Synthesis of fluorescent molecular switches based on porphyrinoids covalently linked with redox active ligands

Article abstract of DOI:10.1016/j.molstruc.2021.131407

5,15-bis(1-hydoxy-2,6-di-tert-butyl-phenyl)10,20-bis-(pentaphenyl-phenyl)phenyl porphyrin and its Nickel complex were synthesized. Oxidation with NOBF₄ gave 5,15bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-bis(pentaphenyl-phenyl)phenyl-porphodimethene and Ni bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-bis-(pentaphenylphenyl)phenyl porphodimethene. The porphyrin conjugates were characterized by NMR, mass, and UV–visible spectroscopy for structural elucidation. The density functional theory studies revealed a decrease in the highest occupied and the lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps in the oxo-porphodimethenes. The photochromism and fluorescence characteristics were also studied. The porphyrin moieties were found to be desirable for the reversible light-driven fluorescent molecular switches.

Full text of DOI:10.1016/j.molstruc.2021.131407

Journal of Molecular Structure 1247 (2022) 131407
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Synthesis of fluorescent molecular switches based on porphyrinoids
covalently linked with redox active ligands
∗
Kharu Nisa, Gaurav Kumar Mishra, M. Thirumal, Shive M.S. Chauhan
Department of Chemistry, University of Delhi, New Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
5,15-bis(1-hydoxy-2,6-di-tert-butyl-phenyl)10,20-bis-(pentaphenyl-phenyl)phenyl porphyrin and its Nickel
complex were synthesized. Oxidation with NOBF4 gave 5,15bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-
bis(pentaphenyl-phenyl)phenyl-porphodimethene and Ni bis(1-oxo-2,6-ditertairy-butyl-phenyl)10,20-bis-
Received 26 June 2021
Revised 27 August 2021
Accepted 29 August 2021
Available online 2 September 2021
(
pentaphenylphenyl)phenyl porphodimethene. The porphyrin conjugates were characterized by NMR,
mass, and UV–visible spectroscopy for structural elucidation. The density functional theory studies re-
vealed a decrease in the highest occupied and the lowest unoccupied molecular orbital (HOMO-LUMO)
energy gaps in the oxo-porphodimethenes. The photochromism and fluorescence characteristics were also
studied. The porphyrin moieties were found to be desirable for the reversible light-driven fluorescent
molecular switches.
Keywords:
Redox chemistry
Photochromism
Substituent effects
Pentaphenyl-phenyl- benzene
Porphyrinoids
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
lation of the π-π interactions would lead to so-called ‘‘toroidal
delocalization [22–28]. (Scheme 1). The intramolecular electron
transfer between redox active porphyrin center and 2,6-di-tert-
butyl-phenol is a new promising mechanistic archetype for the de-
sign of 2-qubit molecular systems. By choosing redox-active lig-
ands, one can tune up the electronic configuration of the mixed
ligand complexes to either low or high spin state structures.
Mixed ligand complexes are a synthetic challenge for tuning the
properties of transition metal complexes. Two 2,6-di-tert-butyl-
phenolic groups are connected through the π-conjugated por-
phyrin rings and bear properties of both porphyrin and hydro-
quinone systems. Our investigations suggested that the introduc-
tion of hydroxy group in 5,15-bis-(2,6-di-tert-butyl-phenyl)-10,20-
bis-(pentaphenyl-phenyl)phenyl porphyrin (1a) favored their corre-
sponding quinoidal structures and not cyclisation [29]. This pat-
tern made the system to perform excellent photochemical dual-
mode switches. Porphyrin derivatives having 1-hydroxy-2,6-di-tert-
butylphenyl substituents at their 5,15-positions undergo reversible
photo-redox switching between porphyrin and porphodimethene
[30] moieties. The changes in the electronic absorption and the
fluorescence emission spectra are observed when 1a changes into
1b. Further, the change in the conformation of macrocycles is ob-
served when 1c changes into 1d (Fig. S24). The optical and elec-
trochemical properties of quinoidal porphyrinoids are unique due
to the small highest occupied and the lowest unoccupied molecu-
lar orbital (HOMO–LUMO) energy gaps and perturbed aromaticity.
Our molecular orbital diagrams are in conformity that the HOMO-
LUMO gap decreases when conversion takes place from enol to
keto form (tautomerism) (Figs. 5,6).
