Design of AIEgen‐based porphyrin for efficient heterogeneous photocatalytic hydrogen evolution

Tetraphenylethylene (TPE)‐conjugated porphyrin TPE‐ZnPF is synthesized in high yield and characterized by single‐crystal X‐ray diffraction. The propeller‐shaped TPE groups not only enable exceptional aggregation‐induced emission (AIE) in the solid state but also abolish the strong π‐π stacking of porphyrin moieties and thus prohibit aggregation‐caused quenching (ACQ). TPE‐ZnPF aggregates feature long‐lived photoexcited states, which subsequently suppress non‐radiative decay channels and enhance emission intensity. Moreover, its aggregates show more efficient light‐harvesting ability due to the Förster resonance energy transfer from the TPE energy donor to the porphyrin core energy acceptor, well‐defined nanosphere morphology, and more efficient photoinduced charge separation than the porphyrin Ph‐ZnPF, which possesses ACQ and agglomerated morphology. As a result, an excellent photocatalytic hydrogen evolution rate (ηH2) of 56.20 mmol g‒1 h‒1 is recorded for TPE‐ZnPF aggregates, which is 94‐fold higher than that of the aggregates of Ph‐ZnPF (0.60 mmol g‒1 h‒1) without the TPE groups.


INTRODUCTION
[3] Typically, irradiation of solar light on a photocatalytic system containing a photosensitizer, sacrificial donor, cocatalyst, and water produces hydrogen.[6][7][8][9][10][11][12][13] In this context, porphyrin-based sensitizers received great interest due to their high light-harvesting ability in the UV-Visible region, multiple redox states for attaining efficient photoinduced charge separation, and high photostability during photocatalysis. [1]Besides, their optoelectronic properties can be easily adjusted by integration of chromophores at four meso-positions of porphyrin macrocycle and insertion of various metals inside the porphyrin ring and consequently tailorable PHE properties. [1]However, most porphyrins exhibit aggregation-caused quenching (ACQ) of photoluminescence in the solid state due to the strong π-π stacking of planar porphyrin moieties. [14,15]This tendency activates the nonradiative decay channels which further shorten the electron lifetimes (τ PL ) of photoexcited states and subsequently low photoluminescence quantum yields (Φ PL ) and therefore inferior PHE performance of porphyrin-based photosensitizers.
[18] Moreover, TPE enhances the photoluminescence of its derivatives in the solid state by suppressing non-radiative decay channels via restriction of intramolecular motions and consequently leads to improved Φ PL. [19] Hence, conjugation of TPE moieties to the meso-positions of the porphyrin ring through a suitable πlinker would suppress ACQ and enhance emission intensity through AIE which subsequently provides longer τ PL of photoexcited states in porphyrins in their aggregated states and hence expectable efficient electron transfer from photoexcited porphyrin aggregates to the proton for water reduction in the PHE process.Additionally, due to the emission range of TPE chromophore lying within 400-450 nm and Soret-band absorption of porphyrin ring falls in the range 400-450 nm, an efficient Förster resonance energy transfer (FRET) from TPE moieties to porphyrin ring can be possible and hence improved light-harvesting nature in the entire UV-Vis region for TPE-conjugated porphyrin can be obtained.More notably, though an iridium motif-conjugated porphyrin with AIE was developed for PHE, this porphyrin did not exhibit AIE completely in the aggregated states. [20]Furthermore, synthesis of such noble-metal-based iridium motif-conjugated porphyrin involves complicated synthetic protocol and is also not much photostable and hence is not economically viable for efficient and long-term PHE technology.Thus, we presumed that an easily synthesizable and cost-effective TPE-conjugated porphyrin is an ideal approach to advance the PHE technology.Keeping all these points in mind, we have developed a TPE-conjugated porphyrin TPE-ZnPF (Figure 1) through an easy synthetic protocol (Scheme 1) to study the effect of AIE on the PHE of TPE-ZnPF porphyrin aggregates.Controlled ACQ porphyrin Ph-ZnPF with phenylene moieties was also synthesized for comparison purposes.

RESULTS AND DISCUSSION
The synthesis of TPE-ZnPF and Ph-ZnPF is shown in Scheme 1.Both TPE-ZnPF and Ph-ZnPF were unambiguously characterized by nuclear magnetic resonance and matrix-assisted laser desorption ionization-time-of-flight. Especially, TPE-ZnPF was confirmed by single-crystal X-ray diffraction (CCDC 2219954) (Figure 2).Free-base porphyrin (TPE-PFNH2) of TPE-ZnPF was also confirmed by singlecrystal X-ray diffraction (CCDC 2219955) (Figure S3).The top and side views of single-crystal structures, the dihedral angles between moieties in single-crystal structures, and the packing diagram in the unit cell of single-crystals are shown in Figures S1-S8.

