Cyclophane-Sustained Ultrastable Porphyrins

Article abstract of DOI:10.1021/jacs.0c02311

We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1 complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on account of its tricyclic nature. The photophysical properties and chemical reactivities of the encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative energy transfer processes highlight the emergent photophysical enhancements. The encapsulated porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and solvolysis under extremely acidic conditions are completely blocked. We anticipate that the ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find applications in single-molecule materials, artificial photodevices, and biomedical appliances.

Full text of DOI:10.1021/jacs.0c02311

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Article
Cyclophane-Sustained Ultrastable Porphyrins
Wenqi Liu, Chenjian Lin, Jacob Andrew Weber, Charlotte L. Stern,
Ryan M. Young, Michael R. Wasielewski, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02311 • Publication Date (Web): 03 Apr 2020
Downloaded from pubs.acs.org on April 4, 2020
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Cyclophane-Sustained Ultrastable Porphyrins
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Wenqi Liu,1 Chenjian Lin,1 Jacob A. Weber,1 Charlotte L. Stern,1 Ryan M. Young,1
Michael R. Wasielewski,1 J. Fraser Stoddart*1,2,3
1. Department of Chemistry, 2145 Sheridan Road, Northwestern University, Evanston, IL 60208, USA
2. Institute for Molecular Design and Synthesis, Tianjin University, Tianjin 300072, China
3. School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
ABSTRACT
We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor
with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic
effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the
substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1
complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on
account of its tricyclic nature. The photophysical properties and chemical reactivities of the
encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved
fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative
energy transfer processes highlight the emergent photophysical enhancements. The encapsulated
porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and
solvolysis under extremely acidic conditions are completely blocked. We anticipate that the
ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find
applications in single-molecule materials, artificial photodevices and biomedical appliances.
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INTRODUCTION
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Molecular recognition is utilized comprehensively by nature for the regulation of biological
processes.1,2 One of the goals in the supramolecular chemistry community is to make3,4 synthetic
receptors that can hold a candle to the binding affinities and functionalities of bioreceptors. In
recent years, several wholly synthetic receptors have been reported5,6 with substrate-binding
affinities exceeding the performance of naturally occurring receptors. These high affinity synthetic
receptors have shown7 promising applications in drug delivery, membrane functionalization and
protein purification. Advances in these biotechnologies create new and demanding requirements
for synthetic receptors with, not only high binding affinities, but also with integrated8–15
functionalities. It is desirable to develop high affinity receptors for functional substrates such as
dye molecules.16–19 Although there have been numerous reports20–22 on dye encapsulations by
several well-known receptors such as cyclodextrins, calixarenes, cucurbiturils and pillararenes,
most of them fail to encapsulate dyes at nanomolar concentrations on account of their low binding
affinities. Examples of high affinity receptors for functional dye molecules23–27 are rare and are
urgently needed28 to meet the demanding requirement of biotechnologists and scientists working
in related fields.
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Porphyrins are indispensable dyes in biology and fulfill many crucial biological functions, such as
oxygen transport, photosynthesis and metabolism.29 Most porphyrins in nature exist as
noncovalent complexes and are buried deep inside the superstructures of porphyrin-binding
proteins, where their microenvironments, not only govern the versatile functions of porphyrins,
but also protect them from direct interactions with solvents and solutes.30 Much effort has been
devoted to making synthetic mimics of these porphyrin-containing devices30–32 and engineer them
to express functions in artificial photodevices,33,34 model enzymes35–40 and biotechnologies41–43
.