Molecular fluorescent light-driven switches have attracted
great interest in stimuli-responsive materials and molecular opto-
electronics [1–3]. Porphyrins are considered of special significance
because of the possibility of coupling their various redox states
with different optical states or for physical coupling with other
chromophores leading to the important phenomena including op-
tical switching [4] and charge separation [5–7]. Porphyrin moieties
containing quinonoid, [8,9] TCNQ, [10,11] and ketone [12]struc-
tural features have been reported. These systems possess redox
and photochemical properties that vary from those of conventional
porphyrins. Porphyrins bearing phenolic [4,5,6,7,13–20] or hydrox-
ylic [5,7,13–20] substituents are subjected to changes depending
on their states of protonation and relative susceptibilities to oxida-
tion which can be further compounded with tautomeric processes
[
21]. New examples of valence tautomeric systems is important for
further development of the area for improving basic understanding
of the phenomenon and also for providing a greater diversity of co-
ordination environments and geometries suitable for practical ap-
plications. In this respect, pentaphenyl-phenylbenzene(PPP) is con-
sidered as one of the promising 3D topologies and is known to ex-
hibit unique electronic features, because (PPP) adopts a propeller-
like conformation which enables π–π interaction between the pe-
ripheral aromatic rings facing each other and sextuple accumu-
∗
Corresponding author at: Department of Chemistry, University of Delhi, 110 007,
India.
E-mail address: smschauhan@chemistry.du.ac.in (S.M.S. Chauhan).
https://doi.org/10.1016/j.molstruc.2021.131407
0
022-2860/© 2021 Elsevier B.V. All rights reserved.

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
Scheme 1. Synthesis of 5,15-bis-(oxo) porphodimethenes.
2
. Materials and methods
riod 0.5 sec and increment E at 40 mV. The quantum chemical cal-
culations for different porphyrin conjugates were carried out using
DFT with the B3LYP (Becke-3-parameters, Lee−Yang−Parr) [31,32]
hybrid exchange correlation functional, which uses correction for
both gradient and correlation. For all of the systems, the elec-
tronic configurations of all of the elements were described using
the LANL2DZ [33] basis set as implemented in Gaussian 09 W [34].
2
.1. Experimental section
Chemicals were sourced from Spectrochem (India), Earth (In-
dia), and Thomas Baker (India) and were used as received. Thin
layer chromatography (TLC) was carried on aluminum plates
coated with silica gel mixed with fluorescent indicator and
was sourced from Merck, Germany. Silica gel chromatography
was performed on Silica Gel 60–120 mesh size(spherical, neu-
tral) purchased from Ekta Marketing Corporation. Dry solvents
dichloromethane(DCM), UV–vis grade acetonitrile and chloroform
were also purchased from Ekta Marketing Corporation.
2.3. General procedure for synthesis of precursors and porphyrin
conjugates
2.3.1. Synthesis of precursors of 1a
1-formyl-diphenylacetylene
thesized in the laboratory
(Tolan)
by
[35]
was
syn-
4-
the
reaction
of
2
.2. Equipment and instrumentation
bromobenzaldehyde(272 mg,1.47 mmol) with phenyl acetylene
0.2 ml, 2 mmol) in the presence of piperidine (0.3 ml,3 mmol) as a
(
NMR ( H, 13C) spectra were recorded on a Jeol 400 MHz spec-
1
base and the palladium bis triphenyl phosphine (14 mg,0.02 mmol)
as reagent in catalytic amount. The reaction mixture was heated
in a pre-heated oil bath to 70 °C for 10 min. The organic layer
was collected and then washed with acid (15 %v/v HCl) un-
til the washings were neutral. The organic layer was washed
again with the water, dried over Na2SO4 and purified by col-
umn chromatography yielding pure substituted diphenylacety-
lene. Physical state: Yellow solid; Yield: 230 mg (80%); Rf: 0.5
trometer in CDCl3 with TMS as a standard. Maldi mass was cal-
culated by Bruker autoflex Speed instrument (N2 laser: 355 nm,
pulse energy 100 μJ,1 ns; acceleration voltage: 19 kV, reflector pos-
itive and negative mode) at IIT Bombay Powai. Trans 2- [3-
(
4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile (DCTB)
were used as matrices. UV-visible spectra were recorded on a
Perkin Elmer (850 B luminescence spectrometer). All the UV-visible
spectra were carried out using UV grade chloroform. All UV-visible
measurements were carried out in 1 cm quartz cuvette with 1 cm
optical pathlength. Electrochemical experiments cyclic voltamme-
try (CV) and differential pulse voltammetry(DPV) were carried out
by using multi-Autolab M204 having NOVA 2.1 software consist-
ing of a standard three electrode arrangement having platinum
coil as counter electrode, glassy carbon as working electrode and
the Ag/AgCl as a reference electrode. All the electrochemical mea-
1
(1:9;ethylacetate:hexane); HNMR(400 MHz,Chloroform-D) δppm:
7.32–7.34(m,3H,CH), 7.61–7.63(s,2H,CH),7.82(s,2H,CH),9.9(1 s,CHO).
