Temperature and Viscosity Dependence of Gold ... - Wiley Online Library

DOI: 10.1002/ejic.201700722

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Luminescent Gold Nanodots | Very Important Paper |

Temperature and Viscosity Dependence of Gold Nanodot Luminescence Sungmoon Choi,[a] Yujin Jeong,[a] and Junhua Yu*[a] Abstract: Strong chelation by ligands is essential for the stabilization of gold clusters used to obtain stable gold nanodots. Higher viscosity and lower temperature increase the emission intensity and blueshift the emission wavelength of nanodots, as observed in rigid environments. This indicates that the gold nanodot may consist of a flexible core structure. The strong dependence of nanodot luminescence on the rigidity of the medium suggests that the luminescence quantum yield of gold nanodots can be further improved when a rigid framework is

applied for the stabilization of gold nanodots. Gold nanodots can potentially act as sensors for the temperature and viscosity of a medium. Well-designed short peptides not only impart extraordinary stability to gold nanodots in cell culture media, but also assist them in approaching cell surfaces for efficient cellular staining. Interestingly, gold nanodots regain temperature sensitivity upon binding to the cell surface, which is useful for increasing the signal-to-noise ratio.

Introduction

yield.[8] This type of structure prompts further investigation into whether the reported gold nanodots share the same characteristics as AuIn clusters that show luminescence properties as well.[9] Furthermore, it is essential to understand what factors determine the photophysical properties of gold nanodots. Due to their large size, proteins are widely used in gold nanodot production. A high signal-to-noise ratio in the emission of protein-stabilized gold nanodots has been observed in cellular matrices.[10] However, the large size of the protection group (protein) may interfere with the labeling efficiency of gold nanodots with protein stabilization. Peptides are excellent alternatives to proteins for noble metal cluster protection,[11] because they are relatively small but retain the necessary functional groups. In this report, we designed a series of peptides to produce luminescent gold nanodots and investigated the factors that determine the photophysical properties of gold nanodots. We found that the vibrational decay of excited gold nanodots significantly influenced their brightness.

The unique photophysical properties of noble metal clusters have inspired significant research focused on improving the stability of these clusters not only in solid matrices, but also in aqueous solution.[1] Contrary to recent achievements for luminescent silver clusters (silver nanodots) that emit at versatile spectral windows with excellent emission brightness, goldbased luminescent species still have room to improve.[2] Gold clusters are commonly obtained under the stabilization of a monolayer of small molecules, but many of these show only strong absorption and rarely emission.[3] Large proteins, mercapto species, and DNA molecules have been observed to assist the preparation of luminescent gold clusters (gold nanodots).[1e,4] The low toxicity of gold species has prompted their wide biological application.[5] Though a comprehensive structure paradigm for luminescent gold clusters (gold nanodots) has not yet been fully obtained, it is believed that partially oxidized gold clusters in AuI–thiolate complexes are the key components in these small-molecule-stabilized gold nanodots.[6] In addition, gold nanodots show significantly lower emission quantum yield (< 8 %) compared to silver nanodots (20–60 %).[6b,7] The low quantum yield of gold nanodots and their reported structure are reminiscent of AuIn clusters that exhibit a triplet excited state and less than 10 % quantum [a] Department of Chemistry and Education, Seoul National University, 1 Gwanak-Ro, Gwanak-Gu, Seoul 08826, South Korea E-mail: [email protected] http://pml.snu.ac.kr/ Supporting information and ORCID(s) from the author(s) for this article are available on the WWW under https://doi.org/10.1002/ejic.201700722. © 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. · This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Eur. J. Inorg. Chem. 2017, 4696–4701

Results and Discussion It is almost traditional that a thiol group is essential for the synthesis of gold clusters.[6b,12] Herein, we designed peptides with no thiol groups and examined the capability of these peptides to stabilize gold nanodots. In theory, these peptides should be able to stabilize gold nanodots in the following manner: electron-rich groups in the peptides form stable five- or sixmembered rings with gold atoms (Scheme 1). In addition, the thiol group in cysteine (C) and the amino group in lysine (K) strongly bind to gold species. Among the combinations of such amino acids, for example, aspartic acid (D), glutamic acid (E), asparagine (N), lysine (K), and histidine (H), HDCNKDKHDCNKDKHDCN (HDCN) showed the highest emission intensity after reducing a mixture of the peptide (0.5 mg)

