Design and synthesis of a fused porphyrin dimer for enhanced visible-to-near-infrared-driven photocatalytic hydrogen evolution

Abstract

A new acceptor–linker–donor–linker–acceptor (A–π–D–π–A) structured porphyrin dimer, F-C19ZnP, was designed and synthesized as a visible-to-near-infrared (Vis-NIR) light-harvesting photosensitizer for photocatalytic hydrogen evolution (PHE). F-C19ZnP features a bimetallic Zn(II)-coordinated fused porphyrin dimer as the donor, a phenylene–ethynylene π-linker, and a 3-ethylrhodanine acceptor. Its photophysical, electrochemical, thermal, and morphological properties, as well as photocurrent response, water contact angle, and PHE performance, were systematically investigated and compared with those of C19ZnP, a structurally similar monometallic porphyrin. Density functional theory calculations provided further insight into the optoelectronic and charge separation characteristics of both porphyrins. F-C19ZnP exhibited markedly broader light absorption (400–1100 nm) and stronger Vis-NIR light-harvesting ability than C19ZnP. F-C19ZnP also exhibits a higher surface area and a larger pore size than C19ZnP, providing more accessible channels for electron transport. It also showed a longer photoexcited electron lifetime (4.9 ns vs. 2.1 ns), enabling more efficient electron transfer to water and resulting in enhanced PHE performance. F-C19ZnP demonstrated a higher photocurrent response, signifying improved charge separation and transport. Its lower water contact angle, attributed to its well-dispersed 300 nm nanosphere morphology, enabled better interfacial interaction with water. Under optimized PHE conditions, F-C19ZnP achieved a PHE rate (ηH₂) of 7.93 mmol g⁻¹ h⁻¹, which is eight times higher than that of C19ZnP (1.01 mmol g⁻¹ h⁻¹), and outperformed C19ZnP at all tested irradiation wavelengths (420, 630, and 810 nm). Furthermore, F-C19ZnP displayed excellent photostability and reusability. As the first Vis-NIR fused porphyrin dimer for efficient PHE, F-C19ZnP paves a promising path for the development of advanced porphyrin-based photosensitizers.

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Introduction

Photocatalytic hydrogen evolution (PHE) from water splitting has become a green and sustainable technology alternative to fossil fuels due to its ability to convert solar energy into chemical energy.^1–3 A photocatalytic system for water splitting typically consists of a photosensitizer, a sacrificial electron donor (SED), a cocatalyst, and water as a proton (H⁺) source. Together, these components generate hydrogen (H₂) in the presence of light.^4,5 In recent years, a variety of photocatalytic systems using inorganic semiconductors,^6–11 inorganic complexes,^12,13 graphitic carbon nitride and its hybrids,^14–17 graphene-derived composites,^18,19 organic dyes,^20–22 conjugated polymers, such as conjugated porous polymers,^23–25 covalent organic frameworks,^26–28 covalent triazine frameworks,^29–31 and linear conjugated polymers,^32–35 metal–organic frameworks,^36–38 perovskite materials^39,40 and tetrapyrrole-based derivatives^5,41 as photosensitizers for PHE have been developed. Among them, tetrapyrrole-based photosensitizers such as porphyrins,⁴² phthalocyanines,^43,44 and corroles,⁴⁵ on the other hand, have recently received significant attention from the scientific community due to their synthetically tunable properties, which allow them to absorb UV-to-visible (UV-Vis) light while retaining suitable energy levels to drive various photocatalytic processes. Particularly, porphyrin derivatives are excellent UV-Vis absorption materials and, like chlorophylls, can act as natural light harvesters in photosynthesis.^5,46 In recent years, we and several other groups have diligently worked to develop effective porphyrin-based photosensitizers for PHE due to their easily adjustable optical and electrical properties through peripheral functionalization and variation of core metals, resulting in improved PHE performance.^5,47–53

However, most of the reported porphyrins with tetra-meso-substituted structures have demonstrated poor visible-to-near-infrared (Vis-NIR) absorption. Since the IR region constitutes 51% of the total solar spectrum, developing Vis-NIR light-harvesting porphyrins could potentially capture up to 98% of the solar spectrum, making them highly useful for practical PHE applications. Inspired by the design of push–pull configuration-based porphyrins in photovoltaics,^54–56 we have recently reported porphyrins with an acceptor–linker–donor–linker–acceptor (A–π–D–π–A) structural configuration for enhanced PHE.⁵⁷ These A–π–D–π–A-based porphyrins exhibited light-harvesting ability in the Vis-NIR region due to intramolecular charge transfer (ICT) from the porphyrin donor moiety to the rhodanine acceptor moiety. The presence of ICT in A–π–D–π–A porphyrins not only decreases the band gap by extending the light absorption range in the solar spectrum but also promotes the photoinduced charge separation and migration of charge carriers in the porphyrins. As a consequence, A–π–D–π–A porphyrins exhibited much higher PHE performance than typical tetra-meso-substituted zinc(II)-coordinated tetraphenylporphyrin (ZnTPP), which possesses inferior separation and migration of photoinduced charge carriers and absorption in the UV-Vis region only due to the lack of D–A interactions.