13CNMR(400 MHz,CDCl3)δppm: 191.1(-CHO), 135.3(-C), 131.6(-CH),
129.4(-CH), 128.8(-CH),128.3(-CH),122.4(-CH),93.3(-C), 88.4(-C);
ESI-MS; 206.9.
2.3.2. Synthesis of 1-formyl-(penta-phenylphenyl)benzene (HPB-CHO)
In a round-bottom flask, tetraphenylcyclopentadienone (460 mg,
1.2 mmol) and 1-formyldiphenylacetylene (247 mg, 1.2 mmol) were
added to diphenylether (6 mL) and the whole reaction mixture
was refluxed for 36 h under nitrogen atmosphere. Upon comple-
tion of the reaction, the purple color faded to brown, then the
reaction mixture was cooled at room temperature. With drop-
wise addition of methanol, the solid product was precipitated, fil-
surements were carried out in Ar-purged solvents with n-Bu NPF
4
6
as the supporting electrolyte at 25 ± 0.1 °C. The CV studies of the
molecules 1a, 1b, 1c and 1d were performed in degassed DCM un-
der Ar atmosphere using 0.49 mM concentration and the scan rate
for the measurements was typically 100 mV/s. The DPV was carried
out keeping the peak amplitude 25 mV, peak width 0.15, pulse pe-
2

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
tered under suction, and was purified by column chromatography.
refluxed in toluene (15 ml) for 24 h. The product was precipitated
from DCM/methanol to give nickel(II) porphyrin. The extent of
the reaction was monitored by TLC. The mixture was allowed
to cool, then the crude mixture was loaded onto a silica plug
(DCM). The product was collected and the solvents were removed.
Yield: 80%, Physical state: green solid, 1H NMR (CD COCD ):
Physical state: off-white solid; 64% yield; Rf = 0.67 (1:9 ethyl ac-
1
etate/hexane); mp>250 °C; H NMR (400 MHz, CDCl -D) δ = 9.78
3
(
s, 1H, CHO), 7.42 (d, 2H, Ar), 7.11 (d, 2H, Ar), 6.91 (m, 25H, Ar);
1
3
C NMR (400 MHz, CDCl -D) δ: 191.0, 134.3, 130.4, 129.2, 127.6;
3
ESI-MS; 562.23.
3
3
δ = 9.49 (s,8H, β-pyrollic), 9.30 (d, 6H,J = 1.6 Hz), 8.65(10H,s),
2
.3.3. Synthesis of 5-(2,6-di-tert-butyl 1-hydroxy-phenyl)
8
5
.47(m,26H), 8.42(s,9H), 7.17 (s,3H), 6.80(s,2H), 6.22(s, 3H),
.17(s,2H), 2.88(s,18H, tert.butyl), 2.76(s,18H,tert.butyl).13
dipyrromethane
C NMR
To a stirred solution of 2,6-di-tert-butyl 1-hydroxy benzalde-
hyde (15 mmol, 3.50 g) and pyrrole (30 mmol, 2.07 mL), trifluo-
roacetic acid (1.5 mmol, 171.03 μL) was added at room temper-
ature. The reaction mixture was stirred for half an hour and
the reaction progress was monitored by thin layer chromatog-
raphy. The pinkish white solid obtained was dipyrromethane:
pinkish white solid; 95% yield; Rf = 0.33 (1:1 ethyl acetate/
petroleum ether); 1H NMR (400 MHz, chloroform-D) δppm:7.58–
(
CD COCD ): 194(C=C),153, 130, 140, 125, 115, 80, 37, 47,32(C-
3
3
−1
−1
C,tert.butyl),15(C-C,tert.butyl),λmax (nm) [ε (M cm )] 438
[
[
+
3000], 548(600). Experimental: (MALDI-TOF , linear mode, m/z):
+
+
M- CH3] = C133H111 N NiO2 ), calcd. 1854.8133, found.1854.0441.
