Solar driven photocatalytic activity of porphyrin sensitized TiO2: Experimental and computational studies

Article abstract of DOI:10.3390/molecules26113131

The absence of a secure long-term sustainable energy supply is recognized as a major world-wide technological challenge. The generation of H₂ through photocatalysis is an environmentally friendly alternative that can help solve the energy problem. Thus, the development of semiconductor materials that can absorb solar light is an attractive approach. TiO₂ has a wide bandgap that suffers from no activity in the visible spectrum, limiting its use of solar radiation. In this research, the semiconductor absorption profile was extended into the visible region of the solar spectrum by preparing porphyrin-TiO₂ (P-TiO₂) composites of meso-tetra(4-bromophenyl)porphyrin (PP1) and meso-tetra(5-bromo-2-thienyl)porphyrin (PP2) and their In(III), Zn(II) and Ga(III) metal complexes. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed on the porphyrins to gain insight into their electron injection capability. The results demonstrate that P-TiO₂ systems merit further in-depth study for applications that require efficient photocatalytic H₂ generation.

Full text of DOI:10.3390/molecules26113131

molecules
Article
Solar Driven Photocatalytic Activity of Porphyrin Sensitized
TiO : Experimental and Computational Studies
2
Sebastian Otieno 1 , Anabel E. Lanterna 4 , John Mack 5 , Solomon Derese 3 , Edith K. Amuhaya * ,
,2,3
2,
5
,
1,
Tebello Nyokong * and Juan C. Scaiano *
1
Centre for Advanced Materials Research (CAMaR), Department of Chemistry and Biomolecular Sciences,
University of Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5, Canada; seba.otieno@gmail.com
School of Pharmacy and Health Sciences, United States International University—Africa,
Nairobi 00800, Kenya
Department of Chemistry, University of Nairobi, Nairobi 00100, Kenya; sderese@uonbi.ac.ke
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK;
Anabel.Lanterna1@nottingham.ac.uk
2
3
4
5
Institute for Nanotechnology Innovation, Rhodes University, Makhanda 6139, South Africa; j.mack@ru.ac.za
Correspondence: eamuhaya@usiu.ac.ke (E.K.A.); t.nyokong@ru.ac.za (T.N.); juan.scaiano@uottawa.ca (J.C.S.)
*
Abstract: The absence of a secure long-term sustainable energy supply is recognized as a major world-
wide technological challenge. The generation of H through photocatalysis is an environmentally
2
friendly alternative that can help solve the energy problem. Thus, the development of semiconductor
materials that can absorb solar light is an attractive approach. TiO has a wide bandgap that suffers
2
from no activity in the visible spectrum, limiting its use of solar radiation. In this research, the
semiconductor absorption profile was extended into the visible region of the solar spectrum by
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
preparing porphyrin-TiO2 (P-TiO ) composites of meso-tetra(4-bromophenyl)porphyrin (PP1) and
2
Citation: Otieno, S.; Lanterna, A.E.;
Mack, J.; Derese, S.; Amuhaya, E.K.;
Nyokong, T.; Scaiano, J.C. Solar
Driven Photocatalytic Activity of
Porphyrin Sensitized TiO2:
meso-tetra(5-bromo-2-thienyl)porphyrin (PP2) and their In(III), Zn(II) and Ga(III) metal complexes.
Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations
were performed on the porphyrins to gain insight into their electron injection capability. The results
demonstrate that P-TiO2 systems merit further in-depth study for applications that require efficient
Experimental and Computational
Studies. Molecules 2021, 26, 3131.
https://doi.org/10.3390/
photocatalytic H generation.
2
Keywords: photocatalysis; porphyrin; electron injection; hydrogen generation; titanium dioxide
molecules26113131
Academic Editor: Łukasz Sobotta
1
. Introduction
Rapid growth in world population and increased industrial activities have led to a
steady rise in global energy consumption [ ]. According to the BP Statistical Review of
World Energy, global energy consumption stands at 13.9 MTOE (Million Tonnes of Oil
Equivalent). This figure is expected to nearly double by 2050 [ ]. Currently, ~85% of global
energy running the transport and industrial sector is supplied by fossil fuels. This has
led to increased greenhouse gas (GHG) emissions and substantial depletion of the most
easily exploited carbon-based energy resources that could alternatively have been used
Received: 23 March 2021
Accepted: 18 May 2021
Published: 24 May 2021
1
1
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
to produce valuable chemicals [
SOx, and GHGs) is to increase the efficiency of energy production and consumption while
reducing the fossil fuel component in the energy mix [ ]. A transition to hydrogen fuel
is considered an ideal alternative to fossil fuels [ ]. Hydrogen has potentially higher
energy efficiency, high energy density and low generation of pollutants [ ]. The world
in the recent years has seen a steady transition from solid to liquid to gas fuels. Each