To this end, one of our goals is to develop artificial receptors that bind strongly with porphyrins in
confined microenvironments, in which we can modulate the photoelectrical properties and
chemical reactivities of the encapsulated porphyrins.4,8,44
Binding of porphyrins has been explored using chemically modified proteins and peptides,30,31,45
nucleotides46,47 and other naturally derived compounds48,49. Porphyrins have also been substrates
for intense targeting in the supramolecular community, where cyclodextrins,50–52 calixarenes,53
cucurbiturils,54,55 cyclophanes,56,57 foldamers58 and coordination metal cages39,59 have all been
developed in order to interact with porphyrins with various functions in mind. Despite all these
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advances in mimicking porphyrin-binding proteins, the challenge remains to design a monomeric
high-affinity receptor that can fully encapsulate porphyrins on account of their large sizes which
exceed the cavity sizes of current synthetic receptors.57
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Recently, we designed27 an X-shaped octacationic cyclophane, XCage8+, which features a large,
rigid binding cavity. The constitution of XCage8+ exhibits high stereoelectronic complementarity
toward perylene diimide (PDI) dyes with picomolar binding affinities in water. Low level
molecular modeling suggests that the porphyrin core is a good fit with the binding cavity of
XCage8+, where multiple [π···π] and [CH···π] interactions come into play upon binding. This
stereoelectronic complementarity has motivated us to explore the possibility of using XCage8+ as
a porphyrin receptor in water. Herein, we report the encapsulation of free-base porphyrin and Zn-
porphyrin using XCage8+ as a receptor with subnanomolar binding affinities. These ultrahigh
affinities can be attributed to multiple [CH···π] interactions in addition to large [π···π] stacking
surfaces between the substrate porphyrins and the receptor XCage8+. Two types of co-
conformational isomers, in which the porphyrin substrates are orientated differently inside the
binding cavity of XCage8+, were uncovered by 1H NMR spectroscopy in D2O. The photophysical
properties of the encapsulated porphyrins turn out to be modulated by XCage8+. Improved
fluorescence quantum yields, red-shifted absorptions and emissions, and a nearly quantitative
energy transfer process are all observed. In addition to these physical attributes, the encapsulated
porphyrins show remarkable chemical stabilities, reflected in the fact that their protonation, D/H
exchange, and solvolysis under extremely acidic conditions, are blocked.
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RESULTS AND DISCUSSION
X-Ray Crystallographic Analysis
A preliminary evaluation of the porphyrin binding capability using XCage8+ was performed by X-
ray crystallography. A mixture of the model compounds mPorp-2H(Zn) with XCage8+ results
(Figure 1) in the solubilization of these porphyrins in water –– a good indication of complex
formation. Single crystals were obtained by slow diffusion of iPr2O into Me2CO solutions of these
8+
⊂
complexes. In the superstructures of mPorp-2H(Zn) XCage , both mPorp-2H and mPorp-Zn
are positioned (Figure 2) horizontally with respect to the binding cavity of XCage8+. The diphenyl
roof and floor of XCage8+ show large areas of [π···π] stacking with the porphyrin cores.
Furthermore, there are multiple [CH···π] interactions between the four p-xylylene pillars of
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XCage8+ and the porphyrin. These [CH···π] distances range from 2.9 to 4.4 Å. The noncovalent
bonding interactions were visualized (Figure S23) by using the independent gradient model (IGM)
analysis.60
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NMR Spectroscopy in Solution
Both mPorp-2H and mPorp-Zn are insoluble in water, preventing the carrying out of quantitative
binding studies. In order to evaluate the receptor substrate binding in solution, two water-soluble
porphyrins (Porp-2H and Porp-Zn), flanked by polydispersed PEG chains, were synthesized
using standard protocols. Upon mixing XCage8+ with Porp-2H(Zn) in D2O, the complexes
formed quantitatively as indicated by the 1H NMR spectra. Surprisingly, two sets of proton signals
for the encapsulated porphyrins are observed (Figure 3), indicating the presence of two co-
conformational isomers. One set of the 1H NMR signals corresponds to co-conformer H as defined
1
by X-ray crystallography. The other set of H NMR signal most likely originates from co-
conformer V in which the porphyrin substrate is located vertically in relation to the binding cavity
of XCage8+. The meso protons (1) of the porphyrin are obscured by XCage8+ in co-conformer H,
and their chemical shift appears at 8.8 ppm as a result of the shielding effects by the diphenyl units.