4
2
.3.7. Synthesis of Ni 5,15-bis-(2,6-di-tert-butylquinone)-10,20-bis-
(hexaphenylbenzene)-porphyrin (1d)
To a stirring solution of 1c (49 mg, 0.027 mmol, 1 equiv.) in
7.90(m,4H, β pyrrolic), 7.58(br,s,NH),7.02 (s, 2H, Ar),6.63(m, β
CH Cl solvent (20 mL), NOBF4 (71.75 mg, 0.27 mmol, 10 equiv.)
2
2
pyrrolic,2H),5.90 (s, 1H, meso-CH), 5.03(s,1H,OH),1.39(s, 18H, tert-
butyl),1.27(s, 18H,tert.butyl).
was added at 25 °C. The reaction was vigorously stirred until
full conversion of starting material was monitored by TLC. The
reaction was slowly quenched with saturated NaHCO3 solution
2
.3.4. Synthesis of 5,15-bis-(2,6-Di-tert-butyl-phenyl-1-hydroxy-
(
5 mL) and extracted with CHCl3 (15 mL). The combined organic
phenyl)-10,20-bis-(hexaphenylbenzene)-porphyrin (1a)
layers were dried over Na SO4 and the reaction mixture was
2
In a three neck round bottom flask, dipyrromethane of 1-
hydroxy(2,6-ditert-butyl-phenyl)benzaldehyde(315 mg,0.9 mmol),1-
dried with slow evaporation of the solvent at room temperature.
Yield:(90%), Physical state: reddish green solid, 1H NMR (C D N):
6
5
(
formyl-pentaphenyl-phenyl)benzene
(506 mg,0.9 mmol)
were
δ = 8.69(s,8H), 7.61(m,43H), 7.29(m,4H), 5.71 (s, 20H),1.68(s,6H),
dissolved in dry dichloromethane (150 ml). The reaction
mixture was purged with argon for 15 minutes and then
.30 (s, 12H,t-bu),1.26 (s, 18H, t-bu). 13C NMR (C D N): 180(C=O),
1
6
5
1
76.03(C=C), 164.37(C=C), 110.76(C=C), 123.31, 118, 55.56, 51.56,
CF COOH (0.094 mmol) was added. The second major spot
3
+
0.8, 35.26, 33.26, 27.28, 16.48; Experimental: (MALDI-TOF ,
5
isolated was trans A B
porphyrin which was character-
2
2
+
+
DCTB linear mode, m/z): [M+NH ] = C132H110 N NiO ), calcd.
ized by means of UV-visible spectroscopy, 1H NMR spec-
4
5
2
1
854.8008. found: 1855.7407.
troscopy, and the Maldi mass spectrometry. Physical state:
1
Purple solid; Yield: 25%; Rf: 0.50(65%CHCl /Pet.ether); H
3
3. Results and discussion
NMR(400MHZ,CDCl )δppm:8.82(s,8H,β-pyrollic),
8.16(s,13H),
.007(s,10H), 7.93(s,3H, HPB-H),7.71(m,1H), 7.23(s, 24H-HPB-
H),7.17(s, 9H), 6.62(s,1H-Ar-H), 5.24(s,2H,OH), 1.29(s,18H,-t-
3
8
3
.1. Synthesis and characterization
butyl),
CDCl3:
1.24(s,18H-tbutyl),
-2.77(s,2H,NH).13CNMR:400MHZ
5
,15-bis-(1-hydoxy-2,6-di-tert-butyl-phenyl)-10,20-
162.60(C=C),
150.35, 148.64,
139.19(C=C),
bis(pentaphenylphenyl)phenyl porphyrin 1a and its Ni substi-
tuted porphyrin 1c having (PPP) groups at meso positions were
synthesized. Introduction of 1-hydroxy-2,6-di-tert-butyl-phenyl
groups increases the solubility of porphyrin and the stability of
radical intermediates [36–39]. The synthesis of targeted porphyrin
conjugates were carried out after the synthesis of precursor
129.68,122.76, 120.56,58.72, 60.02, 36.51, 31.74,29.70,18.43(tert-
butyl carbon). All the chemical shift values are in
+
ppm.
Yield:25%.
(MALDI-TOF, linear
mode,m/z):
[M-
+
+
2,
CH4] = C133H114 N O calcd.1798.8629,found.1798.7715.Uv/vis
4
CH Cl ;rt):λmax [nm]εmax [M 1cm ]421(3380), 517.