Optoelectronic and morphological studies
The absorption and emission spectra of porphyrins and TPE-OH are shown in Figure 3 and the corresponding data are noted in Table 1.As seen in Figure 3A, both porphyrins showed typical Soret-band (ca.420 nm) and Q-band (ca.500-650 nm) absorption peaks of porphyrin moiety in tetrahydrofuran (THF) solution.In addition, the peak at 320 nm belongs to the TPE moiety in TPE-ZnPF.This is witnessed as the appearance of absorption peaks at 320 nm for TPE-OH.Noteworthily, the absorption profile of TPE-ZnPF is highly red-shifted and broadened in THF/water (H 2 O) mixture solution compared to that in THF solution, implying the formation of J-type aggregates in THF/H 2 O. [15] In the case of Ph-ZnPF, the absorption profile is slightly redshifted and broadened in THF/H 2 O in comparison to that in THF.Obviously, TPE-ZnPF aggregates cover a much broader absorption spectrum from 290 to 920 nm than Ph-ZnPF aggregates.It indicates that the conjugation of UV light-absorbing TPE chromophores to the porphyrin core is a facile molecular design to effectively improve the lightharvesting property of porphyrins in the UV-Vis region.This result further signifies that the TPE-ZnPF can be employed as a potential photosensitizer for heterogeneous PHE (vide infra).The morphology of such aggregates was further studied by scanning electron microscopy (SEM) (Figure 4A,B and Figures S11 and S12).The aggregates of TPE-ZnPF possessed well-defined nanosphere morphology (150-250 nm), while agglomerates were observed for Ph-ZnPF.The h Estimated from the intersection of the normalized absorption and emission spectra.nano-sized aggregates of TPE-ZnPF are useful to provide a large active surface area and thereby more photocatalytic reactive sites on the surface of TPE-ZnPF and simultaneously enhanced PHE than agglomerated morphology of Ph-ZnPF. [21]Moreover, dynamic light scattering analysis (Figure S13) indicates that the average size of TPE-ZnPF aggregates is 189.0 nm which also matches the SEM analysis.As seen in Figure 3B, both porphyrins exhibited two emission peaks in the range of 600-700 nm under the excitation of Soret-band absorption (420 nm), revealing that they are related to the porphyrin ring.Importantly, when the emission spectrum of TPE-ZnPF was recorded under the excitation of TPE absorption (320 nm), the emission peak at around 370 nm which corresponds to TPE chromophore disappeared, and the intensity of porphyrin emission peaks was enhanced by 2-fold.This could be attributed to the complete FRET from the TPE energy donor to the porphyrin energy acceptor.Since the emission spectrum of TPE-OH and the absorption spectrum of Ph-ZnPF overlapped extremely well, there is an expected FRET from TPE groups to the porphyrin ring in TPE-ZnPF (Figure S18A). [22]The FRET occurrence is also much beneficial to enhance the light-harvesting ability and thereby stabilize the photoexcited states of TPE-ZnPF. [18,20]he AIE properties of TPE-ZnPF were examined in THF/H 2 O solvent mixture.As shown in Figure 3C, the emission intensity of TPE-ZnPF is gradually enhanced with increasing water fraction (f w ) from 0% to 95% in THF/H 2 O mixture.It indicates that integration of propeller-shaped TPE groups to the porphyrin ring would efficiently reduce the strong π-π stacking of TPE-ZnPF in the solid state and thus prohibit ACQ, which subsequently suppresses the nonradiative decay channels and induces the AIE property for TPE-ZnPF.The AIE property could further stabilize the photoexcited states of TPE-ZnPF and consequently lead to longer τ PL in the aggregated state.This was further confirmed by measuring τ PL of TPE-ZnPF in THF/H 2 O mixture and THF solvent.It was found that the calculated τ PL of TPE-ZnPF in f w = 95% of THF/H 2 O solvent mixture is 3.6 ns which is 2-fold higher than τ PL (1.7 ns) recorded in THF solvent.On the other hand, the calculated Φ PL of TPE-ZnPF in f w = 95% of THF/H 2 O mixture is also higher than that in THF solvent (Table 1).This result is also consistent with the τ PL values.Similarly, the free-base porphyrin TPE-PFNH2 of TPE-ZnPF also showed the AIE property in THF/H 2 O solvent mixture (Figure S19B).On the contrary, the control porphyrin Ph-ZnPF possessing only phenolic moieties showed decreased emission intensity when increasing f w in THF/H 2 O mixture (Figure 3D), which indicates that Ph-ZnPF severely suffers from ACQ.The τ PL and Φ PL values of Ph-ZnPF also depict the strong ACQ (Table 1).On the other hand, the TPE derivative TPE-OH has shown excellent AIE in THF/H 2 O mixture (Figure S19A).All these results clearly demonstrated that the integration of TPE moieties to the porphyrin core in TPE-ZnPF can provide long-lived photoexcited states by reducing the non-radiative decay channels and thus higher PHE performance compared to Ph-ZnPF (vide infra).
The excited-state oxidation (E (P + /P * ) ) and reduction (E( P * /P -) ) potential values of TPE-ZnPF and Ph-ZnPF were estimated by performing cyclic voltammetric experiments (Figure S20A). [23,24]The data is shown in Table 1.As shown in Figure S20B, the energy levels of E (P + /P * ) of both porphyrins are high-lying than H 2 /H + reduction potential and the energy levels of E( P * /P -) are low-lying than TEA redox potential.It suggests that electron transfer from photoexcited states of porphyrins to proton and regeneration of oxidized porphyrins by TEA are thermodynamically favorable during the PHE cycle.