Therefore, we were motivated to develop a porphyrin dimer, F-C19ZnP, with an A–π–D–π–A configuration featuring a bimetallic Zn(II)-coordinated fused porphyrin dimer donor entity and enhanced Vis-NIR absorption for PHE (Fig. 1). Compared to the porphyrin monomer C19ZnP, which contains a monometallic Zn(II)-coordinated porphyrin donor moiety, the porphyrin dimer F-C19ZnP exhibits enhanced light-harvesting ability in the Vis-NIR region, a longer photoexcited state lifetime, and suitable redox potentials due to its rigid, extended π-conjugated core and bimetallic system. Furthermore, F-C19ZnP demonstrates efficient separation and migration of photoinduced charge carriers, as well as a lower water contact angle than C19ZnP.^58–62 F-C19ZnP also possesses a higher surface area and a larger pore size than C19ZnP, offering more accessible pathways for electron transport. The porphyrin dimer F-C19ZnP achieves a PHE rate (ηH₂) of 7.93 mmol g⁻¹ h⁻¹, which is much higher than that of the control porphyrin monomer C19ZnP, which has an ηH₂ of 1.01 mmol g⁻¹ h⁻¹. Notably, although fused copper(II)-coordinated porphyrin-based conjugated polymer thin films have been developed for both electrocatalytic H₂ evolution (EHE) and PHE,⁶³ and directly fused copper(II)-coordinated porphyrin dimers have been reported for EHE,⁶⁴ these materials do not possess an A–π–D–π–A configuration. To the best of our knowledge, no research on the use of A–π–D–π–A-based porphyrin dimers with a bimetallic Zn(II)-coordinated fused porphyrin dimer donor entity as photosensitizers in PHE has been reported so far.

Fig. 1 Structures of A–π–D–π–A-based porphyrins used in this study.

Results and discussion

Experimental section

The synthesis of F-C19ZnP is shown in Scheme 1. F-C19ZnP was unambiguously characterized by nuclear magnetic resonance (NMR) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF). Notably, in the ¹H NMR spectrum of porphyrin monomer 3, the meso proton resonance is observed at 10.06 ppm, while the corresponding β-pyrrolic protons appear in the 9.88–9.33 ppm region (Fig. S14). Upon formation of the triply fused porphyrin 4, the meso proton signal disappears, consistent with C–C bond formation at the meso positions, and the β–β and β′–β′ proton resonances exhibit slight shifts to the 9.89–9.29 ppm range (Fig. S16). F-C19ZnP is highly soluble in chloroform, dichloromethane, and tetrahydrofuran (THF) solvents. Porphyrin scaffold 1 was synthesized and characterized according to the previous report. C19ZnP was also synthesized for comparison.⁶⁵

Scheme 1 Synthetic route of the fused porphyrin dimer F-C19ZnP.

Photophysical and electrochemical properties

Fig. 2(a) shows the absorption spectra of C19ZnP and F-C19ZnP recorded in THF solution, with the corresponding data presented in Table 1. Both porphyrins exhibit two main types of absorption peaks. The absorption peaks appearing in the shorter wavelength regions (ca. 400–550 nm) correspond to the Soret-band of the porphyrin moiety, while the peaks located in the longer wavelength regions (ca. 600–850 nm) correspond to the Q-band of the porphyrin moiety. Moreover, the Soret- and Q-band peaks of F-C19ZnP are significantly more red-shifted than those of C19ZnP. This could be attributed to the strong electron-donating nature of the bimetallic fused porphyrin dimer donor moiety, which can enhance the ICT from the bimetallic fused porphyrin donor moiety to the rhodanine acceptor through phenylene-ethylene π-linkers in the A–π–D–π–A architecture. Notably, F-C19ZnP covers a broad absorption profile from 400 to 1100 nm, while C19ZnP exhibits an absorption profile from 400 to 800 nm with a deep valley in the range of 550–650 nm. This indicates that F-C19ZnP features stronger Vis-NIR light absorption than C19ZnP, which is far more beneficial for capturing more photons from the solar spectrum and, therefore, enhances the solar-driven applications of F-C19ZnP. However, when the absorption spectra of both porphyrins were recorded in the solid state (Fig. 2(b)), they displayed red-shifted Soret- and Q-bands compared to those in the THF solution, illustrating the formation of J-type aggregates in the solid state. This is commonly observed for rod-shaped A–π–D–π–A porphyrins in the solid state.⁵⁷

Fig. 2 Absorption spectra of C19ZnP and F-C19ZnP recorded in (a) THF solution (10 µM) and (b) the solid state (100 µM) at room temperature, (c) emission spectra of porphyrins recorded in THF solution (10 µM), (d) photoluminescence lifetime decay spectra of porphyrins recorded in THF solution (10 µM) (λ_ex = 380 nm), (e) cyclic voltammograms of C19ZnP and F-C19ZnP recorded in THF solution (200 µM) and (f) energy level alignment of components used in the PHE systems.

