A series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.
A series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.
The design and synthesis
of new push–pull organic materials
has emerged as a hot area of research over the past two decades because
of their potential application in organic light-emitting diodes (OLEDs),
nonlinear optics, photovoltaic cells and bioimaging.[1−4] These push–pull semiconducting materials show unique electronic
and photonic features which may be tuned and improved by easy synthetic
modifications. In these push–pull chromophores, heterocyclic
derivatives (which contain nitrogen, oxygen, and sulfur) were mainly
introduced in the π-conjugated systems to modulate their photophysical
and electrochemical properties.[5−7] Many studies report on the preparation
of new organic push–pull materials and their good device or
biological performance. However, a deep understanding of their successful
application by studying their detailed excited-state dynamics and
mechanism is rarely reached. This could be extremely valuable in order
to get feedback and guidance about new optimal design and synthetic
strategies.Perylenediimide (PDI),[8] naphthalenediimide
(NDI),[9] and diketopyrrolopyrrole (DPP)[7] have been largely used as electron-acceptor units
in new materials for optoelectronic and biochemical technologies because
of their excellent chemical, thermal, and photostability. However,
compared to the PDI, NDI, and DPP analogues, 1,8-naphthalimide (NPI)
derivatives show the same positive properties while being even more
promising dyes,[10−19] because they are less affected by aggregation issues. More recently,
phenothiazine (PTZ) was often used as the active component in push–pull
chromophores because of its strong electron-donating capability.[20] The PTZ unit is an electron-rich tricyclic heteroarene
with nonplanar butterfly structure, characterized by the presence
of powerful electron-donor sulfur and nitrogen atoms.[21] In the literature, the photophysical properties and the
HOMO–LUMO energy levels of PTZ derivatives have been easily
modulated by substitutions at the nitrogen and the 3,7-positions of
the phenothiazine unit.[22,23] Our group has functionalized
the 3,7-positions of the phenothiazine unit by using polycyclic aromatic
hydrocarbons of increasing complexity[24] or strong acceptors such as benzothiadiazole[25] or tetracyanobutadiene.[26,27] In the literature,
push–pull phenothiazine–naphthalimide systems have been
successfully employed in some optoelectronic applications.[28−38] However, the research on varying the oxidation state of the sulfur
atom (sulfides, sulfoxides, and sulfones) in the thiazine ring of
phenothiazine is still very limited.[39−41] In this study, we have
synthesized new naphthalimide and phenothiazine-based systems, in
which we have changed the oxidation state of the sulfur atom on the
thiazine ring to investigate its effect on the photonic properties
of the obtained materials.In particular, herein we report the
synthesis of five phenothiazine
and naphthalimide-based compounds, with both dipolar (D−π–A) and quadrupolar (A−π–D−π–A) structures, shown in Chart . In these phenothiazine derivatives, we
alter the oxidation state (i.e., S(II), S(IV), and S(VI)) of the sulfur
atom. In the push–pull chromophores NPI-PTZ1 and NPI-PTZ4, phenothiazine was used as the donor, whereas in NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5, phenothiazine 5-oxide and phenothiazine 5,5-dioxide were used as
donor units. With this study, we investigate the effect of the phenothiazineoxygen functionalization on the spectral, photophysical, and electrochemical
features of these molecules. To reach this goal, we employ not only
cyclic voltammetry and steady-state spectroscopy but also advanced
time-resolved spectroscopic techniques, such as nanosecond and femtosecond
transient absorption as well as broadband fluorescence up conversion
to gain a deep understanding of the excited-state behavior.