Theoretical calculations
Theoretical calculations were carried out for the rationalization of the photophysical properties of the porphyrins.The optimized ground state geometries of TPE-ZnPF and Ph-ZnPF are shown in Figure 5A,B.For both cases, the dihedral angles between fluorinated aryl rings and porphyrin core are almost the same (71.7 • -73.6 • ), which is typical for mesotetra(phenyl)porphyrin.However, in TPE-ZnPF the dihedral angles between TPE units and fluorinated phenyl rings are in the range of 62.7 • -65.5 • , which are smaller than the dihedral angles between the adjacent phenyl moiety and fluorinated phenyl rings in Ph-ZnPF (82.1 • ).This smaller dihedral angle in TPE-ZnPF may induce larger electronic coupling between the two units.This result is in accordance with the efficient FRET from TPE units to porphyrin moiety (vide supra).Moreover, the highest occupied molecular orbital (HOMO) of TPE-ZnPF is completely confined to the porphyrin moiety, and the lowest unoccupied molecular orbital (LUMO) is localized on the TPE unit; therefore, the transition of HOMO to LUMO consists of charge transfer feature from TPE units to porphyrin center (Figure S10).On the contrary, both HOMO and LUMO are localized on the porphyrin ring in Ph-ZnPF indicating a lack of charge transfer ability (Figure S10).The TPE-ZnPF showed a higher dipole moment value of 1.08 D than Ph-ZnPF with 0.67 D. Since TPE-ZnPF possesses charge transfer character and high dipole moment value, it is expected to have a more efficient photoinduced charge separation than Ph-ZnPF and hence higher PHE for the former (vide infra). [25]