Table 1 Photophysical, electrochemical and thermal data of C19ZnP and F-C19ZnP

Porphyrin	λabs^a (nm) (ε, ×10^4 M^−1 cm^−1)	λabs^b (nm)	λem^a (nm)	τPL^a (ns)	ΦPL (%)^a	knr (×10^8 s^−1)	Td (°C)	EOX (V)	EHOMO^c (V) (vs. NHE)	ELUMO^d (V) (vs. NHE)	E0–0^e (eV)
a THF solution.b Spin-coated thin films.c EHOMO (vs. NHE) = 0.77 + EOX (vs. ferrocene).d ELUMO (vs. NHE) = EHOMO − E0-0.e Determined from the intersection of the tangent of the absorption peak and the x-axis by 1240/λ.
F-C19ZnP	516 (9.90), 692 (7.31), 752 (12.11)	540, 695, 760	804	4.9	15	1.69	400	0.39	1.16	−0.27	1.43
C19ZnP	472 (8.41), 500 (10.0), 696 (9.23)	518, 734	702	2.1	10	4.23	345	0.44	1.21	−0.44	1.65

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The emission spectra of F-C19ZnP and C19ZnP, recorded under excitation at 500 nm, which correspond to the Soret band of the porphyrin moiety, are shown in Fig. 2(c). Both porphyrins, F-C19ZnP and C19ZnP, showed emission peaks at 800 nm and 700 nm, respectively. Notably, F-C19ZnP exhibited an intensified emission peak with a 100 nm red-shift compared to C19ZnP. The red-shifted emission wavelength of F-C19ZnP compared to C19ZnP aligns well with the trends observed in the absorption peaks. The intensified emission peak of F-C19ZnP compared to C19ZnP indicates that the insertion of a fused porphyrin donor moiety in the A–π–D–π–A structure can suppress non-radiative and charge recombination channels due to the rigidity of the fused porphyrin donor moiety. The calculated photoluminescence electron lifetime (τ_PL) of F-C19ZnP is 4.9 ns, which is much longer than that of C19ZnP (2.1 ns) (Fig. 2(d)). The photoluminescence quantum yields (Φ_PL) of F-C19ZnP and C19ZnP were calculated to be 15% and 10%, respectively, which is consistent with the τ_PL of the porphyrins. These results precisely match the above description and help provide a long-lived photoexcited state τ_PL value for F-C19ZnP, consequently indicating its potential use as a photosensitizer over C19ZnP in PHE application. Moreover, based on the τ_PL and Φ_PL values, we have calculated the nonradiative decay constant (k_nr) for both porphyrins.⁶⁶ F-C19ZnP shows a k_nr value of 1.69 × 10⁸ s⁻¹, which is much lower than that of C19ZnP (4.23 × 10⁸ s⁻¹). This further demonstrates that F-C19ZnP reduces nonradiative decay channels more effectively than C19ZnP. On the other hand, the Φ_PL and τ_PL values of F-C19ZnP are much higher than those of our previously reported more efficient A–π–D–π–A porphyrin ZnP-CPDT with a cyclopentadithiophene (CPDT) π-linker (Φ_PL = 8.0%, τ_PL = 1.51 ns).⁵⁷ Thus, designing and synthesizing A–π–D–π–A porphyrins using a bimetallic fused porphyrin donor moiety is a very important strategy for achieving long-lived photoexcited states in A–π–D–π–A porphyrins.

The cyclic voltammograms of F-C19ZnP and C19ZnP recorded in THF solution are shown in Fig. 2(e), and the corresponding data are presented in Table 1. F-C19ZnP and C19ZnP exhibited oxidation potential (E_OX) values of 0.39 V and 0.44 V, respectively. The lower E_OX value of F-C19ZnP compared to C19ZnP can be attributed to the greater reducing ability of the electron-rich bimetallic fused porphyrin donor moiety compared to the monometallic porphyrin donor moiety. Based on the E_OX values, the calculated E_HOMO values of F-C19ZnP and C19ZnP porphyrins are 1.16 V and 1.21 V, respectively. On the other hand, the E_LUMO values of F-C19ZnP and C19ZnP were calculated using the equation: E_LUMO = E_HOMO − E_0–0.^67,68 The E_LUMO values of F-C19ZnP and C19ZnP were calculated to be −0.27 V and −0.44 V, respectively. The results indicate that F-C19ZnP and C19ZnP possess suitable E_HOMO and E_LUMO values for efficient charge separation and migration of holes and electrons, which is followed by PHE in the presence of a Pt cocatalyst and an ascorbic acid (AA) SED (Fig. 2(f)).

Morphological and thermal properties

The solid state morphology of F-C19ZnP and C19ZnP was studied using scanning electron microscopy (SEM) analysis. Porphyrin-coated thin films were prepared by drop-casting THF solutions of the porphyrins onto silica substrates. As shown in Fig. 3(a)–(d), both porphyrins formed nanospheres in the solid state. However, it was found that F-C19ZnP formed well-dispersed nanospheres with a size of approximately 300 nm, whereas C19ZnP exhibited stacked nanospheres with variable sizes ranging from 50 to 500 nm. This indicates that F-C19ZnP possesses a more controlled morphology in terms of both size and shape. Such morphology is much more beneficial for facilitating contact between water and a greater number of F-C19ZnP molecules. In contrast, the stacked nanosphere morphology of C19ZnP can limit the exposure of its molecules to water. Thus, a higher PHE performance is expected for F-C19ZnP than for C19ZnP during photocatalytic water splitting (vide infra). To further understand the morphology of the porphyrins, we performed powder X-ray diffraction (PXRD) analysis. As shown in Fig. 3(e), neither F-C19ZnP nor C19ZnP exhibited any sharp peaks. This indicates that both F-C19ZnP and C19ZnP possess an amorphous nature.