Chart 1
Molecular
Structures of the Investigated Compounds
Results
and Discussion
Synthesis and Characterization
The
detailed synthetic
routes for 1,8-naphthalimide-functionalized phenothiazine-based chromophores
are shown in Scheme . The 1,8-naphthalimide substituted NPI-PTZ1 chromophore
was synthesized by the Sonogashira cross coupling reaction of 3-ethynyl-10-propyl-10H-phenothiazine 2 with one equivalent of 6-bromo-2-butyl-1H benzo[de]isoquinoline-1,3(2H)-dione 1 in the presence of Pd(PPh3)4 as the
catalyst in 60% yield. The reaction of NPI-PTZ1 with
1.4 equiv of 3-chloroperbenzoic acid in dichloromethane solution at
room temperature for 1 h resulted in NPI-PTZ2 with 85%
yield, whereas the dioxide derivative NPI-PTZ3 was synthesized
in 78% yield by using three equivalents of 3-chloroperbenzoic acid
in the same conditions. The push–pull chromophores NPI-PTZ4 and NPI-PTZ5 were synthesized by the Sonogashira cross
coupling of 3,7-diethynyl-10-octyl-10H-phenothiazine 3 with two equivalents of 6-bromo-2-butyl-1H benzo[de]isoquinoline-1,3(2H)-dione 1, and of 3,7-diethynyl-10-octyl-10H-phenothiazine 5,5-dioxide 5(42) with two equivalents of 6-bromo-2-(2-ethylhexyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione 4, in 63% and 58% yields, respectively. The chemical structures
of the synthesized molecules were confirmed by 1H and 13C NMR, HRMS, and MALDI-TOF mass spectrometry techniques,
and the products are readily soluble in common organic solvents (see
the Supporting Information).
Scheme 1
Synthesis
of the Compounds under Investigation
Spectral and Fluorescence Properties
Figure shows the absorption and emission
spectra of the investigated compounds in toluene. The lower energetic
absorption band and the emission spectrum appear structured for the
oxygen-functionalized compounds NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5 but broader and structureless for the
oxygen-free NPI-PTZ1 and NPI-PTZ4. In all
cases, the oxygen functionalization of the sulfur atom of the phenothiazine
implies a blue shift of the absorption and emission spectra. For instance,
the absorption maximum is at 437 nm for NPI-PTZ1, 407
nm for NPI-PTZ2, and 395 nm for NPI-PTZ3. An analogous trend was also observed when considering the emission
maxima or the two branched systems (see Table ). The bathochromically shifted spectra observed
for the oxygen-free relative to the oxygen-functionalized derivatives
may indicate that phenothiazine is a stronger donor unit relative
to phenothiazine 5-oxide and phenothiazine 5,5-dioxide.[41]
Figure 1
Normalized absorption (left) and emission (right) spectra
of the
investigated compounds in Tol.
Table 1
Spectral (Absorption and Emission
Maxima, Molar Absorption Coefficient, Optical Band Gap, and Stokes
Shift) and Fluorescence (Quantum Yield (ϕF), Lifetime
(τF), and Rate Constant (kF)) Properties of the Investigated Compounds in Tola
compound
λAbs (nm)
ε (M–1 cm–1)
Eg,opt (eV)
λEm (nm)
Δν
(cm–1)
ϕF
τF (ns)
kF (s–1)
NPI-PTZ1
437
17200
2.28
619
6730
0.51
5.17
9.7 × 107
NPI-PTZ2
407
23200
2.63
478
3650
0.87
2.22
3.9 × 108
NPI-PTZ3
395
27100
2.71
456
3390
0.71
1.69
4.1 × 108
NPI-PTZ4
438
23100
2.28
580
5590
0.66
3.50
1.9 × 108
NPI-PTZ5
421
64900
2.72
448
1430
0.75
1.24
6.0 × 108
The radiative
rate constants
were obtained from the experimentally observed fluorescence quantum
yields and lifetimes as kF = ϕF/τF.
Normalized absorption (left) and emission (right) spectra
of the
investigated compounds in Tol.The radiative
rate constants
were obtained from the experimentally observed fluorescence quantum
yields and lifetimes as kF = ϕF/τF.The optical band gap (Eg,opt), estimated
from the onset wavelength of the absorption spectrum, increases upon
increasing the number of oxygen atoms attached to the phenothiazinesulfur (2.28, 2.63, and 2.71 eV for NPI-PTZ1, NPI-PTZ2, and NPI-PTZ3, respectively) while being unaffected
by the dipolar versus quadrupolar structure (2.28 and 2.72 eV for NPI-PTZ4 and NPI-PTZ5, respectively). The Stokes
shift values are quite large for the oxygen-free NPI-PTZ1 and NPI-PTZ4 molecules (6730 and 5590 cm–1, respectively) and are significantly reduced in the phenothiazineoxide and dioxide derivatives (3650, 3390, and 1430 cm–1 for NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5, respectively). These results suggest a more rigid molecular structure
for the oxygen-functionalized relative to the oxygen-free compounds.
