primarily specified value and the ratio of quantum
yields of the forward and backward reactions preliminary measured using Fisher method. The
quantum yield values were optimized by the iteration method using the program in the Visual
Basic language incorporated into Microsoft Excel.
The absolute light intensity ꢊꢑ was measured using the ferrioxalate actinometer41,42 before
each photolysis experiment (for details see also Supplementary Information). The measurement
accuracy does not exceed 20%.
X-ray diffraction analysis. The crystals suitable for Xꢀray analysis were prepared by
slow evaporation of solutions of compounds NI1–3 in solvent mixtures benzene –
dichloromethane (v/v=1:1), ethanol – ethylacetate (v/v=1:1) and DMF respectively. The
measurements were carried out using SMART APEX II CCD diffractometer (MoKα irradiation,
graphite monochromator, ωꢀscanning). The structures were solved by direct method and refined
by the fullꢀmatrix leastꢀsquares technique against ꢕꢖꢙꢗꢘ in the anisotropic approximation.
Hydrogen atoms were placed in calculated positions and refined in the riding model using
isotropic approximation. The main crystallographic parameters can be found in Supplementary
Information (Table S1). All calculations were performed using SHELX software package.43 The
Xꢀray data of NI1–3 have been deposited with the Cambridge Crystallographic Data Centre as
supplemental publications CCDC 1037166 – 1037168.
Computational details. Quantum chemical calculations were carried out by the MOPAC
2012 program package using the PM6 semiempirical method.44 CI calculations were performed
at optimized geometries, which reached gradient variations less than 0.01 kcal/mol. The solvent
effect was included in geometry optimizations following the «COnductorlike Screening Model»
(COSMO) implemented in MOPAC 2012. A dielectric constant of
ε = 20 and a refraction index
of solvent (
n
) such that n2 = 2 were used for geometry optimization and calculations of electron
distribution of HOMO and LUMO in compounds NI1–3. The CI included eight occupied and
eight unoccupied MOs.
1
NMR measurements. H NMR spectra were recorded on Avance 600 spectrometer
(Bruker) operating at 600.22 MHz. The measurements were performed in tolueneꢀd8 solution at
room temperature. The chemical shifts (given as
ppm relative to the signals corresponding to the residual solvent and recalculated to the internal
standard (TMS); the spinꢀspin coupling constants ( ) were measured with an accuracy of 0.1 Hz.
δ) were determined with an accuracy of 0.01
J

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The numbering of carbon atoms in the naphthalimide moiety,
N
ꢀbutyl group and styryl fragment
of compounds NI1–3 used for the description of 1H NMR spectra is shown on Fig. 6. The signals
of ꢀisomers were identified using the preliminary recorded spectra of ꢀisomers and the spectra
of photostationary states collected after 20 min irradiation of tolueneꢀ 8 solutions of NI1–3 with
Z
E
d
436 nm light (DRK 120 mercury lamp, 120 W) The assignment of signals is based on 2D NMR
1
experiments (HMBC, HSQC and H COSY), which were performed using standard pulse
sequences from the Bruker library. 2D NMR spectra are presented in Supplementary Information
(Fig. S11–28).
(E)ꢀ2ꢀbutylꢀ6ꢀ(4ꢀmethoxystyryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
1H NMR: 0.92 (t, 3Н, СН3(1),
= 7.3), 1.35 ꢀ 1.43 (m, 2Н, CН2(2)), 1.75 – 1.83 (m, 2Н,
CН2(3)), 3.33 (s, 3Н, ОСН3), 4.25 (t, 2Н, CН2(4),
6.98 (d, 1Н, Н(18), = 16.0), 7.18 (dd, 1H, H(15),
= 8.7), 7.44 (d, 1Н, Н(17),
8.51 – 8.56 (m, 2Н, Н(10), Н(16)).
(Z)ꢀ2ꢀbutylꢀ6ꢀ(4ꢀmethoxystyryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
1H NMR: 0.89 (t, 3Н, СН3(1),
CН2(3)), 3.13 (s, 3Н, ОСН3), 4.20 (t, 2Н, CН2(4),
6.54 (d, 1Н, Н(18), = 12.3), 6.64 (d, 1Н, Н(17),
7.11 (dd, 1H, H(15), = 7.4,
= 1.1), 8.37 (d, 1Н, Н(10),
(E)ꢀ2ꢀbutylꢀ6ꢀ(3,4ꢀdimethoxystyryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
1H NMR: 0.92 (t, 3Н, СН3(1), = 7.3), 1.31 – 1.46 (m, 2Н, CН2(2)), 1.72 – 1.86 (m, 2Н,
CН2(3)), 3.44 (s, 3Н, ОСН3), 3.54 (s, 3Н, ОСН3), 4.26 (t, 2Н, CН2(4), = 7.6), 6.62 (d, 1Н,
H(21), = 8.3), 6.93 – 7.13 (m, 3Н, H(18), H(20), H(24)), 7.14 – 7.21 (m, 1Н, Н(15)), 7.50 (d,
1Н, Н(11), = 8.0), 7.51 (d, 1H, H(17), = 15.3), 8.08 (d, 1Н, Н(14), = 8.6), 8.49 – 8.59 (m,
2Н, Н(10), Н(16)).
(Z)ꢀ2ꢀbutylꢀ6ꢀ(3,4ꢀdimethoxystyryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
1H NMR: 0.88 (t, 3Н, СН3(1),
= 7.4), 1.28 – 1.37 (m, 2Н, CН2(2)), 1.69 – 1.76 (m, 2Н,
CН2(3)), 3.04 (s, 3Н, ОСН3), 3.22 (s, 3Н, ОСН3), 4.20 (t, 2Н, CН2(4), = 7.4), 6.24 (d, 1Н,
H(21), = 8.4), 6.48 (s, 1H, H(24)), 6.53 – 6.59 (m, 2Н, H(18), H(20)), 6.66 (d, 1Н, Н(17),
12.3), 7.07 – 7.13 (m, 1Н, Н(15)), 7.36 (d, 1Н, Н(11), = 7.6), 8.04 (d, 1Н, Н(14), = 8.4), 8.39
(d, 1Н, Н(10), = 7.6), 8.49 (d, 1Н, Н(16), = 7.4).
(EꢀNI1).
J
J
J
= 7.9), 6.80 (d, 2Н, H(21), H(23),
= 8.5,
J
J
= 8.7),
J
J
= 7.3), 7.32 (d, 2Н, H(20), H(24),
J
J
= 16.0), 7.46 (d, 1H, H(11),
J
= 8.0), 8.03 (d, 1Н, Н(14),
= 8.6),
(ZꢀNI1).
J
= 7.3), 1.30 – 1.37 (m, 2Н, CН2(2)), 1.69 ꢀ 1.76 (m, 2Н,
J
= 7.9), 6.42 (d, 2Н, H(21), H(23),
J
J
= 8.7),
= 8.7),
J
J = 12.3), 6.87 (d, 2Н, H(20), H(24),
J
J
= 8.4), 7.29 – 7.32 (m, 1Н, Н(11)), 8.01 (dd, 1Н, Н(14),
J = 8.4, J
J
= 7.6), 8.50 (d, 1Н, Н(16), = 7.4).
J
(EꢀNI2).
J
J
J
J
J
J
(ZꢀNI2).
J
J
J
J =
J
J
J
J
(E)ꢀ2ꢀbutylꢀ6ꢀ(4ꢀ(dimethylamino)styryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
1
(EꢀNI3). H NMR: 0.92 (t, 3Н, СН3(1),
J
= 7.3), 1.35 – 1.43 (m, 2Н, CН2(2)), 1.76 – 1.83 (m,
= 7.8), 6.55 (d, 2Н, Н(21), Н(23), J =
2Н, CН2(3)), 2.54 (s, 6Н, N(СН3)2), 4.26 (t, 2Н, CН2(4),
J