Fig. 3 Typical SEM images of (a) and (b) F-C19ZnP and (c) and (d) C19ZnP (note: all the images were taken for drop-cast thin films of porphyrins from their THF solution (100 µM)), (e) typical PXRD patterns of the porphyrins and (f) TGA curves of F-C19ZnP and C19ZnP.

Furthermore, to evaluate the relative porosities of C19ZnP and F-C19ZnP, N₂ adsorption–desorption isotherms were collected at 78 K (Fig. S3, SI). C19ZnP exhibits very low N₂ uptake across the entire pressure range, indicating a slightly nonporous solid. In contrast, F-C19ZnP shows higher gas uptake, consistent with the presence of greater porosity. Brunauer–Emmett–Teller (BET) analysis gives specific surface areas of 6.47 m² g⁻¹ for C19ZnP and 18.12 m² g⁻¹ for F-C19ZnP, confirming the higher accessible surface area of the fused porphyrin dimer. Correspondingly, pore size distribution analysis using the Horváth–Kawazoe method revealed dominant micropores of approximately 6.13 Å for F-C19ZnP and 2.98 Å for C19ZnP. The higher BET surface area and larger pore size of F-C19ZnP compared to C19ZnP can provide more accessible channels for electron transport and, subsequently, more efficient H⁺ reduction and thus enhanced PHE (vide infra).

The thermal decomposition temperatures (T_d) of F-C19ZnP and C19ZnP were evaluated using thermogravimetric analysis (TGA) measurements. As shown in Fig. 3(f), F-C19ZnP and C19ZnP exhibited T_d values of 400 °C and 345 °C, respectively. This demonstrates that F-C19ZnP, with its bimetallic fused porphyrin donor moiety, possesses higher thermal stability than C19ZnP, which has a monometallic porphyrin donor moiety. This result clearly indicate that designing A–π–D–π–A porphyrins with a rigid fused porphyrin donor moiety is an ideal strategy for constructing thermally stable photoactive systems.

Density functional theory (DFT) calculations

To understand the electronic structure, possible ICT behavior, and photoinduced charge separation properties of F-C19ZnP and C19ZnP, we performed DFT calculations. The front and side views of both porphyrins, F-C19ZnP and C19ZnP, exhibited planar conformations with varying angles (Fig. 4(a)). The dihedral angles between the two fused porphyrin moieties in F-C19ZnP are 0.29° and −0.22°. The dihedral angles at the edge of the π-linker are 0.05° and 0.17° for F-C19ZnP and C19ZnP, respectively. Moreover, the bond angle between the fused porphyrin donor moiety and the linker part is lower in F-C19ZnP compared to that in C19ZnP. This can enhance the ICT between the fused porphyrin donor moiety and the acceptor part through the π-linker. Thus, the calculated wavelengths of the absorption peaks of F-C19ZnP are red-shifted compared to those of C19ZnP (Fig. S1 and Table 2). This result is consistent with the experimentally observed red-shifted absorption peaks of F-C19ZnP relative to C19ZnP.

Fig. 4 (a) Ground state optimized molecular geometries of F-C19ZnP and C19ZnP (the hydrogen atoms are omitted for clarity and the long alkyl chains are simplified to methyl groups to reduce the calculation time) and (b) selected frontier molecular orbitals of F-C19ZnP and C19ZnP. Note: calculations were performed using the Gaussian 16 Rev B01 program package at the DFT-CAMB3LYP/GENECP/LANL2DZ level in the gas phase.

Table 2 Calculated electronic transition properties of the low-lying electronic excited states of F-C19ZnP and C19ZnP at the B3LYP/GENECP/LANL2DZ level

Porphyrin	State	Electronic transition	Energy^a (eV/nm)	f^b	Composition^c	CI^d
a Only the selected low-lying excited states are presented.b Oscillator strength.c Only the main configurations are presented.d The CI coefficients are in absolute values. Note: H and L indicate the HOMO and LUMO, respectively, and S and T indicate the singlet and triplet states, respectively.
F-C19ZnP	Singlet	S0 → S1	1.16/1068	1.6259	H → L	0.68367
		S0 → S6	1.91/647	1.5350	H → L+2	0.61919
		S0 → S13	2.30/537	1.4471	H−2 → L+1	0.53836
	Triplet	S0 → T1	0.90/1377	0.0000	H−1 → L	0.67844
		S0 → T2	1.24/994	0.0000	H−2 → L	0.56899
		S0 → T3	1.44/856	0.0000	H → L+3	0.65680
C19ZnP	Singlet	S0 → S1	1.68/733	2.2447	H → L	0.69292
		S0 → S6	2.48/499	0.5025	H−3 → L	0.61432
		S0 → S11	2.65/466	0.5416	H−2 → L+1	0.40831
		S0 → S14	2.91/425	0.7744	H−7 → L	0.45020
	Triplet	S0 → T1	1.08/1143	0.0000	H → L	0.66896
		S0 → T2	1.48/835	0.0000	H → L+1	0.44665
		S0 → T3	1.52/813	0.0000	H−1 → L	0.45921

