Photo-switchable iron-terpyridine complexes functionalized with styrylbenzene unit

Article abstract of DOI:10.1016/j.jphotochem.2020.113059

The manuscript deals with synthesis, characterization, photophysics, and reversible trans-cis photoisomerization behaviours of three homoleptic Fe(II)-terpyridine complexes, ([Fe(tpy-pvp-X)₂]²⁺ where X[dbnd]H, Me, and NO₂)

Full text of DOI:10.1016/j.jphotochem.2020.113059

Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photo-switchable iron-terpyridine complexes functionalized with
styrylbenzene unit
Shruti Mukherjee, Poulami Pal, Anik Sahoo, Sujoy Baitalik*
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The manuscript deals with synthesis, characterization, photophysics, and reversible trans-cis photoisomerization
behaviours of three homoleptic Fe(II)-terpyridine complexes, ([Fe(tpy-pvp-X)2]2+ where X H, Me, and NO )
–
–
Iron
2
Terpyridine-styrylbenzene
Photo-isomerization
Emission switching
DFT and TD-DFT
covalently coupled with photo-active styrylbeneze moiety. The complexes underwent trans-trans to cis-cis
isomerization upon the action of both visible and UV light with remarkable change in their absorption and
emission spectral profiles. The isomerization studies were performed in four different solvents, viz. DCM, MeCN,
MeOH and DMSO with both UV and visible light sources. The reversal from cis-cis to trans-trans isomerization
also proceeds, albeit very slowly, on keeping and can be accelerated upon heating. The rate, rate constant and
quantum yield of photoisomerization were determined in all the solvents. The rate and quantum yield of
isomerization found to be much higher in presence of visible light source in all solvents than that of UV light. To
better understand the electronic structures and correctly assign the experimental optical spectral bands, DFT and
TD-DFT computational studied were also carried out in their trans-trans, trans-cis and cis-cis forms. Reasonably
good correlation between the experimental and calculated bands could lead us to assign the main absorption
spectral bands of the compounds.
1. Introduction
In order to fulfill our objective, we report herein the synthesis,
characterization, photophysics, and reversible trans-cis photo-
Functional molecules with switchable spectral properties induced by
light have received great attention due to their crucial roles in the design
of optical materials, photoswitches and memory devices [1–6]. To this
end, photoisomerization behaviours of a wide variety of compounds
such as azobenzenes, stilbenes, diarylethenes, spiropyrans, and spi-
rooxazines were thoroughly studied [7–12]. Transition metal based
compounds are very attractive in this regard over their organic coun-
terparts as the complexes possess additional features in the context of
better tunability of their electronic properties by varying both inorganic
and organic components [13–24]. For coordination complexes, modu-
lation of their properties by inducing conformational changes on the
photo-responsive component appended on ligands has received most of
the attentions. Majority of studies in this regard are mainly centered on
noble metals which are very expensive and their synthetic procedures
often required very drastic conditions [13–24]. Our aim in this work is to
design suitable base-metal complexes which could exhibit similar
behavior and can lead to the development of a new class of low-cost and
easily synthesizable photo-switches.
isomerization behaviours of three homoleptic Fe(II)-terpyridine com-
plexes ([Fe(tpy-pvp-X)2]2+, where X H, Me, and NO ) covalently
–
–
coupled with photo-active styrylbeneze moiety (Chart 1 2). Iron is the
most abundant low cost element in Earth’s crust and also plays crucial
role in transport and storage of oxygen and in electron transport in
diverse metalloenzymes of living organisms [25–27]. Compared to their
low spin d6 analogues (such as Ru2+ and Os2+), Fe(II)-polypyridine
complexes usually don’t function as effective sensitizers because of
low lying 3/5MC states localized on Fe(II) induces very fast non-radiative
deactivation leading to remarkable lowering of excited state lifetime
[28–35]. Although the Fe(II)-terpyridine complexes don’t exhibit Fe
(II)-centred emission but they exhibit strong 1MLCT absorption at
longer wavelength region (~575 nm) compared with both Ru(II)- and
Os(II)-terpyridine motif [37,32–34]. Photoinduced isomerization
studies of styrylbenzene appended heavier transition metal complexes
(particularly Re, Ru, Ir and Pt) have been performed by several research
groups in recent times [36–43]. These studies contribute significantly
for designing of photochemical molecular devices [44–52]. On the other
* Corresponding author.
