Azomethine phthalimides fluorescent E→Z photoswitches

Article abstract of DOI:10.1016/j.jphotochem.2020.112443

Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2). These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity. Steady-state fluorescence measurements were performed in various solvents and the results show strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was found for EAMP1 R = 6.95 %, k = 8.87 × 10^?4 s^?1 and EAMP2 R = 88.72 %, k = 4.00 × 10^?4 s^?1, respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state (S₀) and first excited state (S₁) by DFT/TD-DFT calculations with B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. It was found that E-isomers in the S₀ have nonplanar conformation, while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped conformation is energetically advantageous compared to the twisted one. The reason is the weak H….π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 °C. The fluorescence E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as E-isomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar solvents, red-shifted emissions with increased fluorescence quantum (Φ_fl) yields have been observed at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the environmental viscosity. Time-resolved fluorescence decay measurements were performed in AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers, especially in less polar 1,4-DOX.

Full text of DOI:10.1016/j.jphotochem.2020.112443

Journal Pre-proof
Azomethine Phthalimides Fluorescent EnullZ Photoswitches
Anton Gergiev, Dancho Yordanov, Deyan Dimov, Ivailo Zhivkov,
Dimana Nazarova, Martin Weiter
PII:
S1010-6030(20)30242-2
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112443
JPC 112443
Reference:
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Journal of Photochemistry & Photobiology, A: Chemistry
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Accepted Date:
27 November 2019
4 February 2020
8 February 2020
Please cite this article as: Gergiev A, Yordanov D, Dimov D, Zhivkov I, Nazarova D, Weiter M,
Azomethine Phthalimides Fluorescent Esrarr;Z Photoswitches, Journal of Photochemistry &
amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112443
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Azomethine Phthalimides Fluorescent E→Z Photoswitches
Anton Gergiev a,b*, Dancho Yordanov c , Deyan Dimov b, Ivailo Zhivkov b,d, Dimana Nazarova b,
Martin Weiter d
a Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756
Sofia, 8 St. Kliment Ohridski Blvd, Bulgaria.
bDepartment of Optical Metrology and Holography, Institute of Optical Materials and
Technologies, Bulgarian Academy of Science, 1113 Sofia, 109 "Acad. G. Bonchev” Blvd., Bulgaria.
c Laboratory of Chemistry and Biophysics of Proteins and Enzyme, Institute of Organic Chemistry
with Centre of Phytochemistry, Bulgarian Academy of Science, 9 "Acad. G. Bonchev" Blvd., Sofia
1113, Bulgaria.
d Materials Research Centre, Faculty of Chemistry, Brno University of Technology, Purkyňova 118,
612 00 Brno, Czech Republic.
a,*Corresponding author: Anton Georgiev, Department of Organic Chemistry, University of
Chemical Technology and Metallurgy, 1756 Sofia, 8 “St. Kliment Ohridski” Blvd, Bulgaria, e-mail:
antonchem@abv.bg
Graphical abstract
1

Highlights
 Two azomethine phthalimides dyes have been synthesized.
 The kinetics of E→Z photoswitching has been investigated.
 Steady-state and time-resolved fluorescence E→Z photoswitching behavior have been
studied.
 The fluorescence quantum (Φfl) yield at photostationary state has been increased compared
to the E-isomers.
Abstract
2

Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine
phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2).
These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence
properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity.
Steady-state fluorescence measurements were performed in various solvents and the results show
strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was
studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light
activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching
behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was
found for EAMP1 R = 6.95 %, k = 8.87×10-4 s-1 and EAMP2 R = 88.72 %, k = 4.00×10-4 s-1,
respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was
analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state
(S0) and first excited state (S1) by DFT/TD-DFT calculations with B3LYP/6-31+G(d,p) level of
theory using IEFPCM in AcCN. It was found that E-isomers in the S0 have nonplanar conformation,
while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped
conformation is energetically advantageous compared to the twisted one. The reason is the weak
H…..π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide
ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in
dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the
same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 oC. The fluorescence
E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as E-
isomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar
solvents, red-shifted emissions with increased fluorescence quantum (Φfl) yields have been observed
at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture
of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the
environmental viscosity. Time-resolved fluorescence decay measurements were performed in
AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental
fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers,
especially in less polar 1,4-DOX.
Keywords: phthalimides, fluorescence photoswitches, E→Z photoisomerization, rate constant,
molecular rotors.
3

1. Introduction
Photoactive compounds containing isomerazable >X=Y< units (X and Y might be equal or different
substituted atoms of C or N) can be reversibly switched on and off upon light irradiation. According
to the isomerazable units, the materials are mainly classified as: (i) azobenzenes; (ii) azomethines;
(iii) stilbenes, spiropyrans, and diarylethenes. Aromatic azomethines or Schiff bases undergo
reversible trans (E)→cis (Z)→trans (E) photoisomerization cycle like azobenzenes [1][2][3].
Photoinduced changes of molecular geometry between E- and Z- switched states allow to control
over the electron rearrangement of fluorophore units resulting in changed levels of frontier
molecular orbitals, which are responsible for the emission. The large review by D. Kim and S.Y.
Park provides a very insightful systematization of multicolor fluorescence photoswitches [4]. They
have classified them according to their fluorescence switching principles and behavior as: (i) color-
correlated and, (ii) color-specific photoswitches. The first switching behavior is explained that one
emission is switched off upon light irradiation and simultaneously the other emission is switched on
correlatedly. The latter is that one emission is switched on and off upon light irradiation, while the
other emission is unchanged. T. Fukaminato has published а critical review on the design and
synthesis of various types of photoswitchable fluorescent molecules with an application as single-
molecule optical memory and super-resolution fluorescence microscopy [5]. The nature of the
fluorescence bands of E/Z photoswitching system is described by twisted intramolecular charge
transfer (TICT), which is a fundamental action of fluorescent molecular rotors (FMR’s). Simplify
description of the process is a rotation of the donor (D) via a single C-C bond around acceptor (A).
Typically D-A connection in the fluorophore consists of the push-pull π-conjugated bridge and the
photophysical properties (absorption and emission) can be modified by the variation of electron
donor and acceptor substituents and the length of the π-conjugated system [6]. Upon light excitation,
the electrons transit from the ground state (S0) to the first excited state (S1) and locally excited (LE)
state with a partial D-A charge transfer (CT) is formed. In this state, the molecule is still planar that
able to produces LE emission or following by a concomitant rotation of the D until twisted to the
right angle concerning the A, which gives red-shifted TICT emission (Fig. 1). The TICT state is
characterized by the total charge separation between D-A and conjugation is destroyed. The
overcoming of the barrier transferring from LE to TICT state critically depends on the
environmental polarity and microviscosity of the solvents, which are responsible for dual
fluorescent properties of the fluorophore (Fig. 1) [7] [8] [9] [10] [11]. Many researchers have studied
the TICT phenomenon long time ago related to its application as FMR’s [6][12][13]. A recent paper
4

