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Abstract

The optical limiting properties of a series of free base porphyrin dyes and their Sn(IV) complexes were analyzed on the nanosecond timescale in the visible region at 532 nm with the second harmonic of an Nd:YAG laser by the open-aperture Z-scan technique. The effect of introducing different meso-aryl groups was investigated with 2-bromophenyl (1), 3-bromophenyl (2), 4-bromophenyl (3), and methyl ester phenyl (4) groups and the corresponding Sn(IV) complexes (1–4-Sn). The most significant optical limiting parameters, including the effective nonlinear absorption coefficient (β_eff), the imaginary component of the third-order susceptibility (Im[χ⁽³⁾]), the second-order hyperpolarizability (γ), and the limiting fluence (I_lim) values, were analyzed to identify trends in the structure-property relationships. More favorable reverse saturable absorption responses were observed for dyes with para-substituted meso-aryl rings, and for the Sn(IV) complexes relative to the free base dyes.

Keywords

porphyrins Sn(IV) complexes Z-scan optical limiting photophysics

Introduction

Over the last few decades, a growing issue has been faced in the field of aviation safety due to irresponsible laser pointer use when aircraft are approaching runways. The use of lasers in a military context has also grown considerably. This has led to considerable research interest in developing optical limiting (OL) materials so sensitive optics and the human eye can be protected from intense incident laser pulses across the visible region and neighboring regions of the UV and NIR.¹ The goal is to identify materials that are largely transparent under ambient light conditions but attenuate the transmission of intense laser pulses. Molecular dyes, such as phthalocyanines and porphyrins, have been particularly significant in this context. The second harmonic of an Nd:YAG laser at 532 nm provides a way to test the OL properties of the molecular dye in the region where the human eye is most sensitive.¹ Several different mechanisms, such as nonlinear refraction (NLR), scattering (NLS) and absorption (NLA), can attenuate the transmission of light through OL materials.

The focus in this study is on the use of Z-scan measurements^2–4 to analyze the NLA properties of a series of porphyrin dyes. At higher incident light intensities, a reverse saturable absorption (RSA) response is observed.¹ In the context of molecular dyes, this can be caused by two-photon absorption (TPA) and excited state absorption (ESA), and its magnitude can be used to quantify the effective nonlinear absorption coefficient (β_eff), the imaginary component of the third-order susceptibility (Im[χ⁽³⁾]), and second-order hyperpolarizability (γ) values of the OL materials so that key trends in the structure-property relationships can be identified. The use of a nanosecond pulsed laser with a pulse width of 7 ns for the Z-scan analyses means that only an effective nonlinear absorption coefficient (β_eff) can be measured, since, unlike on the femtosecond timescale, multiphoton absorption processes other than simultaneous two-photon absorption can contribute to the observed RSA response.^5,6 ESA from either the S₁ or T₁ state are expected to be the dominant factor on the nanosecond timescale.

Porphyrins are potentially suitable for use as optical limiters due to their extended π-conjugation systems, resulting in a large number of ππ* excited states in both the singlet and triplet manifolds, which can facilitate both TPA and ESA in a manner that generates large RSA responses.^7,8 The optical spectroscopy of porphyrin ligands can be understood on the basis of molecular orbitals (MOs) in an M_L = 0, ±1, ±2, ±3, ±4, ±5, ±6, ±7, 8 sequence associated in a 16-atom 18 π-electron system corresponding to the inner ligand perimeter.^9,10 This results in a HOMO and LUMO with M_L = ±4 and ±5 angular nodal properties, and allowed and forbidden ΔM_L = ±1 and ±9 transitions, which give rise to an intense B (or Soret) band at ca. 420 nm and weaker Q bands in the 500−700 nm region. This results in minimal absorbance across most of the visible region under ambient light, consistent with the requirements for an optical-limiting material. The effect of introducing different meso-aryl groups was investigated with 2-bromophenyl (1), 3-bromophenyl (2), 4-bromophenyl (3), and methyl ester phenyl (4) groups and the corresponding Sn(IV) complexes (1-4-Sn), so structure-property relationships can be identified. A diamagnetic main group Sn(IV) metal ion was introduced into the central cavity of 1-4 to determine whether increased intersystem crossing to the triplet manifold due to the heavy atom effect results in enhanced optical limiting properties. The effect of having a bromine atom at the ortho, meta and para-positions of the meso-phenyl groups was also explored.

