Supramolecular porphyrin as an improved photocatalyst for chloroform decomposition

Abstract

In this work, the outlying decoration of the free-base meso-(4-tetra) pyridyl porphyrin (H₂TPyP) with the RuCl(dppb)(5,5′-Me-bipy) ruthenium complex (here named Supra-H₂TPyP) is observed as an improved molecular photocatalyst for dye-mediated chloroform (CHCl₃) decomposition via one-photon absorption operating in the visible spectral range (532 nm and 645 nm). Supra-H₂TPyP offers a better option for CHCl₃ photodecomposition when compared to the same process mediated by pristine H₂TPyP, which requires either excited-state- or UV absorption. The chloroform photodecomposition rates for Supra-H₂TPyP as well as its excitation mechanisms are explored as a function of distinct laser irradiation conditions.

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1. Introduction

Chlorine-derived disinfectants like sodium hypochlorite, chloramine, and chlorine dioxide are important structures for the treatment of drinking water and swimming pools.^1–5 Despite their good effectiveness in killing pathogenic microorganisms, unfortunately, when these substances interact with natural organic matter (e.g., urine, sweat, and saliva) non-negligible amounts of undesired byproducts can be generated. Among them, chloroform (CHCl₃) brings significant concerns to health due to its cytotoxic effects.^1,6–8 In recent years, two major efforts have been conducted to deal with this problem: (1) the development of novel molecular sensors to detect chloroform in water,^9–11 and (2) the creation of methodologies to decompose this molecule in solution. Concerning the effort (2), although the biotechnological^12,13 and the pulsed-corona-discharges-assisted¹⁴ approaches have been lately tested, the dye-catalyzed photochemical-based approaches stand as low-cost, non-invasive, selective, and controllable alternatives.^15–20

The excited-state dynamics exhibited by porphyrins^21–25 set them as versatile molecular photocatalysts^15,26–29 that have been explored in fields like dye-sensitized solar cells,^30–33 photomedicine,^34–36 and artificial photosynthesis.^37–39 In the last decades, these macrocycle molecules have been explored as photocatalysts for dye-mediated decomposition of chloroform (CHCl₃) under one-photon absorption (OPA) and/or excited-state absorption (ESA).^15,16 Differently from visible light OPA, ultra-violet OPA (λ_exc = 266 nm) accesses the porphyrin's photooxidative excited states enabling the decomposition of CHCl₃ through an oxi-reduction reaction, which yields hydrochloric acid (HCl) and other products.¹⁵ More recently, ESA¹⁶ became an alternative approach to employ visible radiation for CHCl₃ photodecomposition.

The tunning of the excitation energy threshold for photooxidation could allow the photo-reaction to occur at lower- (higher-) energy OPA (ESA) but is still poorly explored. The synthesis of novel porphyrinic structures could facilitate such energy threshold controllability. Examples of synthetic approaches are: (1) the inner and outer macrocycle decoration;^21,40,41 (2) oligomer formation involving one or more types of porphyrins;^38,42–44 and (3) supramolecular porphyrin structures.^26,45–51 The attachment of metallic complexes (e.g., ruthenium-, rhenium- and iridium-derived) at the outer positions of porphyrins generates supramolecular structures that offer novel excited states for achieving the photo-reaction threshold while keeping original states from both the pristine porphyrin and the complexes.^{46,48,49,52–56}

In this work, a supramolecular structure originated from the attachment of the RuCl(dppb)(5,5′-Me-bipy) ruthenium complex at each (4-pyridyl) site of the meso-tetra(4-pyridyl) free-base porphyrin (H₂TPyP) is used as a molecular photocatalyst for CHCl₃ decomposition, allowing the reaction to occur via visible-light OPA. The influence of the excitation wavelength and mechanisms on the photo-reaction rates is also explored.