by S. Sasaki and co-authors updates the mechanisms and nature of TICT in terms of applicability of
the phenomena in device fabrications such as solar cells and OLED’s [7]. They have concluded that
there are several shortcomings: (i) the extent of pretwisting required to form the TICT state is
unclear; (ii) the data on the excited-state CT equilibria are still incomplete due to the difficulty of
discerning excited-state species from fluorescence spectra; (iii) the influence and contribution of
other transfer effects such as exciplexes remains unclear; (iv) various isomeric substitution effects
have not yet been fully elucidated.
Fig. 1. Mechanism of the LE and TICT emissions according to the Jablonski diagram.
The D-A interaction via –CH=N− group in the excited state is performed by rotation of aromatic
substituents, where the π→π* (S0→S2) and n→π* (S0→S1) transitions have different contributions.
This is essential to their emissions behavior to enhance the quantum yield the π→π* energy must
be reduced at the expense of n→π* transitions [14]. In general, the fluorescent properties of 4-
substituted phthalimides with amino or dialkylamino groups are widely studied [15] [16] [17] [18]
[19]. They show remarkable Stoke shifts upon different solvent environments and are used as
fluorescent probes in cell biology, protein labeling as well as fluorescent polymers for flexible
OLED’s devices. For example, 4-aminophthalimide (4AP) having electron-donor amino group,
5

exhibited large quantum yield in THF (Φfl = 0.70), DCM (Φfl = 0.76) and acetone (Φfl = 0.68),
while replacing the amino group with dimethylamino one to 4-dimethylaminophthalimide (DAP)
exhibits very weak fluorescence. The reason is the large torsion at the bond between the
dimethylamino group and the phthalimide resulting in TICT emission [14]. R. Orita and co-workers
have studied 4-substituted phthalimides with five- and six-membered alicyclic electron-donating
amino groups [14]. Based on DFT calculations and lifetime measurements, they have concluded
that the alicyclic amino group produces planar intramolecular charge transfer with improved
quantum yields, especially when the dispersion in polymer matrices such as PMMA occurs (Φfl =
0.99). Several examples in the literature could be found for the investigation of fluorescence
photoswitching properties of phthalimide and naphthalimide dyes [20] [21] [22]. The paper from P.
Panchenko describes the E/Z fluorescence photoswitching performance of styrylnaphthalimides by
TICT [21]. They have found that the TICT of some dyes is stabilized in polar media ensuring rather
high fluorescence and long wavelength emission near to 620 nm. The E/Z photochemistry of the
aromatic azomethines has been studied by many researchers in terms of their reversible optical
memory [23][24][25][26][26]. The tuning of light induced isomerization is achieved by ortho- and
para-substitution of phenyl rings with electron-donating and electron-accepting groups [23].
The motivation of our study for the design and synthesis of two novel 4-substituted azomethine
phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties was to investigate their
bidirectional photoswitching behavior. We have investigated the kinetics of their E/Z
photoconversion by Vis-light activation and long wavelength UV-light, as well as back Z/E thermal
relaxation. The fundamental fluorescence bands of E-isomers and at the photostationary state (PSS)
were studied by steady-state and time-resolved spectroscopy in AcCN and 1,4-DOX. In the switched
state, the chromophores have shown increased fluorescence quantum yield.
Experimental
2.1.Materials Used
The phthalimide, 9-anthracenecarboxaldehyde, 4-(Dimethylamino)benzaldehyde, ethyl bromide,
and triethylamine were purchased from Sigma-Aldrich and used without further purification. The
nitric acid, sulphuric acid, potassium carbonate, DMF, and MeOH were delivered from local
supplier Valerus Ltd. For spectroscopy measurements, all solvents were spectroscopy grade purity.
2.2.Synthesis of the N-ethyl-4-arylazomethine phthalimides
6

The synthetic pathway of the azomethine dyes is presented on Scheme 1. In order to synthesis 1 (4-
nitrophthalimide), 2 (4-nitro-N-ethylphtalimide) and 3 (4-amino-N-ethylphthalimide) we have
followed the methodology of early published paper [27]. The 1H- and 13C-NMR spectra of 3 used
as an intermediate compound to Schiff bases synthesis are presented in the Supporting information.
Scheme 1. Synthetic pathway of the EAMP’s.
2.3.General procedure for condensation reaction between 4-amino-N-ethylphthalimide and
aromatic aldehydes to azomethines
The N-ethyl-4-aminophthalimide (2.628 mmol, 1.0 equiv) and aromatic aldehydes (2.628 mmol, 1.0
equiv) were suspended in dry MeOH (25 mL). Subsequently, 0.5 mL of Et3N was added and the
mixture was heated to reflux without stirring for 7 hours. The reaction was controlled by TLC
(CHCl3 : EtOAc = 2:3). After cooling to room temperature the reaction mixture was concentrated
by evaporation and then stored in refrigerator to precipitate, then filtered off and washed with cold
MeOH:H2O (1:1). The Schiff bases were recrystallized by ethanol to obtain pure products. The 1H-
and 13C-NMR spectra are presented in the Supporting information.
2.3.1. Synthesis of the (E)-5-((anthracen-9-ylmethylene)amino)-2-ethylisoindoline-1,3-dione,
(EAMP1)
The compound was synthesized from 0.5 g (2.628 mmol) 4-amino-N-ethylphthalimide and 0.541 g
(2.628 mmol) 9-anthracenecarboxaldehyde, yield 71 % as yellow-orange solids, m.p. = 147-149 ºC.
ATR-FTIR cm-1: 3048 (ν Ar-H), as 2972, s 2934 (ν -CH3 and >CH2), s 1762, as 1701 (ν >C=O),
1623 (ν –CH=N-), 1580,1519 (ν Ar C=C), as 1449, s 1390 (δ -CH3 and >CH2), 1345 (C-N-C
stretching vibration of imide ring). 1H-NMR (DMSO-d6) ppm: 9.85 (1H, s); 8.92-8.83 (3H, t, J = 10
Hz), 8.19 (2H, d, J = 9 Hz), 7.93-7.79 (3H, d, J = 14 Hz), 7.67-7.59 (4H, d, J = 11 Hz), 3.65-3.62
(2H, q, J = 3 Hz), 1.21-1.19 (3H, t, J = 5 Hz). 13C-NMR (DMSO-d6) ppm: 167.93, 167.86, 157.75,
7