E-mail address: sujoy.baitalik@jadavpuruniversity.in (S. Baitalik).
https://doi.org/10.1016/j.jphotochem.2020.113059
Received 14 October 2020; Received in revised form 24 November 2020; Accepted 28 November 2020
Available online 2 December 2020
1010-6030/© 2020 Elsevier B.V. All rights reserved.

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
The rate constant of isomerization process in all solvents were
assessed from absorption titration data by following Eq. 1 [58,59].
ln{(A0 -A∞)/(At-A∞)}= kiso
t
(1)
where A0, At, and A∞ show the absorbance at time t = 0, t, and ∞,
respectively. kiso is rate constant of isomerization and t is the time
required for execution of isomerization process. Both rate constant (kiso
)
and absorption at ∞ time (A∞) were estimated by nonlinear least-square
method. The light source intensity of the lamp was 0.11 W. Quantum
̃
yields (φ) of isomerization process were derived by using the Eq. 2 [60],
ν
= (φI0/V)(1ꢀ 10ꢀ Abs
)
(2)
where v is the rate of trans-to-cis isomerization, I0 is the photon flux at
the front of the cell, V is the volume of the solution, and Abs is the initial
absorbance at the irradiation wavelength. We have estimated the photo-
isomerization quantum yield of complexes by a relative method using
azobenzene which was extensively studied in literature. We performed
photoisomerization experiments of azobenzene in our experimental
setup and upon considering quantum yield of trans to cis photo-
isomerization of azobenzene in MeOH as 0.15 according to literature
report (wavelength of irradiation is 334 nm) [61–62], we have calcu-
lated the quantum flux. Then by using quantum flux, we have calculated
the quantum yield of photoisomerization of the present complexes by
using Eq. 2.
Chart 1. Chemical structures of the complexes.
hand, related studies incorporating styrylbenzene appended terpyridine
ligands and 3d metals are relatively sparse in literature.
Photo-isomerization behaviours of selected Fe(II) complexes based on
terpyridine-azobene conjugate were reported by Nishihara and
co-workers [53–53–54]. Prior to this work, only a single report on
photo-isomerization behaviour of one Fe(II)-terpyridine complex cova-
lently coupled with styrylbenzene moiety was reported by Araki and
co-workers [55]. But to our knowledge, no detailed discussions on op-
tical switching behaviours of Fe(II)-terpyridine complexes covalently
coupled with photo-active styrylbeneze moiety were reported in the
literature. In the present work, we thoroughly studied the effect of sol-
vents and influence of both electron donating and electron withdrawing
substituent (X) on the photophysics as well as thermodynamic and ki-
netic aspects of isomerization process of complexes. Effect of excitation
wavelength on thermodynamic and kinetic aspects of photo-
isomerization process were also addressed in this work. Recently, we
reported photophysics and photoisomerization behaviours of a homo-
2.3. Computational investigation
Computational details were also provided in the electronic supple-
mentary information.
3. Results and discussions
3.1. Synthesis and characterization
leptic ([Ru(tpy-pvp-X)2]2+
)
as well as
a
heteroleptic series
([(tpy-PhCH3)Ru(tpy-pvp-X)]2+) of Ru-terpyridine complexes with
same styrylbenzene-terpyridine ligands [56–56–57]. In this study, we
will also be interested to compare the photo-isomerization behaviours of
present Fe(II) complexes with previously reported analogous complexes
of Ru(II). Finally, DFT and TD-DFT calculations were also performed on
various forms of the complexes (trans-trans, trans-cis and cis-cis) to get
insight about the electronic structures as well as for appropriate
assignment of their optical spectral band.