2. Materials and methods

2.1 Samples and spectroscopic measurements

The synthesis of H₂TPyP and its supramolecular H₂TPyP[RuCl(dppb)(5,5′-Me-bipy)]₄ structure (Supra-H₂TPyP, see Fig. 1) follows the procedures described in the literature.^47,57 The spectroscopic measurements were conducted with samples dissolved in CHCl₃ stabilized with amylene (NEON Inc.) used as received. Solution concentrations were kept below 10.0 μM to prevent spontaneous aggregation and inner filter effects. Absorption spectra were acquired with a JASCO V-670 spectrophotometer whereas steady-state photoluminescence (PL) spectra were measured using a setup composed of a Xenon lamp (ACTON); a monochromator (ACTON, model 300i) and a portable spectrophotometer (Ocean Optics), with the PL signal being detected in a 90° geometry relative to the excitation beam direction. Time-resolved PL experiments were conducted using a Time-Correlated Single Photon Counting (TCSPC) system (Horiba, Delta-Flex model with 27 ps of temporal resolution), equipped with a pulsed excitation source (λ_exc = 352 nm with 8.0 MHZ of repetition rate). The PL decays were collected at the maximum of each spectrum. Emission quantum yields^58,59 were calculated adopting H₂TPyP dissolved in CHCl₃ as standard²² and applying eqn (1):where, Φ_sa and Φ_st stand for the quantum yields of the sample (sa) and standard (st) solution, respectively. The quantities Abs_st (F_st) and Abs_sa (F_sa) are the absorbances at 420 nm (the integrated PL spectrum for the corresponding excitation) from the sample and standard solutions.

Fig. 1 (a) H₂TPyP and (b) Supra-H₂TPyP, with RuCl₂(dppb)(5,5′-Mebipy) ruthenium complexes attached at each (4-pyridyl) site of H₂TPyP. Here, 5,5′-Mebipy stands for 5,5′-dimethyl-2,2′-bipyridine (N–N), and dppb stands for 1,4-bis(diphenylphosphine) butane (P–P).

2.2. Photochemistry assays

For OPA in porphyrins, two sources (purchased from Laserline Inc), green-OPA (532 nm; ∼4.0 W cm⁻², 19 200 J cm⁻² total incident fluence, 90.0 minutes of irradiation) and red-OPA (635 nm; ∼5.0 W cm⁻², 30 000 J cm⁻² total incident fluence, 90.0 minutes of irradiation) were utilized. Here, ESA is promoted by a frequency-doubled Q-switched Nd-YAG laser (Quantel, model Q-smart 100, 6.0 ns FWHM, 532 nm excitation, 20.0 Hz of repetition rate, total incident fluence regime of ∼2500 J cm⁻²). The quantitative analysis of all irradiation conditions was performed by considering the effectively absorbed fluence instead of incident fluence. The relation between incident (F_I), transmitted (F_T) and absorbed (F_A) fluences is written as F_I = F_T + F_A, or .⁵⁸ From the Lambert–Beer law,⁵⁸ (with A(λ) corresponding to absorbance at the incident wavelength, for the non-irradiated solution) resulting in eqn (2):

F_A = F_I(1 − 10^−A(λ)) (2)

During all the spectroscopic measurements and irradiation processes, the samples were placed in sealed quartz cuvettes of 1.0 cm path length with four polished windows.

Control samples of H₂TPyP and Supra-H₂TPyP dissolved in CHCl₃ were kept in a dark environment at room temperature (∼22.5 °C) to evaluate their stability. Absorption spectra confirm that no spontaneous modifications occur in both samples for 240.0 minutes of storage, endorsing their stability. This implies their durability is at least three times greater than the time demanded to perform the photo-induced reactions discussed herein.