Experimental section

Materials

Spectroscopic grade dichloromethane and dimethylsulfoxide (DMSO) were obtained from Merck.

Synthesis

Porphyrin dyes 1-4 have been reported previously^11–14 and were prepared according to the Adler-Longo method.¹⁵ 3-Sn has been reported previously¹⁶ and was prepared along with 1-2-Sn and 4-Sn according to a previously reported literature procedure for the insertion of a dichloro Sn(IV) ion into the central cavity of a free base tetraarylporphyrin.¹⁷

1-Sn. Yield (75%); ¹H NMR (400 MHz, CDCl₃) δ_H / ppm 8.75 (s, 8H pyrrole), 8.25 (s, 4H), 8.00 (s, 4H), 7.60 (s, 4H), 7.10 (s, 4H). MALDI TOF-MS, calc. 1117.93 [C₄₄H₂₄Br₄Cl₂N₄Sn], m/z, found 1083.37 m/z, [M−Cl + H]⁺.

2-Sn. Yield (65%); ¹H NMR (400 MHz, CDCl₃) δ_H / ppm 9.00 (s, 4H), 8.85 (s, 4H), 8.35 (s, 4H), 8.00 (s, 4H), 7.85 (s, 4H), 7.60 (s, 4H). MALDI TOF-MS, calc. 1117.93 [C₄₄H₂₄Br₄Cl₂N₄Sn], m/z, found 1083.255 m/z, [M−Cl + H]⁺.

4-Sn. Yield (70%); ¹H NMR (400 MHz, CDCl₃) δ_H / ppm 8.75 (s, 8H), 8.50 (s, 8H), 8.25 (s, 8H), 4.12 (s, 12H). MALDI TOF-MS, calc. 1034.51 [C₅₂H₃₆N₄O₈Cl₂Sn], m/z, found 999.51 m/z, [M−Cl]⁺.

Instrumentation

The ¹H NMR spectra of 1-4 and 1-4-Sn were recorded in CDCl₃ with a Bruker AMX 600 instrument at 298 K. MALDI-MS data were measured in positive ion mode with a Bruker AutoFLEX III Smartbeam TOF/TOF instrument. α-Cyano-4-hydroxycinnamic acid was used as the matrix. UV-visible absorption spectra were recorded with a Thermo Scientific Evolution 350 instrument. The comparative method¹⁸ was employed to calculate fluorescence quantum yield (Φ_F) values with zinc tetraphenylporphyrin used as the standard (Φ_F = 0.039,¹⁹ DMSO). A similar method was used to determine the singlet oxygen quantum yield (Φ_Δ) values of 1-4-Sn in DMSO. Diphenylisobenzofuran (DPBF) was used as the singlet oxygen scavenger in DMSO. The photooxidation of DPBF was monitored by UV-visible absorption spectroscopy at 419 nm. Tetraphenylporphyrin was used as a standard (Φ_Δ = 0.52,²⁰ DMSO). An Edinburgh Instruments LP980 spectrometer and pump beams within the range 420−434 nm provided by an Ekspla NT-342B laser fitted with an OPO were used to measure triplet-state lifetimes. The Soret band absorbance of the sample solutions was maintained at ca. 1.5 in nitrogen-saturated DMSO by degassing for 20 min before the triplet excitation and absorbance measurement at 500 nm. The triplet state decay curves were fitted exponentially using OriginPro 9 software.

All Z-scan measurements were performed with a 15 cm focal length lens at 532 nm with a frequency-doubled Quanta-Ray Nd:YAG laser (1.5 J / 7 ns pulse length at FWHM) in near-Gaussian transverse mode at a fixed incident laser pulse energy of 35 µJ in a manner described previously.²¹ A repetition rate of 10 Hz was used to avoid cumulative thermal non-linearity effects. The circular beam was spatially filtered to eliminate higher-order modes. A 2 mm quartz cuvette was used to move the sample on a translation table through the lens focal point.