− −1
(
2 2
1
-formyl-pentaphenyl-phenyl-benzene as previously reported
40]. The targeted porphyrin conjugates were synthesized by
condensation between 1-formyl-(pentaphenylphenyl)benzene
2
.3.5. Synthesis of 5,15-bis-(2,6-di-tert-butylquinone)-10,20-bis-
[
(
hexa-phenyl-benzene)-porphyrin (1b)
To a stirring solution of 1a (49 mg, 0.027 mmol, 1 equiv.) in
CH Cl solvent (20 mL), NOBF4 (71.75 mg, 0.27 mmol, 10 equiv.)
and 1- hydroxy(2,6-di-tert-butyl-phenyl)dipyrromethane in dry
dichloromethane (CH Cl ) solvent followed by the oxidation with
2
2
was added at 25 °C. The reaction was vigorously stirred until full
conversion of starting material was monitored by TLC. The reaction
was slowly quenched with saturated NaHCO3 solution (5 mL) and
extracted with CHCl3 (15 mL). The combined organic layers were
2
2
2
,3-dichloro-5,6-dicyanobenzoquinone (DDQ) yielding porphyrin
1
a as the major product with 25% yield(Fig. S3). The subsequent
metallation of 1a porphyrin with Ni(NO ) •6H O was achieved
3
2
2
by refluxing 1a in toluene solvent which further yielded 1c por-
dried over Na SO and the reaction mixture was dried with slow
2
4
phyrin(Fig. S3). Further, the addition of 10 equivalents of NOBF4
to 1a and 1c resulted in the formation of 1b and 1d The 1H NMR
evaporation of the solvent at room temperature. Yield: 90%, Physi-
cal state: greenish solid, 1H NMR (CD OD): δ = 8.97 (s, 8H), 8.61
3
spectrum of 1a in CDCl3 is shown in (Fig.S4). At room temperature,
the macrocycle 1a exhibits singlet at δ = 8.82 ppm corresponding
to the 8 β pyrrolic protons, singlet at 8.16 ppm corresponding to
(
s,3H), 7.54(d,37H,8.4 Hz),7.51(s,10H), 7.32(s,3H),7.29(s,4H),1.48(s,
6H, t-bu).13
3
C
NMR (CD OD):δ = 175.52(C=O), 163.48(C=C),
3
1
52.02, 131.92, 130.32, 127.98, 125.30, 124.02, 48.29, 48.08,
1
3 protons and multiplet at 7.71 ppm. The tertiary butyl protons
4
7.66, 35.67, 30.50, 20.52,7.87.Uv/vis(CH Cl ;rt):λ [nm](εmax
2 2
−1cm−1]) [445(4660), 671(150). Experimental: (MALDI-
were observed in the range of 1.24 ppm to 1.29 ppm. The hydroxyl
protons appear as a singlet at 5.24 ppm. The NH peak resonates at
[
M
TOF , linear mode,DCTB:m/z [M+3H]+ = C132H111 N O ), calcd.
+
+
4
2
-
2.77 ppm corresponding to 2 protons. Only one β pyrrolic proton
1783.8691; found.1783.6671.
resonance was observed which along with the equivalence of two
pentaphenyl-phenyl benzene moieties indicated an effective D4h
symmetry in the solution [41]. The mass spectrometric analysis
of 1a confirmed the composition with molecular ion signal at
m/z 1798.8943 (Fig. S17). The 1H NMR spectra of 1b revealed the
2
.3.6. Synthesis of Ni 5,15-bis-(2,6-di-tert-butyl-1hydroxyphenyl)-
1
0,20-bis-(hexaphenylbenzene)-porphyrin (1c)
To solution of 1a porphyrin (100 mg, 0.027 mmol) was
added Ni(NO3)2 .6H O (0.112 g, 0.451 mmol). The mixture was
a
2
3

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
Fig. 1. Uv visible spectra of 1a and 1b (left) and 1c and 1d(right) in CH2Cl2(5 × 10−5).
disappearance of peak in the range of 5–6 ppm which confirms
the formation of 1b. Meanwhile in 13C NMR, we observed a peak
at 175 ppm which is a indicative of carbonyl (C=O) group in
the quinoid moiety. The heteronuclear multiple bond correlation
color change from purple to green (see Fig. 2). The UV-visible spec-
tra of 1b shows peaks at 445 nm along with the appearance of new
peak at 668 nm with well-defined isosbestic points in the elec-
tronic absorption spectra. The well-defined isosbestic points indi-
cate that the two long-lived species are involved in this process.