The ligands, tpy-pvp-X (X = H, Me and NO2) were prepared upon
treating 4′-(2,2′:6′,2′′-terpyrididyl-4)-benzyltriphenyl phosphonium
bromide (tpyPhCH2PPh3Br) with 4-substituted benzaldehyde in
dichloromethane within the temperature range of 0ꢀ 5 ◦C under argon
protection and characterized by our reported procedure [56–57,63].
The complexes were synthesized by reacting tpy-pvp-X (X = H, Me and
NO2) with Fe(ClO4)2 (1:2 ratio) in CHCl3-MeOH (1:1, v/v) mixture at
room temperature. Purification of the complexes were carried out by
alumina column chromatography {1:10 (v/v) PhCH3-MeCN mixture}
followed by recrystallization from CHCl3-MeOH (1:2, v/v) mixture. All
complexes were characterized by elemental (C, H and N) analyses, high
resolution mass and NMR spectral measurements and characterization
data were presented in electronic supplementary information (Fig.
S1-S6, Supplementary information).
2. Experimental
2.1. Materials
Chemicals and solvents were procured either from Sigma or from
local vendors. Synthesis and characterisation of tpy-pvp-X (X = H, Me,
and NO2) were accomplished by our reported procedure [56]. Detailed
procedure for synthesis, purification and characterization of Fe(II)
complexes are provided in electronic supplementary information.
3.1.1. NMR spectra
1H NMR spectra of 1–3 were acquired in CD3CN and displayed in
Fig. 1. Tentative assignments of all peaks were done with the help of
their COSY spectra together with by comparing the spectra of structur-
ally similar complexes. The singlet at ~2.31 ppm (Fig. S5, Supplemen-
tary information) counting three protons for 2 is clearly due to ꢀ CH3
group of coordinated tpy-PhCH3 moiety. Another singlet which appears
within 9.17–9.24 ppm corresponds to H3′ proton. A pair of doublets
within 7.16–7.90 ppm is assignable as the protons of ethylenic double
bond (H9 and H10) and corresponds to trans-trans conformation. In some
cases, they appeared within the broad multiplet because of the coinci-
dence of other protons.
2.2. Physical measurements
2.2.1. Determination of trans-cis photoisomerization rate constant and
quantum yields
In the photoisomerization measurements, a 1-cm light path length
quartz cell was used. The concentration of solution was within the range
of 1 × 10ꢀ 5 M - 2 × 10ꢀ 5 M and thoroughly degassed with N2 before the
experiments. The whole isomerization processes were carried out in
Lelesil photocatalytic reactor which was designed by Lelesil Innovative
Systems. Here we used both ultraviolet and visible light sources. The
wave length used for UV source was 334 nm and for visible it was
436 nm.
3.2. Computational investigations
Geometry optimization of the complexes was carried out with the aid
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S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Fig. 1. 1H NMR (400 MHz) spectra of [Fe(tpy-pvp-H)2]2+ (a), [Fe(tpy-pvp-Me)2]2+ (b) and [Fe(tpy-pvp-NO2)2]2+ (c) in CD3CN.
of Gaussian 09 program in acetonitrile medium (Fig. S7, Supplementary
information). Selected bond distances and angles are given in Tables S1-
S4 (Supplementary information). In all complexes, Fe(II) is coordinated
in bis-tridentate manner with distorted octahedral geometry. Frontier
molecular orbital sketch are presented in Figs. S8-S10 (Supplementary
information). Among four HOMOs, the first two (HOMO and HOMO-1)
are mainly composed of vinyl phenyl and p-substituted phenyl group,
while the other two HOMOs (HOMO-2 and HOMO-3) are mainly
localized on Fe(II) centre on all of their trans-trans, trans-cis and cis-cis
forms. On the other hand, all four LUMOs are composed predominantly
of tpy moiety with small contribution of vinyl phenyl group with the
exception of [Fe(tpy-pvp-NO2)2]2+, where LUMOs are mainly localized
either on nitrobenzyl group or on tpy moiety (Table S5 and Fig. S10,
Supplementary information).
energy band(s) spanning with 368ꢀ 376 nm is due to phenyl--
vinyl→terpyridine charge transfer transitions. Multiple very intense
peaks within UV region arise from ligand centred π-π* transitions. It is
observed that maximum of MLCT and LLCT band in the complexes varies
to a small extent depending upon electronic nature of the substituent, X.