3. Results and discussions

3.1 Photophysics of Supra-H₂TPyP

As shown in Fig. 2, the absorption spectrum of Supra-H₂TPyP displays red-shifted B- and Q-bands relative to H₂TPyP, in agreement with literature.^16,60 Additionally, signatures belonging to the metallic complex are also observed (see intraligand bands located at around 311 nm).^47,61–65 Due to their superposition with H₂TPyP bands, spectral signatures associated with the complex's MLCT bands centered around 470 nm are not resolved but lead to an overall increase of the spectrum baseline^49,52,56 (Fig. 2b).

Fig. 2 Normalized absorption spectrum at (a) B- and (b) Q-bands. (c) Normalized PL spectrum (λ_exc. = 420 nm) and (d) fluorescence decays (λ_detec. = 650 nm) for H₂TPyP (green curves) and Supra-H₂TPyP (purple curves).

The Supra-H₂TPyP PL spectrum (Fig. 2c) shows a small redshift (∼1 nm) regarding H₂TPyP. This effect is accompanied by obtaining a 72% lower emission quantum yield for Supra-H₂TPyP concerning H₂TPyP²² (see Table 1), in agreement with previous reports.^49–52 The Supra-H₂TPyP also presents a variation in the relative intensity between electronic (Q(0,0)) and vibronic (Q(0,1)) emission bands with relation to H₂TPyP, which indicates that either (1) the involved H₂TPyP vibronic progressions are changing^52,53 or (2) a new band arises and overlaps with H₂TPyP original bands.^52,53 From TCSPC experiments it is observed that the mono-exponential decay (7.26 ns) exhibited by H₂TPyP in CHCl₃⁶⁰ evolves to a bi-exponential decay (τ₁ = 2.01 ns (54%) and τ₂ = 4.54 ns (46%)) in Supra-H₂TPyP (Fig. 2d and Table 1). Based on the literature,^49,52,53 and considering that isolated ruthenium complexes do not exhibit emission bands around the probed wavelength (650 nm), τ₁ is associated with Supra-H₂TPyP, while the lifetime τ₂ is associated with the perturbed H₂TPyP deactivation pathway.^49,52,53 This trend supports the hypothesis of a novel band in the steady-state PL spectrum⁵³ (Fig. 2c).

Table 1 Photophysical features of H₂TPyP and Supra-H₂TPyP, dissolved in CHCl₃. Here, τ₂ (τ₁) refers to the excited state lifetime of the former (newly-formed) relaxation pathway

	Bmax (nm)	Qmax (nm)	PLmax (nm)	τ1 (ns)	τ2 (ns)	Φ (× 10^−2)
H2TPyP	417	513	650	—	7.26 (100%)	1.61
Supra-H2TPyP	424	517	651	2.01 (54%)	4.54 (46%)	0.45

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3.2 Green-OPA irradiation of Supra-H₂TPyP

In chloroform, H₂TPyP is photostable under green-OPA irradiation.^15,16 This is not true for Supra-H₂TPyP under the same irradiation condition. Fig. 3 shows that the spectroscopic signatures of Supra-H₂TPyP are significantly modified as a function of the incident fluence. Because the modifications occur in different stages, the results are separately discussed in three fluence ranges: (i) 0–2400 J cm²; (ii) 2400–6000 J cm²; and (iii) 6000–19 200 J cm⁻².

Fig. 3 Evolution of the absorption spectrum of Supra-H₂TPyP under green-OPA excitation (532 nm, 0–19200 J cm⁻²) at (a) the UV bands (from 280 nm to 350 nm), (b) the B-band, and (c) the Q-band.