133.91, 132.17, 131.25, 130.83, 129.52, 129.03, 128.26, 127.68, 126.48, 126.16, 125.37, 124.68,
115.92, 32.94, 14.19. MS (ESI+) calculated for C25H18N2O2, [M+]: 378.1368; found: 378.3634.
2.3.1. Synthesis of the (E)-5-((4-(dimethylamino)benzylidene)amino)-2-ethylisoindoline-1,3-
dione, (EAMP2)
The compound was synthesized from 0.5 g (2.628 mmol) 4-amino-N-ethylphthalimide and 0.392 g
(2.628 mmol) 4-(Dimethylamino)benzaldehyde, yield 78 % as red solids, m.p. = 163-165 ºC. ATR-
FTIR cm-1: 3065 (ν Ar-H), as 2978, s 2939 (ν -CH3 and >CH2), s 1766, as 1701 (ν >C=O), 1643 (ν
–CH=N-), 1593, 1551 (ν Ar C=C), as 1463, s 1396 (δ -CH3 and >CH2), 1348 (C-N-C stretching
vibration of imide ring). 1H-NMR (DMSO-d6) ppm: 9.66 (1H, s), 7.69-7.67 (2H, d, J = 7 Hz), 7.47-
7.45 (1H, d, J = 8 Hz), 6.91-6.90 (1H, d, J = 4 Hz), 6.79-6.76 (3H, m), 3.52-3.48 (2H, q, J = 6 Hz),
13
3.04 (6H, s), 1.11-1.09 (3H, t, J = 7 Hz). C-NMR (DMSO-d6) ppm: 190.36, 168.56, 168.27,
155.33, 154.65, 135.01, 132.00, 125.24, 124.96, 117.17, 116.94, 111.52, 107.40, 40.22, 30.33,
14.32. MS (ESI+) calculated for C19H19N3O2, [M+]: 321.1477; found 321.2817.
2.4. Quantum Chemical Calculations
The molecular geometry and electronic structure optimization in the ground state (S0) of the studied
Schiff bases were performed by GAUSSIAN 09D.01 software package using density-functional
theory (DFT) with B3LYP exchange-correlation hybrid functional combined with the standard 6-
31+G(d,p) basis set both in vacuo and using solvation in AcCN as E- and Z- isomers [28]. The
interactions between the molecules and the solvent were evaluated on the same basis set by the
polarizable continuum model using the integral equation formalism variant (IEFPCM). Natural
Bonding Orbital (NBO) and charges distribution were computed after optimization of geometry on
the same basis set in vacuo. The molecular geometry optimization in the first excited (S1) state as
E- and Z-isomers were computed by TD-DFT using the same basis set the in AcCN solvent. The
frequency analysis was made at the same level of theory to characterize the stationary points on the
potential surface and to obtain thermodynamic parameters such as absolute enthalpy (H) and Gibbs
free energy (G) at 298 K.
2.5. Photoisomerization Experiment
The photoinduced E→Z isomerization of the studied Schiff bases was performed in AcCN solution
at room temperature on custom-built optical set-up with high-resolution USB-650 Red Tide
spectrometer (Ocean Optics) operating in the range of 200 - 1000 nm. As the light source was used,
a pulsed xenon light source (PX-2) directed to pass through the dye solutions and to get directly to
8

the spectrometer through optical output fiber. Special care has been taken to eliminate all possible
sources of stray light. The source of Visible (Vis) light was halogen lamp (power 10 W) equipped
with bandpass filter at 410 nm of EAMP1 as well as long wavelength UV irradiation was broadband
lamp with λmax = 350 nm (power 10 W) of EAMP2, perpendicular directed to the probe beam. The
illumination time was set at 60 min (EAMP1) and 200 min (EAMP2) until the photostationary state
(PSS) was reached. After exposure, the probes undergo Z→E relaxation in darkness for 60 min at
room temperature (EAMP1) and 90 min thermal relaxation at 60 ºC ±1 ºC (EAMP2) using cuvette
thermostat. The spectra were collected at every 1 or 4 min during the entire experiment by
integration time set at 100 ms and scans to average at 15. Before the experiments start, the solutions
of the dyes were stored in the dark overnight.
2.6. Spectral Measurements
The NMR spectra were recorded on a Bruker Avance II+ spectrometer operating with frequency
1
13
600 MHz for H and 125 MHz for C in DMSO-d6. ATR-FTIR spectra of the compounds were
recorded on a Bruker Tensor 27 FTIR spectrophotometer in the range of 4400-600 cm-1 with a
resolution of 2 cm-1 at room temperature. The external reflection diamond crystal has been used and
the samples were scanned 128 times. The UV-VIS spectra were carried out on the Cary 5E- UV-
VIS-NIR spectrophotometer. The steady-state fluorescence spectra were recorded via a FluoroLog
3-22 (HORIBA) spectrofluorometer in the range 200-800 nm with a resolution of 0.5 nm and
double-grating monochromators by excitation wavelength set near to the absorption maxima of
dyes. The emissions at PSS were measured immediately after Vis light irradiation at 410 nm for
EAMP1 and UV irradiation at 350 nm for EAMP2 in AcCN and 1,4-DOX. Absolute fluorescence
quantum yields (Φfl) of the dyes were determined using calibrated integrated sphere Quanta-φ
(HORIBA) as E-isomers and at PSS in AcCN, 1,4-DOX and glycerol. The low temperature (at liquid
nitrogen ~ 77 K) emissions were measured in AcCN and 1,4-DOX as E-isomers and at PSS. The
cuvette sample was placed in Dewar and before the measurement, the sample was tempered for 30
min (Fig S8 in the supporting information). To investigate the viscosity impact on molecular rotors,
the emissions of binary mixtures of glycerol:ethanol were carried out in different ratios. Time-
resolved fluorescence decay was measured on the TBX Picosecond Photon Detection Module
(HORIBA) using pulsed Nano-LED laser diodes with excitation wavelengths at 372 nm for EAMP1
as E-isomer and at PSS, 329 nm for EAMP2 as E-isomer and 360 nm for EAMP2 at PSS. The
concentrations of the dyes were CM ~ 1×10-5.
9

3. Results and Discussion
3.1.Synthesis and molecular geometry optimization
Two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl
π-donor moieties were synthesized via four-step reactions. The first step was classical nitration of
phthalimide to 4-nitrophthalimide (1) and the second one was alkylation of NH imide by ethyl
bromide in DMF in the presence of K2CO3 to N-ethyl-4-nitrophthalimide (2) [27]. The ethyl group
was introduced to increases solubility. The third step was catalytic hydrogenation of −NO2 to –NH2
by Pd/C catalyst in MeOH to N-ethyl-4-aminophthalimide (3) and the final step was condensation
of 3 with 9-anthracenecarboxaldehyde and 4-(Dimethylamino)benzaldehyde by refluxing in dry
MeOH in the presence of Et3N (Scheme 1) to corresponding 4-substituted phthalimide Schiff bases
(EAMP1 and EAMP2). The synthesized dyes are new and unstudied of their photochromic behavior,
which will be promising for spatial and temporal control of fluorescence photoswitching, TICT-
active molecular rotors as well as in dynamic light modulation between two switched states. Before
the spectral study of switching behavior, we focus our attention on the optimized molecular
geometry of E- and Z-isomers in S0 and S1 states to understand the rearrangements of the aromatic
substituents around –CH=N- group between both isomers related to their spectral properties. As E-
isomers in the S0 state, the dyes are characterized by the nonplanar configuration of anthracenyl/4-
(Dimethylamino)phenyl moieties respect to the –CH=N− bond. A similar observation has been done
by Y. Luo and coworkers carrying out a detailed investigation of a large number of N-
benzylideneaniline derivatives [23]. Depending on the aromatic π-donors nonplanarity is
determined by C(27)-C(25)-C(19)-N(11) dihedral angle (φ): 29.19 º for EAMP1 and 2.16 º for
EAMP2 (Fig. 2). It can be accepted that 4-(Dimethylamino)phenyl part of EAMP2 has a nearly
planar configuration around –CH=N− bond. The reason for the bigger angle of EAMP1 is the
repulsion between azomethine H(20) and H(35) of anthracene fused ring compared to the EAMP2.
The other φ respect to the phthalimide ring via –CH=N− is C(19)-N(11)-C(1)-C(2), which is slightly
10