The molar absorption coefficients (ε in Table and Figure S19) are found to increase upon increasing the number of oxygen atoms
linked to the phenothiazine (17200, 23200, and 27100 M–1cm–1 for NPI-PTZ1, NPI-PTZ2, and NPI-PTZ3, respectively) and upon passing from
the monobranched to the two-branched systems (23100 and 64900 M–1cm–1 for NPI-PTZ4 and NPI-PTZ5, respectively).The fluorescence quantum yields
are significant (51–87%)
in toluene (Table ). The obtained values are generally higher for the oxygen-functionalized
than for the oxygen-free derivatives and enhanced in the quadrupolar
systems relative to the dipolar analogues. Fluorescence lifetimes
of several nanoseconds were measured through time correlated single-photon
counting measurements. The lifetime values are shorter in the phenothiazineoxide and dioxide compared to the phenothiazine derivatives (5.17,
2.22, and 1.69 ns for NPI-PTZ1, NPI-PTZ2, and NPI-PTZ3, respectively) and in the two-branched
relative to the monobranched molecules (3.50 and 1.24 ns for NPI-PTZ4 and NPI-PTZ5, respectively). As a result,
the radiative rate constants (kF = ϕF/τF) are enhanced in the oxygen-functionalized
derivatives and in the quadrupolar systems (see Table ). Our findings clearly demonstrate the positive
effect of the oxygen functionalization on the light absorption and
emission capability of these phenothiazine–naphthalimide systems.
Electrochemical Properties
Cyclic voltammetry was used
to explore the redox behavior and potentials of the investigated samples.
The electrochemical properties are depicted in Figure S16. The electrochemical data of all the derivatives
are collected in Table S1. The mono and
di-1,8-naphthalimide-based phenothiazine derivative NPI-PTZ1 and NPI-PTZ4 exhibit a single reversible reduction
wave at −1.23 V corresponding to the 1,8-naphthalimide acceptor
unit. Similarly, NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5 show a reversible reduction wave at −1.20,
−1.21, and −1.23 V corresponding to the reduction of
the same unit. The oxygen-free compounds, NPI-PTZ1 and NPI-PTZ4, exhibit a single reversible oxidation wave at 0.78
and 0.83 V attributed to the phenothiazine strong donor. In contrast, NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5 exhibit a single irreversible oxidation wave at 1.41, 1.50, and
1.56 V corresponding to the phenothiazine 5-oxide and phenothiazine5,5-dioxide units. Therefore, on the anodic side, the oxidation waves
of the oxygen-functionalized phenothiazine derivatives are shifted
toward more positive values compared to NPI-PTZ1 and NPI-PTZ4 because of the increase of the sulfur oxidation state
on the thiazine ring. The HOMO and LUMO energy levels of NPI-PTZ1, NPI-PTZ4, NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5 were estimated by using the first onset
potentials of oxidation and reduction waves at −5.05, −5.12,
−5.72, −5.84, and −5.85 eV and −3.30,
−3.26, −3.29, −3.22, and −3.25 eV, respectively.
These results indicate that increasing of sulfur oxidation state has
more effect on the HOMO compared to the LUMO energy levels.
Theoretical
Calculations
The structural and electronic
properties of the NPI-PTZ molecules (containing methyl substituents
instead of the alkyl ones) were investigated by DFT calculations.[43] The simpler methyl substituents were used in
order to save computational time. Molecular geometries and frontier
molecular orbitals of all the investigated compounds are shown in Figure . They exhibit nonplanar
structures due to the presence of phenothiazine, phenothiazine 5-oxide,
and phenothiazine 5,5-dioxide units as a central core with the typical
butterfly structure (see also Figure S17). In the case of NPI-PTZ1 and NPI-PTZ4, the HOMOs are localized over the phenothiazine strong donor unit
while LUMOs are mainly concentrated on the naphthalimide acceptor.
In the case of the oxygen-functionalized systems, due to the presence
of phenothiazine 5-oxide and phenothiazine 5,5-dioxide weaker donor
units, the HOMOs are spread over the whole molecule while the LUMOs
are mainly concentrated on the 1,8-naphthalimide unit, acting as a
powerful acceptor.
Figure 2
Optimized ground-state geometry and frontier HOMO and
LUMO orbitals
obtained by DFT calculations (B3LYP functional/6-31G** basis set).