The optical characteristics of 1b are quite different from those of
1a, a broad and intense peak appears around the 668 nm. (band
gap decreases in 1b as revealed by DFT calculations). The UV-
Visible absorption spectroscopy of Ni metallated hydroxy porphyrin
(1c) displays strong intense Soret peaks at 438 nm with weak Q
bands at 548 and 600 nm. Meanwhile, the absorption spectra of
1d shows intense peak at 445 nm along with the two peaks at
555 nm and 611 nm respectively (Fig. 1). We also noticed the isos-
bestic points in the spectra of 1b and 1d Our UV-visible studies
are in agreement with the time-dependent density functional (TD-
DFT) calculations.
(
HMBC) NMR is quite indicative for the formation of 1b. The
NH peak disappeared because NOBF4 also generates radicals, and
these delocalized radicals show silent NMR spectra due to the fast
electronic relaxations. Also in CD OD, the NH proton exchanges
3
with deuterium. The Maldi MS spectra of 1b showed peaks at
1
783.7922 (Fig.S18). The 1H NMR spectra of 1c was recorded in
CD COCD , and the spectra exhibits the absence of NH peak which
3
3
is the indicative of metallation. The downfield shifting of peaks
were observed in the deuterated acetone. The β pyrrolic protons
resonate at 9.49 ppm. One doublet was observed at 9.30 ppm with
very weak coupling, and one multiplet at 8.42 ppm corresponding
to the PPP(pentaphenyl-phenyl-benzene) protons. The OH peak
was observed at 5.17 ppm. The tertiary butyl protons were ob-
served at 2.88 ppm and 2.76 ppm corresponding to the 36 tertiary
butyl protons.
3
.2.2. Fluorescence studies
The 1H NMR spectrum of 1d exhibits six peaks at the chem-
ical shifts (in ppm) 8.69, 7.54, 5.71 and the three tertiary butyl
protons resonate from 1.15 to 1.68 ppm (Fig.S11) representing the
complex dissociation in the solution. The NMR was carried out in
deuterated pyridine. Nickel porphyrins readily oxidize in the pres-
ence of traces of radicals which consequently broaden the NMR
spectra [42]. Due to the low solubility, some carbon signals were
detected indirectly via 1H−13C HMBC NMR experiments. Oxidation
of 1a to 1b entails loss of aromaticity and an extension of conjuga-
tion over a greater number of atoms similar to the case of hydro-
quinone/quinone redox couple. A vital property of Nickel(II) com-
plexes [42–52] is that they show the thermochromism induced by
the transformation of the nickel(II) spin state between a low-spin
The fluorescence performances of the two target compounds 1a
and 1b in the DCM solvent were investigated at room temperature.
Compound 1a emitted green fluorescence under 365 nm light. As
shown in Fig. 3, two strong fluorescence emission peaks at 798 nm
and 817 nm are observed when 1a was excited by 365 nm and
4
21 nm light. Interestingly, with the extension of the UV-irradiation
(
excitation at 445 nm), the fluorescence intensity decreases gradu-
ally and the photoisomerization to 1b took place and the fluores-
cence phenomenon is completely quenched.
This quenching is attributed to the photoinduced electron trans-
fer from (2HP-PPP)OH (1a) to yield P .
+
O
2.−
diradical anion as a
charge-separated state. However, the fluorescence intensity re-
stored slowly when further irradiated with visible light (>445 nm).
It indicates that the irradiation with the different wavelengths
of light can effectively control the fluorescence intensity. This
feature has potential application in the molecular-level stimuli-
responsive materials and the non-destructive readout system for
photon storage. The fluorescence of 1b is observed at 798 nm,
(
S = 0) state with a tetracoordinate square planar structure and a
high-spin (S = 1) state with a hexacoordinate octahedral geome-
try. Such a behavior was noticed in Ni(II) complex, as the green
color of Nickel complex (1c) changed when heated in the DMF sol-
vent. Controlling the nickel(II) spin state assists in the development
of molecular memories, switches and this leads to further under-
standing of biological systems [53].
6
43 nm and 850 nm when the sample is excited at 395 nm, 422 nm,
and 445 nm, respectively. When the sample is excited at 445 nm
in the visible region, the emission still occurs. From this phe-
nomenon. we have concluded that the quinoid type moiety isomer-
izes back to the hydroquinone one. We noticed isosbestic points
in both absorption and the emission spectra of 1a and 1b. When
compound 1c is excited by 395 nm light, the two fluorescence
emission peaks are observed at 434 nm and another intense peak
at 796 nm. The same fluorescence behavior is also observed for 1d
The fluorescence intensity decreases gradually when 1c was irra-
3
3
4
.2. Photophysical properties
.2.1. Absorbance studies
The UV-Visible absorption spectra of 1a shows Soret peak at
21 nm and weak Q bands at 517 nm and 554 nm with some vi-
bronic structures (Fig. 1). Subsequent irradiation of 1a in the pres-
ence of CH Cl2 with ultraviolet light (λ= 365 nm) resulted in the
2
4

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
Fig. 2. Photochromism shown by 1a(simple visualization of 1a por. Under short Uv light. The color fades and changes back to purple when exposed to day light.