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8.7), 7.11 (d, 1Н, Н(18),
Н(24), = 8.7), 7.50 (d, 1Н, Н(17),
= 8.3), 8.52 – 8.57 (m, 2Н, Н(10), Н(16)).
(Z)ꢀ2ꢀbutylꢀ6ꢀ(4ꢀ(dimethylamino)styryl)ꢀ1Hꢀbenzo[d,e]isoquinolineꢀ1,3(2H)ꢀdione
ꢀNI3). 1H NMR: 0.89 (t, 3Н, СН3, = 7.4), 1.28 – 1.36 (m, 2Н, CН2(2)), 1.68 – 1.77 (m, 2Н,
CН2(3)), 2.33 (s, 6Н, N(СН3)2), 4.22 (t, 2Н, CН2(4), = 7.7), 6.18 (d, 2Н, Н(21), Н(23), = 8.9),
6.50 (d, 1Н, Н(18), = 12.4), 6.72 (d, 1Н, Н(17), = 12.4), 6.92 – 6.97 (m, 2Н, Н(20), Н(24)),
7.09 – 7.13 (m, 1Н, Н(15)), 7.44 –7.48 (m, 1Н, Н(11)), 8.07 – 8.14 (m, 1Н, Н(14)), 8.42 (d, 1Н,
Н(10), = 7.7), 8.50 (dd, 1H, H(16), = 7.4, = 1.0).
J
= 16.0), 7.17 (dd, 1H, H(15),
J
= 7.3,
J
= 8.3), 7.42 (d, 2Н, Н(20),
J
J
= 16.0), 7.52 (d, 1H, H(11),
J = 7.3), 8.11 (d, 1Н, Н(14), J
(
Z
J
J
J
J
J
J
J
J

Page 11 of 27
Physical Chemistry Chemical Physics
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DOI: 10.1039/C6CP07255K
3. Results and Discussion
Steady-state spectroscopic properties. The wavelengths of maximum absorption (ꢚ
ꢋꢌꢍ
)
ꢛꢋꢜ
and emission (ꢚꢝꢛꢞ ꢋꢜ), molar absorptivity (ꢟꢠ), fluorescence quantum yields (ꢀꢝꢞ) and Stokes shifts
(Δꢡꢢ) of compounds NI1–3 in protic and aprotic organic solvents covering a wide polarity range
are listed in Table 1. As an example, Fig. 1 shows the spectra of NI1–3 in ethyl acetate.1
Comparison of electron density distribution in frontier molecular orbitals (Fig. 2) reveals that the
long wavelength absorption can be attributed to an intramolecular charge transfer (ICT) from
either dimethylaminoꢀ, monoꢀ or dimethoxystyryl fragment towards the carbonyl groups of
naphthalimide moiety. In accordance with typical behavior of ICT bands, the increase in electron
donating character of styryl fragment on going from NI1 to NI3 causes red shift of the
absorption and emission maxima (Fig. 1).
34.0
25.5
17.0
8.5
1.10
0.88
0.66
0.44
0.22
0.00
4
3
5
6
2
1
λ
ε
0.0
300
400
500
600
700
λ / nm
Figure 1. Absorption (1, 2, 3) and normalized emission (4, 5, 6) spectra of compounds NI1 (1, 4), NI2 (2, 5) and
NI3 (3, 6) in ethylacetate.
NI1
NI2
NI3
LUMO
HOMO
Figure 2. Frontier orbitals of compounds NI1–3 by the PM6 method.
1 Absorption and fluorescence spectra of NI1–3 in all solvents from Table 1 are listed in Supplementary Information
(Fig. S29–76).