successive source has increased the ratio of hydrogen to carbon. Hydrogen is an attractive
chemical energy carrier that on reaction with oxygen, produces only water, thus a clean
energy source.
x
2,3]. The key to reducing problematic emissions (NO ,
4,5
4–6
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
7,8
Advanced oxidation processes (AOPs) such as photocatalytic hydrogen generation
from sacrificial electron donors (SEDs) in aqueous media is one of the green sources being
4
.0/).
Molecules 2021, 26, 3131. https://doi.org/10.3390/molecules26113131
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 3131
2 of 15
explored. This is due to the mild reaction conditions required, using only sunlight and a
semiconductor [
the focus of appreciable research interest in hydrogen generation under UV-visible light
excitation [11 14]. This application is based on the generation of an electron-hole (e-h) pair
9,10]. Transition metal oxide systems such as TiO and ZnO have been
2
–
upon photoexcitation and the subsequent transfer of photoinduced electrons to electron
acceptors such as protons through the metal oxide conduction band (CB) [10,15,16]. TiO2,
in particular, has gained widespread applications in sunscreens, paints, photovoltaics and
environmental remediation [17–21]. These applications are credited to its wide distribu-
tion in terms of its commercially exploitable reserves, inertness, favorable photocatalytic
properties, low toxicity, and high photo efficiency and activity [22–24].
Despite many advantages, TiO is a wide bandgap semiconductor. Thus, it predomi-
2
nantly absorbs light in the UV region (<400 nm) of the solar spectrum, taking advantage
of only 5% of the solar energy reaching the earth surface [22]. Low sensitivity to photons
in the visible region, which constitutes about 41% of solar energy, poses a significant
disadvantage to its use in solar panels [25]. On the other hand, rapid e-h recombination
severely hinders practical efficiencies derived from any mechanism aimed at harvesting
electrons or holes [18–26]. With these shortcomings in mind, researchers have focused on
developing photoactive systems that will extend the photoresponse spectrum of TiO into
2
the visible region and prolong the life span of charge-separated states [14,15]. Doping with
narrow bandgap semiconductors, decorating TiO surface with nanoparticles, and dye
2
sensitization are some of the modification strategies that have been used to shift the optical
response of TiO -based photocatalysts [18,22,27,28].
2
Recently, there has been a significant focus on dye sensitization, due to the difficulty in
preparing doping materials and the need for lattice exchange at very high temperatures [22].
Upon photoexcitation of the dye, sufficient energy is produced that prompts surface
electron transfer between the dye and TiO interface populating the conduction band
2
(
CB) [
practical applications have been limited due to their undesirable environmental impact
leaving non-toxic organic dyes such as porphyrins as the most desirable alternative [30 31].
Here, we investigate the photocatalytic suitability of different porphyrins when ad-
9,29]. Ruthenium-based sensitizers were extensively used initially. However, their
,
sorbed onto the TiO surface. This study aims to gain insights about the sensitization
2
process using different porphyrin systems that could enhance the material’s efficiency.
For this purpose, meso-tetra(4-bromophenyl)porphyrin (PP1) and meso-tetra(5-bromo-2-
thienyl)porphyrin (PP2) and their In(III), Zn(II) and Ga(III) metal complexes were syn-
thesized and characterized to examine whether modifying the meso-aryl groups has a
significant impact on photocatalytic activity. The structural difference between meso-
phenylporphyrins and meso-5-bromothien-2-ylporphyrins is known to result in signifi-
cantly different photophysical and electrochemical properties [32]. Zinc, indium, and
gallium metals were used to better understand trends in photocatalytic activity related to
the central coordinated ion. Theoretical calculations were carried out using the density
functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods
to develop a deeper understanding of the intrinsic optical properties of the porphyrins.
The porphyrins were adsorbed on nanometric TiO to yield P-TiO nanocomposites. The
2
2
photocatalytic activity of the
tigated under irradiation by a solar simulator (AM 1.5). The results suggest that
photocatalysts merit further in-depth study for use in hydrogen generation applications.
P-TiO nanocomposites in hydrogen generation was inves-
2
P-TiO2
2. Results and Discussion
2.1. Electronic Absorption Spectra of PP1 and PP2 and Their Metal Complexes
The details for the synthesis of porphyrins are provided in the Supporting Information
1
with H NMR and mass spectra provided in Figures S1–S4 and synthesis is shown in
Scheme 1. The results suggest PP1 and PP2 are obtained as monomers and no evidence for
dimerization/polymerization was detected.