Fig. 3. Fluorescense Spectra of 1a(left) and 1b (right) (5 × 10− M).
5
Fig. 4. Fluorescense Spectra of 1c(left) and 1d (right) (5 × 10− M).
5
diated with the visible light. Moreover, 1d showed emission even
peaks at -0.72 V and -1.39 V corresponding to the one oxidation
peak at -1.40 V respectively. The narrow potential gap [60–62] of
after irradiating within the wavelengths of visible light.
0.68 V between first oxidation peak and the reduction peak is in
3
.2.3. Electrochemical studies
line with the radical character of porphyrin moiety. From the CV
and DPV plots of 1a, it is concluded that 1a is relatively diffi-
cult to reduce (two poorly reversible reduction peaks are observed
at -0.72 V and -1.39 V in DPV in its free-base form.(see Fig. S21)
Highly reversible one-electron oxidation peaks at 1.62 V and 1.77 V
are also observed in DPV in 1a. These observations led us to test
In order to understand the redox properties of synthesized por-
phyrins, the oxidation potentials of synthesized porphyrin conju-
gates were evaluated using (CV) and the (DPV)(Fig. S21). The ob-
served redox waves are fairly reversible and their potentials are
summarized in (Table 1). 1a exhibits two one electron reduction
5

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
Fig. 5. Molecular orbital diagram of 5,15-bis-(1-hydroxy-2,6-di-tert-butylphenyl)-10,20-bis-(pentaphenylphenyl)phenylporphyrin(left) and 5,15-bis-(1-oxo-2,6-di-tert-
butylphenyl)-10,20-bis-(pentaphenylphenyl)phenyl porphyrin (1b).
Fig. 6. Molecular orbital diagram of 1c (left) and formation of diquinone of 1c (right).
Table 1
redox couple and the dioxo/monooxo couple respectively. The Ni(II)
Summary of the electrochemical potentials for free base and metallated
porphyrin complex (e.g., NiTPP, [54–58] tetraphenylporphyrin) has
porphyrins.
+
a reduction peak at -1.75 V (vs.Fc/Fc , in CH Cl , using 0.1 M TBAP
2
2
Mol.
E1°x
E2°x
E1Red
E2Red
ꢀE(electrochemical gap)
as supporting electrolyte). The one electron-reduced species of the
Ni(II) porphyrin complex (1c) exists as an equilibrated form be-
tween the Ni(I) porphyrin anion and the Ni(II) porphyrin π anion
radical. [59] The positive shift of the nickel-centered redox peak of
Ni-1 (1c) relative to that of NiTPP indicates that reduction of the
nickel center in Ni-1 occurs more easily than reduction of NiTPP.
On the other hand, the first and the second reversible waves of 1d
at Ea = -0.95 V and Epc = -0.85 V correspond to the monooxo/dioxo
and Ni(II)/Ni(III) redox couples, respectively.
1
1
1
1
a
b
c
-1.40
-0.80
0.38
-
-
-0.72
-0.93
-0.91
-0.85
-1.39
0.68
0.08
1.04
0.1
-
-1 0.42
-
d
-0.95
-
ox
red
ELUMO = −[eE + 4.44] in eV and EHOMO = −[eE + 4.44] in eV [60–62] (a-
c).
that even a mild oxidant (NOBF ) can convert 1a into 1b. 1b dis-
4
plays one oxidation peak at -0.80 V corresponding to one reduc-
tion peak at -0.93 V. The cyclic voltammogram of the Ni(II) com-
plex (1c) exhibits two reversible redox waves at Ea1/2 = 0.98 V and
two reduction waves at E1/2 red = -0.60 V (vs. Fc/Fc+), as shown in
3.2.4. DFT studies
To further have
a look into the structural, electrochemi-
cal, and the spectral properties of porphyrin conjugates 1a,
1c and their oxo derivatives, the DFT studies were carried
out. The DFT studies revealed that both the meso substituted
(
Fig.S21). These redox waves of 1c are assigned to the Ni(III)/Ni(II)
6