PHE studies
PHE was evaluated in a photocatalytic system consisting of porphyrin photosensitizer (PS), TEA sacrificial donor, Pt cocatalyst, and THF/H 2 O proton source.Typically, we used a THF/H 2 O (0.5:9.5, v/v) mixture to maintain the highest degree of AIE property and correlate the obtained PHE data with photophysical properties.As shown in Figure 6A, TPE-ZnPF produced a PHE rate (ηH 2 ) of 56.20 mmol g −1 h −1 .Under the same conditions, Ph-ZnPF delivered a ηH 2 of 0.60 mmol g −1 h −1 which is much lower than that of TPE-ZnPF.This result undoubtedly indicates that the AIE phenomenon played an important role to enhance the PHE of TPE-ZnPF.The effect of the degree of AIE on the PHE performance of TPE-ZnPF was studied as well.The ηH 2 of the TPE-ZnPF photocatalytic system increased with increasing f w in THF/H 2 O mixture (Figure 6B).This result clearly demonstrates that TPE groups reduced the strong π-π interactions between planar porphyrin moieties in the aggregated state of TPE-ZnPF which resulted in suppressed non-radiative decay channels and thus enhanced photoluminescence and thereby long-lived photoexcited states.Subsequently, this leads to an efficient electron transfer from long-lived photoexcited states of TPE-ZnPF to Pt cocatalyst for proton reduction.Moreover, the ηH 2 of the photocatalytic system of TPE-ZnPF is the highest value among noble-metal-free porphyrin small molecules reported so far. [1,26,27]The photostability of TPE-ZnPF was verified by calculating ηH 2 for four consecutive PHE experiments of the photocatalytic system under 20 h light irradiation (Figure 6C).From Figure 6C, the ηH 2 of the photocatalytic system does not show any obvious degradation in each cycle.The nanosphere morphology of TPE-ZnPF also did not change before and after 20 h light irradiation (Figure S14).It implies that TPE-ZnPF holds good photostability.
To further improve the PHE of TPE-ZnPF, we also varied the different sacrificial donors (Figure 6D) and the concentration of TEA (Figure S21), and the amount of Pt in the photocatalytic systems (Figure S22).Obviously, these results imply that the PHE data shown in Figure 6A corresponds to the most optimized PHE for TPE-ZnPF.Moreover, under the optimized PHE conditions of TPE-ZnPF, freebase porphyrin TPE-PFNH2 showed a lower ηH 2 than that of TPE-ZnPF.It indicates that Zn metal is very important for enhanced PHE.However, the ηH 2 of TPE-PFNH2 is 8-fold higher than Ph-ZnPF, indicating the importance of TPE conjugation to the porphyrin macrocycle.Nevertheless, the photocatalytic system of TPE-ZnPF in the presence of the chloro(pyridine)cobaloxime (CoPyCl) cocatalyst exhibited ηH 2 of 9.12 mmol g -1 h -1 which is 6-fold lower than that of the Pt cocatalyst-based photocatalytic system (Figure S23).The photoinduced charge separation and migration of charged carriers of photosensitizers also dictate the performance of the PHE system. [28]As the response in the photocurrent-time (i-t curves) relates to the photoinduced charged species separation of photosensitizer, the porphyrin aggregates coated on indium tin oxide (ITO) were prepared.As shown in Figure 7A, the TPE-ZnPF aggregates showed higher current density in the i-t curves than that of Ph-ZnPF aggregates indicating more efficient photoinduced charge separation and migration of charged species such as holes and electrons for the former porphyrin aggregates.This result is in accordance with the higher PHE of the photocatalytic system of TPE-ZnPF porphyrin aggregates than the Ph-ZnPF porphyrin aggregates photocatalytic system.A feasible PHE mechanism of the photocatalytic systems of porphyrins is shown in Figure 7B.Under light illumination, the ground state porphyrins undergo photoexcitation which results in photoexcited porphyrins and these highly reactive photoexcited porphyrins are further reduced by gaining electrons from the TEA.Subsequently, the reduced porphyrins transfer electrons to the Pt nanoparticles where protons are converted to hydrogen and the porphyrins finally return to the ground state.

CONCLUSIONS
A simple molecular design tactic was explored to convert porphyrins from ACQ to AIE in the solid state by integrating AIEgen TPE moieties into the porphyrin core.The developed TPE-ZnPF significantly showed FRET between TPE and porphyrin ring and thereby improved light-harvesting ability.
The AIE property further prompted long-lived photoexcited states of TPE-ZnPF in the solid state and thus longer τ PL value than Ph-ZnPF with ACQ phenomenon.Photocurrenttime correlation and DFT studies indicated that TPE-ZnPF possesses much more efficient photoinduced charge separation than Ph-ZnPF.TPE-ZnPF aggregates also possess well-defined nanosphere morphology while agglomerated morphology was assigned to Ph-ZnPF which indicates more photocatalytic reactive sites for TPE-ZnPF aggregates.As a consequence of high light-harvesting ability, long-lived photoexcited states, and elevated photoinduced charge separation, TPE-ZnPF aggregates produced a very high ηH 2 of 56.20 mmol g -1 h -1 , which is 94-fold higher than that of Ph-ZnPF (0.60 mmol g -1 h -1 ).

F I G U R E 1
Structures of porphyrins used in this study.

S C H E M E 1
Synthetic scheme of TPE-ZnPF and Ph-ZnPF.TA B L E 1 Photophysical and electrochemical properties of TPE-ZnPF, Ph-ZnPF, and TPE-OH.

F I G U R E 2
Single-crystal structure of TPE-ZnPF (hydrogen atoms are omitted for clarity).

F I G U R E 7
(A) I-t response spectra of porphyrins recorded on ITO substrate and (B) feasible PHE mechanism of photocatalytic systems: PS is a photosensitizer, ET refers to electron transfer, and Red represents reduction.