Molecules 2021, 26, 3131
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Scheme 1. Synthetic pathway of PP1, PP2 and their metal complexes.
UV-visible absorption spectra are one of the most informative spectroscopic methods
in the context of porphyrin chemistry. PP1 PP2 and their metal complexes all exhibit ab-
sorption spectra characteristic of etio type porphyrins with an intense Soret (or B) band that
lies between 418 434 nm and weaker Q bands in the 500 700 nm region (Figures 1 and 2).
,
−
−
Gouterman’s four-orbital model [33] can be used to rationalize trends in the electronic
excitation spectra of free base porphyrins and their metal complexes. The main trends
in the optical and redox properties can be rationalized through a consideration of four
frontier orbitals that are derived from the highest occupied molecular orbital (HOMO) and
2
−
lowest unoccupied molecular orbital (LUMO) of a C H
16
parent hydrocarbon perimeter.
−a and −s nomenclature (Figure 3) for these orbitals depending
on whether there is a nodal plane ( and -a) or large coefficients ( and -s) aligned with
the y-axis [34]. For example, the HOMO 1, HOMO, LUMO and LUMO+1 of PP1 can
be assigned as the
−a and −s MOs, respectively (Figure 3). The MOs derived from
the HOMO and LUMO of the C H
16
Michl introduced an a, s,
a
s
−
a, s,
2
−
parent perimeter have M values of
±
4 and
±
5,
16
16
L
respectively. Four spin allowed one-electron transitions are possible within the four-orbital
model. It is on this basis that Gouterman predicted the possibility of allowed B transition
(
∆
M =
±
1) at high energy and a forbidden Q transition (
∆
M =
±
9) at lower energy [33,35].
L
L
Free base porphyrins (PP1 and PP2) have D2h symmetry so there is a splitting of the main

Molecules 2021, 26, 3131
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electronic Q00 transition and a Q01 overtone into x- and y-polarized components, so that
four Q bands are observed [33,36]. However, metalloporphyrins have D4h symmetry and
hence have only two Q bands due to the main electronic transition and a vibrational
overtone [33]. This reduction in the number of the Q bands is a characteristic feature
of metalloporphyrins, further confirming the successful insertion of metal ions into the
porphyrin core.
Figure 1.
(Left) Normalized absorption spectra of PP1 and the corresponding metal complexes in toluene. (Right) Spectra
zoom in the Q band region.
B
Figure 2.
(Left) Normalized absorption spectra of PP2 and the corresponding metal complexes in toluene. (Right) Spectra
zoom in the Q band region.

Molecules 2021, 26, 3131
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Figure 3. Angular nodal patterns and energies of the
HOMO LUMO gaps for TiO2 [31], PP1 PP2 and the metal porphyrin complexes from the B3LYP geometry optimizations
BOTTOM). The
−a and −s MOs are highlighted with thicker black lines, while other occupied MOs are denoted with
a small black square. Black circles are used to highlight the LUMO gaps are denoted with
and −a MOs. The HOMO
vertical black dashed lines and the values are provided in red text.
a,
s,
−a and −s MOs of PP1 (TOP). Calculated MO energies and
−
,
(
a, s,
a
−
The experimental data provided in Table 1 demonstrate that there are only relatively
minor shifts in the absorption bands of PP1 relative to those of tetraphenylporphyrin
(
H TPP), since the only structural difference is the bromine atom at the para-position of the
2
phenyl rings. Phenyl moieties have only a minimal influence on the electronic structure
of the porphyrins, due to the unfavorable steric interaction between hydrogens of the
β
pyrrole ring and ortho hydrogens of the phenyl group. This positions the phenyl substituent
orthogonal to the porphyrin core, thus resulting in an increased rotational barrier [32,37].
However, meso-5-bromothien-2-yl-substituted PP2 has spectral bands that are red
shifted by 10 nm compared to the spectrum of PP1 (Table 1). Smaller 5-membered ring
meso-substituents such as the thienyl groups do not have ortho hydrogens, thus making it
easier for the meso-aryl rings to adopt a coplanar orientation with the porphyrin core [38].
This enhances the resonance interactions with the tetrapyrrole ring, which results in a
significant red shift of the Q and B bands of PP2 [39,40]. This trend is also observed in the
spectra of the metal complexes of PP2 and PP1 (Table 1).
The B bands for the metal derivatives of PP1 and PP2 are red shifted by 2–9 nm
compared to those of their respective free base monomers (Table 1). Metal insertion can
slightly alter the
at the porphyrin core improves their structural symmetry since the inner NH protons
tilt the pyrrole ring out of plane [34 38]. Therefore, greater resonance interaction with
π electron delocalization of the porphyrin ring. Co-ordination of metals
,