A large disagreement between experimental and theoretical absorption
spectra is noticed in Fig. 2. In particular, the presence of a large band
within the spectral domain of 400ꢀ 600 nm in the computed spectra is
not reflected in the experimental spectra. This disagreement gives an
indication of an inappropriate level of calculations for 1st row transition
metal complexes. Our method is based on very limited basis sets leading
to very localized Kohn-Sham orbitals that do not necessary reflect the
real electronic densities in play.
On excitation at the lowest energy absorption band (~575 nm),
complexes do not exhibit any luminescence. The photophysics of Fe(II)-
polypyridine complexes gets complicated by the presence of low-lying
metal-centered (3/5MC) excited states which get populated upon sur-
face crossing from MLCT state within ultrafast time domain [28–35].
This phenomenon is frequently happened in Fe(II)-polypyridine com-
plexes which is responsible for their non-emitting characteristics
TD-DFT calculated results of the complexes in their trans-trans, trans-
cis and cis-cis forms are summarized in Table S6-S8 (Supplementary
information) and the involvement of FMOs in their lowest energy band
are displayed in Fig. S11 (Supplementary information). The lowest en-
ergy band for Fe(II) complexes is an admixture of FeII(d
π)→π*(tpy-pvp-
X) metal-to-ligand charge transfer (MLCT) and phenyl-vinyl-
→terpyridine charge transfer (LLCT) transitions in the visible region.
The next higher energy band is found to be an admixture of both LLCT
[28–35]. Upon excitation at the LLCT or
π-π* band, the complexes
display an intense emission band in spectral domain of 463ꢀ 503 nm,
probably due to LLCT transition (Fig. S12, Supplementary information).
It is to be mentioned here that the free ligands, upon excitation at their
LLCT band (spanning within the range of 325ꢀ 361 nm), display intense
emission band in the wavelength range of 398ꢀ 420 nm. Thus, coordi-
nating influence of Fe2+ leads to red-shift of ligand-centered emission in
the Fe(II)-terpyridine complexes.
and π-π* transitions with the exception of the nitro-derivative (3), where
substantial MLCT character is also observed.
3.3. Absorption and emission spectra
Comparison between the calculated and experimental absorption
spectra of the complexes in acetonitrile are presented in Fig. 2. All
In-situ formation of Fe(II) complexes were also monitored through
absorption and emission spectroscopy (Fig. 3. and Fig. S13-S14, Sup-
plementary information). It is observed that a new band is evolved at
~575 nm and intensity of the band increases linearly with Fe2+ addition
till the [Fe2+]/[tpy-pvp-X] ration reaches 0.5. Addition of Fe2+ beyond
0.5 equiv does not produce any further change (Fig. 3a and Fig. S13a-
S14a, Supplementary information). The titration profile based on
absorbance at 575 nm and several clean isosbestic points imply single
complexes show a very intense band at ~575 nm due to FeII(d
π)→π*
(tpy-pvp-X) MLCT transition. TD-DFT calculations also indicate finite
contribution of phenyl-vinyl→terpyridine charge transfer (LLCT) char-
acter to the said MLCT band. The band is shifted to lower energy region
compared with the parent [Fe(tpy)2]2+ (551 nm) complex probably
because of charge delocalization induced by additional phenyl-vinyl
group at 4′-position of terpyridine moiety [51–56]. The next higher
3

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
3.4. Photo-isomerization behaviours
All three complexes possess two isomerizable strylbenzene units. We
are thus interested to investigate their spectral behaviours upon action
of light. We carried out photoisomerization studies of the complexes in
few selected solvents, viz. dichloromethane, acetonitrile, methanol and
dimethysulfoxide at room temperature (25 ◦C). In addition, both UV
(334 nm) and visible (436 nm) light source were used for irradiating the
solutions of the complexes. At first, isomerization studies are carried out
in dichloromethane in presence of UV light. The intensity of both MLCT
band at ~575 nm and LLCT at ~375 nm was found to decrease to a small
extent for 1 and 2, whereas increase for the nitro-derivative (3) (Fig. 4a-
c.). In the emission side, photo-irradiation induces remarkable decrease
of ligand-centred emission band intensity at ~500 nm with the excep-
tion of nitro-derivative (3) where small enhancement is noticed (Fig. 4d-
f).