In range (i), both B- and Q-bands undergo a blue shift. The main Supra-H₂TPyP B-band, originally centered at 424 nm (0 J cm⁻²) (purple solid line), downshifts to 419 nm at 2400 J cm⁻² (olive solid line) while the Q_y(0,1) downshifts from 517 nm to 515 nm. This behavior suggests that Supra-H₂TPyP is undergoing a similar photodissociation as reported for the H₂TPyP[RuCl₂(CO)(PPh₃)₂]₄ tetraruthenated porphyrin.⁵³ The occurrence of photodissociation in the fluence range (i) would imply: (1) the decomposition of ruthenium complexes; and (2) the absence of processes associated with the fluence ranges (ii) and (iii).^15,16 However, our data show that (1) and (2) do not happen, see Fig. 3. It is seen that in the fluence range (i), the RuCl(dppb)(5,5′-Me-bipy) complexes in Supra-H₂TPyP, with intraligand bands spanning from ∼280–360 nm, undergo a sort of transformation, forming a novel supramolecular species, baptized PP-Supra-H₂TPyP. The determination of the type of transformation that is photomodifying RuCl(dppb)(5,5′-Me-bipy) is out of the scope of this work but we understand that photooxidation,^66,67 photoisomerization,^68–70 and photoinduced ligand loss^71,72 are possible processes driving the phenomenon.

In range (ii), the peak at 419 nm no longer blueshifts and starts developing into a novel spectroscopic signature centered at 444 nm, see Fig. 3b. The rise of this new signature occurs simultaneously with the enhancement of the Q_x(0,0)-band (around 648 nm). These are signatures associated with photo-protonation of PP-Supra-H₂TPyP.^{15,16,73–75} Moreover, the PP-Supra-H₂TPyP 419 nm peak redshifts in ≈2 nm, which is a signature for the protonation of the photomodified RuCl(dppb)(5,5′-Me-bipy).^16,76 In fact, a closer inspection of RuCl(dppb)(5,5′-Me-bipy) intraligand bands shows that the peak at ∼327 nm is enhanced (Fig. 3a). It is important to recall that protonation requires acid environments. This is reported for complexes similar to RuCl(dppb)(5,5′-Me-bipy), in which an acidic solution was created through the addition of aliquots of HCl.^77,78 Since we do not add any acid to our solutions, the observation of protonation of both the porphyrin and the photomodified RuCl(dppb)(5,5′-Me-bipy) confirms the release of HCl in the solution as a consequence of CHCl₃ photodecomposition via green-OPA.

In the range (iii), it is observed that protonated PP-Supra-H₂TPyP signatures, B-(≈444 nm) and Q_x(0,0)-(≈648 nm) bands, disappear, which is followed by the rise of new bands at ≈466 nm and ≈658 nm, respectively. The new features indicate that the protonated PP-Supra-H₂TPyP is forming aggregates (possibly J-aggregates), similar to those previously reported for H₂TPyP under ESA excitation.¹⁶ It is also observed that the intraligand band of the photomodified RuCl(dppb)(5,5′-Me-bipy) at 327 nm undergoes a decrease in intensity with no indications of further transformations.

The protonation of Supra-H₂TPyP also induces new signatures in the steady-state PL spectrum, which are in agreement with the results discussed above for UV-Vis absorption. Fig. 4a shows that the Supra-H₂TPyP PL spectrum (purple solid line) evolves to the protonated PL spectrum (red solid line).¹⁶ After protonation, the PL signal continuously decreases with increasing fluence, becoming almost null at 19 200 J cm⁻². This decrease in the PL magnitude is associated with a reduction in the emission quantum yield of the protonated species, which is characteristic of J-aggregation in range (iii).^{16,74,79–83}

Fig. 4 (a) PL spectra evolution with increasing fluence (λ_exc. = 444 nm). Supra-H₂TPyP PL decay (λ_detec. = 650 nm) under green-OPA (λ_exc. = 532 nm) for: (b) fluence range (i), and (c) fluence range (ii). (d) Fluence dependence of the τ₍₀₎/τ ratio for PP-Supra-H₂TPyP (green spheres) and its protonated counterpart (red spheres).