Molecules 2021, 26, 3131
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the meso-substituents is observed due to the reduced steric hindrance. The extended
conjugation is reflected in bathochromic shift of the Q and B bands [37
metallated porphyrins, indium complexes exhibited the largest shifts of 8
that has been reported previously [42]. Similar observations were observed in previous
studies [41 43 45]. It is noteworthy that InPP1 and InPP2 were predicted to have the
smallest HOMO LUMO gap values in the geometry optimization calculations (Figure 3),
which is consistent with the observed red shifts of the major spectral bands.
,41]. Among the
−
9 nm, a scenario
,
–
−
Table 1. The UV-visible absorption data for PP1, PP2 and their metal complexes in toluene. Please
note: the assignment of the Q bands (Qyo1, Qy , Qxo1, Qx00) is shown in Figure 2.
00
3
−1
−1
)
Q Bands (nm) (ε × 10 ) (L mol cm
Soret Band (nm)
ε × 10 ) (L mol cm
5
−1
−1
−1
(
(
)
)
Qy01
Qy00
Qx01
Qx00
H2TPP
PP1
PP2
421
421 (2.1)
431 (2.6)
515
517 (9.4)
524 (13.5)
549
550 (4.5)
561 (7.0)
591
649
650 (2.1)
665 (3.7)
593 (2.7)
600 (4.5)
3
−1
−1
Q Bands (nm) (ε × 10 ) (L mol cm
)
Soret Band (nm)
5
−1
ε × 10 ) (L mol cm
Q01
Q00
ZnPP1
InPP1
GaPP1
425 (9.6)
430 (5.4)
424 (6.9)
550 (52.8)
563 (31.0)
553 (29.0)
589 (10.6)
602 (13.2)
592 (6.2)
ZnPP2
InPP2
GaPP2
433 (8.0)
439 (3.0)
434 (3.9)
559 (51.9)
570 (15.7)
561 (22.7)
602 (8.5)
613 (6.6)
604 (4.7)
2
.2. Photo-Physical Characterization of PP1, PP2 and Their Metal Complexes
The synthesized porphyrins exhibit two characteristic porphyrin emission peaks that
arise from the Q00 and Q01 transitions [35 46 47]. The emission spectra of PP1 PP2 and their
,
,
,
metal conjugates (Figures S5 and S6) are generally similar but with differing maxima. The
spectra of the metalloporphyrins contain less intense emission bands that are blue-shifted
compared to those of the free bases. The ΦF values for the metalloporphyrins are lower
than those of PP1 and PP2 (Table 2), due to the heavy atom ensuring strong spin-orbit
coupling and enhanced intersystem crossing (ISC) [47
triplet states lowering fluorescence quantum yield. The ΦF values of ~0.01 for InPP1 and
InPP2 are the lowest. The values for the metalloporphyrins were shorter than those of
–49]. Improved ISC populates the
τ
F
free base porphyrins (Table 2) due to the heavy atom effect.
Table 2. Photophysical parameters of PP1, PP2 and their metal complexes in toluene. Qyo1, Qy00,
Qxo1, Qx00) is shown in Figure 2. The equations for the determination of fluorescence quantum
yield (ΦF) and lifetime (
τ
r
F), the radiative (k ) and non-radiative (knr) rate constant are provided in
Section 3.4.
λem (nm)
Stokes
Φ
τF (ns)
8
−1
8
−1
)
F
kr (10 s
)
knr (10 s
−
1
Shift cm
Q00
Q01
PP1
654
717 (sh)
649
650
8460
8120
8200
8100
0.019
0.010
0.013
0.006
1.55 ± 0.02
0.38 ± 0.01
0.48 ± 0.01
0.24 ± 0.02
0.123
0.255
0.269
0.233
6.33
26.1
20.6
41.4
ZnPP1
GaPP1
InPP1
600 (sh)
597 (sh)
610 (sh)
660
PP2
670
621 (sh)
624 (sh)
633
730 (sh)
659
663
8280
7920
7960
6980
0.017
0.012
0.013
0.010
1.37 ± 0.02
0.54 ± 0.01
0.53 ± 0.01
0.29 ± 0.02
0.124
0.219
0.240
0.331
7.18
18.3
18.6
34.2
ZnPP2
GaPP2
InPP2
673 (sh)

Molecules 2021, 26, 3131
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2.3. Density Function Theory Calculations
Density functional theory (DFT) computations were performed to help identify suit-
able compounds for photosensitization and compute the photophysical properties of the
synthesized porphyrins for comparison with experimental values. The spectral bands
observed in the visible region of the absorption spectra of PP1
plexes can be readily assigned to the forbidden Q and allowed B transitions of Gouterman’s
-orbital model on the basis of the TD-DFT calculations (Figure 4 and Table 3). Only the Q
and B bands are predicted to lie in the visible region when a systematic over-estimation
of the band energies is taken into account, since other -MOs are well separated in en-
ergy terms from the
−a, and −s MOs (Figure 3). The calculated Q and B band
maxima for meso-5-bromothien-2-yl-substituted PP2 ZnPP2 GaPP2, and InPP2 are red
shifted compared to those of meso-phenyl-substituted PP1 ZnPP1 GaPP1, and InPP1
, PP2 and their metal com-
4
π
a, s,
,
,
,
,
(
Figure 4 and Table 3), thus exhibiting the same trend observed in the experimental values
(
Figures 1 and 2, Table 1). The major spectral bands of InPP1 and InPP2 are more red
shifted than those of Zn(II) and Ga(III) complexes (Table 3). This is also consistent with
the trend observed experimentally (Figures 1 and 2, Table 1) due to a narrowing of the
HOMO
is predicted, which has very large MO coefficients at the meso-carbons (Figure 3), while
there is a large stabilization of the
−a, and −s MOs due to the electron-withdrawing
effect of the trivalent central In(III) ion.
−
LUMO gap (Figure 3). A relative destabilization of the s MOs of InPP1 and InPP2
a
,
Figure 4. TD-DFT spectra for PP1, PP2, and the metal complexes using the CAM-B3LYP functional
with either SDD or 6-31G(d) basis sets with green diamonds highlighting the oscillator strength
−
1
values for bands predicted to lie below 35,000 cm . Large red diamonds are used to highlight the Q
3
3
and B bands of Gouterman’s 4-orbital model. Simulation of the spectra was carried out using the
−
1
Chemcraft software package, Version 1.8 (build 536b) with fixed bandwiths of 1000 cm . Details of
the calculations are provided in Table 3.