In contrast, the meso protons (1) in co-conformer V are beyond the coverage of XCage8+; thus,
their chemical shift shows up at 10.1 ppm. By comparing integrations, we found that the ratio of
8+
⊂
co-conformer V to co-conformer H is 6:4 and 4:6, respectively for Porp-2H XCage and Porp-
8+
⊂
Zn XCage . Co-conformer V represents a kinetically trapped metastable state, which is
gradually transformed into co-conformer H over time. It takes 72 h at room temperature to
8+
⊂
⊂
complete the transformation in the case of Porp-Zn XCage . The transformation of Porp-2H
XCage8+ is more difficult to achieve and requires additional heating at 70 ˚C for 24 h to form the
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⊂
co-conformer H. This observation differs from the previously reported PDI XCage complex,
where the substrate PDI is only observed as being positioned vertically with respect to the binding
cavity of XCage8+. The co-existence of co-conformers H and V can be attributed to the square-
shaped porphyrin core, which presents a similar overlapping surface area with XCage8+ in both
co-conformers. The phenyl groups in the porphyrin are expected to experience unfavorable steric
strain in co-conformer V, making it a less stable species when compared with co-conformer H,
where the phenyl groups actually contribute to the overall stability of the complex by supporting
several [CH···π] interactions with XCage8+. While controlling the transformation of these two co-
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conformers is beyond the scope of this investigation, it is worth noting that this type of co-
conformational isomerization could lead to new opportunities to manipulate multiple binding
states within a multicyclic receptor.
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The H NMR spectrum of the equilibrated Porp-2H XCage in D2O reveals (Figure 3)
distinctive peaks for porphyrin units as co-conformer H.61 Protons D, E, F on XCage8+ experience
the deshielding effect of the aromatic porphyrin ring and are downfield shifted. Protons A and C,
which are positioned within the porphyrin shielding region, experience upfield shifts. Protons B,
facing the shielding center of the porphyrin ring, experiences the most dramatic upfield shift (∆#
= –3.6 ppm). A NOESY experiment confirmed (Figure 4) the encapsulated structure by showing62
the expected through-space correlation peaks between Porp-2H and XCage8+. It is worthy of note
that the triazole rings are also likely to participate in binding with XCage8+, as revealed by the
through-space correlations between the triazole ring protons 7 and protons F. Such a guest-
backfolding phenomenon has been reported23,63,64 previously to stabilize noncovalent complexes.
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1
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⊂
The corresponding H NMR spectroscopic analysis of Porp-Zn XCage is described in the
Supporting Information.
Photophysical Properties
The association between Porp-2H(Zn) and XCage8+ induces characteristic changes in their optical
properties. Red-shifted absorption and emission (Figure 5a and 5b) of the encapsulated Porp-2H
were observed, and its fluorescence quantum yield was enhanced from 16 to 25 %, benefiting from
the porphyrin being isolated in the hydrophobic binding pocket of XCage8+. In comparison,
previously reported porphyrin receptors either quench57 the fluorescence or fail to induce any
photophysical response.55 The encapsulation of Porp-Zn by XCage8+ decreases the fluorescence
quantum yield from 5 to 0.6 %.
There is an efficient energy transfer process from XCage8+ to Porp-2H. When excited at 290 nm,
8+
⊂
the complex exhibits (Figure 5c) strong emission peaks for the Porp-2H XCage complex at
650 nm. The energy transfer efficiency was estimated by comparing (Figure 5d) the fluorescence
8+
8+
⊂
emission spectra of XCage and Porp-2H XCage excited at 330 nm. The close-to-complete
8+
8+
⊂
fluorescence quenching of XCage in the complex of Porp-2H XCage is a compelling sign
of the efficient energy transfer, which is calculated to be > 96%. Time-dependent DFT calculations
8+
⊂
carried out on Porp-2H XCage reveal that the HOMO is localized on Porp-2H and the LUMO
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on XCage . The calculated UV-Vis absorption spectrum of Porp-2H XCage is red-shifted
compared with that of Porp-2H, which is in agreement with experimental observations.