Results and discussion

Synthesis

Porphyrins 1-4 and their Sn(IV) complexes were prepared and purified according to literature procedures^15,17 and ¹H NMR spectroscopy and MALDI-TOF MS measurements were conducted to confirm their successful synthesis and ensure their purity (Figure 2).

Photophysicochemical properties

Porphyrins are potentially suitable for use as optical limiters, since they absorb weakly across most of the visible region (Figure 2). Free base tetraarylporphyrins are typically only weakly fluorescent and have moderately high singlet oxygen quantum yields.²² The Sn(IV) complexes that were prepared are only weakly fluorescent due to the presence of the heavy central Sn(IV) ion and enhanced intersystem crossing rates with Φ_F values in the 0.01−0.02 range in DMSO (Table 1). 1-4-Sn were also found to have moderately high singlet oxygen quantum yield values with triplet state lifetimes on the microsecond timescale (Table 1). ESA associated with their optical limiting properties is therefore expected to be associated primarily with the triplet manifold.

Table 1.

Photophysicochemical data for 1-4-Sn in DMSO.

	λ_max
	B band	Q bands	Φ_F	Φ_Δ	τ_F (µs)
1-Sn	431	563, 606	0.01	0.24	22
2-Sn	425	552, 590	0.01	0.21	16
3-Sn	429	563, 605	0.02	0.53	59
4-Sn	428	564, 603	0.02	0.44	5.0

Optical limiting properties

Porphyrin dyes and their Sn(IV) complexes are potentially suitable for use as optical limiters since they absorb relatively weakly across most of the visible region, including at 532 nm, the second harmonic of Nd:YAG lasers (Figures 1 and 2). Herein, we employ the Z-scan technique to investigate the NLA properties of 1-4 and 1-4-Sn in CH₂Cl₂ at an incident laser pulse energy of 35 µJ (Figures 3–5). 7 ns laser pulses were used. The B band absorbance of the solution was adjusted to 2.0 in each case. A significant RSA response was observed in the normalized transmission (T(z)) data measured for each sample (Figure 3). All of the dyes absorb in the Q band region at 532 nm (Figure 2). The linear absorption coefficient (α) values associated with this are incorporated into the Z-scan calculations (Table 2). At the incident laser pulse intensity used, the one-photon absorption associated with the non-zero α values results in a subsequent sequential ESA process from either the S₁ or T₁ states.^6,21 The OL properties were quantified from the T(z) data sets by fitting the RSA responses that are observed when the sample cuvette is translated in the z-direction of the laser light through a lens focal point. Equation 1, reported previously by Sheik-Bahae and co-workers, can be used for this^2,3:

T (z) = \frac{1}{\sqrt{π} q_{0} (z)} \int_{- \infty}^{\infty} \ln [1 + q_{0} (z) e^{- τ^{2}}] d τ

(1)

q₀(z) corresponds to the size of the RSA response obtained with a circular-shaped laser pulse as described in Equation 2:

q_{0} (z) = \frac{2 β_{eff} P_{0} L_{eff}}{π ω (z)^{2}}

(2)

Figure 1.

Structures of free base porphyrins 1-4 and their Sn(IV) complexes 1-4-Sn that were selected for study.

Figure 2.

(a) UV-visible absorption spectra of 2 and 2-Sn in CH₂Cl₂, MALDI-TOF MS data for 2-Sn, and (c) the ¹H NMR spectrum for 2-Sn as typical examples of the characterization data for the dyes and complexes studied. 532 nm is highlighted in the UV-visible absorption spectra, since this wavelength is used for the Z-scan measurements in Figures 3–5.

Figure 3.

Open aperture Z-scans for 1-4 and 1-4-Sn in CH₂Cl₂ at a fixed laser pulse intensity of 35 µJ.

Figure 4.