Molecules 2021, 26, 3131
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Table 3. Electronic excitation bands of the synthesized porphyrins at the TD-DFT level of theory using the CAM-B3LYP
functional with either SDD or 6-31G(d) basis sets.
Band a
# b
Calc. c
Exp. d
15.4
17.1
3.7
Wave Function e
Q
Q
B
1
2
3
4
16.7
18.4
26.9
27.6
599
543
372
363
0.01
0.04
1.44
1.85
650
586
59% s → −s; 40% a → −a; . . .
59% s → −a; 40% a → −s; . . .
57% a → −a; 32% s → −s; . . .
58% a → −s; 39% s → −a; . . .
PP1
2
421
B
Q
B
1, 2
3, 4
18.3
27.5
545
364
0.01
1.63
17.0
23.5
587
425
53% s → −a/−s; 46% a → −a/−s; . . .
ZnPP1
GaPP1
InPP1
46% a → −a/−s; 44% s → −a/−s; . . .
Q
B
1, 2
3, 4
18.0
27.1
554
368
0.01
1.48
17.0
23.6
589
424
50% s → −a/−s; 41% a → −a/−s; . . .
55% a → −a/−s; 42% s → −a/−s; . . .
Q
B
1, 2
3, 4
17.7
27.0
565
370
0.03
1.63
16.6
23.3
601
430
60% s → −a/−s; 39% a → −a/−s; . . .
58% a → −a/−s; 37% s → −a/−s; . . .
Q
Q
B
1
2
3
4
16.4
18.0
26.1
26.9
609
555
383
372
0.01
0.06
1.40
1.80
15.0
16.6
665
602
57% s → −s; 42% a → −a; . . .
59% s → −a; 40% a → −s; . . .
54% a → −a; 34% s → −s; . . .
58% a → −s; 38% s → −a; . . .
PP2
23.2
431
B
Q
B
1, 2
3, 4
18.0
26.7
556
374
0.01
1.54
16.6
23.1
602
433
50% s → −a/−s; 48% a → −a/−s; . . .
ZnPP2
GaPP2
49% a → −a/−s; 44% s → −a/−s; . . .
Q
B
1, 2
3, 4
17.9
26.7
560
375
0.00
1.43
16.6
23.0
603
434
50% s → −a/−s; 49% a → −a/−s; . . .
49% a → −a/−s; 48% s → −a/−s; . . .
Q
B
1, 2
3, 4
17.4
26.3
573
380
b
0.02
1.57
16.2
22.8
616
439
55% s → −a/−s; 43% a → −a/−s; . . .
InPP2
53% a → −a/−s; 40% s → −a/−s; . . .
a
c
Band assignment referred to in this work. Energy states assigned in ascending order within the TD-DFT calculation. Calculated
Calc) band energies (10 cm ), wavelengths (nm) and oscillator strengths in brackets (f). d Experimental (Exp) energies (10 cm ) and
3
−1
3
−1
(
e
wavelengths (nm), The wave functions based on the eigenvectors predicted by TD-DFT. One-electron transitions between the
s MOs are highlighted in bold. Only contributions greater than 10% are provided.
a,
s
,
−a and
−
2
.4. Suitability for Electron Injection
Figure 3 compares the relative HOMO and LUMO energies of the PPs studied here
with the energies of the valence and conduction band (VB and CB respectively) edges of
TiO . It can be observed from the DFT calculations (Figure 3) that the HOMO and LUMO
2
energy levels of the photosensitizers fit the requirements for an efficient photosensitizer [50],
since the LUMO energies of the photosensitizers lie between
−
2.48 and
−
3.34 eV, well above
the TiO CB (ca. −4.0 eV), implying that efficient electron injection from the photoexcited
2
sensitizer into the TiO CB should be viable [51]. It was anticipated that the stabilization of
2
the frontier π-MOs associated with the incorporation of Ga(III) and In(III) ions (Figure 3)
would better match the TiO CB due to the electron-withdrawing effect of the trivalent ions
2
and that this would be further enhanced by the electron-withdrawing properties of the
meso-5-bromothien-2-yl rings of PP2.
2
.5. Characterization of the Porphyrin-TiO (P-TiO ) Photocatalysts by Diffuse
2 2
Reflectance Spectroscopy
UV-visible diffuse reflectance spectra (Figure 5) provide evidence for the presence of
porphyrins on the TiO surface. TiO does not exhibit any absorbance in the visible region
2
2
at >400 nm, while the spectra of the
P
-TiO photocatalysts contain the characteristic B and
2
Q bands of the porphyrins. The presence of these bands demonstrates that porphyrins were
successfully loaded onto the surface of TiO . The photocatalysts are therefore anticipated to
2
exhibit a significantly broader absorption range. To infer the amount of porphyrin loaded
in each system, the intensity of the porphyrins Soret bands were compared using the nor-
malized diffuse reflectance spectra (Figures S7 and S8). The analyses suggest that in general
PP1 porphyrins attach better to the surface of TiO than their PP2 counter parts. Within the
2
both PP1 and PP2 series, the attachment follows the trend Ga-PP > PP~In-PP >> Zn-PP.