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Moreover, the frontier molecular orbitals of both porphyrins are shown in Fig. 4(b). In both compounds, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are mainly localized over the central porphyrin macrocycle, indicating that the core unit dominates the primary optical transitions. To better assess the potential for ICT upon photoexcitation, particularly in the context of the molecular complexity of F-C19ZnP, we further examined the neighboring orbitals. The HOMO−1 remains localized on the porphyrin core, while the LUMO+1 orbital shows significant localization on the peripheral substituents, especially on the π-linker and the 3-ethylrhodanine acceptor group. This spatial distinction implies that while the initial HOMO–LUMO transition is localized on the central porphyrin unit, subsequent relaxation enables ICT from the electron-rich porphyrin core towards the electron-deficient 3-ethylrhodanine group. Given the strong electron-withdrawing nature of the rhodanine unit, it influences the stabilization of the LUMO+1 orbital, which participates in ICT, especially in a polar microenvironment. Moreover, the π-conjugated linker between the porphyrin donor macrocycle and rhodanine acceptor groups facilitates electronic communication, supporting multi-orbital-mediated charge separation pathways. This highlights the role of the π-linker and the 3-ethylrhodanine acceptor in modulating the photophysical behavior.

To gain more insight into charge distribution and donor–acceptor interactions within the F-C19ZnP and C19ZnP porphyrins, electrostatic potential (ESP) maps were constructed (Fig. S2). These maps visualize the spatial distribution of electron-rich and electron-deficient regions and the direction of ICT. In both molecules, the electron-rich regions (indicated by blue surfaces) lie on the central porphyrin core, whereas the electron-deficient regions (shown by red surfaces) are situated on the phenylene–ethynylene π-linker and 3-ethylrhodanine edges on both sides of the central porphyrin core. This distribution demonstrates electronic polarization across the conjugated framework, indicating a net electron transfer from the porphyrin donor towards the terminal acceptor moieties upon photoexcitation. Efficient electron delocalization from the porphyrin core to the outer acceptors facilitates the stabilization of the charge-separated state, which can promote the H⁺ reduction reaction for cocatalyst-supported PHE efficiency. This delocalization can further suppress the charge recombination process, which is beneficial for photocatalytic performance (vide infra).

The calculated dipole moments of F-C19ZnP and C19ZnP are 2.94 D and 2.93 D, respectively, which are very close. This implies that both porphyrins possess nearly identical ground-state polarity despite structural modification. This suggests that the incorporation of a fused porphyrin–linker framework in F-C19ZnP does not significantly change the overall direction and magnitude of the ground-state charge distribution compared with C19ZnP. In other words, the spatial arrangement of electron-donating and electron-withdrawing units leads to a comparable dipole vector orientation and extent.

PHE studies

The PHE performance of F-C19ZnP and C19ZnP porphyrins was evaluated by employing them as photosensitizers/photocatalysts in a heterogeneous photocatalytic system. In this setup, THF is used as a water-miscible solvent to achieve a facile dispersion of the porphyrins in water. Water serves as the H⁺ source, AA acts as the SED, and platinum (Pt) nanoparticles are employed as the cocatalyst. The entire system is irradiated with full-spectrum LED white light. To prepare the PHE systems, THF solutions of the porphyrins are added to water, resulting in the dispersion of the porphyrins, which subsequently self-assemble into nanoaggregates within the THF/water (1 : 1, v/v) solvent mixture. These nanoaggregates impart a heterogeneous character to the photocatalytic system. The formation of such aggregates is crucial, as they provide active catalytic sites similar to those found in conventional heterogeneous powder photocatalysts. This aggregation greatly improves the interaction between the photocatalyst and water, thereby facilitating efficient PHE under photocatalytic conditions. From Fig. 5(a), it can be observed that F-C19ZnP produced an ηH₂ of 7.93 mmol g⁻¹ h⁻¹, which is 8-fold higher than that of C19ZnP (1.01 mmol g⁻¹ h⁻¹). Moreover, the ηH₂ of F-C19ZnP is also 4.5-fold higher than that of our previously reported more efficient A–π–D–π–A porphyrin ZnP-CPDT with a CPDT π-linker (1.80 mmol g⁻¹ h⁻¹).⁵⁷ The higher ηH₂ of F-C19ZnP compared to C19ZnP can be attributed to the following factors: (i) strong light-harvesting ability in the Vis-NIR region, (ii) long-lived photoexcited state τ_PL value, (iii) well-defined nanosphere morphology with uniform size (vide supra), (iv) high BET surface area and large pore size (vide supra), (v) efficient separation and migration of photoinduced charge carriers, (vi) low electron transport resistance (R_ct2), and (vii) lower water contact angle (vide infra). Besides, the photocatalytic system of F-C19ZnP did not produce any H₂ in the absence of the AA SED and the Pt cocatalyst. This indicates that AA is crucial for reducing F-C19ZnP, and the Pt cocatalyst helps enhance charge separation in the photoexcited states of F-C19ZnP and the reduction of H⁺ to H₂. To further optimize the PHE of F-C19ZnP, the ηH₂ of F-C19ZnP photocatalytic systems was recorded at different concentrations of AA and variable amounts of Pt loading while keeping the F-C19ZnP concentration constant (0.1 mM) in a THF/water mixture (1 : 1, v/v). As shown in Fig. S4(a), the ηH₂ of F-C19ZnP photocatalytic systems increases with increasing AA concentration from 0.1 M to 0.4 M, but further increases in AA concentration decrease ηH₂. On the other hand, the ηH₂ of the photocatalytic system of F-C19ZnP increases with increasing Pt loading from 1 to 3 wt%, but further increases in Pt loading reduce ηH₂ (Fig. S4(b)). These results indicate that the F-C19ZnP photocatalytic system containing 0.4 M of AA and 3 wt% of Pt is the best-optimized condition for achieving high PHE. The PHE performance of F-C19ZnP was also tested in the presence of different SEDs such as triethylamine (TEA), triethanolamine (TEOA), and ethylenediaminetetraacetic acid (EDTA). Fig. S5 shows that the ηH₂ of photocatalytic systems containing TEA, TEOA, and EDTA is much lower than that of photocatalytic systems containing AA. This indicates that AA is the ideal SED for achieving enhanced PHE performance of the F-C19ZnP porphyrin. Using the optimized conditions mentioned above (0.4 M AA and 3 wt% Pt) for the F-C19ZnP photocatalytic system, the PHE performance of F-C19ZnP was further tested under different ratios of THF/water mixtures (Fig. S6). The results show that a 1 : 1 (v/v) THF/water mixture is the ideal solvent composition to achieve efficient PHE for F-C19ZnP.