The input fluence plotted against the output fluence for 1-4 and 1-4-Sn in CH₂Cl₂ at a fixed laser pulse intensity of 35 µJ.

Figure 5.

Calculation of the I_lim values for 1-4 and 1-4-Sn in CH₂Cl₂ at a fixed laser pulse intensity of 35 µJ.

Table 2.

Optical limiting data for 1-4 and 1-4-Sn in CH₂Cl₂ at a fixed laser pulse intensity of 35 µJ.^a

	α (cm⁻¹)	β_eff (cm.GW⁻¹)	I_lim (J.cm⁻²)	Im[χ⁽³⁾] (esu)	γ (esu)
1	8.8	10	-	2.1 × 10⁻¹¹	–
2	4.5	7.0	-	1.5 × 10⁻¹¹	–
3	0.28	35	4.0	7.5 × 10⁻¹¹	1.6 × 10⁻²⁹
4	0.29	40	3.6	8.8 × 10⁻¹¹	1.8 × 10⁻²⁹
1-Sn	4.1	41	-	8.8 × 10⁻¹¹	–
2-Sn	1.6	32	-	6.8 × 10⁻¹¹	–
3-Sn	0.25	140	0.8	3.0 × 10⁻¹⁰	9.8 × 10⁻²⁹
4-Sn	0.23	85	1.4	1.8 × 10⁻¹⁰	6.0 × 10⁻²⁹

The absorbance at the B band maximum was kept fixed at 2.0.

where P₀ provides the peak power, and L_eff is the effective pathlength derived from the 2 mm cuvette pathlength L and the α value of the solution at 532 nm:

L_{eff} = \frac{1 - e^{(- α L)}}{α}

(3)

The beam width in Equation 2 is calculated with Equation 4 from the sample position (ω(z)):

ω (z) = ω_{0} \sqrt{1 + {(\frac{z}{z_{0}})}^{2}}

(4)

z is the distance on the translation table from the focal point of the lens, while the Raleigh length (z₀) is defined as πω₀²/λ. In this context, λ corresponds to the 532 nm wavelength of the second harmonic of the Nd:YAG laser, and ω₀ is the beam waist at z = 0, the distance from the center of the laser beam at which incident intensity is reduced to 1/e² of the on-axis value.

An analytical version of Equation 1 was used to calculate q₀(z) directly from the experimental T(z) data²¹:

\begin{aligned} T (z) = & 0.363 e^{(\frac{- q_{0} (z)}{5.60})} + 0.286 e^{(\frac{- q_{0} (z)}{1.21})} + 0.213 e^{(\frac{- q_{0} (z)}{24.62})} \\ + 0.096 e^{(\frac{- q_{0} (z)}{115.95})} + 0.038 e^{(\frac{- q_{0} (z)}{965.08})} \end{aligned}

(5)

When Equation 4 is substituted into Equation 2, q₀(z) is defined as:

q_{0} (z) = \frac{Q_{0}}{1 + \frac{z^{2}}{z_{0}^{2}}}

(6)

The maximum value of the fitted Gaussian-shaped curve, Q₀, is provided as:

Q_{0} = \frac{2 β_{eff} P_{0} L_{eff}}{{π ω}_{0}^{2}}

(7)

When the experimental RSA response is fitted with Equation 5, the FWHM and peak maxima values correspond to z₀ and Q₀, respectively. Equation 8 can then be used to derive the β_eff value:

β_{eff} = \frac{λ z_{0} Q_{0}}{2 P_{0} L_{eff}}

(8)

The imaginary component of third-order nonlinear susceptibility (Im[χ⁽³⁾]) can be derived directly from the β_eff value. This parameter quantifies how rapidly an optical limiter attenuates the incident laser beam. It can be determined using Equation 9²³:

Im [χ^{(3)}] = \frac{η^{2} ε_{0} c λ β_{eff}}{2 π}

(9)

In which η is the linear refractive index of the solvent, and ɛ₀ and c are the permittivity of free space and speed of light.