Physical Chemistry Chemical Physics
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Table 1. Absorption and Fluorescence characteristics of compounds NI1, NI2, NI3 in various solvents
εꢣ ∙ 10ꢒꢤ
/
ꢧꢨ ∙ 10ꢒꢩ
/
M–1ꢁcm–1
Δꢡꢢ / cm–1
s–1
ꢝꢞ
ꢚꢋꢛꢌꢋꢍꢜ / nm
ꢚ
(ꢚꢥꢦ) / nm
ꢀꢝꢞ
ꢇ / ns
№
Solvent (ꢟ)
ꢛꢋꢜ
1
2
3
4
5
6
7
8
9
Propylene carbonate (65.0)
Dimethyl sulfoxide (46.7)
Acetonitrile (37.5)
411
421
406
408
409
410
408
405
402
405
398
413
413
415
415
415
418
429
412
415
416
417
416
413
408
412
405
417
419
421
420
421
22.1
23.1
22.8
23.9
22.9
21.9
22.7
23.2
23.5
21.9
21.8
23.2
23.7
22.2
22.7
21.8
30.2
28.0
28.3
28.5
28.2
27.4
28.1
28.6
30.1
27.1
33.3
29.2
28.1
27.8
27.0
26.7
579 (370)
589 (420)
570 (365)
557 (400)
549 (400)
544 (435)
544 (350)
527 (365)
504 (365)
495 (415)
450 (365)
603 (350)
588 (350)
577 (350)
585 (350)
568 (350)
610 (400)
620 (400)
606 (380)
592 (350)
579 (370)
574 (370)
574 (360)
551 (400)
523 (360)
510 (415)
461 (400)
638 (390)
621 (400)
619 (400)
605 (390)
578 (400)
7060
6775
7087
6556
6235
6008
6127
5716
5034
4489
2903
7629
7206
6765
7002
6491
7530
7181
7770
7204
6767
6559
6617
6064
5389
4664
2999
8307
7763
7598
7281
6452
0.33
0.41
0.33
0.28
0.25
0.24
0.26
0.25
0.21
0.23
0.33
0.79
0.57
0.44
0.44
0.41
0.51
0.40
0.43
0.50
0.51
0.54
0.50
0.41
0.42
0.35
0.27
0.040
0.16
0.30
0.47
0.57
2.51 1.31
3.05 1.34
2.35 1.40
2.10 1.33
1.95 1.28
1.80 1.33
1.85 1.41
1.45 1.72
1.08 1.94
1.04 2.21
1.02 3.24
2.67 2.96
2.87 1.99
2.62 1.68
2.53 1.74
2.48 1.65
3.06 1.67
3.15 1.27
3.80 1.13
4.02 1.24
3.83 1.33
3.70 1.46
3.43 1.46
3.13 1.31
2.08 2.02
1.73 2.02
1.29 2.09
0.33 1.21
1.22 1.31
2.04 1.47
2.81 1.67
3.20 1.78
Acetone (20.7)
3ꢀMethylbutanoneꢀ2 (16.1)
4ꢀMethylpentanoneꢀ2 (13.1)
1,2ꢀDimethoxyethane (7.2)
Ethylacetate (6.0)
Diethyl ether (4.3)
10 Toluene (2.4)
11 Cyclohexane (1.9)
12 Methanol (32.7)
13 Ethanol (24.6)
14 nꢀButanol (17.5)
15 nꢀHexanol (13.3)
16 nꢀDecanol (8.1)
1
2
3
4
5
6
7
8
9
Propylene carbonate (65.0)
Dimethyl sulfoxide (46.7)
Acetonitrile (37.5)
Acetone (20.7)
3ꢀMethylbutanoneꢀ2 (16.1)
4ꢀMethylpentanoneꢀ2 (13.1)
1,2ꢀDimethoxyethane (7.2)
Ethylacetate (6.0)
Diethyl ether (4.3)
10 Toluene (2.4)
11 Cyclohexane (1.9)
12 Methanol (32.7)
13 Ethanol (24.6)
14 nꢀButanol (17.5)
15 nꢀHexanol (13.3)
16 nꢀDecanol (8.1)
1
2
3
4
5
6
7
8
9
Propylene carbonate (65.0)
Dimethyl sulfoxide (46.7)
Acetonitrile (37.5)
30.7
32.1
31.3
31.3
31.0
29.9
30.0
31.3
32.4
28.2
27.9
27.9
29.3
26.6
27.8
28.5
0.27 0.78
0.29 0.66
0.38 0.84
1.05 0.88
1.90 0.95
2.56 0.98
1.68 1.13
3.65 1.04
3.03 1.19
2.15 1.53
0.88 2.39
0.025 2.52
0.082 2.32
0.20 1.85
0.44 2.95
1.05 1.43
469
485
462
465
468
468
465
460
453
460
446
470
473
476
476
475
755 (469)
776 (480)
743 (400)
721 (400)
705 (400)
695 (400)
689 (400)
660 (425)
608 (510)
585 (460)
511 (450)
746 (430)
736 (470)
750 (470)
702 (470)
668 (470)
8077
7732
8186
7636
7183
6979
6992
6588
5628
4645
2852
7872
7555
7675
6763
6083
0.021
0.019
0.032
0.092
0.18
Acetone (20.7)
3ꢀMethylbutanoneꢀ2 (16.1)
4ꢀMethylpentanoneꢀ2 (13.1)
1,2ꢀDimethoxyethane (7.2)
Ethylacetate (6.0)
0.25
0.19
0.38
0.36
0.33
0.21
Diethyl ether (4.3)
10 Toluene (2.4)
11 Cyclohexane (1.9)
12 Methanol (32.7)
13 Ethanol (24.6)
14 nꢀButanol (17.5)
15 nꢀHexanol (13.3)
16 nꢀDecanol (8.1)
0.0063
0.019
0.037
0.13
0.15