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Communication and the optimized amount of gold ion (140 μM) in an aqueous solution (1 mL). This is followed in emission level by HDCHLHLHDCHLHLHCDH (HDCH) and KECDKKECDKKECDK (KECD) (Figure 1). The emission intensity of the gold nanodots stabilized with a peptide (LSHTKCKHLLHKCKTHSL, LSHT) with less powerful chelating residues, such as leucine (L), serine (S), and threonine (T), significantly decreased. Even in the presence of cysteine residues, peptides such as SCPTGCNSDDKCPCGN (SCPT) and VHLPPPVHLPPP (VHLP) have lost the capacity to stabilize gold nanodots, because the peptide contains only a few chelating residues. We excluded the possibility that the weak stabilization of nanodots by the peptide SCPT was due to the poor solubility of SCPT in water. No emission was detected when more hydrophobic solvents, specifically acetonitrile, ethanol, and toluene, were used. This suggests that stabilization of gold nanodots by strong chelation of protection groups is critical for the successful synthesis of luminescent gold nanodots. However, the presence of thiol–gold bonding is not indispensable to the generation of luminescent gold nanodots. When all the cysteine residues in HDCN were replaced with serine, the new peptide HDSNKDKHDSNKDKHDSN (HDSN) showed improved capability to stabilize gold nanodots, compared to the cysteine-rich KECD (Figure 1). These emission peaks exhibited a maximum between 650 and 670 nm, and excitation peaks were centered at 370 to 470 nm. However, there was no obvious peak, but merely a mild bump in the region of the absorption spectrum corresponding to the excitation spectrum, indicating that the synthetic yield of gold nanodots was low. The emission quantum yield of the HDCN-stabilized gold nanodot was calculated after subtracting the background absorption of the impu-

rities and found to be 8 %, among the highest reported results.[6b] These gold nanodots exhibit a microsecond lifetime (0.5 μs, Supporting Information Figure S1). All these photophysical properties of gold nanodots are reminiscent of those of the AuIn cluster that exhibits a triplet excited state.[8] However, different from the AuIn clusters, oxygen did not quench the emission of the peptide-stabilized gold nanodots, but enhanced the synthetic yield of nanodots (Supporting Information Figure S2). This indicates that the excited state of the gold nanodot is not a triplet state, since there was no triplet–triplet annihilation between the gold nanodots and oxygen molecules. It is more likely that the Laporte rule takes effect between the excited state and ground state. The slow transition between the excited and ground state of gold nanodots implies that the radiative rate for depopulation of the excited states strongly depends on the surrounding environment. Vibrational deactivation is a common pathway that lowers the luminescence quantum yield of triplet dyes and luminescent complexes.[13] The rigidity of the molecule structure per se, or that induced by a rigid matrix in which the molecule resides, may suppress the non-radiative decay and subsequently increases the luminescence quantum yield.[14] When the HDCN-stabilized gold nanodot was dissolved in the aqueous glycerol solution, its emission intensity increased with the concentration of glycerol (Figure 2a and 2b). The emission intensity of nanodots in the 80 % (v/v) glycerol solution was 1.6fold higher than that in water, indicating that the increasing viscosity of glycerol slowed the vibrational deactivation of the excited gold nanodot. We found that the DNA-stabilized gold nanodots exhibited the same emission intensity dependence

Scheme 1. Five- and six-membered ring chelation of the residues in a peptide to stabilize gold nanodots.

Figure 1. Gold nanodots under stabilization by various peptides. (a) Absorption spectrum of HDCN-stabilized gold nanodots. (b) Excitation spectrum of HDCNstabilized gold nanodots and emission spectra of gold nanodots in the presence of various peptides. Eur. J. Inorg. Chem. 2017, 4696–4701

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Communication on solvent viscosity, whereas the DNA-stabilized silver nanodots did not (Supporting Information Figure S3). This suggests that viscosity dependence is a common character of the luminescence of gold nanodots. A similar trend was observed in reverse

micelles. The gold nanodot showed a brighter emission in the Igepal Co-520/cyclohexane reverse micelles at a lower waterto-oil (w/o) ratio, in which the water nanopool was more rigid (Figure 2c and 2d).[15] The intensity of the gold nanodot de-

Figure 2. Gold nanodots in rigid environments. (a) Emission (right) and excitation (left) spectra of the HDCN-stabilized gold nanodot in aqueous glycerol solution. The glycerol/water ratios (v/v) are given in the legend. (b) Plot of the emission intensity of gold nanodots vs. glycerol concentration. (c) Emission spectra of the above nanodots dissolved in reverse micelles at various water to oil ratios, as indicated in the legend. (d) Plot of emission intensity vs. water/ oil ratio.