K. Nisa, G.K. Mishra, M. Thirumal et al.
Journal of Molecular Structure 1247 (2022) 131407
porphyrins 1a and 1c maintain
a
geometry in which sub-
4. Conclusion
stituents (pentaphenyl-phenyl)benzene (PPP) and 1-hydroxy-2,6
di-tert.butyl-phenyl groups are aligned in the plane of core por-
phyrin moiety. Owing to the rigidity of (PPP), the core porphyrin
moiety maintains its planarity with respect to the “mean plane”
defined by the four meso carbon atoms. Oxidation of 1a and
In summary, we have synthesized the porphyrin pentaphenyl-
phenyl benzene hydroquinone and quinone moieties which ex-
hibits the photochromism. The 1-hydroxy-2,6 di-tert.butylphenyl
groups were introduced at the two meso positions of the por-
phyrin ring to construct the two light-driven fluorescent molecu-
lar switches. The synthesis, photochromism, and the fluorescense
performances were further investigated. The changes in the UV–
vis absorption and the fluorescence emission spectra of the target
molecules were studied. We have introduced Ni in the core of por-
phyrin and analyzed the effect on the tautomerism (conversion of
hydroquinone to quinone state). We observe that the metal is not
affecting the tautomerism. These type of molecules can help in the
development of single molecule magnets. The reversible intercon-
version of tautomers is a desirable feature for the potential use in
the optoelectronic devices and other applications.
1c using NOBF4 as an oxidizing agent results in the formation
of mono and di-Oxo porphodimethenes for both 1a and 1c por-
phyrin moieties. The time dependent density functional theory
(TDDFT) calculations suggest that 1a shows absorptions in range
of 300–600 nm and λmax at around 400 nm. The UV–vis spec-
trum of 1c shows absorption maxima at λmax = 392 nm, whereas
the formation of oxo derivative increases the λmax to 900 nm for
1d Antiaromatic porphyrinoids typically exhibit weak and broad
absorption bands in the near infrared region (>800 nm). The os-
cillator strengths which is the measure of the intensity of the
absorption peaks of the Q-band in 1a is higher as compared
to that of the 1c porphyrin. This indicates that the introduc-
tion of Ni metal decreases the oscillator strength of the Q-band.
The broadening and the red-shift of the absorption bands to-
gether with an increase in the intensity of the Q-bands for the
oxo derivatives of 1a and 1c are promising approaches for con-
structing the molecular devices and machines. TDDFT calcula-
tions reproduce well the experimental results of the electronic
spectra.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
The ten lowest-energy singlet and triplet electronic transitions
of all the porphyrin conjugates were computed with the TDDFT
calculations based on the optimized ground-state geometries. The
simulated UV–vis spectra for all porphyrin conjugates are shown in
Fig. S23 and the calculated transition energies, oscillator strength
and the molecular orbital excitations for the most relevant transi-
tions of the electronic absorption bands of porphyrins are given
in Table S2. TDDFT calculations suggest that 1a shows absorp-
tions in range of 300–600 nm and λmax at around 400 nm, these
bands are in sharp contrast to Q-bands of porphyrins, which are
located at shorter wavelengths [63]. The UV–vis spectrum of 1c
shows absorption maxima at λmax = 392 nm, whereas formation of
quinone derivative increases the λmax to 900 nm for 1d More im-
portantly, the oscillator strengths (which is the measure of the in-
tensity of the absorption peaks) of the Q-band in 1a is higher as
compared to that of the 1c porphyrin. This indicates that the intro-
duction of Ni metal decreases the oscillator strength of the Q-band.
The broadening and the red-shift of the absorption bands together
with an increase in the intensity of the Q-bands for the quinone
derivatives of 1a and 1c are promising strategies for constructing
molecular devices and machines. We note that the TDDFT calcu-
lations reproduce well the experimental results of the electronic
spectra.
Kharu Nisa: Formal analysis, Methodology, Investigation, Val-
idation, Writing – original draft, Writing – review & editing.
Gaurav Kumar Mishra: Software, Visualization. M. Thirumal: Su-
pervision. Shive M.S. Chauhan: Project administration, Resources,
Funding acquisition, Conceptualization.
Acknowledgments
Kharu Nisa acknowledges Dr. Hemant Kumar Kashyp and Vikas
Khatri of IIT Delhi for encouragement and providing facilities for
computational studies. S.M.S Chauhan acknowledges UGC-India for
BSR Faculty Fellowship. S.M.S Chauhan is also thankful to Prof.
Rama Kant from University of Delhi for providing electrochemical
experimental facility. G.K. Mishra thanks UGC-India for providing
senior research fellowship.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.131407.
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