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Physical Chemistry Chemical Physics
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Considering the pushꢀpull character of the studied molecules, we proposed that the
solvent polarity growth would enhance the ICT in the excited state, which would lead to
pronounced changes in the dipole moments upon excitation. Indeed, the absorption maxima
showed a slight red shift with increasing solvent polarity, normally interpreted as indicating that
the ground state and FrankꢀCondon (FC) excited state have similar dipole moments, whereas the
fluorescence spectra demonstrated strong solvatochromic effect (Table 1) indicative of a very
high relaxed ICT excited state dipole moment as compared to that of the ground state. In order to
demonstrate this hypothesis, we estimated the changes in the dipole moment by LippertꢀMataga
plot, which is essentially a plot of the Stokes shift of the fluorescence emission versus the solvent
polarity.45–46 The equation is expressed as followed:2
2(ꢁe − ꢁg )2
2


ε −1
n −1
~
~
~
ꢂν = νabs − νfl =
−
+ const




hca3
2ε +1 2n2 +1
(8)
(9)
ε −1
2ε +1
n2 −1
2n2 +1
ꢂf =
−
where ꢡꢢꢋꢌꢍ and ꢡꢢꢝꢞ are the wavenumbers of the absorption and fluorescence maxima; Δꢡꢢ denotes
the Stokes shift; ꢪꢫ and ꢪꢬ are ground and excited states dipole moments respectively; and n
ꢟ
are the dielectric constant and refractive index of the medium respectively; h is Planck’s
constant, c is the velocity of light, and a is the radius of the solvent cavity in which the
fluorophore resides (Onsager cavity radius). The symbol Δꢭ is called the orientation
polarizability and accounts for the spectral shifts due to reorientation of the solvent molecules. In
general, the reorientation of the solvent molecules is expected to result in substantial Stokes
shifts.
The LippertꢀMataga plots of compounds NI1–3 are depicted in Fig. 3. Sixteen solvents
were used to explore the change in the dipole moment on excitation and good linearity was
found for the overall regressions with correlation coefficients higher than 0.95. Toluene (№ 10)
was not considered in the fitting because correlation with this solvent was poor. Obviously, the
interaction between toluene and fluorophore molecules is complex and stands out of the
simplified model used in the Eq (8) where only nonꢀspecific solvation effects are taken into
account. Yet, in toluene (ε = 2.4), the emission spectra of all three compounds NI1–3 were
structureless, whereas in cyclohexane (a solvent with very close polarity, ε = 1.9) a clear vibronic
bands were observed (see Fig. S39b, Fig. S55b and Fig. S71b in Supplementary Information).
This result is very close to that obtained in the work47 where the photophysical properties of Nꢀ
2 The Eq (8) is written in CGS system of units

Physical Chemistry Chemical Physics
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alkylꢀ1,8ꢀnaphthalimides in various solvents including alkylated benzenes were studied. The
latter was found to form charge transfer complexes in the excited state (exciplexes) upon
interaction with the naphthalimide moiety. Probably, the same effect is responded for the specific
solvation in toluene in our case.
9000
a
8000
12
13
15
14
7000
6000
5000
4000
3000
2000
3
1
2
16
8
4
5
7
6
9
10 X
11
0.0
0.1
0.2
0.3
ꢀf
9000
8000
7000
6000
5000
4000
3000
2000
12
b
13
14
3
1
4
15
2
7
16
8
5
6
9
X
10
11
0.0
0.1
0.2
0.3
ꢀf
9000
8000
7000
6000
5000
4000
3000
2000
c
3
1
2
4
12
14
13
7
5
8
6
15
16
9
X
10
11
0.0
0.1
0.2
0.3
ꢀf
Figure 3. Dependence of Stokes shift of NI1 (a), NI2 (b) and NI3 (c) on the solvent polarity function Δꢭ. The
solvents are numbered according to the sequence presented in Table 1. Protic solvents are depicted by triangles and
aprotic ones – by spots. The straight line represents the best leastꢀsquare fit to the data (excluding the value obtained
for toluene, 10). Correlation coefficients are 0.955, 0.974 and 0.980 for NI1, NI2 and NI3 respectively.