It is observed that prolonged irradiation leads to gradual decrease of
both MLCT and LLCT bands in all three complexes with concomitant
increase of π-π* band intensities in the UV region and eventually the
MLCT band is completely removed which is indicative of de-
coordination of Fe2+ centre from the complex backbone (Fig. S15-S17,
Supplementary information). The final spectrum in each case looks very
similar to that of the photolyzed product (cis-form) of free ligand [63].
Continued photo-irradiation, also leads to gradual increase of emission
intensity in the second step and finally reaches at saturation at the end of
photolysis (Fig. S15b-S17b, Supplementary information). Enhancement
of emission intensity in the second step is again indicative of
de-coordination of Fe2+ from complex architecture. In presence of
visible light source, we observed almost similar trend (with small vari-
ation of spectral profile) but the rate is much faster than with UV light
(Fig. S18-S19, Supplementary information). Thus, in dichloromethane,
the first-step change is due to trans→cis isomerization while the
second-step change corresponds to de-coordination Fe2+ from complex
architecture.
As photo-irradiation, irrespective of irradiation wavelength, of DCM
solution of the complexes leads to de-coordination, we are interested to
see the behaviours of the complexes in other solvents. To this end, we
have taken a representative complex, [Fe(tpy-pvp-H)2]2+ and carried
out isomerization experiments in three additional solvents, viz. aceto-
nitrile, methanol and dimethysulfoxide. Absorption and emission spec-
tral changes of [Fe(tpy-pvp-H)2]2+ upon action of visible light are
presented in Fig. 5–7, while in presence of UV light are displayed in Fig.
S20-S22 (Supplementary information). In contrast to the behaviours in
DCM, all the three complexes exhibit a distinct one-step spectral change
in presence of both UV and visible light source and no signature of Fe2+
de-coordination is observed even after photolysis of more than three
hours. One-step spectral change is observed in all three solvents,
although the pattern and extent of change differ slightly from each
Fig. 2. Overlay of the calculated (dotted lines) and experimental (solid lines)
UV–vis absorption spectra of trans-[Fe(tpy-pvp-X)2]2+ with X= H (1), Me (2),
and NO2 (3) in acetonitrile. Calculated results are also presented in the
sticks form.
conversion of free tpy-pvp-X to [Fe(tpy-pvp-X])2]2+. The composition of
the complexes was also confirmed by high resolution mass spectra. The
change of emission intensity of the LLCT band on gradual addition of
Fe2+ is delineated in Fig. 3b and Fig. S13b-S14b (Supplementary infor-
mation) and the insets show that complete quenching of emission occurs
upon addition of 0.5 equiv of Fe2+ ion.
Fig. 3. UV–vis absorption (a) and emission (λex =330 nm) (b) spectral changes of tpy-pvp-H in dichloromethane upon incremental addition of Fe2+. Inset to figure (a)
shows the change of absorbance at 575 nm, while inset to figure b indicates the change in emission intensity at 398 nm vs. equivalent of Fe2+
.
4

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Fig. 4. Absorption and emission (λex =330 nm) spectral change of [Fe(tpy-pvp-H)2]2+ (a and d, respectively), [Fe(tpy-pvp-Me)2]2+ (b and e, respectively) and [Fe
(tpy-pvp-NO2)2]2+ (c and f, respectively) in dichloromethane upon irradiation with UV light. Insets to the figures a-f indicate the irradiation time.
Fig. 5. UV–vis absorption (a) and emission (λex =330 nm) (b) spectral changes of [Fe(tpy-pvp-H)2]2+ (1) in acetonitrile upon irradiation with visible light. Inset to
figure (a) indicates the irradiation time.