Optimized ground-state geometry and frontier HOMO and
LUMO orbitals
obtained by DFT calculations (B3LYP functional/6-31G** basis set).The theoretical HOMO–LUMO band gaps for NPI-PTZ1, NPI-PTZ4, NPI-PTZ2, NPI-PTZ3, and NPI-PTZ5 are 2.75, 2.77, 3.10, 3.18, and 2.97
eV, respectively
(Figure S18). The oxygen-free phenothiazine
derivatives thus show a decreased band gap relative to the oxygen-functionalized
ones. In order to investigate the spectroscopic properties, TD-DFT
calculations were carried out in dichloromethane, and the results
are shown in Table S2. The electronic absorption
spectra calculated by TD-DFT are in reasonable agreement with the
experimental spectra.
Solvent Effect and Hyperpolarizability
The effect of
the solvent on the absorption and emission spectra was investigated
for NPI-PTZ1–5. One representative example is
shown in Figure (NPI-PTZ5), while all the other data are reported in detail
in Figures S20–S22. The solvent
effect is negligible on the absorption spectra, while being very significant
on the emission spectra. For all the samples, a large red shift of
the emission maximum was observed upon increasing the solvent polarity.
In the case of the oxygen-free molecules, the emission spectra always
appear bell-like shaped (Figures S20 and S22) and are red-shifted beyond 850 nm in the most polar media. For
the oxygen-functionalized compounds, the solvent plays the role to
tune the spectral shape: the fluorescence spectrum is structured in
the low polar solvents but becomes broad and bell-like shaped in the
more polar media (Figure ).
Figure 3
Solvent effect on the absorption (left) and emission (right) spectra
of NPI-PTZ5.
Solvent effect on the absorption (left) and emission (right) spectra
of NPI-PTZ5.The Stokes shift (Δυ) values were plotted as a function
of the solvent properties (f(ε, n2)) according to the McRae equation (see Figure S23 and Table S3). From the slope of the linear fits performed
on these trends, the difference between the excited- and ground-state
dipole moment (Δμ) was obtained (see Table S4). The Δμ values are significant in agreement
with the positive fluorosolvatochromism. In particular, Δμ
is generally higher for the oxygen-free compared to the oxygen-functionalized
systems (29.4, 26.9, and 26.1 D for NPI-PTZ1, NPI-PTZ2, and NPI-PTZ3, respectively), in line with the phenothiazine
being a stronger electron donor relative to phenothiazine-5-oxide
and phenothiazine-5,5-dioxide. An estimate of the frequency-dependent
(βCT) and frequency-independent (β0) hyperpolarizability was obtained through the Oudar equation. For
the dipolar molecules, the obtained β0 is roughly
the same, around 60 × 10–30 esu–1 cm5. Enhanced hyperpolarizabilities are found for the
quadrupolar molecules, particularly for the oxygen-functionalized
chromophore (β0 = 146 × 10–30 esu–1 cm5 for NPI-PTZ5 and 71.2 × 10–30 esu–1 cm5 for NPI-PTZ4). Thus, the effect of the oxygen
functionalization of the phenothiazine is positive not only on the
linear but also on the nonlinear optical properties of these molecules.The effect of the solvent on the fluorescence quantum yield was
also investigated (Table ). A completely different behavior has been observed for the
oxygen-free relative to the oxygen-functionalized molecules. In the
case of NPI-PTZ1 and NPI-PTZ4, the fluorescence
quantum yield drastically decreases upon increasing the solvent polarity:[44,45] it is reduced by 2 orders of magnitude on going from Tol to DMF.
In contrast, the fluorescence efficiency is significant in all the
investigated solvents for the oxygen-functionalized molecules. For NPI-PTZ1, an apparent viscosity effect is revealed: ϕF is 1 order of magnitude higher in the viscous BCN solvent
relative to other solvents of similar polarity. This suggests that
structural rearrangements may occur during excited-state deactivation.