Page 15 of 27
Physical Chemistry Chemical Physics
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Ground state geometry optimization with PM6 Hamiltonian of MOPAC was used to
calculate the farthest distance between the carbonyl oxygen and amino nitrogen or metoxy
oxygen of styryl fragment in molecules NI1–3. Onsager cavity radii of 6.85 Å, 6.80 Å, 6.89 Å
were obtained as a half of this distance and the dipole moment changes on excitation estimated
from the slops of linear fit lines in Fig. 1 were found to be around 20.78 D, 21.96 D, 22.95 D for
NI1, NI2, NI3 respectively. It is known that the value of 4.8 D corresponds to the dipole moment
appearing when two unit charges are separated each other by the distance of 1 Å. Hence, the
changes in the dipole moment of 20.78 D, 21.96 D and 22.95 D correspond unit charges
separation by 4.33 Å, 4.58 Å and 4.78 Å, which is a clear evidence of a charge transfer state
formation, and also is in a good agreement with the (ꢪꢬ − ꢪꢫ) values obtained for 1,8ꢀ
naphthalimides with different electronꢀdonating groups at 4th position of naphthalene ring.32,48–52
Fluorescence quantuim yields of compounds NI1–3 were dramatically influenced by the
solvent polarity (Table 1), however, the character of changes was quite complex to be
understood only on the basis of the steadyꢀstate spectroscopy data. Apparently, to get a deeper
insight into the processes occurring after the photoexcitation, timeꢀresolved experiments were
strongly needed. So, we further analyzed the effects of solvent and molecular structure on the
transient absorption (TRABS) spectra.
Transient absorption spectroscopy. Timeꢀresolved TRABS spectral map of compound
NI1 in cyclohexane is shown in Fig. 4a. As it can be seen, the spectral profile of negative signal
at 425ꢀ500 nm exhibits vibronic structure which highly resembles the steadyꢀstate fluorescence
of NI1 in this solvent (for comparison with the steadyꢀstate spectrum see Fig. S77 in
Supplementary Information). This negative band can be attributed to stimulated emission of
styrylnaphthalimide chromphore from the relatively rigid locally excited (LE) state with mostly
planar geometry. The same LE character of the excited states could be also suggested for
dimethoxystyryl and dimethylaminostyryl derivatives NI2 and NI3 because in these cases the
similar features of negative stimulated emission bands are observed when cyclohexane is used as
a solvent (Fig. S83 and Fig. S89 in Supplementary Information).
Upon the increase of solvent polarity on going from cyclohexane to diethyl ether, a new
redꢀshifted negative band at 575ꢀ600 nm emerges in TRABS spectra of all three compounds
NI1–3 (Fig. 4b–d), indicating the existence of a lower lying emissive state. Typically to pushꢀ
pull fluorophores containing stilbene fragment, an additional deactivation pathway (besides
fluorescence, internal conversion and intersystem crossing) of LE state is open in principle,
which involves twisting of one of the single bonds adjacent to the central double bond and leads
to a relaxed intramolecular charge transfer state (TICT state) with higher dipole moment and
nonꢀplanar geometry.34–36 The probability of TICT state formation for the studied

Physical Chemistry Chemical Physics
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styrylnaphthalimides is consistent with the local character of HOMO and LUMO orbitals of
NI1–3 (Fig. 1) as well as it can be clearly seen from the calculated plot of the first singlet excited
state (S1) energy of compound NI1 versus angle of rotation around C12–C17 (see Fig. 6 for
numbering of atoms) ꢮꢀbond in solvents of different polarity (Fig. 5a). When a nonꢀpolar solvent
is used (ε = 1) the angle change from 0 to 90° leads to an increase in energy, whereas in a polar
solvent (ε = 20) it reaches maxima at 40° and then drops rapidly at higher angle values, the effect
being understood on the basis of stabilization of TICT state in a more polar environment.
b
a
0.1100
0.1012
0.06800
0.06312
10
0.07938
0.05750
0.03563
0.01375
-0.008125
-0.03000
0.05094
0.03875
0.02656
0.01437
0.002188
-0.01000
10
1
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength / nm
Wavelength/nm
c
d
0.1380
0.1620
0.1339
0.1589
0.1298
0.1558
0.1256
0.1527
0.1215
0.1496
0.1174
0.1465
0.1133
0.1434
0.1091
0.1403
0.1050
0.1373
0.1009
0.1342
0.09675
0.09263
0.08850
0.08438
0.08025
0.07612
0.07200
0.06788
0.06375
0.05963
0.05550
0.05138
0.04725
0.04312
0.03900
0.03488
0.03075
0.02663
0.02250
0.01838
0.01425
0.01012
0.006000
0.001875
-0.002250
-0.006375
-0.01050
-0.01462
-0.01875
-0.02287
-0.02700
-0.03113
-0.03525
-0.03937
-0.04350
-0.04763
-0.05175
-0.05587
-0.06000
-0.06413
-0.06825
-0.07237
-0.07650
-0.08063
-0.08475
-0.08887
-0.09300
-0.09713
-0.1013
-0.1054
-0.1095
-0.1136
-0.1177
-0.1219
-0.1260
0.1311
0.1280
0.1249
0.1218
0.1187
0.1156
0.1125
0.1094
0.1063
0.1032
0.1001
0.09703
0.09394
0.09084
0.08775
0.08466
0.08156
0.07847
0.07537
0.07228
0.06919
0.06609
0.06300
0.05991
0.05681
0.05372
0.05063
0.04753
0.04444
0.04134
0.03825
0.03516
0.03206
0.02897
0.02588
0.02278
0.01969
0.01659
0.01350
0.01041
0.007313
0.004219
0.001125
-0.001969
-0.005062
-0.008156
-0.01125
-0.01434
-0.01744
-0.02053
-0.02362
-0.02672
-0.02981
-0.03291
-0.03600
10
10
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength / nm
Wavelength / nm
Figure 4. Timeꢀresolved TRABS maps of compounds NI1–3: a) NI1 in cyclohexane; b) NI1 in diethyl ether; c) NI2
in diethyl ether; d) NI3 in diethyl ether. In order to adjust the best visibility of kinetics of various processes on the
logarithmic scale some artificial time delays represented as numbers +1.6/+1.5/+0.2/+0.4 are added on the time
delay axis legend.
Generally, one could suggest several ways of formation of TICT state for the studied
compounds NI1–3. Thus, in the case of dimethylaminostyryl derivative NI3 torsion of molecule
may proceed not only around C12–C17 bond (Naphthalimide group twist), but also C18–C19
(Phenyl group twist) and N2–C22 (Dimethylamino group twist). To speculate on the most
probable nature of TICT state S1 state energy calculations for changing the twist angle of the