Molecules 2021, 26, 3131
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Figure 5. Diffuse Reflectance spectra of the P-TiO photocatalysts.
2
2.6. Photocatalytic Activity of the Photocatalysts
The activity of the -TiO photocatalysts in the generation of hydrogen in aqueous
P
2
solution was evaluated for each of the photocatalysts. Eight photocatalysts, namely:
ZnPP1-TiO2, GaPP1-TiO2, GaPP2-TiO2, InPP2-TiO2 ZnPP2-TiO2, InPP1-TiO2, PP1-TiO2,
and PP2-TiO , were assessed in 1% methanol solution. All the photocatalysts exhibited the
2
ability to generate H (Figure 6), including bare TiO . PP1-TiO2, PP2-TiO and GaPP2-TiO2
2
2
2,
showed a superior photocatalytic activity rate for H generation relative to bare TiO . In
2
2
the case of native PPs, H production from PP1 is at least 30% higher than that found
2
for PP2. Although this could be related to slightly higher PP1 loading, differences in
the porphyrin loadings cannot explain the decrease of in production of H when using
2
metalloporphyrins, particularly Ga-PP and In-PP that show higher to similar loadings
compared to their native PPs. Therefore, the variation in the activity of the systems could
be a result of different electron dynamics between the surface of the dye and TiO . Freebase
2
porphyrins (PP1 and PP2) had better photosensitization than their metal complexes due
to more electronic coupling with the TiO surface. Both PP1 and PP2 being metal free
2
are flexible with fairly non-planar conformations thus can afford multiple adsorption
configurations due to variable dihedral angles. The freebase cavity can co-ordinate more
effectively with TiO by flat attachment favoring electron injection in comparison to the
2
metal complexes [52]. Additionally, metallophorphyrins have greater non-radiative decays
compared to their native porphyrins suggesting fast singlet state decay, which can hamper
electron injection into TiO2.
Figure 6. Hydrogen generation rates for the P-TiO photocatalysts in 1% methanol solution under
2
simulated solar light.

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2
.7. Mechanism of Photocatalysis
The proposed mechanism for the photochemical reactions of the sacrificial molecules
is illustrated in Scheme 2. Methanol was used as a sacrificial electron donor to facilitate the
generation of H , for simplicity Scheme 2 shows only the first oxidation step (formation
2
of hydroxymethyl radical) although further oxidation is expected [53]. Under simulated
solar irradiation, UV and visible light energy can be absorbed by the
P-TiO composite. In
2
addition to the direct excitation of TiO creating electron-hole pairs, injection of electrons
2
from the LUMO of photoexcited porphyrins into the TiO conduction band is also energeti-
2
cally favorable (Figure 3) for all
P
-TiO systems studied. This could generate a continuous
2
+
flux of electrons into the TiO CB that are readily available for the reduction of the H ion.
2
Significantly, more efficient H production was obtained when PP1-TiO and PP2-TiO2
2
2
composites were used. With the exception of GaPP2-TiO , the composites prepared from
2
metal porphyrin complexes have lower H production efficiencies than that obtained for
2
bare TiO . This could be related to the ability of the heavy metals to favor ISC and hence
2
populate triplet states (laying at lower energy levels). This is likely to hamper rather than
enhancing the electron flow from the LUMO of the photoexcited porphyrin to the TiO CB.
2
The presence of the heavy sulfur atoms of the meso-aryl rings of PP2 could explain why the
photocatalytic properties of the PP1-TiO composite were superior to those of PP2-TiO2.
2
Interestingly, however, the MPP2-TiO composites are consistently more efficient than the
2
analogous MPP1-TiO materials. This is consistent with the higher ΦF and
τ values for
F
2
these complexes in Table 2, which is consistent with greater population of the S state that
1
can be expected to enhance electron injection from the photoexcited sensitizer into the
TiO CB. In a similar manner, the ΦF and
τ values for PP1 are higher than those for PP2
F
2
(Table 2). It is noteworthy that although the InPP1 and InPP2 are predicted to lie closer in
energy to the TiO CB (Figure 3), it is the GaPP1-TiO and GaPP2-TiO materials that have
2
2
2
superior photocatalytic effects (Figure 6) among the metallophorphyrins studied.
Scheme 2. Proposed mechanism for H generation by TiO on sensitization with porphyrins [53–55].
2
2
3. Materials and Methods
3.1. Materials
Pyrrole, 4-bromobenzaldehyde, dichloromethane (DCM), methanol (MeOH), high
purity grade silica gel (35–60 mesh particle size, thin layer chromatography (TLC) plates,
sodium acetate, 5-bromo-2-thiophenecarboxaldehyde, indium(III) chloride, tetraphenyl-
porphyrin (H TPP), zinc tetraphenylporphyrin (ZnTPP) and gallium (III) chloride were
2
purchased from Sigma Aldrich (Munich, Germany). Hexane, propionic acid, sodium hy-