Fig. 5 (a) The ηH₂ of photocatalytic systems of C19ZnP and F-C19ZnP under irradiation for 5 h: C19ZnP/F-C19ZnP (0.1 mM) + AA (0.4 M) + THF/water (1 : 1, v/v) + Pt (3 wt%), white LED light (full spectrum, 148.5 mW cm⁻²), (b) ηH₂ of photocatalytic systems of C19ZnP and F-C19ZnP under irradiation for 5 h: C19ZnP/F-C19ZnP (0.1 mM) + AA (0.4 M) + THF/water (1 : 1, v/v) + Pt (3 wt%), white LED light (420 nm (68 mW cm⁻²), 630 nm (49 mW cm⁻²) and 810 nm (46 mW cm⁻²)), (c) H₂ evolution of different cycles of the F-C19ZnP photocatalytic system: F-C19ZnP (0.1 mM) + AA (0.4 M) + THF/water (1 : 1, v/v) + Pt (3 wt%), white LED light (full spectrum); 0.4 M AA was re-added after the third cycle, (d) I–t response spectra of porphyrins; (e) water contact angle measurements of F-C19ZnP (top) and C19ZnP (bottom) in water; and (f) feasible PHE mechanism of photocatalytic systems. PS represents the photosensitizer, ET refers to electron transfer, and “Red” represents reduction.

To further understand the relationship between light-harvesting ability and PHE performance, we explored the wavelength-dependent ηH₂ of both F-C19ZnP and C19ZnP (Fig. 5(b)). These measurements were conducted under optimized conditions and full-spectrum white LED light irradiation for both photocatalytic systems of F-C19ZnP and C19ZnP. For F-C19ZnP, the ηH₂ values achieved were 0.41, 0.50, and 0.30 mmol g⁻¹ h⁻¹ at the wavelengths of 420 nm, 630 nm, and 810 nm, respectively, under white LED light irradiation. In contrast, C19ZnP produced significantly lower ηH₂ values of 0.10, 0.05, and 0.01 mmol g⁻¹ h⁻¹ at the same wavelengths. These results clearly establish the superior photocatalytic performance of F-C19ZnP across the Vis-NIR region compared to C19ZnP. More notably, the PHE performance of F-C19ZnP in the NIR region (specifically at 810 nm) is comparable to its activity in the visible region (420 nm and 630 nm). This signifies that F-C19ZnP maintains efficient PHE even under longer-wavelength light irradiation, a feature that is highly required for solar energy applications. In contrast, C19ZnP shows negligible PHE activity in the NIR region, demonstrating its limited light-harvesting range. These conclusions provide convincing evidence that the design and development of A–π–D–π–A-based porphyrins, predominantly those incorporating a bimetallic fused porphyrin donor moiety, represent a highly effective strategy for extending PHE activity into the NIR region. Such molecular engineering not only broadens the light absorption spectrum but also significantly enhances the PHE performance.

The apparent quantum yield (AQY) of F-C19ZnP was calculated to be 1.64%, 2.0%, and 1.2% at 420, 630, and 810 nm, respectively, while the AQY values of C19ZnP were 0.39%, 0.20%, and 0.04% at the same wavelengths (Fig. S7). These results are highly consistent with their observed PHE performance, further demonstrating the superior ability of F-C19ZnP to utilize NIR photons compared to C19ZnP. The photostability of F-C19ZnP was tested by recording the PHE of the F-C19ZnP photocatalytic system over 40 h of light irradiation. As shown in Fig. S8, the PHE of the F-C19ZnP photocatalytic system continuously increased with time, indicating excellent photostability of F-C19ZnP under prolonged light irradiation conditions. Additionally, the reusability of F-C19ZnP was tested by analyzing the PHE of the F-C19ZnP photocatalytic system over four consecutive photocyclic experiments under 20 h of light irradiation (Fig. 5(c)). In each cycle, the F-C19ZnP photocatalytic system produced similar ηH₂, indicating the good reusability of F-C19ZnP. Furthermore, we analyzed the morphology data of F-C19ZnP before and after 20 h of light irradiation in its photocatalytic system. Fig. S9 shows that the nanosphere morphology of F-C19ZnP did not change before and after photocatalysis, further confirming the photostability of F-C19ZnP under PHE conditions.⁵¹ To further assess the stability of F-C19ZnP during PHE, we recorded the UV-Vis absorption spectra of the photocatalytic system before and after 40 h of irradiation, as shown in Fig. S10. The absorption profile of F-C19ZnP remained nearly unchanged after prolonged light irradiation, confirming its excellent photostability. This result indicates that the porphyrin structure, including the Zn(II) center, remains intact under the reaction conditions. While we did not directly measure the Zn ion concentration in solution, the preserved absorption features strongly suggest that no significant decomposition or demetallation of the catalyst occurred during the photocatalytic process.