In order to gain a better understanding of the influence of molecular encapsulation on
photophysical properties, transient absorption (TA) experiments were performed at femtosecond
and nanosecond resolutions. Femtosecond TA studies, exciting the Soret band at 414 nm, reveal
(Figure 6) a significant enhancement of the lifetime of intersystem crossing when Porp-2H is
encapsulated in the cavity of XCage8+. This result corroborates the enhanced fluorescence
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8+
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⊂
⊂
XCage shows
quantum yield of Porp-2H
XCage . Compared to Porp-2H, Porp-2H
improved stability of the triplet state as revealed by the nanosecond TA spectra. The energy
8+
⊂
transfer within Porp-2H XCage was investigated by femtosecond TA spectroscopy using an
excitation wavelength of 330 nm. Under these conditions, we only observe (Figure S43) the
excited state of Porp-2H, and no excited state of XCage8+ could be detected (Figure S41) within
0.4 ps, suggesting an ultrafast rate of the energy transfer, which corroborate the efficient energy
⊂
transfer process observed by the fluorescence emission spectroscopy. In contrast to Porp-2H
XCage8+, femtosecond TA spectra of Porp-Zn XCage shows a charge-separated state,
8+
⊂
accounting for the decreased fluorescence of this complex.
Binding Thermodynamics and Kinetics
The changes in optical properties upon porphyrin encapsulation enable a facile study of the binding
events. Fluorescence titrations of Porp-2H and Porp-Zn with ExBox4+ yielded directly their
binding constants in water. Since the binding affinities for XCage8+ with Porp-2H and Porp-Zn
are too high to be determined directly, competitive titrations were performed by displacing
ExBox4+ with XCage8+ from the complex Porp-2H(Zn) ⊂ ExBox4+. The binding affinities (Table
1) between ExBox4+ and the two porphyrins are in the order of 107 M–1. Compared with ExBox4+,
XCage8+ shows around a 1000-fold enhancement in the binding affinities, which are around
1010 M–1 (Kd = 0.1 nM). The highest affinity (Ka = 1.7 × 1010 M–1) was achieved in the binding
between XCage8+ and Porp-2H. It should be noted that these Ka values are interpreted as a lower
limit to the stability constant, as the measurement was performed under conditions where
co-conformers V and H coexist65 in solution. The equilibrated co-conformer H is expected to have
a higher stability with the absence of the metastable species.
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Since the high binding affinity and aggregation of the two porphyrins prevent the accurate
measurement of binding constants by isothermal titration calorimetry (ITC), a single injection
experiment was performed in order to determine the binding enthalpy. The Gibbs free energy of
the receptor-substrate complexation was estimated directedly from the corresponding fluorescent
titrations, providing a value for T∆S. Compared with ExBox4+, the binding enthalpies of XCage8+
are in the range of 6–7 kcal mol–1 larger, a major contributing factor to the enhanced affinity.
Surface area overlap analysis reveals66 that XCage8+ provides 1.5 times more binding surface areas
for the porphyrin core compared with that of ExBox4+: 80% of the porphyrin core overlaps with
XCage8+, whereas only 50 % of the porphyrin core overlaps in the case of ExBox4+. Compared
with Porp-2H, Porp-Zn shows a significant drop in binding enthalpy toward both XCage8+ and
ExBox4+, an observation which agrees well with the titration results which show that the binding
of Porp-Zn is generally three times weaker compared with that of Porp-2H. This result implies
that the dehydration of the Zn ion upon binding is a high energy demanding process.
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The kinetics of porphyrin encapsulation by XCage8+ can be tracked by the change in fluorescence
over time. The resulting kinetic profiles are fitted (Figure S53 and S54) using a second order
kinetics equation. The threading rate constants of XCage8+ with Porp-2H and Porp-Zn are
determined67 to be 7.2 ×104 and 4.6 ×104 M–1s–1, respectively. The remarkably rapid threading
kinetics agrees well with previously reported24 results where threading a long polymer chain over
a macrocyclic receptor is a rapid process. It is necessary to note that the rapid threading kinetics
measured here represent the formation of the Porp-2H(Zn) ⊂ XCage8+ complexes, in which co-
conformers V and H coexist as a mixture. The transformation of co-conformer V into H is a slow
process and requires days to reach completion. Furthermore, the slower threading kinetics of Porp-
Zn matches well with the observed co-conformer distribution, where a less amount of the
metastable co-conformer V is formed when compared with Porp-2H. The dissociation rate
constants (koff) for the Porp-2H(Zn) ⊂ XCage8+ complexes can be calculated using the equation
koff = kon / Ka and reveals extremely slow dissociation processes with the rate constants and half-
lives (t1/2) calculated at 4.2 ×10–6 (t1/2 = 46 h) and 7.4 ×10–6 s–1 (t1/2 = 26 h) for Porp-2H ⊂ XCage8+
and Porp-Zn ⊂ XCage8+, respectively. The slow dissociations of Porp-2H(Zn) ⊂ XCage8+
endow the complexes with kinetic stabilities, wherein considerable amounts of the 1:1 complexes
can still exist for days, even in the presence of a competitor that has a stronger binding affinity
with XCage8+.