Page 17 of 27
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selected bonds (C12ꢀC17; C18ꢀC19 or N2ꢀC22) of NI3 in polar solvent (ε = 20) were carried out.
As it can be seen from Fig. 5b, the twist of Phenyl in the excited state indicates a possible
formation of the TICT state, however, the twist of Naphthalimide forms more energetically
favorable TICT state. The twist of Dimethylamino group in the excited state is not energetically
favorable and can be excluded from further discussion. Thus, the most probable twisting in the
excited state occurs for Naphthalimide fragment as the potential energy of such twisted state is
lower compared with any other one. Moreover, after placed on the excited state surface, the
rotation of Naphthalimide decreases only potential energy instead of Phenyl, where the state
energy during rotation must overcome a potential barrier and hence, such process is much more
time consuming.
36
a
ε
ε
= 1
b
40
36
32
28
24
4
N2
−
C22 twist
C19 twist
C17 twist
= 20
C18
−
C12
−
32
28
24
2
ε
ε
0
ε
-2
-4
0
20
40
60
80
100
0
20
40
60
C17, C18 C19, N2
80
100
Angle of rotation around C12
−
C17 bond / °
Angle of rotation around C12
−
−
−
C22 bond / °
Figure 5. Plot of energy of NI1 (a) and NI3 (b) first singlet state vs. angle of rotation around various single bonds.
The calculations were performed using MOPAC 2012.
Table 2. Characteristic times of disappearance of LE state signal (ꢇꢙ) and appearance of TICT state signal (ꢇꢯ) in
TRABS spectra of the compounds NI1 and NI2 in different solvents.
NI1
NI2
ꢇꢯ / ps
1.90
ꢇꢙ / ps
2.20
ꢇꢯ / ps
2.32
ꢇꢙ / ps
2.13
Diethyl ether
1,2ꢀDimethoxyethane
Acetonitrile
1.23
1.46
1.10
not determineda
0.83
0.14
0.14
0.14
TICT state band is partially consumed by the positive signal of excited state absorption (Supplementary Information,
Fig. S86).
The analysis TRABS kinetics showed that the disappearance of LE state signal in the
case of compounds NI1 and NI2 proceeded with almost the same characteristic times (ꢇꢙ) as the

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appearance of TICT state absorption in various solvents (
ꢇ
ꢯ).3 As expected, the increase of
solvent polarity resulted in higher rates of formation of TICT states (Table 2). A little difference
between ꢇꢯ and ꢇꢙ can be explained by the fact that the time ꢇꢯ is affected to some extent by the
dynamic solvation of twisted form which shifts TICT absorption band to longer wavelengths on
a timescale of excited state relaxation map. Dynamic solvation effects were very easily
detectable when polar solvents with low viscosity (like acetonitrile) were used. The full data on
TRABS spectral maps, TRABS spectra at different time delays and time profiles of the signals
are presented in Supplementary Information (Fig. S77–94).
A very interesting particularity was found in the TRABS spectra of NI1 and NI2
recorded in diethyl ether. In these cases, the residual signals of LE states were not completely
quenched during the lifetime of TICT forms. This observation allows to conclude that a quasiꢀ
equilibrium between twisted and planar forms is possible, which could be a result of a low
TICT–LE energy gap due to relatively weak solvation in Et2O. As the comparison of Fig. 4b and
Fig. 4c shows, the fraction of LE state was reduced for the compound NI2 where the dipole
moment and thus, solvation energy of the excited state is higher with respect to those of NI1. In
more polar than diethyl ether solvents, a complete disappearance of LE state absorption signal
with 100% yield of twisted form was observed (see Supplementary Information).
Transient absorption spectrum of compound NI3 in Et2O (Fig. 4d) was essentially
different from NI1 and NI2. In this case, formation of a planar locally excited state was not
observed. The excitation of NI3 directly produced local excited state with a twisted geometry, as
evident from the appearance of the redꢀshifted negative stimulated emission band of TICT form
at around 600 nm immediately followed by the excitation pulse. Indeed, the obtained TRABS
data are in a very good agreement with the Xꢀray structures of naphthalimides NI1–3 (Fig. 6),
which show that NI3 molecule in the ground state seems to have some preorganization for rapid
yielding of TICT state as dihedral angle C11–C12–C17–C18 of compound NI3 is much higher
(32.5°) compared with that of NI2 (1.6°) and NI1 (10.8°). It should be also noted that the TICT
state in the case of NI3 is more polar, as it can be realized considering the strongest electron
donating character of the dimethylaminostyryl fragment. Experimentally, this was expressed in
the dynamic solvation of TICT state, which caused a slight slope of stimulated emission band at
600 nm on the TRABS map (Fig. 4d) with a concomitant appearance of a small negative signal
at 720 nm. Obviously, both these negative signals correspond to the same TICT state likely
possessing a broad emission region, which is partially consumed by the positive signal of excited
state absorption.
3 Time evolution of TRABS signals and fitting kinetic curves at different wavelengths can be found in
Supplementary Information.