affected from the aromatic π-donors: 42.22 º (EAMP1) and 39.72 º (EAMP2) due to the orbital
repulsion between azomethine H(20) and H(12) phthalimide. The aromatic moieties can act as rotors
around TICT-active C(25)-C(19) and N(19)-C(1) bonds, which spectral properties further discussed
in subsection 3.3. As can be expected, E-isomers ensure free rotation of D by twisted D-A charge
transfer, which is strongly sensitive to the polarity and viscosity environments. To verify the rotation
of substituents around the above-mentioned σ-bonds the calculations of S1 state indicate changing
of the φ to 8.40 º and 17.01 º for EAMP1 and 0.01 º and 2.36 º for EAMP2, which suggest almost
planar structure, especially for EAMP2 (Fig. 2). As E-isomer the φ around Me2N(35)-C(32) bond
remains unchanged. The Z-isomer of EAMP1 in the S0 state is characterized by the twisted
conformation of anthracene ring regard to the phthalimide one via –CH=N- bond by the φ 63.94 º
of C(27)-C(25)-C(19)-N(11) fragment. Such structure ensures face-to-face (sandwich) like π-
interaction between aromatic rings enabling the charge transfer (CT) from anthracene (donor) to
phthalimide (acceptor). Moreover, an insignificant change of φ to 66.89 ° is observed in the S1 state,
which means the rotation of the anthracenyl rotor off due to the steric hindrance. In other words, the
TICT emission will be blocked at the expense of LE one. The geometry of Z-isomer of EAMP2 has
a T-shaped conformation of 4-(Dimethylamino)phenyl ring in the S0 state regard to the phthalimide
one via –CH=N− bond by the φ -5.66 º of C(27)-C(25)-C(19)-N(11) fragment. The shape of Z-
isomer suggests that is possible to exist a weak H…..π noncovalent interaction (NCI). This makes
Z-isomer relatively favorable and energetically preferable compared to the twisting structure. Such
structure of Z-isomer restricts the rotation of 4-(dimethyamino)phenyl part, hence the TICT
emission will be decreased. After excitation to the S1 state, the φ is changed to 36.35 ° and the other
specific rotation around Me2N(35)-C(32) dihedral angle from 0.06 ° (S0) to 9.75 ° (S1) confirm the
assumption for the molecular rotor behavior of the dye as Z-isomer (Fig. 2). Photoinduced changes
of the molecular geometry always cause rearrangement of the electron densities and specific
configuration of Z-isomers of EAMP’s provides different photophysical properties as will be
discussed later about their photoswitching behavior. Similar systems of azoheteroarenes with high
stability of Z-isomer have been studied by J. Calbo et.al. [29]. The authors have investigated several
phenylazo pyrazoles and pyrrazolones of their photoswitching behavior and based on DFT
calculations and experimental kinetic data, they have concluded that T-shaped conformation via –
N=N- bond is preferable than the twisted one.
11

Fig.2. Optimized molecular geometry of the EAMP1 (A) and EAMP2 (B) as E- and Z-isomers in the
ground state S0 and first excited state S1 by DFT/TD-DFT /B3LYP/6-31+G(d,p) level of theory using
IEFPCM in AcCN. The change of dihedral angels (φ) around –CH=N– and –N(Me)2 from S0 to S1
are depicted in the diagram.
In general, the energy barrier of Z/E back relaxation of azomethines (70 kJ mol-1) is smaller than
this of the azo compounds (95 kJ mol-1) [23]. One of the advantages of the low barrier for the
relaxation process is the opportunity for rapid control of the reversible geometrical changes in light-
driven molecular devices. Thermodynamic parameters provide information about the relative
energy between two switched states via photoinduced changes of the geometry. Table 1 summarizes
the calculated ΔEE→Z, ΔμE→Z, ΔHE→Z, ΔGE→Z and ΔSE→Z of EAMP’s, where the photoinduced E/Z
conversion is the difference between final state (f) Z-isomers (products of the photochemical
reaction) and initial state (i) E isomers (reactants). As can be seen, EAMP1 has lower ΔGE→Z and
ΔμE→Z values compared to the EAMP2. The calculated ΔS describes the order of the macroscopic
system and the probability of photochemical reactions. EAMP1 is characterized by a negative value,
while EAMP2 has a positive one. The negative change of the entropy for E/Z isomerization means
12

that the photoconversion is favored by the kinetic factor, as opposed to positive change that indicates
a thermodynamically controlled process.
Table 1. Theoretically calculated total electronic energy E (RB3LYP), dipole moment μ and
thermodynamic parameters of the EAMP1 and EAMP2 as E- and Z- isomers by DFT/ B3LYP/6-
31+G(d,p) level of theory using IEFPCM in AcCN.
E (RB3LYP)
[kcal mol-1]
ΔE
μ
[D]
Δμ
[D]
H
ΔH
G
ΔG
*ΔS
[kcal mol-1]
[kcal mol-1] [kcal mol-1] [kcal mol-1]
[kcal mol-1] [cal K-1
mol-1]
Compounds
E-EAMP1
Z-EAMP1
E-EAMP2
Z-EAMP2
-767810.71
-767806.52
-659061.56
-659056.13
6.89
5.85
-767565.08
-767560.97
-658829.42
-658823.81
-767614.82
-767610.10
-658877.89
-658872.67
4.19
5.43
-1.03
-1.79
4.10
5.60
4.71
5.22
-2.06
1.29
12.01
10.22
ΔH−ΔG
*ΔS =
푇
3.2.Kinetics study of E/Z photoswitching behavior
The E/Z photoswitching performance of the dyes was studied in AcCN by a different wavelength of
activation depending on the absorption maxima of π→π* transitions. EAMP1 shows maximal
response to a Vis-light at 410 nm, while EAMP2 was activated by long wavelength UV-light at 350
nm (Fig. 3). The PSS of EAMP1 was reached within 60 min (Fig.3A1) of the main absorption band
at λmax = 409 nm with a relatively small response. EAMP2 was switched to PSS after 140 min
illumination with a large response of π→π* transition band at λmax1 = 337 nm and small changes of
weak n→π* transition band as a shoulder at λmax2 = 392 nm (Fig.3B1). New blue-shifted bands have
appeared at λmax1 = 300 nm (π→π*) and λmax2 = 365 nm (n→π*), which characterize the features of
Z-isomer. The degree of E/Z photoisomerization, R at PSS, was determined according to Eq. 1 and
photoisomerization kinetics data, rate constant k were fitted by Eq’s 2 (E/Z) and 3 (Z/E) [30]
[31][32][33]:
퐴 −퐴
0
푅 =
∞ × 100 (1)
퐴
0
퐴 −퐴
0
ln(
ln(
∞) = 푘. 푡 (2)
퐴 −퐴
푡
∞
퐴 −퐴
∞
0) = 푘. 푡
(3)
퐴 −퐴
∞
푡
13