Figure 3. The influence of temperature on the emission of gold nanodots. (a,b) Emission (right) and excitation (left) spectra of the HDCN-stabilized gold nanodot in reverse micelles (a) and aqueous glycerol solution (b). The temperature of the medium is given in the legend. (c) Plot of the emission wavelength of the gold nanodot in the glycerol solution vs. the temperature and its linear fit. (d) Normalized excitation spectra of the above nanodot in glycerol solution at various temperatures as indicated in the legend. Eur. J. Inorg. Chem. 2017, 4696–4701

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Communication creased 1.3-fold when water was added into the reverse micelles, reaching a w/o ratio of 2.8. Even though the effects of the viscosity on the luminescence gold nanodots can be primarily ascribed to vibrational deactivation, conformational changes in gold nanodots by internal rotation may be possible.[16] A lack of the information on the structure of gold nanodots hinders an affirmative description of such conformational changes. However, if such an internal rotation took place, a more flexible structure of the gold nanodot would be possible. The strong dependence of nanodot luminescence on rigidity suggests that the luminescence quantum yield of gold nanodots can be further improved when a rigid framework is used for the stabilization of gold nanodots. The observed large Stokes' shift for the gold nanodots may imply a wide emission spectral shift in the nanodots as temperature changes. However, we did not observe a unidirectional relationship between the temperature and the emission maximum of the gold nanodots in an aqueous solution (Supporting Information Figure S4). Interestingly, the emission spectrum of the gold nanodots in reverse micelles blueshifted as the tem-

perature decreased (Figure 3a). The emission peak of the HDCNstabilized gold nanodots changed from 655 nm at 40 °C to 614 nm at 14 °C. However, Texas red, a common organic dye, did not show a similar response to temperature in reverse micelles (Supporting Information Figure S5). This may be ascribed to the unique structure of gold nanodots, which may respond to the high rigidity of reverse micelles. Therefore, we examined the temperature response of gold nanodots in an aqueous glycerol solution. The emission intensity of the HDCN-stabilized gold nanodots increased almost sevenfold when the temperature decreased from 55 to –80 °C. Their emission peak blueshifted from 663 to 640 nm (Figure 3b, c, and Supporting Information Figure S6), whereas there was no peak shift in the normalized excitation spectrum (Figure 3d). This indicates that the energy gap between the excited electronic state and the ground state remains the same during temperature/viscosity changes. The variation in the emission spectrum was due to the processes after the Frank–Condon absorption, such as vibrational relaxation and internal conversion. Usually these two processes are faster than the radiative decay from the excited

Figure 4. Stability of the peptide-stabilized gold nanodots and cellular staining. (a) Emission (left) and excitation (right) spectra of aqueous solution of the HDCN-stabilized gold nanodot obtained a week and a year after its synthesis. (b) Emission (655 nm) intensity of the above gold nanodot in DMEM at 37 °C. The error bars indicate the standard error. (c, d) Bright field (c) and emission (d) images of live Hela cells incubated with the above gold nanodots for 2 h. (e, f) Emission images of formaldehyde-fixed Hela cells stained with the above gold nanodot for 2 h. Images were taken either at 30 °C (e) or 15 °C (f). Scale bar, 10 μm. Eur. J. Inorg. Chem. 2017, 4696–4701