5

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Fig. 6. UV–vis absorption (a) and emission (λex =330 nm) (b) spectral changes of [Fe(tpy-pvp-H)2]2+ (1) in dimethysulfoxide upon irradiation with visible light.
Inset to figure (a) indicates the irradiation time.
Fig. 7. UV–vis absorption (a) and emission (λex =330 nm) (b) spectral changes of [Fe(tpy-pvp-H)2]2+ (1) in methanol upon irradiation with visible light. Inset to
figure (a) indicates the irradiation time.
other. Furthermore, the extent of change is much greater with visible
light than that of UV light. The spectral change in MeCN and DMSO is
very neat and well defined compared with MeOH in presence of visible
light. Gradual decrease in intensities for MLCT and LLCT band occurred
and at their expense concomitant increase of π-π* band intensities are
observed in both solvents. Well defined isosbestic points are observed in
each case. By contrast, the extent of change in MeOH is much less. The
intensity of the ligand-centred emission band at ~400 nm for [Fe(tpy-
pvp-H)2]2+ in MeCN gradually quenched and at the end of photolysis,
the band is substantially red-shifted to ~500 nm. In DMSO, similar
quenching of emission takes place but no red-shift of the band is noticed.
The emission spectrum in MeOH is different from both MeCN and
DMSO. Instead of quenching, substantial emission enhancement is
observed in MeOH in presence of both UV and visible light source.
Well-defined spectral changes (both absorption and emission) clearly
indicate the occurrence of isomerization across the double bond. But we
are not very sure whether isomerization taking place from trans-trans to
trans-cis or to cis-cis form. In spite of our low level of calculations, TD-
DFT results indicate that the MLCT band of trans-cis form is red-shifted
compared with their trans-trans form, while blue-shifted in case of cis-
cis forms. In the final form of our experimental absorption spectra, the
MLCT band also gets blue shifted Fig. 8 and Fig. S23 (Supplementary
information). That’s why we speculate that trans-trans to cis-cis con-
version are occurring for these complexes upon photo-irradiation. While
going from trans-trans to cis-cis form of the complexes, obviously there is
an intermediate trans-cis state. But we are unable to locate trans-cis form
Fig. 8. Overlay of the calculated (dotted lines) and experimental (solid lines)
absorption spectra of trans-trans (black), trans-cis (blue) and cis-cis (red) form of
[Fe(tpy-pvpꢀ Me)2]2+ in acetonitrile.
during the course of isomerization. The reverse process that is isomeri-
zation from cis-cis to trans-trans form also proceeds very slowly on
keeping and accelerated upon heating the solution of the complexes. The
complexes are heated around 40 ◦C in each of three solvents. The
reversible changes are very prominent and we got cis-cis to trans-trans
form of the complexes (Fig. 9, Fig. S24-S26, Supplementary
information).
It would be better if we could also monitor the isomerization process
6

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Fig. 9. UV–vis absorption (a) and emission (λex =330 nm) (b) spectral changes of [Fe(tpy-pvp-H)2]2+ (1) in acetonitrile upon keeping heating for times. Inset to
figure (a) and (b) indicates the heating time at 40 ◦C.
through NMR spectroscopy. To this end, we attempted isomerization
studies in the said solvents. Unfortunately, the rate of isomerization
process is extremely slow and we are unable to see any observable
change even after photolysis for about 12 h. Concentration of the com-
plexes for NMR experiments were 10ꢀ 3 M compared with 10-5 M used
for absorption and emission spectral studies.
to evolution of a strong band in the visible (~575 nm due to Fe(II)→tpy
MLCT transition) and induces significant quenching of ligand-centered
emission in [Fe(tpy-pvp-X)2]. Photo-isomerization rate constants of Fe
(II) complexes retarded dramatically (varying between 0.8 and
32.4 × 10-4 s-1, depending upon irradiating wavelength and nature of
solvents). This is a consequence of the presence of a low lying MLCT
electronic excited state that efficiently quenches the higher energy
phantom p* state. In addition, presences of several low energy vibra-
tional states that are created upon complexation, restrict the molecule
from reaching the crossing point for isomerization.