where A0 is the initial absorbance, At is the absorbance at the moment t during the irradiation and
A∞ represents absorbance at the PSS. The degree of E/Z photoisomerization of EAMP1 is
considerably low (R = 6.95 %) with fast conversion (Table 2). In contrast, the EAMP2 is
characterized by large photoisomerization (R = 88.72 %) and a two-fold lower speed of conversion
compared to the EAMP1 (Table 2). Let‘s consider the back Z/E relaxation, where EAMP1 fully
returned to E-isomer up to 60 min at room temperature in the dark, while EAMP2 at these conditions
is stable for 600 min (Fig. 3A1 and 3B1inset graphics) that is unusual for Schiff bases. After rising
temperature up to 60 oC the recovery of EAMP2 becomes up to 90 min. The kinetics of thermal Z/E
relaxation of EAMP1 is faster than the E/Z reaction (Table 2), which is typical for the Schiff bases.
As we pointed out in the previous section the T-shaped conformation of EAMP2 is preferable
resulting in H…..π NCI of Z-isomer at PSS (Fig. 2), which stabilize switched state at room
temperature compared to the EAMP1. The Z/E relaxation is favorable at 60 oC, where the NCI’s are
destroyed. From the degree of photoisomerization and kinetics data, the following conclusions can
be drawn: (i) aromatic substituents at 4-position of phthalimide ring connected by –CH=N- have a
strong influence on E/Z photoconversion and back thermal relaxation; (ii) anthracene fused ring of
EAMP1 is a π-donor leads to absorbance maximum in the visible range, where the kinetic factor
dominates the E→Z→E photoisomerization cycle (Fig. 3A2), therefore can be referred to as a T-
type photochromic switch; (iii) 4-(dimetylamino)phenyl donor moiety of EAMP2 stabilizes Z-
isomer by T-shaped conformation, therefore bidirectional switching is thermodynamically
controlled process (Fig. 3B2) and can be referred to as a P-type photochromic switch [34][35][36].
14

Fig. 3. E→Z→E photoisomerization cycle in AcCN of: EAMP1 by Vis-light at 410 nm up to 60 min
(A1), dark relaxation at room temperature up to 60 min (inset graphic A1) and kinetic curves ΔA at
409 nm as a function of the time (A2); EAMP2 by UV-light at 350 nm up to 200 min (B1), dark
relaxation at room temperature up to 600 min, thermal dark relaxation at 60 ºC up to 90 min (inset
graphics B1) and kinetic curves ΔA at 337 nm as a function of the time (B2). The absorbance
changes
are
presented
as
∆퐴 = 퐴0 − 퐴푡
(퐴0 ꢀ푠 푡ℎ푒 ꢀ푛ꢀ푡ꢀ푎푙 푎푏푠표푟푏푎푛푐푒 푎푛푑 퐴푡 ꢀ푠 푡ℎ푒 푎푏푠표푟푏푎푛푐푒 푎푡 푡ℎ푒 푚표푚푒푛푡, 푡) as a function of the
time of illumination and dark relaxation.
15

Table 2. Experimentally determined values of the photoisomerization degree (R), rate constants
(k) and the half-life of reaction (t1/2) of E/Z photoisomerization and Z/E thermal relaxation of the
EAMP’s dyes in AcCN (The graphics are presented on Fig.S9 in the Supporting information).
Compounds
E→Z EAMP1
Z→E EAMP1
E→Z EAMP2
Z→E EMAP2
R [%]
6.95
6.86
88.72
88.63
k [s-1]
t1/2 [s]
781.4
608
1732.8
653.9
*8.87 ± 0.07×10-4
*1.14 ± 0.12×10-3
*4.00 ± 0.04×10-4
**1.06 ± 0.26×10-3
*Determined at room temperature
**Determined at 60 ºC
The analysis of TD-DFT spectrum of the E-isomer of EAMP1 shows that vertical π→π* (S0→S2)
transition has a major contribution, while n→π* (S0→S1) transition has minor one (Fig.S10 in the
Supporting information). The experimental spectrum of EAMP1 in AcCN is characterized by
broadband at λmax = 409 nm, where both transitions are overlapped. Upon excitation, strong
overlapping of frontier molecular orbitals and mixture of HOMO→LUMO and
HOMO→LUMO+1electron transitions between anthracene and phthalimide rings are observed
(Fig. 4). As a result, there is no effectively charge separation, which leads to a low
photoisomerization degree of EAMP1. As, well known the photoisomerization of such system
becomes when the electrons populated S2 excited state and after relaxation to the S1 switched to the
ground state of Z-isomer. The back process is happened by excitation to the S1 state and relaxation
to the ground state of E-isomer or by thermal relaxation [37]. The calculated spectrum of pure Z-
isomer is characterized by a blue shifting and decreasing oscillator strength of π→π* transition,
while for n→π* transition slightly increases. In the switched state upon excitation, the separation of
HOMO→LUMO+1 orbitals leads to CT from anthracene fused ring to the phthalimide one (Fig. 4),
which is essential to fluorescence behavior as we discussed later. The TD-DFT spectrum of EAMP2
as E-isomer displays a major contribution of π→π* (S0→S2) transition attributed to the
HOMO→LUMO+1 and n→π* (S0→S1) related to the HOMO→LUMO (Fig.S10 in the Supporting
information and Fig. 4). The HOMO level mainly occupied the 4-(dimethylamino)phenyl ring,
while LUMO and LUMO+1 mainly occupied the phthalimide one, which leads to a separation of
the charges and a reasonable degree of photoisomerization. The experimental spectrum in AcCN
before isomerization is characterized by the bands at λmax1 = 337 nm and shoulder at λmax2 = 392
nm. The calculated spectrum of the Z-isomer displays decreasing oscillator strength and blue shifts
of the π→π* transition, resulting in an increased contribution of the n→π* transition. The
16