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Communication state to the ground state of chromophores (Vavilov's law). However, when the temperature is low and the solvent is viscous, the time for solvent reorientation increases, and consequently the vibrational decay of the excited state from higher vibrational states to the lowest vibrational state slows down, resulting in possible radiative decay from the unrelaxed or higher vibrational states of the lowest excited state.[17] This is in agreement with our observations that the emission wavelength of gold nanodots was dependent on the temperature only in rigid environments, such as a water pool of reverse micelles or aqueous glycerol solution, but not in pure water. The gold nanodots are particularly sensitive to temperature changes. The gold nanodots were also extraordinarily stable. The HDCN-stabilized gold nanodots were stable in aqueous solution for a year (Figure 4a). This also indicates excellent stability in phosphate-buffered saline (PBS) and cell culture media, such as Dulbecco's Modified Eagle's Medium (DMEM). The emission intensity remained for more than two days in DMEM at 37 °C (Figure 4b) and more than 19 days in PBS (Supporting Information Figure S7), clearing the way for biological applications. The excellent stability of the peptide-stabilized gold nanodots can be ascribed to the multiple-chelating and strong bonding peptide. Upon incubation of the gold nanodot with Hela cells at 37 °C for two hours, nanodots were adsorbed on the surface of cells due to the peptide configuration (Figure 4c, 4d, and Supporting Information Figure S8). The HDCN peptide has two more basic groups than acid groups on it, suggesting that it is likely positively charged after binding gold species in DMEM. This positive charge enables it to adsorb onto the negatively charged cell surface more efficiently. In agreement with this idea, the negatively charged DNA-stabilized gold nanodots did not show obvious staining of Hela cells under similar conditions (Supporting Information Figure S9). Interestingly, the gold nanodots regained temperature sensitivity upon binding to the cell surface. The dependence of the emission of the gold nanodots on temperature disappeared in water and was only observed in rigid environments, such as viscous glycerol solutions and water pools in reverse micelles. However, when formaldehyde-fixed Hela cells were stained with the HDCN-stabilized gold nanodots and imaged, we observed that the average emission intensity of the cells increased by 40 % when the temperature of cells was changed from 30 to 15 °C (Figure 4e and 4f ). This indicates that cellular matrices act as viscous environments, and the emission intensity of the gold nanodot can be enhanced by binding to the cellular matrices, as well as by decreasing the sample temperature. Even though decreasing the imaging temperature is not a conventional method during live cell imaging, our experiment demonstrates an efficient method to significantly increase the signal-to-noise ratio of emission images in optical imaging with luminescent gold nanodots.

Experimental Section Preparation of Peptide-Stabilized Gold Nanodots: Peptide (0.5 mg) and gold ions (140 μM) were mixed in water (1 mL), followed by reduction with aqueous sodium borohydride (1 mg/mL, 10 μL). Gold nanodots were used as probes seven days after the chemical reduction of the mixture. The above solution (1 mL) may be lyophilized in a freeze dryer to yield one dose of lyophilized gold nanodots. Preparation of Reverse Micelles: Igepal Co-520 reverse micelles were prepared by dissolving Igepal Co-520 in cyclohexane followed by the addition of concentrated aqueous gold nanodots to obtain the desired value of wo. The concentration of Igepal Co-520 in isooctane was kept at 0.4 M throughout the experiments. One dose of the lyophilized gold nanodots was dissolved in the above reverse micelles. Cell Culture and Staining: Hela cells were incubated under 5 % carbon dioxide/air at 37 °C, in DMEM with 4.5 g/L glucose, Lglutamine, and pyruvate, supplemented with 10 % fatal bovine serum and 1 % penicillin/100 units streptomycin mixture. Cells were fixed with formaldehyde (4 %) in PBS for 15 min at room temp., followed by two quick washes with PBS, and then stained with 1 mL of the above HDCN-stabilized gold nanodots. In terms of live cell imaging, live Hela cells growing on glass coverslips were quick washed with warm DMEM and incubated in DMEM supplemented with the lyophilized HDCN-stabilized gold nanodots (one dose of the gold nanodots per 1 mL of DMEM). The cells were imaged on an Olympus IX-81 microscope installed with an Andor LucaEM S 658M camera. Low-Temperature Spectra: Samples dissolved in aqueous glycerol solutions were stored in a –80 °C freezer and their spectra were recorded with a precooled fluorometer as quick as possible. Strong scattering occurred when the glycerol solution froze. The emission intensity and peak locations were corrected by subtracting the scattering signals.