Fig. 4b shows that the Supra-H₂TPyP PL decay at 0 J cm⁻² (purple solid line), which initially displays two characteristic lifetimes (4.54 ns (46%) and 2.01 ns (54%)), continuously evolves into the PP-Supra-H₂TPyP decay profile at 2400 J cm⁻², with a dominant characteristic lifetime of ≈7.01 ns (91%). A minor contributing lifetime of ≈ 3.17 ns (4%) is also measured and understood to belong to the remaining Supra-H₂TPyP. In addition, a new PL decay with a characteristic lifetime of 0.98 ns (5%) associated with the protonated PP-Supra-H₂TPyP is observed, see Table 2. The results in Fig. 4c and Table 2 support that in the range (i) both Supra-H₂TPyP and PP-Supra-H₂TPyP coexist. When the range (ii) starts (2400 J cm⁻²), PP-Supra-H₂TPyP becomes predominant, coexisting with both Supra-H₂TPyP and the protonated PP-Supra-H₂TPyP. From 3600 J cm⁻² on, the percentage contribution of the lifetime associated with Supra-H₂TPyP becomes negligible.

Table 2 Excited state decay parameters showing the evolution of Supra-H₂TPyP under green-OPA. The values in parentheses in the lifetime columns represent the percentage contribution of each fitting exponential. χ² estimates the fitting precision

Fluence (J cm^−2)	τ1 (ns)	τ2 (ns)	τ3 (ns)	χ^2
0	2.01 (54%)	4.54 (46%)	—	1.06
1200	2.16 (17%)	6.39 (83%)	—	1.03
2400	3.17 (4%)	7.01 (91%)	0.98 (5%)	1.07
3600		7.08 (89%)	0.93 (11%)	1.04
4800		7.00 (82%)	0.75 (18%)	1.06
6000		6.94 (65%)	0.73 (35%)	1.02
7200		6.84 (42%)	0.66 (58%)	1.12

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At 7200 J cm⁻², the PP-Supra-H₂TPyP (protonated PP-Supra-H₂TPyP) lifetime decreases from 7.01 ns (0.98 ns) to 6.84 ns (0.66 ns). Fig. 4d shows that, at 7200 J cm⁻², the quenching of these lifetimes, τ₂₍₀₎/τ₂ (PP-Supra-H₂TPyP) and τ₃₍₀₎/τ₃ (protonated PP-Supra-H₂TPyP), are 1.02 and 1.47, which originates from exciplex formation.²² The quenching of the protonated PP-Supra-H₂TPyP is 144% more effective in comparison with that for PP-Supra-H₂TPyP. This result supports the presence of electrostatically-enhanced exciplex formed between the positively charged protonated PP-Supra-H₂TPyP and Cl⁻ ions dispersed in the solution.¹⁶ No discussion is addressed for range (iii) because the decrease in PL intensity invisibilized the PL decay measurements.

As discussed in the literature,^15,16 free base porphyrins (H₂TPyP and H₂TPP) under UV-OPA (λ_exc = 266 nm) and ESA (λ_exc = 532 nm) excitations are able to photodecompose CHCl₃ and form HCl with excitation energy thresholds located above the B-band energy (≈2.97 eV).^15,16 This implies that these free-base porphyrins never underwent photoprotonation under OPA operating at 475 nm (2.61 eV) or 532 nm (2.33 eV).^15,16 In this work, we were able to photoprotonate PP-Supra-H₂TPyP dissolved in CHCl₃ under green-OPA (≈2.33 eV), meaning HCl is being formed as a consequence of CHCl₃ decomposition. This process should not happen if the ground and the excited states of PP-Supra-H₂TPyP were similar in energy to the ground and the excited states of H₂TPyP. Since the excitation gap is fixed by the CW laser energy employed in green-OPA (≈2.33 eV), our results indicate two possible scenarios: (1) the involved ground state in PP-Supra-H₂TPyP must be higher in energy in comparison to the correspondent state in H₂TPyP and/or (2) the photooxidative excited state in PP-Supra-H₂TPyP must be lower in energy concerning H₂TPyP.