Molecules 2021, 26, 3131
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drogen carbonate, acetic acid and zinc acetate were purchased from Loba Chemie PVT Ltd,
Mumbai, India. Toluene and ethyl acetate were purchased from Fisher Scientific, Leices-
tershire, UK. Distilled water was obtained from an ultrapure filtration system (Millipore,
−
1
resistivity 18 mΩ cm ). TiO P25 was purchased from Univar Canada.
2
3.2. Instrumentation
The ground state electronic absorption spectra were obtained at room temperature
from an Agilent Technologies Cary 60 UV-Vis spectrophotometer in 1.0
cuvettes, while corrected emission spectra were obtained with a PTI Quanta Master spec-
trofluorimeter (Photon Technology International, of London, ON, Canada) and 0.4 1.0 cm
×
1.0 cm quartz
×
cuvettes. The IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Madison, WI,
USA), and centrifugation was carried out on an Eppendorf centrifuge 5804 R. Fluorescence
lifetimes were measured using a time-correlated EasyLife fluorescence lifetime fluorimeter
(Photon Technology International, of London, ON, Canada). The instrument response
function (IRF) was measured at the excitation wavelength (395 nm) using Ludox as the
scatterer. Mass spectrometry data were obtained from a Bruker AutoFLEX III Smartbeam
MALDI-TOF instrument (Billerica, MA, USA). The elemental analyses were carried out on
a Vario EL III Microcube CHNS Analyzer. H NMR spectra were obtained from a Bruker
AVANCE II 400 MHz NMR spectrometer (Burlinton, Ontario, Canada). with CDCl as
3
the solvent. Diffuse reflectance spectroscopy (DR) data were collected on a Varian Cary
1
00 UV-Vis spectrophotometer (Santa Clara, CA, USA) coupled to an integrating sphere
accessory. Samples were irradiated using a Luzchem SolSim CCP instrument (Ottawa,
Canada) providing simulated solar radiation approximately matching in intensity the
1.5 air mass (AM) solar spectrum. The hydrogen generated was detected and quantified
with a Perkin Elmer, Clarus 480 GC-TCD (Madison, WI, USA) using argon as a carrier gas
and a 5 A zeolite molecular sieve column.
3.3. Synthesis of Porphyrins
The aromatic meso-substituted porphyrins PP1 and PP2 were synthesized by a single
step acid-catalyzed condensation reaction of pyrrole with an appropriate aldehyde based
on the Adler-Longo method [56 57]. Their metal complexes were synthesized according to
literature methods [58 59]. Scheme 1 gives a summary of the synthesis procedure used and
the reaction conditions.
,
,
3.4. Photophysical Parameters
As illustrated in Equation (1), comparative methods were used to determine flu-
orescence quantum yield (ΦF) values. Tetraphenylporphyrin (H TPP) in toluene was
2
Std
used as a standard (
Φ
= 0.11) for the metal-free porphyrins (PP1 and PP2), while zinc
F
Std
tetraphenylporphyrin (ZnTPP) in toluene was the standard (
porphyrins [60].
Φ
= 0.033) for the metalated
F
F A n2
Std
Std
Φ =
Φ
F
(1)
F
ꢀ
ꢁ
2
FStdA n
Std
In this equation, A and AStd are the optical densities for the sample and standard
solutions respectively, while F and F are the integrated areas under the emission curves
for sample and standard, n and n are the refractive indices of the solvents. The radiative
Std
Std
r
(k ) and non-radiative (knr) rate constant data were obtained by calculation according to
Equations (2) to (3) [61].
Σ k = τ1
(2)
F
kr = Φ × Σ k
knr = Σ k − kr
(3)
(4)
F