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Chemical Stability
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It is well-known that porphyrins and metalloporphyrin are susceptible to acidic environments.
Protonation occurs at the pyrrole subunits and leads to changes in photophysical properties, which
limit their performance in certain technical scenarios. When added to a solution of HCl (1 M),
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Porp-2H is protonated instantly, as judged from the change of its color from brown to green and
8+
⊂
a red-shifted absorption in the UV-Vis spectrum (Figure 7a). In contrast, Porp-2H XCage
resists protonation and no change is observed (Figure 7b) under the same conditions, i.e., the fact
that the high charge density of XCage8+ plus its strong affinity with Porp-2H provide protection
from H+ attack in aqueous solution. Encapsulation facilitated protonation (positive pKa shifts) is
well documented,68–71 whereas examples of frustrated protonation (negative pKa shifts), induced
by synthetic receptors, are rare.72 There is no example, to our knowledge, where protonation can
be totally shut down by molecular encapsulation, a property which would require a high binding
affinity and the protection of the protonation site deep inside the binding cavity. As a comparison,
4+
⊂
the Porp-2H ExBox complex with four positive charges and a micromolar binding affinity
fails to provide these kinds of protection and instantly decomposes (Figure S60) into the
corresponding protonated species, namely Porp-4H2+ and ExBox4+, under the same conditions.
On the other hand, Porp-Zn suffers (Figure S59) from solvolysis in the presence of HCl (1M) as
2+
8+
⊂
judged by the appearance of Porp-4H in its absorption spectrum. Porp-Zn XCage remains
stable in HCl solution.
Considering the excellent performance of XCage8+ which prevents H+ from attacking the
porphyrin core, we envisioned that D/H exchanges, involving the pyrrole subunits in deuterated
⊂
solvents, should also be blocked. In order to test this hypothesis, Porp-2H
XCage8+ was
1
prepared, first of all in H2O, and subsequently re-dissolved in D2O. The H NMR spectrum of
8+
⊂
Porp-2H XCage shows (Figure 7c) clearly NH signals at –5.6 ppm, resulting from the
shielding effect provided by both the porphyrin core and the biphenyl units in XCage8+. A
comparison of the NH integration, with respect to other porphyrin proton signals, indicates73 no
sign of D/H exchange.
CONCLUSIONS
The tricyclic cyclophane serves as an excellent receptor for both the free-base and Zn-porphyrins
with subnanomolar affinity in water. The tricyclic nature of XCage8+ permits the formation of two
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co-conformationally isomeric complexes with both porphyrins, as revealed by H NMR
spectroscopy. XCage8+ is able to modulate both the photophysical properties and chemical
reactivities of the encapsulated porphyrins. The isolation of both porphyrins by XCage8+ with
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ultrahigh stabilities provides us with a new platform to investigate porphyrins at the single-
molecule level.74–76 We speculate that the encapsulation characterizing the Porp-Zn XCage
8+
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⊂
complex could be quite general for a library of metalloporphyrins with a wide range of properties,
leading to applications in nanotechnology,43,77 artificial photodevice fabracation78,79 and
biomedical science.80,81
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/
Experimental procedures, chemical synthesis and characterization, mass spectral data, NMR
spectra, X-ray crystal data, computational analysis, photophysical data, binding studies, and
chemical stabilities studies (PDF)
⊂
Crystallographic data for mPorp-2H XCage•8PF6 (CIF)
⊂
Crystallographic data for mPorp-Zn XCage•8PF6 (CIF)
AUTHOR INFORMATION
Corresponding Author
* stoddart@northwestern.edu
ORCID
Wenqi Liu: 0000-0001-6408-0204
Chenjian Lin: 0000-0002-8817-5133
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Ryan M. Young: 0000-0002-5108-0261
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Michael R. Wasielewski: 0000-0003-2920-5440
J. Fraser Stoddart: 0000-0003-3161-3697
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Notes
The authors declare no competing financial interest
ACKNOWLEDGEMENTS
We thank Northwestern University (NU) for their support of this research. We also thank the
personnel in the Integrated Molecular Structure Education and Research Center (IMSERC) at NU
for their assistance in the collection of the data. This project was supported in the Wasielewski
laboratory by the National Science Foundation under grant number DMR-1710104 (M.R.W.).