Acknowledgments

Conclusion In summary, we have found that thiol groups are not essential for the synthesis of gold clusters. Instead, strong chelating ligands are essential for the stabilization of gold clusters to obEur. J. Inorg. Chem. 2017, 4696–4701

tain stable gold nanodots. Short peptides that can form fiveand six-membered rings with gold species have been applied to stabilize gold clusters, yielding gold nanodots with excellent luminescence quantum yields. These nanodots are sensitive to medium viscosity and temperature. A more viscous medium, such as aqueous glycerol solution or water pool in reverse micelles at lower temperature, slows the vibrational relaxation of the gold nanodots and consequently enhances the emission from a higher vibrational excited state, resulting in a higher emission intensity and a blueshift of the emission peak. Moreover, these gold nanodots are extraordinarily stable. They show a strong stain on the live cell surfaces likely due to electrostatic interactions. The cell surface acts as a viscous medium, which induces emission enhancement and temperature-sensitive emission of the gold nanodots. These gold nanodots can potentially act as sensors for temperature and viscosity of a medium.

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This work was supported by the Korean National Research Foundation (NRF) [2013R1A1A2061063, 2015R1A2A1A15055721]. S. Choi thanks the Korean National Research Foundation (NRF) [2013R1A1A3012746, 2015R1D1A1A01057710].

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Communication Keywords: Rigidity · Cluster compounds · Imaging agents · Solvent effects · Nanoparticles

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[1] a) S. Fedrigo, W. Harbich, J. Buttet, J. Chem. Phys. 1993, 99, 5712–5717; b) S. Choi, R. M. Dickson, J. Yu, Chem. Soc. Rev. 2012, 41, 1867–1891; c) J. T. Petty, J. Zheng, N. V. Hud, R. M. Dickson, J. Am. Chem. Soc. 2004, 126, 5207–5212; d) J. Zheng, P. R. Nicovich, R. M. Dickson, Annu. Rev. Phys. Chem. 2007, 58, 409–431; e) J. P. Xie, Y. G. Zheng, J. Y. Ying, J. Am. Chem. Soc. 2009, 131, 888–889; f) J. H. Yu, S. Choi, R. M. Dickson, Angew. Chem. Int. Ed. 2009, 48, 318–320; Angew. Chem. 2009, 121, 324; g) M. Höldrich, S. Liu, M. Epe, M. Lämmerhofer, Talanta 2017, 167, 67–74. [2] a) S. Choi, J. Yu, Chem. Eur. J. 2016, 22, 12660–12664; b) S. Choi, J. Yu, S. A. Patel, Y.-L. Tzeng, R. M. Dickson, Photochem. Photobiol. Sci. 2011, 10, 109–115; c) W. W. Guo, J. P. Yuan, Q. Z. Dong, E. K. Wang, J. Am. Chem. Soc. 2010, 132, 932–934; d) T. Vosch, Y. Antoku, J.-C. Hsiang, C. I. Richards, J. I. Gonzalez, R. M. Dickson, Proc. Natl. Acad. Sci. USA 2007, 104, 12616– 12621; e) L. A. Peyser, A. E. Vinson, A. P. Bartko, R. M. Dickson, Science 2001, 291, 103–106. [3] a) J. Chen, Q.-F. Zhang, T. A. Bonaccorso, P. G. Williard, L.-S. Wang, J. Am. Chem. Soc. 2014, 136, 92–95; b) Y. Negishi, T. Tsukuda, J. Am. Chem. Soc. 2003, 125, 4046–4047; c) W. P. Wuelfing, S. M. Gross, D. T. Miles, R. W. Murray, J. Am. Chem. Soc. 1998, 120, 12696–12697; d) H. G. Boyen, G. Kastle, F. Weigl, B. Koslowski, C. Dietrich, P. Ziemann, J. P. Spatz, S. Riethmuller, C. Hartmann, M. Moller, G. Schmid, M. G. Garnier, P. Oelhafen, Science 2002, 297, 1533–1536; e) M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz, R. Jin, J. Am. Chem. Soc. 2008, 130, 5883–5885. [4] a) C. A. J. Lin, T. Y. Yang, C. H. Lee, S. H. Huang, R. A. Sperling, M. Zanella, J. K. Li, J. L. Shen, H. H. Wang, H. I. Yeh, W. J. Parak, W. H. Chang, Acs Nano 2009, 3, 395–401; b) C.-L. Liu, H.-T. Wu, Y.-H. Hsiao, C.-W. Lai, C.-W. Shih, Y.-K. Peng, K.-C. Tang, H.-W. Chang, Y.-C. Chien, J.-K. Hsiao, J.-T. Cheng, P.-T. Chou, Angew. Chem. Int. Ed. 2011, 50, 7056–7060; Angew. Chem. 2011, 123, 7194; c) S. Link, A. Beeby, S. FitzGerald, M. A. El-Sayed, T. G. Schaaff, R. L. Whetten, J. Phys. Chem. B 2002, 106, 3410–3415; d) S. Chakraborty, S. Babanova, R. C. Rocha, A. Desireddy, K. Artyushkova, A. E. Boncella, P. Atanassov, J. S. Martinez, J. Am. Chem. Soc. 2015, 137, 11678– 11687; e) S. Choi, Thesis thesis, Georgia Institute of Technology 2010; f)