Molecules 2021, 26, 3131
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Fluorescence lifetime ( F
τ
) values were determined using a time-correlated EasyLife
X filter fluorescence lifetime fluorimeter. Time-resolved fluorescence decay curves were
analyzed by deconvoluting the observed decay with the IRF to obtain the intensity decay
function. Reduced chi-squared values were evaluated and considered to determine the
best fit for experimental data.
3.5. Theoretical Calculations
Quantum calculations were performed using the Gaussian 09 program [62] and the
theoretical model of choice was density functional theory (DFT). DFT is useful in providing
direct information on electronic structure and strikes a favorable balance between compu-
tational cost and accuracy [63]. The gas-phase geometries of the porphyrin monomer struc-
tures were optimized by Becke’s 3-parameter Lee Yang Parr (B3LYP) exchange-correlation
functional. Linear time-dependent DFT (TD-DFT) calculations were performed with the
Coulomb Attenuating Method B3LYP (CAM-B3LYP) functional. Mixed basis sets were used
for these calculations; 6-31G(d,p) and Stuttgart-Dresden (SDD), the latter being preferred
for porphyrin monomers with In(III) or Ga(III) metal ions. Simulation of the spectra was
−
1
carried out using the Chemcraft software package with fixed bandwidths of 1000 cm
.
3
.6. Supporting Porphyrins on TiO P25
2
A procedure reported by Duan and co-workers in 2010 [64] was followed to deposit
the porphyrins on the surface of TiO . The porphyrins (10 mg) were dissolved in DCM
2
(
8 mL) in eight separate vessels. To each solution, TiO (0.4 g) was added, and the mixture
2
was sealed. The resulting suspension was sonicated for 30 min, then stirred at ambient
temperature for 12 h. The photocatalyst was collected by centrifugation after washing three
◦
times with DCM and left to dry at 60 C for 12 h.
3.7. Photocatalytic Hydrogen Generation
Photocatalytic hydrogen generation studies were carried out according to the proce-
dure initially used by Hainer et al. [11]. These studies were carried out in a 10 mL glass vial
sealed with crimp seals and rubber septa. In each experiment, 10 mg of the photocatalyst
and 4 mL aqueous methanol solution (1%) was added to the glass vial. The system was
degassed under argon for 30 min and sealed in the vacuum degassing chamber. The
catalyst suspension was dispersed for 5 min in an ultrasonic bath then irradiated from the
top for 4 h under simulated solar light with magnetic stirring. Headspace gas (4 mL) was
sampled from each vial by a sample lock syringe through the septum and immediately
injected in the GC-TCD. The H signal was detected at ~4.0 min. A calibration curve for
2
H gas detection in the GC-TCD instrument was used for quantification.
2
4. Conclusions
In conclusion, meso-tetra(4-bromophenyl)porphyrin PP1 and meso-tetra(5-bromo-2-
thienyl)porphyrin PP2 and their Zn(II), (Cl)In(III) and (Cl)Ga(III) complexes were suc-
cessfully synthesized and characterized. Diffuse reflectance spectroscopy demonstrated
that the porphyrins were successfully loaded onto the surface of TiO P25. PP2-TiO2,
2
PP1-TiO and GaPP2-TiO exhibited superior hydrogen generation compared to bare TiO
2
2
2
demonstrating their potential utility for photocatalytic H generation. A wide range of
2
porphyrin analogs exist with enhanced absorption in the Q band region, so research will
continue in identifying dyes that are best suited for this application by focusing on dyes
with long-lived and well-populated S excited states.
1
Supplementary Materials: The following are available online: details for the synthesis and character-
ization of the porphyrins as well as their 1H NMR, mass, fluorescence spectra and Diffuse reflectance
spectra (Figures S1–S8).

Molecules 2021, 26, 3131
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Author Contributions: Conceptualization was by E.K.A.; the methodology involves syntheses and
characterization of the porphyrins by S.O., E.K.A. and S.D.; quantum chemical calculations were
done by S.O. and J.M.; the original draft was written by S.O., E.K.A.; A.E.L., J.C.S., J.M. and T.N.
edited the original manuscript; Project administration and funding acquisition were by A.E.L., J.C.S.,
E.K.A. and T.N. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Canada’s International Development Research Centre (IDRC),
(
UID 108569-001), and by the Department of Science and Technology (DST) Innovation and National
Research Foundation (NRF), South Africa, through the DST/NRF South African Research Chairs
Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID 62620).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article or Supplementary Materials.
Acknowledgments: This work was financially supported by Canada’s International Development
Research Centre (IDRC), (UID 108569-001), and by the Department of Science and Technology (DST)
Innovation and National Research Foundation (NRF), South Africa, through the DST/NRF South
African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID
6
2620). Theoretical calculations were carried out at the Centre for High Performance Computing in
Cape Town.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of the compounds are available from the authors.
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