The rate constant (kiso) and quantum yield (Φ) of isomerization were
calculated for both forward and backward process (thermal) in all sol-
vents (Table 1). It is noticed that kiso as well as Φ are dependent upon
irradiation wavelength as well as nature of the solvents. First of all, both
kiso and Φ was found to be much higher in presence of visible light
source in all solvents than that of UV light (Table 1). kiso is also
dependent on polarity as well as bulk viscosity of the solvents; found to
be highest in DCM and least in MeOH. The dependence of kiso and Φ with
different solvent parameters (viz. polarity index, dielectric constant, and
dipole moment) is displayed in Fig. 10 and Fig. S27-S28 (Supplementary
information). The correlation is reasonably well with the exception of
MeOH. Lesser value of kiso in more polar solvents is probably because of
greater degree of solvation leading to increase of effective rotor volume
of the complex cations. Free energy of activation (ΔG∕=) of the isomer-
ization processes (both forward and backward) were also calculated
from kiso values and by the use of Eyring’s theory. Calculated ΔG∕= values
are presented in Table 1.
It would be appropriate to give some insight about the isomerization
process. Light-induced ligand substitution of otherwise photochemically
inert complexes is not very unusual in DCM. Although [Ru(bpy)3]2+ is
photochemically inert, there is evidence that it undergoes photo-
substitution in chlorinated solvent (such as DCM). While [Ru(bpy)3]2+
as the PFꢀ6 salt is photo-inert in water but in DCM, the photochemistry of
[Ru(bpy)3]X2, (X = Clꢀ , Brꢀ , NCSꢀ ) is well behaved [64–65], giving rise
to Ru(bpy)2X2 with ϕ lying between 10-1 and 10-2. A crucial difference
between water and DCM solutions is that salts of [Ru(bpy)3]2+ are fully
ion-paired in the latter medium. The probable reason for
photo-substitution is thermally activated formation of a 3MC excited
state which induces large metal-ligand displacement in the excited state
leading to the cleavage of Ru-N bond.
Proper understanding of photo-reactivity and photophysics of first-
row transition metal complexes is still a challenge for both experimen-
talists and theoreticians. Indeed the high density of excited states of
mixed characters with important contribution of metal-centered (MC)
states lead to a rich and extremely complicated photochemistry with
various competing channels of deacativation such as luminescence, non-
radiative processes, dissociation and isomerization. Due to lack of our
expertise, we are unable to calculate appropriately the low-lying triplet
and quintet MC states which is crucial for understanding the photo-
physics of Fe(II)-polypyridine complexes. Usually the pure triplet IL
state localized on the isomerizable ligand plays a central role in the
photoisomerization processes. It should be accessible via 1MLCT/1LLCT
ΔG∕=298k = 298R{ln(kB298/h.kiso)}
(3)
We have previously reported photoisomerization behaviours of free
tpy-pvp-X ligands in presence of UV light [63]. We found that the lowest
energy LLCT absorption band within the spectral domain of
330ꢀ 361 nm gradually decreases with concomitant increase of a band
~355ꢀ 390 nm upon irradiation. Free ligands exhibit strong emission
band within 389ꢀ 420 nm which quenched substantially upon isomeri-
zation. The rate constant of trans→cis isomerization was found to vary
between 1.61 and 11.7 × 10ꢀ 3 s-1. In the present study, we observed that
photophysics and photo-isomerization of tpy-pvp-X unit gets dramati-
cally affected upon coordination with Fe2+. Coordination of Fe2+ leads
Table 1
Rate constants, quantum yield and free energy of activation of trans→cis and cis→ trans in isomerization process of [Fe(tpy-pvp-H)2]2+ in different solvents.