HOMO→LUMO and HOMO→LUMO+1 indicate the separation of the CT from donor 4-
(dimethylamino)phenyl ring to acceptor phthalimide one.
Fig.4. Isodensity surface plots of the frontier molecular orbitals for E- and Z-isomers of EAMP’s
dyes calculated by DFT/B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN.
After UV-irradiation to PSS, quantitative (or near-quantitative) photoswitching to the Z-isomer can
be determined by the area of minimum absorbance at a wavelength where the E-isomer absorbed
and Z-rich PSS can be quantified by UV-VIS spectroscopy. The pure Z-isomer spectrum was
extracted using the E-isomer spectrum and the quantified PSS. PSS ratio was assigned by
interpolating between the E- and Z-isomer spectra using the following equation (4) [29]:
퐴
− 퐴
푍
푃푆푆
푓
푃푆푆
=
(4)
퐴 −퐴
퐸
푍
where fPSS is the fraction of E-isomer present in the PSS, APSS, AE, and AZ are the absorbances at the
π→π* transitions at the PSS, E- and Z-isomers, respectively. The spectrum of the pure Z-isomer was
estimated based on the PSS containing highest concentration of Z-isomer. Initially, the fraction of
E-isomer presented in this PSS was estimated from the absorbance at the E-isomer absorption
maximum. Using this estimation, the spectrum of the authentic E-isomer was subtracted from that
of the PSS, using equation (5) [29]:
17

퐴
− 퐴 ×푓
퐸
푃푆푆
퐴푍 =
(5)
1−푓
where f is the fraction of E-isomer present in the PSS, and A is the corresponding absorbance at the
PSS, E-spectrum. Fig. 5. shows the spectrum of pure Z-isomer of EAMP2 for 11.28 % E-isomer in
PSS. The features of pure Z-isomer obtained by interpolation between the experimental spectra of
the E-isomer and at PSS show a reasonable fit with TD-DFT spectrum (Fig.S10 in the supporting
information).
Fig.5. Determination of pure Z-spectrum of EAMP2 in AcCN.
3.3.Fluorescence E→Z photoswitching behavior and molecular rotors
The broad absorption bands of EAMP1 in five different solvents (1,4-DOX, THF, MeOH, glycerol,
and AcCN) are characterized by overlapped π→π* (S0→S2) and n→π* (S0→S1) transitions
(Fig.6A1). Less polar 1,4-DOX and viscous glycerol increase the energy of electron transitions
compared to the red-shifted absorption in polar protic EtOH and more polar aprotic solvents (THF
and AcCN). Red-shifted absorption in the EtOH and THF (λmax = 416 nm) are associated with
specific solvent-solute interactions by intermolecular hydrogen bonding of EtOH with phthalimide
ring and better solvation by solvent-solute dipole interactions in THF related to the excited state
electronic structure of anthracene fused ring. EAMP2 displays opposite behavior compared to the
EAMP1 due to the 4-(Dimethylamino)phenyl moiety, where the increase of solvents polarity shows
slightly blue-shifting of the absorption (Fig. 6B1). Less polar 1,4-DOX indicates that dipole moment
18

decreases during the electronic transitions and Franck-Condon excited state is more stabilized by
nonpolar solvation, which is the reason for relative higher red-shifting compared to the AcCN and
THF [38]. In contrast, the steady-state emission spectra of EAMP1 display a strong sensitivity of
the environmental polarity and viscosity, where the EtOH red-shifted emission with a large Stokes
shift is observed (Fig.6A2 and Table 3). The reason is in the excited state dye molecules form
specific solute-solvent hydrogen bonding with the polar protic solvents, like MeOH or EtOH (Fig.
7A), which produces large polarization with D-A CT followed by TICT emission [7] [8][18].
Unusual emission band in 1,4-DOX (λem = 587 nm) near 119 nm red-shifted from the emissions in
THF and AcCN (λem = 468 and 476 nm) is observed. Generally, the structure of the studied dyes
can be represented as D−CH=N−A, where A is constant acceptor substituent (phthalimide ring) and
D is variated donors. Consequently, the spectral behavior will be determined by changing the
electronic structure of the D - anthracene fused and 4-(Dimethylamino)phenyl rings. It is well known
that the dual fluorescence of FMR’s is due to the red-shifted TICT emission, which is favored in
polar (protic or aprotic) solvents compared to the blue-shifted LE emission favorable in nonpolar
ones [6][10]. EAMP2 shows similar behavior like EAMP1 by red-shifted emissions in the EtOH
and 1,4-DOX (λem = 588 and 579 nm) and blue-shifted emissions in the THF and AcCN (λem = 554
and 561 nm) (Fig.6B2 and Table 3). The TICT and LE emissions strongly depend on the viscosity
environment, where the increase of viscosity, the rotation is reduced resulting in increased LE
emission [6]. EAMP1 in the glycerol displays red-shifted emission (λem = 555 nm) compared to the
THF and AcCN. On the other hand EAMP2 has the lowest energy emission (λem = 612 nm) in
glycerol compared to the all used solvents. What is the reason for this unspecific dual fluorescence
behavior of the dyes? A reasonable explanation can be given by the electronic structure of –CH=N−
bond and n-π interaction with the phthalimide ring. Based on the second order perturbation theory
analysis by resonance stabilization energy E(2), as E-isomers the dyes have considerable π-π and n-
π conjugation between donor aromatic rings and acceptor phthalimide one via –CH=N- bond (Table
S1 in the Supporting information). The calculated NBO charges of E-isomers presented in Fig.7B
by summing of the atomic charges of the donor and acceptor parts are associated with the resonance
stabilization energy E(2) of both isomers. As E-isomers, significant D−A π-π interaction via –
CH=N− is observed, while as Z-isomers, the conjugation is decreased due to the twisted
conformation resulting in reduced π-donor strength of aromatic substituents and increased acceptor
character of phthalimide ring, which enhances the n→π* conjugation (contribution) of lone pair
(LP) of 4-substituted nitrogen (Fig. 7C). Upon illumination of the dyes to their PSS depending on
19

the polarity of the solvent, the fluorescence quantum yield (Φfl) considerably increases. In the polar
AcCN, the dyes show Φfl = 62.92 % (EAMP1) and Φfl = 57.91 % (EAMP2), compared to the low
yields of E-isomers around 10 % (Table 3). The Φfl in less polar 1,4-DOX also increases at PSS to
40.06 % (EAMP1) and 75.88 % (EAMP2) compared to the E-isomers (Table 3). The reason for the
increased Φfl confirms the statement that reducing the π→π* (S0→S2) transitions of the Z-isomers
enhances the contribution of n→π* (S0→S1) transitions.
Fig.6. Experimentally measured UV-VIS and steady-state fluorescence spectra of EAMP1 (A1 and
A2) and EAMP2 (B1 and B2) in various solvents. The excitation wavelength was set near to the
absorption maxima.
20