During the interaction between the molecular OL dyes and the incident photons, a second-order hyperpolarizability (γ) is induced, since a bias is generated in the average orientation of their permanent dipole moments. The magnitude of the hyperpolarizability can be determined with Equation 10:

γ = \frac{Im [χ^{(3)}]}{f^{4} C_{mol} N_{A}}

(10)

In which N_A and C_mol are Avogadro's constant and the molar concentration of the dye, and f is a Lorentz local field factor derived from Equation 11:

f = \frac{η^{2} + 2}{3}

(11)

Since the γ value is dependent on dye concentration, it provides the most suitable OL parameter for comparing data sets reported in the literature.

It has been reported previously that OL dyes with favorable properties should have Im[χ⁽³⁾] and γ values that lie between 10⁻¹²-10⁻¹⁰ and 10⁻³⁴-10⁻²⁹ esu,^5,23,24 respectively. The values for 3-4 and 3-4-Sn (Table 2) lie within the target ranges. In the context of 1, 2, 1-Sn and 2-Sn, very large α values were obtained at 532 nm, due primarily to aggregation effects. This means that the concentration of the dye could not be accurately determined. Since NLS is also likely to contribute significantly to the observed RSA response in this context, γ values associated with the NLA properties of the dyes could not be determined accurately (Table 2). It is noteworthy, however, that the observed β_eff values for 1, 2, 1-Sn and 2-Sn are significantly lower than those observed for 3, 4, 3-Sn and 4-Sn, where low α values were obtained since the dye remained in the linear range in terms of the Beer-Lambert law. Considerably larger non-linear responses were observed in plots of output fluence against input fluence for 3, 4, 3-Sn and 4-Sn (Figure 4).

The limiting threshold intensity (I_lim) provides the fluence value at the point within the RSA response where transmission decreases to 50% of a linear response (Figure 5). The International Commission on Non-Ionizing Radiation Protection provided guidelines for acceptable exposure to incident laser light.²⁵ It has been calculated previously, by taking into account that the average human blink reflex is 250 ms,⁶ that 0.95 J.cm⁻² is the recommended exposure limit for nanosecond pulsed lasers at 532 nm. The I_lim values for 3-Sn were found to be slightly lower than this (Figure 5 and Table 2), demonstrating that Sn(IV) meso-arylporphyrins are potentially suitable for use as optical limiting materials. This value could be lowered further by increasing the solution concentration or by preparing dye-embedded polymer thin films.^6,21,26

Conclusion

A series of Z-scan measurements was made for 1-4 and 1-4-Sn in CH₂Cl₂, keeping the absorbance value of the B band fixed at 2.0. More favorable RSA responses were observed for dyes and complexes with para-substituted meso-aryl rings due to enhanced solubility relative to the meta- and ortho-substituted dyes, and for the Sn(IV) complexes relative to the free base dyes due to the incorporation of the main group metal ion. An I_lim value of 0.80 J.cm⁻² was obtained for 3-Sn, which is below the recommended threshold for nanosecond timescale Nd:YAG lasers, demonstrating that Sn(IV) porphyrins are potentially suitable for use as optical limiting materials. The trends identified in this study will help guide the future rational design of porphyrin complexes for use as optical limiters.

Footnotes

Acknowledgements

Laser-related equipment and maintenance were provided by the Laser Rental Pool Programme of the Council for Scientific and Industrial Research (CSIR) of South Africa.

ORCID iDs

Gambolo Kabeya

Godfred Sebiawu

Nande Mgedle

Bokolombe P Ngoy

John Mack

Tebello Nyokong

Funding

This work was supported by the South African National Research Foundation (NRF), with grants to TN (uid: 62620) and JM (uid: 119259), and by a grant from the African Laser Centre.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

Data sets are available on request.

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Optical limiting properties of free base porphyrins and their Sn(IV) complexes on the nanosecond timescale

Abstract

Keywords

Introduction

Experimental section

Materials

Synthesis

Instrumentation

Results and discussion

Synthesis

Photophysicochemical properties

Optical limiting properties

Conclusion

Footnotes

Acknowledgements

ORCID iDs

Funding

Declaration of conflicting interests

Data availability

References