Table 2
Fluorescence Quantum Yields of the
Investigated Compounds in Solvents of Different Polarity and Viscosity
solvent
f(ε, n2)
η (cPs)
ϕF NPI-PTZ1
ϕF NPI-PTZ2
ϕF NPI-PTZ3
ϕF NPI-PTZ4
ϕF NPI-PTZ5
Tol
0.0242
0.59
0.51
0.87
0.70
0.66
0.75
Tol/An 50:50
0.143
1.16
0.20
1.27
1.16
0.47
0.73
CHCl3
0.293
0.58
0.017
0.92
0.90
0.12
0.76
EtAc
0.400
0.46
0.0075
0.90
0.93
0.099
0.73
THF
0.441
0.55
0.0079
0.87
0.89
0.067
0.89
DCM
0.474
0.45
0.0030
0.99
0.92
0.049
0.85
BCN
0.586
1.24
0.029
1.06
1.14
0.048
0.73
Ac
0.651
0.32
0.0015
0.50
0.90
0.047
0.68
DMF
0.664
0.92
0.0013
0.15
0.71
0.0084
0.44
Ultrafast Spectroscopic
Investigation of the Intramolecular
Charge Transfer
The singlet excited-state dynamics of the
NPI-PTZ molecules was investigated via femtosecond resolved spectroscopies,
such as fluorescence up conversion and transient absorption. Figure shows the results
of the broadband fluorescence up-conversion measurements carried out
in a nonpolar solvent (Tol) for the oxygen-free NPI-PTZ4 and the oxygen-functionalized NPI-PTZ5, as representative
examples. The exhibited behavior is very different in the two cases.
For NPI-PTZ4, a significant red shift of the time-resolved
emission spectra was observed. The emission spectrum is slightly structured
right after light absorption and becomes bell-like shaped at longer
delays. In the case of NPI-PTZ5 in Tol, a structured
spectrum was recorded at all delays after excitation, and its maximum
does not significantly shift in time. These findings indicate that
in the case of NPI-PTZ4, a population dynamics between
two distinct excited states (the locally excited state, S1(LE), and an intramolecular charge-transfer state, S1(ICT))
may be already operative in a nonpolar solvent. On the other hand,
for NPI-PTZ5, only the S1(LE) state is involved
in the excited-state deactivation in Tol. Details about the results
of the global analysis are given in panel C of Figure and in Table S5. The transient assignments are confirmed by the transient absorption
measurements carried out for the same molecules in Tol (Figure S24 and Table ). Whereas for NPI-PTZ4 an evolution
in time of the transient absorption spectra is observed, the spectra
obtained for NPI-PTZ5 do not change their shape but just
show a decay with time. In the case of NPI-PTZ4, the
spectral shape recorded right after excitation evolves to give a transient
spectrum characterized by two ESA bands at ca. 540 and 750 nm. This
spectral shape is similar to that reported in the literature for the
phenothiazine radical cation absorption.[46−48] These results
confirm that the phenothiazine is a stronger electron donor unit relative
to phenothiazine 5-oxide and phenothiazine 5,5-dioxide, so that an
ICT is observed even in a nonpolar medium for the PTZ derivatives.
The transient absorption measurements have also revealed the population
of a long-lived transient species (Inf) associated with the lowest
excited triplet state (T1) (Table ). Analogous results have been obtained for
the oxygen-free and oxygen-functionalized dipolar molecules in Tol,
and the results are collected in Tables and S5.
Figure 4
Fluorescence
up-conversion spectroscopy of NPI-PTZ4 (left) and NPI-PTZ5 (right) in Tol.
Table 3
Results of Global Analysis of the
Femtosecond Transient Absorption Data for the Investigated Compounds
in Tol and DMFa
Solv.i and Solv.d, inertial and diffusive solvation, respectively; SR, structural
relaxation.
Fluorescence
up-conversion spectroscopy of NPI-PTZ4 (left) and NPI-PTZ5 (right) in Tol.Solv.i and Solv.d, inertial and diffusive solvation, respectively; SR, structural
relaxation.In all cases,
an evolution in time of both the transient emission
and absorption spectra is found in a more polar solvent. This is clearly
shown in Figure for
the case of NPI-PTZ5 in DMF, as a representative example
(see also Figure S25). The S1(LE) to S1(ICT) population dynamics takes place within
the inertial solvation in a polar solvent (Table ). For the case of NPI-PTZ4 and NPI-PTZ5, the ultrafast measurements were carried out in several
solvents of different polarity (Table S6), and the ICT rate solvent dependence was analyzed in the context
of the Marcus theory (see Table S7 and Figure S26).[49] A detailed analysis of the
results in Table allows
discussion of another important difference in the behavior of the
oxygen-free versus the oxygen-functionalized compounds. The S1(ICT) lifetime is long in Tol (a few nanoseconds) and becomes
extremely short in DMF for the oxygen-free derivatives (5.5 and 11
ps for NPI-PTZ1 and NPI-PTZ4, respectively).