Fig. 7. (A) Specific dye-alcohol interactions with imide carbonyl oxygen by intermolecular hydrogen
bonding. (B) NBO charges of the E- and Z-isomers of EAMP dyes calculated at DFT/ B3LYP/6-
31+G(d,p) level of theory in vacuo. Charge transfer on the molecular backbone is presented by
summing the atomic charges of donor and acceptor part of the molecules. (C) Resonance energy of
n→π conjugation of the 4-substituted phthalimide nitrogen as E- and Z-isomers of the dyes.
Table 3. Summarized results of absorption wavelengths (λabs), emission wavelengths (λem) and Stokes
shifts (Δῡ) of the E-isomers and at PSS in different polarity solvents in room temperature (RT). The
emissions of the E-isomers and at PSS in AcCN and 1,4-DOX were measured at liquid nitrogen
(LN), ~ 77 K. The fluorescence quantum yield (Φ) of E-isomers and at PSS was measured in 1,4-
DOX, AcCN and glycerol. Lifetime measurements (τ), radiative (kr) and non-radiative (knr) decay
rate constants were determined in 1,4-DOX and AcCN of E-isomers and at PSS.
λabs
λem
*Δῡ
Φ
kr
knr

Compounds
Solvents
[nm] [nm] [cm-1] [%]
[s-1]
[s-1]
[ns]
1,4-DOX
1,4-DOX at
77 K
372
-
587
622
9845 12.53 23.4
-
5.3×106 3.7×107
-
-
-
-
THF
EtOH
Glycerol
AcCN
AcCN at 77
K
416
416
389
409
468
607
555
476
2670
7563
7689 31.06
-
-
-
-
-
-
-
-
-
-
-
E-EAMP1
3441 13.70 20.5
-
6.6×106 4.2×107
-
-
-
637
587
-
-
1,4-DOX
374
9845 40.06 24.1
1.6×107 2.4×107
21

1,4-DOX at
77 K
-
-
-
-
588
-
-
THF
EtOH
Glycerol
AcCN
AcCN at 77
K
1,4-DOX
1,4-DOX at
77 K
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
PSS-EAMP1
383
411
571
481
8596 42.45
3540 62.92 24.1
-
2.6×107 1.5×107
-
-
-
342
-
632
579
593
-
-
11968 8.59 34.43 2.4 ×106 2.6×107
-
-
-
-
-
THF
EtOH
Glycerol
AcCN
AcCN at 77
K
1,4-DOX
1,4-DOX at
77 K
331
340
354
337
554
588
612
561
12160
12404
11908 23.47
11848 7.36
-
-
-
-
-
-
-
-
-
-
E-EAMP2
-
27.1
-
2.7×106 3.4×107
-
-
-
368
-
614
570
593
-
-
9630 75.88 52.9
-
1.4×107 4.5×106
-
-
-
-
THF
EtOH
Glycerol
AcCN
AcCN at 77
K
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
PSS-EAMP2
370
365
614
589
10740 33.20
10419 57.91 23.7
-
2.4×107 1.7×107
-
-
-
614
-
-
1
1
*Determined by ∆ῡ = 107 × (휆
−
)
휆
푎푏푠
푒푚
In order to support the assumption for the described above unspecific dual fluorescence behavior
and their D-A interactions as E- and Z-isomers (at PSS), we have conducted an investigation by
steady-state fluorescence spectroscopy at room temperature (RT) and liquid nitrogen (LN ~ 77 K)
as well as by lifetime measurements. The increased emission intensity of EAMP1 in AcCN at PSS
in RT compared to the E-isomer is observed due to reducing the rotation of the anthracene ring and
enhanced the n→π* contribution (Fig. 8A1). At LN huge red-shifting emission (λem ~ 637 nm) of
both isomers indicates minor vibrational relaxations and major HOMO→LUMO transition related
to the n→π* (S0→S1) excited state. This can be confirmed by comparison of excitation spectra at
RT and LN, which are given on the Fig S11 in the Supporting information. The behaviour in 1,4-
DOX at RT displays slightly increased emission intensity at PSS and blue-shifted emission at PSS
at LN, which matches with this in RT (λem ~ 588 nm). This means that the n→π* (S0→S1) of LE
excited state is preferable compared to the TICT state (Fig. 8B1 and Fig S11 in the Supporting
information). The behaviour of EAMP2 is similar with EAMP1, except the considerable red-shifting
emission at PSS (λem ~ 589 nm) in AcCN at RT compared to the E-isomer (λem ~ 561 nm) due to
polar D-A interaction of 4-(Dimethylamino)phenyl moiety and good solvation of n→π* (S0→S1)
excited state (Fig. 8A2 and Fig S11 in the Supporting information).
22

Fig. 8. Steady-state fluorescence emissions of E- and Z- isomers at PSS in room temperature (RT)
and liquid nitrogen (LN) ~ 77 K : A1) EAMP1 in AcCN, A2) EAMP2 in AcCN, B1) EAMP1 in 1,4-
DOX and B2) EAMP2 in 1,4-DOX. The E→Z photoisomerization was performed by Vis-light at 410
nm up to 60 min (EAMP1) and UV-light at 350 nm up to 200 min (EAMP2) until PSS and the
samples ware immediately subjected to measurements.
Time-resolved fluorescence decay experiment provides specific information for the excited state
stability (lifetime) related to the D-A interaction. Depending on the donor strength of substituents
in the excited state and nature of the solvents the CT between D and A is characterized by different
lifetime. The obtained results show good fit with a triple exponential approach as E-isomers and at
PSS in AcCN and 1,4-DOX (Fig. 9 and Table 3). The values of the radiative (kr) and non-radiative
(knr) decay rate constants were estimated (Eq’s 6.1 and 6.2) using the quantum yields (Φ) and
lifetimes (τ) based on the previously reported approaches [8][39].
Ф
푘푟 =
6.1

23

100 − Ф
푘푛푟
=
6.2

The average lifetime of EAMP2 is longer than EAMP1 before isomerization in both solvents and
the results could be attributed to the observed reduced emission of EAMP1 due to non-radiative
de-excitation process. At PSS EAMP1 increases its lifetime especially in less polar 1,4-DOX due to
the better contribution of n→π* transition and reduced TICT state (Fig. 9A1 and 9B1), which
confirms by increased fluorescent de-excitation. The longest stability of the excited state of EAMP2
at PSS is observed in 1,4-DOX (Fig. 9A2 and 9B2) and could be assigned to NCI of T-shaped
conformation and intensive CT between the reduced distance of D-A interaction. In summary, the
extended lifetimes at PSS of both compounds in the polar and nonpolar solvents are associated with
reducing the π→π* (S0→S2) transitions and enhance the contribution of n→π* (S0→S2) ones, where
the Φfl and fluorescence de-excitation are considerably increased.
Fig.9. Time-resolved fluorescence decay measurements: A1) and A2) Intensity decay of E-isomers
and at PSS of EAMP1 and EAMP2 in AcCN; B1) and B2) Intensity decay of E-isomers and at PSS
of EAMP1 and EAMP2 in 1,4-DOX: The excitation light sources were 372 (for EAMP1 as E-isomer
24

and at PSS), 329 and 360 nm (for EAMP2 as E-isomer and at PSS) pulsed Nano-LED laser diodes.
The fitting was performed by triple exponential decay function.
In order to investigate the molecular rotors’ behavior, the binary mixtures of glycerol and ethanol
in different ratios as E-isomers were investigated (Fig. 10). The results show that by increasing the
viscosity the emission gradually enhances, which means decreases of TICT state and enhances the
LE emission due to reducing the rotation hence vibrational or non-radiative relaxation. At PSS in
glycerol, EAMP1 has two-fold increased red-shifted emission due to break off the rotation related
to the LE state with increased Φfl = 42.45 % (Fig’s 10A1 and 11 and Table 3). EAMP2 at PSS
indicates weak increasing the emission due to the T-shaped conformation and specific D-A NCI
related to increased Φfl = 33.20 %, where break off is not fully completed.
Fig. 10. Steady-state fluorescence spectra of EAMP’s in binary mixtures of glycerol and ethanol in
different ratios as E-isomers (A1 and B1). The emissions in glycerol at PSS were of EAMP1 (A1)
and EMAP2 (B1). The functional dependences of the emission intensities as E-isomers by increasing
viscosity are presented for EAMP1 (A2) and EMAP2 (B2).
25