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Thus, in solvents more polar than diethyl ether, the planar LE state transforms effectively
into TICT state with lower energy which further undergoes either fluorescence or nonꢀradiative
decay. The existence of two emissive states for NI1–3 has also been revealed by the plots of
fluorescence quantum yields versus the excited state lifetime which were constructed from the
data on ꢀꢝꢞ and
in the excited state lifetime during the solvent change resulted in the increase of the quantum
yield, the ratio ꢀꢝꢞ/ꢇ equal to the radiative relaxation rate (
ꢨ) being kept approximately
ꢇ in Table 1 (Fig. S95, Supplementary Information). In these plots, the increase
ꢧ
constant for the majority of aprotic solvents used (Table 1). Accurately, a good linearity was
observed for all aprotic solvents except cyclohexane and toluene (also diethyl ether in the case of
NI1 and NI2), showing the appearance of another radiative state with a different ꢧꢨ value.
NI1
NI2
NI3
Figure 6. Xꢀray crystallography data for the compounds NI1–3
Photoisomerization studies. As it was mentioned above, photochemical E,Zꢀ
isomerization is a distinctive property of styryl dyes and substituted stilbenes which should
always be taken into consideration once one tries to speculate about the photophysics of such
type of systems. To study photoisomerization the solvents where the nature of the lowest
emitting state of NI1–3 is different were chosen. We used cyclohexane and toluene since in this
solvents LE state for all three compounds is dominant in governing the rates of the main
relaxation pathways. On the other hand, it was interesting to verify whether E,Zꢀisomerization
may occur when the lowest excited state responded for the fluorescence is TICT state. For this
purpose a polar aprotic solvent acetonitrile was used.

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Upon irradiation at 436 nm, solutions of NI1–3 in toluene and cyclohexane exhibited
pronounced decrease of the long wavelength absorption band with a simultaneous increase of
intensity at shorter wavelengths (Fig. S1–6, Supplementary Information), which is typical for the
transformation of Eꢀ into Zꢀisomer. However, when irradiation was carried out in acetonitrile,
very slight changes in the absorption spectra were observed for all three compounds (Fig. S7–9,
Supplementary Information). This result allows to conclude that only planar LE states are
responded for the E,Zꢀ photoisomerization. Considering the above analysis of TRABS spectra, a
suggestion can be made that in all solvents except cyclohexane and toluene (also diethyl ether in
the case of NI1 and NI2) where TICT states are dominant E,Zꢀphotoisomerization does not play
any significant role in the photophysics of studied compounds.
1
Figure 7.Aromatic and aliphatic part of H NMR spectrum of compound NI3 in tolueneꢀd8 before (top) and after
(bottom) irradiation at 436 nm. The signals of Zꢀisomer are marked with color. The concentration of NI3 is 2ꢁ10–2
M. The arrows show the upfield shift of proton signals upon photoisomerization. The numbering of atoms is shown
in Fig. 6.
To confirm the possibility of E,Zꢀisomerization in the excited states 1H NMR
spectroscopy was also used. In tolueneꢀd8 solutions, compounds NI1-3 exist as
thermodynamically stable Eꢀisomers showing spinꢀspin coupling constants of olefinic protons
3JH18–H17 equal to 15.3–16.0 Hz. After the irradiation, the signals of Eꢀisomer were decreased,
whereas a new set of signals with a lower 3JH18–H17 constant (12.3 –12.4 Hz) corresponding to Zꢀ
isomer was detected. As an example, Fig. 7 shows 1H NMR spectra of compound NI3 before and

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after irradiation at 436 nm. The spectra of Eꢀisomers and corresponding photostationary states of
NI1 and NI3 can be found in Supplementary Information (Fig. S10).
The spectral characteristics of phototransformation products (Zꢀisomers) were calculated
by Fisher method39 from the absorption spectra of two photostationary states obtained at
irradiation wavelengths 436 and 365 nm and the spectra of Eꢀ isomers. Also, the quantum yields
of the forward (E → Z) and backward (Z → E) photoreactions were determined. The data are
summarized in Table 3. As the values of ꢀꢰꢍ show, isomerizatiom is the main deactivation
pathway of LE states of EꢀNI1–3 in toluene and cyclohexane.
Table 3. E,Zꢀ Photoisomerization quantum yields and spectral characteristics of Eꢀ and Zꢀisomers of compounds
NI1, NI2 and NI3 in toluene and cyclohexane.
Toluene
Cyclohexane
NI1
NI2
NI3
NI1
NI2
NI3
ꢋꢌꢍ
/nm(ꢟꢠ ∙ 10ꢒꢤ), Eꢀisomer
/nm(ꢟꢠ ∙ 10ꢒꢤ), Zꢀisomer
405 (21.9) 412 (27.1) 460 (28.2) 398 (21.8) 405 (33.3) 446 (27.9)
382 (11.9) 395 (9.3) 438 (6.2) 376 (15.2) 386 (10.3) 436 (7.7)
0.86 / 0.14 0.76 / 0.24 0.61 / 0.39 0.67 / 0.33 0.75 / 0.25 0.71 / 0.29
ꢚ
ꢛꢋꢜ
ꢋꢌꢍ
ꢚ
ꢛꢋꢜ
[Z] / [E]a
ꢀꢁ→ꢂ
ꢀꢂ→ꢁ
0.33
0.44
0.59
0.23
0.82
0.22
0.40
0.37
0.35
0.72
0.22
0.53
0.47
0.33
0.80
0.25
0.49
0.49
0.33
0.82
0.23
0.49
0.45
0.27
0.72
0.30
0.40
0.50
0.21
0.71
b
ꢀꢰꢍ
ꢀꢝꢞ, Eꢀisomer
ꢀꢰꢍ + ꢀꢝꢞ
a[Z] / [E] is the molar ratio of Zꢀ and Eꢀisomers in photostationary state obtained after irradiation at 436 nm
bCalculated as ꢀꢰꢍ = ꢀꢁ→ꢂ/(1 − ꢀꢂ→ꢁ). The value ꢀꢰꢍ corresponds to the fraction of molecules of Eꢀisomer in the
initially formed LEꢀstate relaxing via isomerization pathway.
Fluorescence quantum yields and excited state lifetimes. Aiming to rationalize the
effects of solvent polarity on the fluorescence quantum yields and excited state lifetimes in those
cases where the emission is originating mostly from TICT form we suggested the two factors
that should be considered in the analysis. The first one is the stabilization of TICT state due to
strong solvation in a more polar media. This leads to an increase in the lifetime and emission
quantum yield. On the other hand, a higher solvation in polar solvents (especially specific one by
hydrogen bonding in water and alcohols) results in effective transformation of excitation energy
into multiple vibrational quanta, i.e. internal conversion to the ground state (the second factor),
which in contrast would decrease the values of
ꢇ .
and ꢀꢝꢞ
The opposite spectral effects induced by the two factors were observed for the
compounds NI1 and NI3 in aprotic solvents. Methoxystyryl naphthalimide NI1 demonstrated an
almost linear increase in ꢀꢝꢞ with the growth of
ꢇ when the solvent was changed from