In heterogeneous PHE, the separation and migration of photoinduced charge carriers from the surface of the photosensitizer to the cocatalyst and SED influence the efficiency of H₂ evolution in photocatalytic systems.⁶⁹ To investigate how efficiently charge carrier separation and transport can occur for C19ZnP and F-C19ZnP, we performed photocurrent–time (i–t) response and Nyquist plot studies. Porphyrins coated on indium tin oxide (ITO) substrates were used to analyze the i–t curve responses and Nyquist plots. As shown in Fig. 5(d), F-C19ZnP exhibited a higher photocurrent density compared to C19ZnP, signifying the efficient separation and migration of photoinduced charge carriers, such as hole–electron pairs, for the former porphyrin. Additionally, we recorded a Nyquist plot of porphyrins under illumination conditions to estimate the electron transport rate. From Fig. S11, it can be seen that a smaller radius of the circle was observed for F-C19ZnP in comparison to C19ZnP. This demonstrates that F-C19ZnP possesses a lower R_ct2 than C19ZnP and, therefore, enhanced electron migration within F-C19ZnP under illumination conditions. Since F-C19ZnP possesses efficient hole–electron pair separation and higher electron transport than C19ZnP, the availability of electrons for transfer from the F-C19ZnP surface to the Pt cocatalyst is sufficiently high. This subsequently enhances the reduction of H⁺ to H₂. This result aligns with the higher PHE of the photocatalytic system of F-C19ZnP compared to the C19ZnP photocatalytic system.

Water contact angle measurements were performed to elucidate the interface between the porphyrins and water. As shown in Fig. 5(e), F-C19ZnP exhibited a water contact angle of 122°, which is lower than that of C19ZnP (132°), indicating that F-C19ZnP interacts more readily with water compared to C19ZnP. This enhanced interaction facilitates a more efficient electron transfer rate from photoexcited F-C19ZnP to H⁺, resulting in improved PHE performance. The well-dispersed nanosphere morphology of F-C19ZnP in water likely promotes easier access of water molecules to the photocatalyst surface, whereas the strongly stacked nanosphere morphology of C19ZnP restricts interactions at the interface between C19ZnP and water (vide supra), highlighting the crucial role of porphyrin morphology in enhancing PHE activity. Furthermore, the higher BET surface area and larger pore size of F-C19ZnP (vide supra) compared to C19ZnP offer more accessible channels for electron transport and, consequently, more efficient H⁺ reduction and enhanced PHE (vide supra). The increased surface area provides more active sites for water molecules to interact with the photocatalyst, while the larger pore size allows water to penetrate more easily into the interior part of the material, accelerating the accessibility of active sites within the pores. Both factors contribute to better dispersion of F-C19ZnP than C19ZnP in water and facilitate mass transfer, ultimately enhancing the overall PHE performance of F-C19ZnP. Improved wettability further ensures that the F-C19ZnP surface remains well-hydrated, which is essential for efficient charge transfer and the generation of reactive species during photocatalytic water splitting.

Besides, it should be noted that while our study demonstrates that F-C19ZnP forms well-dispersed nanospheres with uniform size, which likely facilitates better interaction with water and improved PHE performance, we propose that direct evidence connecting morphology to primary photophysical properties, such as longer τ_PL and enhanced charge separation, can be demonstrated based on the following experimental results. Absorption and emission studies (Fig. 2(a–c) and Table 1) reveal that F-C19ZnP exhibits significantly red-shifted and intensified Soret- and Q-band peaks, broader light absorption, and a longer τ_PL compared to C19ZnP. These features, along with a lower k_nr and a higher Φ_PL, indicate suppressed charge recombination and more efficient charge separation. Photocurrent and Nyquist analyses further confirm superior charge carrier separation and electron transport in F-C19ZnP. The well-dispersed nanosphere morphology, higher BET surface area, and larger pore size of F-C19ZnP likely enhance water accessibility and electron transfer, indirectly supporting improved photophysical behavior. Although a direct fundamental relationship between morphology and photophysical properties requires further investigation, our results strongly suggest that the controlled nanosphere morphology of F-C19ZnP complements its intrinsic photophysical advantages, leading to superior PHE performance.