Based on the experimental and theoretical study of E/Z photoswitching performance of the dyes and
its dual fluorescence in different polarity solvents the mechanism of their action can be suggested.
As E-isomers, the rotation of the donors around –CH=N− is characterized by reduced emission in
AcCN and 1,4-DOX with relative low quantum yield (around 10 %), this mode is assigned as
rotation on (Fig. 11). After photoisomerization to the Z-isomers, the rotation off due to the steric
hindrance of donor substituents with the phthalimide ring. Anthracene fused rings of EAMP1 in the
switched mode unable to rotation and its quantum yield considerable increases in AcCN, 1,4-DOX,
and viscous glycerol. In contrast, 4-(Dimethylamino)phenyl moiety of EAMP2 is characterized by
T-shaped conformation and reduced distance between D-A CT. The specific NCI retains the donor
fixed around the phthalimide ring and the rotation is reduced. In terms of the steady-state and time-
resolved fluorescence measurements the origin of fundamental bands is associated with excitation
to LE state. Depending on the nature of solvents and aromatic substituents the CT D-A interaction
is a key factor for the emissions. As E-isomers, the distance between D-A is larger compared to the
Z-isomers and this is an important factor for the TICT emissions. Moreover, the photoinduced
changes of the molecular geometry always cause the electron rearrangement followed by changing
the levels of the frontier molecular orbital. From a practical point of view, the bidirectional
photoswitches allow controlling the emission between the two isomers. The applications of this
phenomenon can be realized in the optical memory storage, OLED’s and high-resolution
microscopy. The major problem of our dyes is low tuning of the n→π* (S0→S1) transitions between
both switched state (especially for EAMP1), which is essential for their performance. Our
measurements show that photoswitch EAMP2 containing 4-(dimethylamino)phenyl ring allows
better tuning capability compared to EAMP1, which is promising for the aforementioned
applications.
26

Fig. 11. Suggested mechanism of the fundamental fluorescence bands and rotation modes of the E-
and Z-isomers of EAMP’s based on the steady-state and lifetime measurements.
4. Conclusions
We present a systematic theoretical and experimental E/Z photoswitching study of two 4-substituted
azomethine phthalimide dyes. EAMP1 is characterized by twisted conformation as Z-isomer, while
for the EAMP2 T-shaped conformation is preferred resulting in weak H…..π noncovalent
interactions (NCI’s) between 4-(dimethylamino)phenyl moiety and phthalimide ring. The Z-isomer
of EAMP2 at room temperature in the dark is stable up to 600 min compared to the EAMP1 at the
same conditions, which undergoes full Z→E relaxation for less than 60 min. The back thermal
relaxation to E-isomer of EAMP2 is achieved up to 90 min at 60 ºC. From the experimental data
following conclusions can be drawn: (i) aromatic substituents at 4-position of phthalimide ring
connected by –CH=N- have a strong influence on E/Z photoconversion and back thermal relaxation;
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(ii) Anthracene fused ring of EAMP1 kinetic factor dominates the E→Z→E photoisomerization
cycle, therefore can be denoted as T-type photochromic switch; (iii) 4-(Dimethylamino)phenyl
donor moiety of EAMP2 stabilizes Z-isomer by T-shaped conformation, therefore the bidirectional
switching is a thermodynamically controlled process and can be referred to as P-type photochromic
switch.
Steady-state fluorescence measurements in various solvents show strong sensitivity of the emission
on the solvent polarity and viscosity. It was found that the dual fluorescence character of the dyes
depends on the electronic structure of the donor substituents. EAMP1 with anthracene fused rings
show red-shifted emission in less polar 1,4-DOX compared to the AcCN. Upon illumination to PSS
the fluorescence quantum yields (Φfl) considerable increases in AcCN and1,4-DOX. The reason for
the increased Φfl is the reduced π→π* (S0→S2) transitions of the Z-isomers and enhanced
contribution of n→π* (S0→S1) transitions. The molecular rotor behaviour was investigated in binary
mixtures of glycerol and ethanol in different ratios as E-isomers The results show that increasing
the viscosity the emission gradually enhances, which means decreases of TICT state and enhances
the LE emission due to reducing the rotation hence vibrational or non-radiative relaxation. Time-
resolved fluorescence decay experiment provides specific information for the excited state stability
related to the D-A interaction. Before isomerization in both solvents, the average lifetime of EAMP2
is longer than EAMP1. The results could be attributed to the observed reduced emission of EAMP1
and high rate of non-radiative de-excitation. At PSS EAMP1 improves its lifetime especially in less
polar 1,4-DOX due to the better contribution of n→π* transition and the reduced TICT state. After
photoisomerization to Z-isomers, the rotation off due to the steric hindrance of donor substituents
with phthalimide ring resulting in increased LE emission. The extended lifetimes at PSS of both
compounds in the polar and nonpolar solvents are associated with reducing the π→π* (S0→S2)
transitions and enhance the contribution of n→π* (S0→S2) ones, where the Φfl considerable
increases.
Author statement
Anton Georgiev: formulated the idea, synthesized the compounds, done the DFT calculations,
done the spectral measurements and written the paper
Dancho Yourdanov: done the spectral measurements and discussed on the results
Deayn Dimov: done the spectral measurements and discussed on the results
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Ivaylo Zhivkov: help for the life-time measurements and read the final version of manuscript.
Dimana Nazarova: discussed on the experiment and check the final paper.
Martin Weiter: check the final version of the paper and provided the helpful discussion.
Conflict of Interest
The authors declared that the article content has no conflicts of interest.
Acknowledgements
This work was financially supported by the Bulgarian National Scientific Fund project ДН 08/10
of the Ministry of Education and Science, Bulgaria. The authors also gratefully acknowledge the
Czech Science Foundation project No. GA19-22783S. Computational resources were provided by
the CESNET LM2015042 and the CERIT Scientific Cloud LM2015085, provided under the
programme "Projects of Large Research, Development, and Innovations Infrastructures". Research
equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap
for research infrastructures) supported by Bulgarian Ministry of Education and Science under
contract D01-284/17.12.2019 was used in this investigation.
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