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ethylacetate to propylene carbonate (Table 1, Fig. S95a in Supplementary Information), showing
the factor 1 to play the major role. A good linearity for the dependence of ꢀꢝꢞ on
also took
ꢇ
place for NI3, however, in this case the increase in solvent polarity reduced the lifetime and
quantum yield (Table 1, Fig. S95c in Supplementary Information), which could be realized by
the priority of the second factor. Thus, one can conclude that the competition between the two
factors depends on the charge separation in the excited TICT state. Following this logic, the
highest dipole moment of TICT form of compound NI3 provides it to rapidly undergo internal
conversion to S0, while less polar TICT state of NI1 seems to have got a stabilizing effect when
the interactions with solvent molecules is raised. In the case of dimethoxystyrylnaphthalimide
NI2, where the electron releasing properties of styryl fragment are higher than in NI1 but lower
than in NI3, the mean situation occurs that is reflected in the interplay between these two factors.
As a result, the fluorescence quantum yield and excited state lifetime are not influenced
dramatically upon the change of the aprotic solvent (Table 1). The similar intermediate case by
all appearance could be realized for compound NI1 in protic solvents (Table 1), in which
solvation effects are intensified by the hydrogen bonding to such extent that the leak of the
excitation energy through internal conversion becomes favorable. Based on the above analysis,
there could be supposed three types of spectral behavior of NI1–3 upon the change in the solvent
polarity: i) factor 1 is dominant (NI1 in aprotic solvents); ii) both factor 1 and factor 2 are in
respond for the relaxation of TICT state (NI1 in protic and NI2 in aprotic solvents); iii) factor 2
is dominant (NI3 in protic and aprotic solvents).

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4. Conclusion
To conclude, we have shown that the main relaxation pathways of the local excited
singlet state of compounds NI1–3 are fluorescence, formation of TICT state and E,Zꢀ
photoisomerization. The interplay between these pathways occurs when either solvent or
electron donating properties of styryl fragment is changed. Isomerization proceeds effectively
only in nonꢀpolar solvents like cyclohexane and toluene where the planar LE states are dominant.
The twisting of stryrylnaphthalimide chromophore is favored by the introduction of extra
methoxy or dimethylamino group in the molecule NI1 as well as by the polar environment. In
the solvents ranged from ethylacetate to methanol the fluorescence emission has mostly a TICT
nature and can also be modulated using these two factors.
As it has been mentioned in the introduction section, biological applications of NIR dyes
as molecular fluorescent probes and markers require them to show several features, the most
important being high Stokes shift values, red shifted absorption and emission maxima as well as
high emission quantum yield. Very often, dyes exhibiting high values of Stokes shift and NIR
fluorescence possess a significant ICT character in the excited state. This means that solvation in
polar media like alcohol or water would lead to fluorescence quenching since the internal
convertion rate would grow since S0ꢀS1 energy gap would decrease and the number of possible
vibrations would also grow in the solvates including multiple solvent molecules. This is a
common disadvantage of ICT fluorophores. Our study illustrates that in the case of NI1 and NI2
molecules the TICT states may be stabilized in polar media ensuring rather high fluorescence. It
was very unusual to observe long wavelength emission (about 620nm) of NI2 which is a typical
ICT system in DMSO or ethanol with a rather high quantum yield 0.16ꢀ0.40. In contrast to the
majority of naphthalimide dyes, the studied compounds NI1–3 have demonstrated a rather high
fluorescence originating from TICT states. Indeed, in most of the cases, TICT states of 4ꢀaminoꢀ
1,8ꢀnaphthalimides are not fluorescent or weakly fluorescent, which is a reason commonly used
to explain the quenching effect of polar protic solvents on the emission intensity.51–53 With this
point of view, compounds NI1 and NI2 appear to be very interesting, because their TICT
fluorescence remains approximately at constant level when the polarity of a protic (for NI1) or
an aprotic (for NI2) solvent grows.
Thus, we have performed a precise analysis of the effects of solvent and molecular
structure on the photophysics of styrylnaphthalimides. We believe that the results presented
herein would be useful for understanding the spectral properties of naphthalimide dyes as well as
would help researchers working in the field of life sciences to properly choose fluorophores
relevant to satisfy a specific system’s requirements.

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Acknowledgements
O.A.F. thanks RSF project № 16ꢀ13ꢀ10226 (synthesis of the compounds, Xꢀray data and
photoisomerization studies), G.J. thanks the Région Aquitaine for financial support (steadyꢀstate
and timeꢀresolved optical measurements).

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