On the other hand, the lifetime revealed for the S1(ICT)
state of the oxygen-functionalized molecules remains long in DMF (1.3–3.8
ns). This difference suggests a twisted intramolecular charge-transfer
nature (TICT) of the relaxed excited state for the oxygen-free molecules,
which are relatively more flexible. Differently, the S1(ICT) state produced for the oxygen-functionalized molecules in polar
solvents should show a planar structure (PICT) in each of the butterfly
branches, as suggested by the long lifetimes and significant fluorescence
quantum yields. Therefore, the oxygen functionalization changes the
nature of the populated ICT state (TICT for oxygen-free and PICT for
oxygen-functionalized compounds) with important consequences for the
emission features (see Chart ).
Figure 5
Femtosecond fluorescence up-conversion (left) and transient absorption
(right) spectroscopy of NPI-PTZ5 in DMF.
Chart 2
Sketch of the Excited-State Dynamics of Oxygen-Functionalized (NPI-PTZ5) and Oxygen-Free (NPI-PTZ4) Molecules
in Nonpolar and Polar Solvents
Femtosecond fluorescence up-conversion (left) and transient absorption
(right) spectroscopy of NPI-PTZ5 in DMF.It is also interesting to compare the behavior of analogous dipolar
and quadrupolar systems. The absorption spectra of the quadrupolar
derivatives are generally red-shifted relative to those of the dipolar
analogues. The emission spectra are structured and different in Tol;
however, they show a surprising coincidence in polar solvents such
as DMF, as shown in Figure for the case of the phenothiazine-dioxide derivatives. This
result suggests the occurrence of excited-state symmetry breaking
(ESSB) for the quadrupolar compound in polar solvents.[50−53] The broadband fluorescence up-conversion measurements give a deep
insight into the dynamics of this ESSB (Figures and S27). The
analogous spectral evolution in time observed for the phenothiazine-dioxide
dipolar and quadrupolar derivatives in DMF indicates that this SB
is ultrafast for NPI-PTZ5, within the occurrence of inertial
solvation (evolution associated spectra, EAS1 in Figure ).
Figure 6
Comparison between the
steady-state absorption and emission spectra
(left) and between the EAS obtained by global analysis of the fluorescence
up-conversion data (right) of NPI-PTZ3 and NPI-PTZ5 in DMF.
Comparison between the
steady-state absorption and emission spectra
(left) and between the EAS obtained by global analysis of the fluorescence
up-conversion data (right) of NPI-PTZ3 and NPI-PTZ5 in DMF.
Triplet Properties
The triplet excited-state dynamics
was investigated via nanosecond transient absorption. The obtained
transient spectra in Tol are shown in Figure (NPI-PTZ2 is not shown for
the lower signal/noise ratio). The transient absorption maximum of
the broad positive band detected is at ca. 535 nm for NPI-PTZ1, and it is blue-shifted for the phenothiazine-oxide and dioxide
monobranched derivatives (490 nm). The absorption band is slighthly
red-shifted when passing to the quadrupolar analogues (600 nm for NPI-PTZ4 and 500 nm for NPI-PTZ5). The lifetime
of this transient species is hundreds of nanoseconds in air-equilibrated
solution and tens of microseconds in nitrogen-purged solutions (Table ). The same transient
absorption signals could be produced by energy transfer in sensitization
experiments where 2,2′-dithienyl ketone (DTK)[54−56] was employed as the high-energy triplet donor and NPI-PTZ1/NPI-PTZ3 were employed as the triplet energy acceptors
(see Figures S28–S30). The oxygen
effect on the lifetime and the sensitization experiments demonstrate
that the revealed transient absorption is relative to the T1 state. The triplet lifetime (τT,N2 in Table ) decreases with the
oxygen functionalization (65 μs for NPI-PTZ1, 34
μs for NPI-PTZ2, and 25 μs for NPI-PTZ3) and in the quadrupolar structures (42 μs for NPI-PTZ4 and 33 μs for NPI-PTZ5).
Figure 7
Nanosecond transient
absorption spectra recorded for the investigated
compounds in nitrogen purged toluene.
Table 4
Triplet Properties Obtained by Nanosecond
Transient Absorptiona
compound
solvent
λT (nm)
τT,air (ns)
τT,N2 (μs)
ϕT
ϕΔ
NPI(56)
Tol
410
370
9.3
0.95
1.12
NPI-PTZ1
Tol
535
266
65
0.31
NPI-PTZ2
Tol
490
34
0.15
NPI-PTZ3
Tol
490
372
25
0.31
NPI-PTZ4
Tol
600
230
42
0.19
NPI-PTZ5
Tol
500
168
33
0.22
Singlet oxygen quantum yields
(ϕΔ) obtained by means of phosphorescence measurements
employing phenalenone in Tol (ϕΔ = 0.99) as
a reference compound.