To gain more insight into the PHE of porphyrins, we proposed a reasonable cyclic mechanism for the photocatalytic systems, as illustrated in Fig. 5(f). In the first step, upon light irradiation, porphyrins undergo photoexcitation; in the second step, the photoexcited porphyrins are reduced by gaining electrons from AA; and in the final step, the reduced porphyrins return to the ground state by donating electrons to the Pt cocatalyst for H⁺ reduction. In our previous studies, we have already demonstrated that the reduction of photoexcited porphyrins by SEDs such as AA and TEA can occur during the cyclic PHE mechanism.^50,69–71 Additionally, our photocatalytic experiments show that H₂ evolution does not occur in the absence of AA, indicating its essential role as an SED. The significant enhancement in ηH₂ with AA, compared to other SEDs, further supports its involvement in the reduction process. Notably, the photophysical properties of F-C19ZnP, such as the longer τ_PL and higher Φ_PL, suggest efficient charge separation and suppressed recombination, which are consistent with effective electron transfer from AA to the photoexcited porphyrin. Thus, based on all these results, we assume that the reduction of photoexcited porphyrins by AA is involved in the cyclic PHE mechanism, as demonstrated in the proposed mechanism in Fig. 5(f).

Conclusions

In summary, we designed and synthesized a new A–π–D–π–A-based porphyrin dimer, F-C19ZnP, featuring a bimetallic Zn(II)-coordinated fused porphyrin dimer donor entity, and studied its optoelectronic, thermal, morphological, and PHE properties. These were compared with those of the C19ZnP, which has a monometallic Zn(II)-coordinated porphyrin donor moiety. F-C19ZnP showed a stronger Vis-NIR light-harvesting ability (400–1100 nm) than C19ZnP, which has visible light absorption ability (400–800 nm). Additionally, F-C19ZnP exhibited a longer photoexcited τ_PL than C19ZnP due to the rigid bimetallic fused porphyrin dimer donor moiety. Moreover, F-C19ZnP formed well-dispersed nanospheres with a size of 300 nm in the solid state, whereas stacked nanospheres with various sizes (50–300 nm) were observed for C19ZnP. Furthermore, the well-dispersed nanosphere morphology of F-C19ZnP in water facilitated easy access of water molecules to the surface of F-C19ZnP. In contrast, C19ZnP, with its strongly stacked nanosphere morphology, restricted the interactions at the interface between C19ZnP and water. This also demonstrates that the morphology of porphyrins plays a key role in understanding the interface between porphyrins and water. F-C19ZnP further demonstrates increased surface area and pore size relative to C19ZnP, which enhance the accessibility of channels for electron transport. I–t and Nyquist plot studies revealed that F-C19ZnP displayed efficient photoinduced charge separation and a lower R_ct2 than C19ZnP, resulting in higher electron transport mobility in its photocatalytic systems. DFT calculations further supported the enhanced light-harvesting capability and photoinduced charge separation of F-C19ZnP. Under PHE conditions, F-C19ZnP produced an η_H2 of 7.93 mmol g⁻¹ h⁻¹, which is 8-fold higher than that of C19ZnP (1.01 mmol g⁻¹ h⁻¹). Additionally, F-C19ZnP also exhibited comparable PHE performance in the NIR region to that in the visible region, whereas negligible PHE performance in the NIR region was observed for C19ZnP. This illustrates that developing bimetallic fused porphyrin dimers represents a highly effective strategy for extending PHE activity from the visible to the NIR region. The PHE system of F-C19ZnP is highly photostable, as evidenced by enhanced H₂ evolution from the initial time to 40 h of light irradiation. Research on the effect of different metal centres in the fused porphyrin donor moiety and various linkers on the PHE performance of fused porphyrin dimers is underway in our laboratory.

Author contributions

G. B. Bodedla and V. Piradi designed, synthesized, and characterized (by NMR and MALDI-TOF) all the materials used in this study. They also performed experiments such as UV-Vis, photoluminescence, cyclic voltammetry, SEM, i–t response and Nyquist plot studies, water contact angle measurements, and PHE experiments, and wrote the original manuscript. M. Imran performed the DFT calculations and analyzed the results. J. Zhao supervised the DFT studies. W.-Y. Wong and X. Zhu supervised the research project and revised the manuscript. All the authors discussed the results and evaluated the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: materials and methods, synthesis, photocatalytic systems under different conditions, DFT calculations, NMR spectra, and MALDI-TOF spectra. See DOI: https://doi.org/10.1039/d5qm00920k.

Acknowledgements

G. B. B. acknowledges the financial support from the Start-up Fund for RAPs under the Strategic Hiring Scheme (P0048725) of the Hong Kong Polytechnic University. W. -Y. W. acknowledges the financial support from the RGC Senior Research Fellowship Scheme (SRFS2021-5S01), the Research Centre for Carbon-Strategic Catalysis (CE01 and CE41), the Hong Kong Polytechnic University (YXA2), the Research Institute for Smart Energy (CDAQ) and Ms Clarea Au for the Endowed Professorship in Energy (847S). X. Z. acknowledges the funding from the General Research Fund (HKBU12304320), the NSFC/RGC Joint Research Scheme (N_HKBU213/22), and the Initiation Grant for Faculty Niche Research Areas (IG-FNRA) (2020/21)-RC-FNRA-IG/20-21/SCI/06 from Hong Kong Baptist University.

Notes and references

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Footnote

† These authors contributed equally.

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