Eur. J. Inorg. Chem. 2017, 4696–4701

www.eurjic.org

[6]

[7] [8] [9]

[10] [11] [12] [13]

[14] [15] [16] [17]

T.-H. Tsoi, Y.-J. Gu, W.-S. Lo, W.-T. Wong, W.-T. Wong, C.-F. Ng, C.-S. Lee, K.L. Wong, ChemPlusChem 2017, 82, 802–809. a) C. Lasagna-Reeves, D. Gonzalez-Romero, M. A. Barria, I. Olmedo, A. Clos, V. M. S. Ramanujam, A. Urayama, L. Vergara, M. J. Kogan, C. Soto, Biochem. Biophys. Res. Commun. 2010, 393, 649–655; b) K. Zheng, M. I. Setyawati, D. T. Leong, J. Xie, ACS Nano 2017, 11, 6904–6910; c) N. Goswami, F. Lin, Y. Liu, D. T. Leong, J. Xie, Chem. Mater. 2016, 28, 4009–4016; d) N. Goswami, Q. Yao, Z. Luo, J. Li, T. Chen, J. Xie, J. Phys. Chem. Lett. 2016, 7, 962–975. a) Z. T. Luo, X. Yuan, Y. Yu, Q. B. Zhang, D. T. Leong, J. Y. Lee, J. P. Xie, J. Am. Chem. Soc. 2012, 134, 16662–16670; b) Y. Yu, Z. T. Luo, D. M. Chevrier, D. T. Leong, P. Zhang, D. E. Jiang, J. P. Xie, J. Am. Chem. Soc. 2014, 136, 1246–1249. J. Sharma, H. C. Yeh, H. Yoo, J. H. Werner, J. S. Martinez, Chem. Commun. 2010, 46, 3280–3282. W.-F. Fu, K.-C. Chan, V. M. Miskowski, C.-M. Che, Angew. Chem. Int. Ed. 1999, 38, 2783–2785; Angew. Chem. 1999, 111, 2953. a) Y. A. Lee, J. E. McGarrah, R. J. Lachicotte, R. Eisenberg, J. Am. Chem. Soc. 2002, 124, 10662–10663; b) V. W. W. Yam, E. C. C. Cheng, Z. Y. Zhou, Angew. Chem. Int. Ed. 2000, 39, 1683–1685; Angew. Chem. 2000, 112, 1749. S. Choi, R. M. Dickson, J.-K. Lee, J. Yu, Photochem. Photobiol. Sci. 2012, 11, 274–278. J. Yu, S. A. Patel, R. M. Dickson, Angew. Chem. Int. Ed. 2007, 46, 2028– 2030; Angew. Chem. 2007, 119, 2074. W. Kurashige, Y. Niihori, S. Sharma, Y. Negishi, Coord. Chem. Rev. 2016, 320–321, 238–250. A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle, A. S. de Sousa, J. A. Gareth Williams, M. Woods, J. Chem. Soc., Perkin Trans. 2 1999, 493–504. N. J. Turro, Modern Molecular Photochemistry, University Science Books, 1991. D. E. Moilanen, N. E. Levinger, D. B. Spry, M. D. Fayer, J. Am. Chem. Soc. 2007, 129, 14311–14318. G. Oster, Y. Nishijima, J. Am. Chem. Soc. 1956, 78, 1581–1584. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer US, 2013.

Received: June 19, 2017

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