3.3 Other irradiation conditions

Fig. 5a and b show the same photo-induced modifications (PP-Supra-H₂TPyP formation, photo-protonation, and photo-aggregation) in the spectroscopic signatures of Supra-H₂TPyP for green-ESA and red-OPA irradiations, respectively. The successful decomposition of CHCl₃ under red-OPA irradiation (photon energy delivered ≈1.96 eV) sets a new lower limit for the excitation energy required to trigger the process.

Fig. 5 Evolution of the absorption spectrum of Supra-H₂TPyP dissolved in CHCl₃ and irradiated under (a) green-ESA (532 nm, 6 ns, 0–2500 J cm⁻²) and (b) red-OPA (633 nm, 0–19 200 J cm⁻²).

To understand the photodecomposition rates under the different laser irradiation conditions, the Supra-H₂TPyP B-band integrated area (from 360 to 490 nm) for each laser absorbed fluence (F_A) was normalized by the B-band integrated area (also from 360 to 490 nm) of the reference non-irradiated Supra-H₂TPyP, and plotted as a function of F_A (see Fig. 6). The exponential profiles obtained under each laser irradiation condition were fitted using A_F = e^(−kF_A), where A_F and k represent, respectively, the aforementioned normalized areas for each F_A and the net photomodification rate (given in cm² J⁻¹). The fitting results show that the rates obey the following hierarchy: green-ESA (k = 4.32 × 10⁻³ cm² J⁻¹) > green-OPA (k = 1.43 × 10⁻³ cm² J⁻¹) > red-OPA (k = 0.50 × 10⁻³ cm² J⁻¹), which shows that green-ESA leads to more efficient photoreaction processes in comparison with the OPA-based irradiations.

Fig. 6 Absorbed fluence-dependent evolutions of normalized B band's area comparing green-ESA (filled olive spheres), green-OPA (filled light-green circles), and red-OPA (filled red triangles) excitations applied to Supra-H₂TPyP.

4. Conclusions

Supra-H₂TPyP places itself as an excellent candidate for a broad-range OPA-visible-light active molecular photocatalyst in dye-mediated chloroform decomposition. This supramolecular structure displays good photoreaction rates under visible OPA irradiation, which is a simpler and more affordable excitation mechanism than visible ESA and UV-OPA. Our results show that Supra-H₂TPyP when combined with different excitation wavelengths can be used to controllably photodecompose CHCl₃ and, consequently, controllably photoprotonate Supra-H₂TPyP itself, which is also interesting for sensing chloroform in solution and other applications in fields like materials science.

Author contributions

J. M. S. Lopes: data curation, writing-original draft, writing – review & editing, investigation, visualization, formal analysis, methodology. A. A. Batista: writing-review & editing, data curation, formal Analysis, project administration, resources, validation. P. T. Araujo: data curation, writing-original draft, writing – review & editing, visualization, investigation, fund acquisition, supervision, project administration, validation, formal analysis, methodology. N. M. Barbosa Neto: data curation, writing – original draft, writing – review & editing, visualization, investigation, validation, formal analysis, methodology, supervision, resources, funding acquisition, project administration.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

Acknowledgements

The Brazilian authors are indebted to the Brazilian National Council for Scientific and Technological Development (CNPq – processes numbers: 306147/2020-3 and 425124/2018-5), São Paulo Research Foundation (FAPESP – process number: 2014/50869-6) and the Education Ministry (CAPES – Process number: 23038.000776/201754) via the projects of the National Institute for Science and Technology on Organic Electronics and with Amazonian Foundation for the Support of Studies and Research (FAPESPA – process number: 88881.159129/2017-01). N. M. B. N. is especially indebted to the Fulbright Foundation for its Visiting Professor Award Grant. P. T. A. acknowledges the National Science Foundation for supporting this work under the CAREER grant # CHE-1848418. The authors are also grateful to Prof. Sanclayton Moreira, of the Graduate Program in Physics of the Federal University of Para, for granting access to his experimental facilities.

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