Solvents
Monit-oring
trans→cis (visible light)
cis → trans (thermal)
trans→cis (UV light)
cis → trans (thermal)
λ/nm
kiso×104
Φ×102
ΔG±/
kiso×105
ΔG±/
kiso×104/
Φ×102
ΔG±/
kiso×105/
ΔG±/
/sꢀ 1
kJMꢀ 1
/sꢀ 1
kJMꢀ 1
sꢀ 1
kJMꢀ 1
sꢀ 1
kJMꢀ 1
DCM
365
361
375
365
32.4
9.1
11.9
3.8
87.2
90.9
91.7
92.2
12.9
8.4
95.2
96.2
103.0
99.2
14.1
2.6
5.2
1.1
1.2
0.5
89.2
100.2
98.9
96.2
9.2
1.7
3.1
0.5
96.0
MeCN
DMSO
MeOH
100.3
98.7
5.3
2.3
5.5
2.8
4.3
1.3
2.5
0.8
103.2
7

S. Mukherjee et al.
Journal of Photochemistry & Photobiology, A: Chemistry 407 (2021) 113059
Fig. 10. Plot of rate constant (kiso) and quantum yield (Φ) vs. dielectric constant of solvents (a and b respectively) with linear least–squares fit to the data.
to 3MLCT/3LLCT ISC and/or 3MLCT/3LLCT-3IL vibronic coupling. This
could help at deciphering the competition between luminescence
/isomerization/dissociation and at understanding why solvent effects
are very important. The present data are not at maturity for pushing too
far for mechanistic interpretation for the isomerization process of pre-
sent Fe(II)-terpyridine complexes. We are still working on this and
several fundamental issues which we are unable to address presently are
open for further investigation for understanding the complete picture.
switches which in turn could be potential building blocks for informa-
tion processing and at the molecular level.
Authors statement
I am submitting the revised manuscript on behalf of all the co-
authors. All the authors are aware of the submission and no one have any
objection.
4. Conclusions
Declaration of Competing Interest
With regard to our aim for designing suitable base-metal complexes
for the development of new class of low-cost easily synthesizable photo-
switches, we designed in this work a new class of homoleptic Fe(II)-
terpyridine complexes by incorporating styrylbenzene moiety as the
photo-switchable unit and thoroughly investigated their photo-
isomerization behaviours through absorption and emission spectro-
scopic techniques. The complexes underwent trans-trans to cis-cis
isomerization upon the action of both visible and UV light with
remarkable change in their absorption and emission spectral profiles.
The isomerization studies were performed in four different solvents, viz.
DCM, MeCN, MeOH and DMSO with both UV and visible light sources.
Apart from DCM, where de-coordination of Fe2+ takes place from the
complex backbone upon prolonged light exposure, isomerization pro-
cess proceeds smoothly in all the other solvents. The reversal from cis-cis
to trans-trans isomerization also proceeds, albeit very slowly, on keeping
and can be accelerated upon heating. The rate, rate constant and
quantum yield of isomerization were determined in all the solvents in
presence of both UV and visible light source. The kiso as well as Φ is
found to be much higher in presence of visible light source in all solvents
than that of UV light. kiso and Φ is also dependent on polarity as well as
bulk viscosity of the solvents and a linear correlation is found between
either kiso or Φ and different solvent parameters (viz. polarity index,
dielectric constant, and dipole moment). The present data are not suf-
ficient for complete understanding of the mechanistic aspects for
isomerization process of Fe(II)-terpyridine complexes. Close proximity
of several excited states with mixed characters together with sizeable
contribution of metal-centered (MC) states complicate the photochem-
istry with various competitive process of relaxation, viz. emission, non-
radiative processes, dissociation and isomerization. We are still working
on this and several fundamental issues which we are unable to address
presently are open for further investigation for understanding the
complete picture. Although we are unable to present proper mechanistic
details for photoisomerization process, present Fe(II)-terpyridine com-
plexes could be useful for the construction low-cost molecular photo-
The authors declare no conflict of interest.
Acknowledgements
Financial assistance received from SERB [Grant No. CRG/2020/
001233] and CSIR [Grant No. 01(7619)/18/EMR-II], New Delhi, India
are gratefully acknowledged. P. Pal and A. Sahoo acknowledge CSIR for
their research fellowship.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2020.
113059.
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