Nanosecond transient
absorption spectra recorded for the investigated
compounds in nitrogen purged toluene.Singlet oxygen quantum yields
(ϕΔ) obtained by means of phosphorescence measurements
employing phenalenone in Tol (ϕΔ = 0.99) as
a reference compound.The
involvement of the triplet excited state in the deactivation
has also been investigated by means of singlet oxygen phosphorescence
measurements (Figure S31). These experiments
allowed quantitative determination of the singlet oxygen quantum yields
(ϕΔ), which may be considered estimates of
the triplet yields (ϕT). The ϕΔ values measured in Tol are reported in Table , together with the yield measured for the
parent NPI compound, whose ϕT was previously obtained
through triplet sensitization.[57] The good
agreement between the NPI triplet (ϕT = 0.95) and
singlet oxygen (ϕΔ = 1.12) yields, within the
experimental error, indicates the reliability of our method. The ϕΔ obtained for the NPI-PTZ compounds in Tol are between
19 and 31%. These values show a trend consistent with the ϕF in Tol (Table ). For the oxygen-functionalized compounds, we find that ϕF + ϕΔ ≈ 1; this suggests that
the excited-state deactivation in a nonpolar solvent is justified
considering just the fluorescence and intersystem crossing. For the
oxygen-free compounds, ϕF + ϕΔ < 1; this points to a role played by internal conversion to the
ground state from the ICT state even in a nonpolar medium.
Conclusions
We have designed and synthesized 1,8-naphthalimide-based phenothiazine
derivatives, both with dipolar and quadrupolar structures, in which
we have increased the oxidation state of the sulfur atom on the phenothiazine
unit by one or two oxygen functionalizations. Our results show that
the oxygen substitution as well as the quadrupolar structural motif
have a positive impact on the optical (absorption extinction coefficient
and fluorescence efficiency) and nonlinear optical (hyperpolarizability)
properties of these new organic materials. The nanosecond time-resolved
spectroscopic experiments revealed a certain involvement of the lowest
triplet excited state in the deactivation (intersystem crossing) of
these compounds. The electrochemical study demonstrates that the phenothiazine5-oxide and phenothiazine 5,5-dioxide show reduced electron-donating
ability relative to the phenothiazine unit. The quantum chemical simulations
predicted HOMOs localized on the phenothiazine strong donor unit for
the oxygen-free derivatives and delocalized over the whole molecular
structure for the oxygen-functionalized derivatives, with the LUMOs
being localized on the naphthalimide strong acceptor in all cases.
The ultrafast spectroscopic measurements, transient absorption, and
fluorescence up conversion uncovered the detailed intramolecular charge-transfer
mechanism, subtly tuned by the molecular structure of these push–pull
chromophores. In particular, a planar intramolecular charge transfer
(PICT) and thus highly fluorescent state was populated upon photoexcitation
of the sulfoxide and sulfone-based compounds in polar solvents. Differently,
the relaxed singlet state exhibits a twisted intramolecular charge
transfer (TICT) nature in the case of the oxygen-free phenothiazines,
leading to significant fluorescence quenching. The advanced broadband
fluorescence up-conversion spectroscopy data suggest that the photoinduced
ICT occurs by breaking the excited-state symmetry in the quadrupolar
chromophores. Our results show that synthetically tuning the sulfur
oxidation state in these molecules leads to either highly efficient
intramolecular charge transfer (phenothiazine derivatives) or highly
efficient emission (phenothiazine-oxide and dioxide derivatives).
These findings establish oxygen functionalization as a new effective
synthetic strategy to tailor the photophysics of phenothiazine-based
organic materials for different optoelectronic applications. The new
materials here thoroughly investigated for their optical and photophysical
properties are thus promising for either photon-to-current conversion
applications in organic photovoltaics (oxygen-free compounds) or current-to-photons
applications in organic light-emitting diodes (oxygen-functionalized
compounds).
Authors: Alena Khadieva; Mansur Rayanov; Ksenia Shibaeva; Alexandr Piskunov; Pavel Padnya; Ivan Stoikov Journal: Molecules Date: 2022-05-08 Impact factor: 4.927
Chart 1
Scheme 1
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