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Table 1. Binding Constants and Thermodynamic Data at 25 ˚Ca
entry Host
Guest
Ka b / M–1 ∆G c / kcal mol–1 ∆H d / kcal mol–1 T∆S / kcal mol–1
1
2
3
4
ExBox4+ Porp-2H 1.4 × 107
ExBox4+ Porp-Zn 5.1 × 106
XCage8+ Porp-2H 1.7 × 1010
XCage8+ Porp-Zn 6.2 × 109
–9.7
–9.1
–9.8
–5.4
–0.2
+3.7
–2.2
+0.6
–13.9
–13.4
–16.1
–12.8
a The standard errors are presented in Supporting Information. bDetermined by fluorescence
titration. cEstimated from fluorescence titration. dMeasured by ITC
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Captions for Figures
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Figure 1. Structural formulas of the compounds relevant to the physical organic investigation
discussed in this paper
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Figure 2. Stick representation of the solid-sate superstructures obtained from single-crystal X-ray
crystallography. (a) Top-down view, (b) side-on view and (c) [CH···π] binding surfaces of mPorp-
2H ⊂ XCage8+. (d) Top-down view, (e) side-on view and (f) [CH···π] binding surfaces of mPorp-
Zn ⊂ XCage8+
Figure 3. Co-conformational isomer transformation in D2O solution tracked by dynamic 1H NMR
spectroscopy. (a) Molecular models illustrating the transformation of co-conformer V to H. (b) 1H
NMR (500 MHz, D2O, 25 ̊
C) Spectra of mPorp-2H ⊂ XCage8+ collected at 0, 48 and 72 h at room
temperature, along with additional heating at 70 ˚C for 5 and 24 h
1
Figure 4. H NMR Spectroscopic investigation of the formation of the Porp-2H ⊂ XCage8+
complex. (a)1H NMR (500 MHz, D2O, 25
̊
1
and (bottom) XCage8+. (b) H–1H NOESY (500 MHz, D2O, 25
equilibrated Porp-2H ⊂ XCage8+. Proton labels are defined on the relevant structural formulas in
Figure 1
Figure 5. Steady-state absorption and emission spectra. (a) Absorption and (b) emission (ex: 440
nm) spectra of Porp-2H (blue, 10 µM) and Porp-2H ⊂ XCage8+ (red, 10 µM). (c) Emission
spectra (ex: 290 nm) of Porp-2H (blue, 1 µM) and Porp-2H ⊂ XCage8+ (red, 1 µM). (d) Emission
spectra (ex: 330 nm) of XCage8+ (black, 1 µM) and Porp-2H ⊂ XCage8+ (red, 1 µM). All spectra
were collected in H2O at 25˚C
Figure 6. Femtosecond transient absorption spectroscopy. Femtosecond TA spectra of (a) Porp-
2H and (c) Porp-2H ⊂ XCage8+ in H2O excited at 414 nm. Species-associated spectra of (b)
Porp-2H and (d) Porp-2H ⊂ XCage8+ obtained by wavelength global fitting to an A → B → C
kinetic model. State A represents the higher singlet excited state S2 1*Porp-2H, state B is the
lowest singlet excited state S1 1*Porp-2H, and state C is the triplet state T1 3*Porp-2H. State C in
(d) is not fully resolved on account of the slow ISC rate
Figure 7. Stability test of Porp-2H and Porp-2H ⊂ XCage8+. Absorption spectrum of (a) Porp-
2H and (b) Porp-2H ⊂ XCage8+ in H2O (blue) and 1M HCl (red). Inserts showing the
corresponding solution in H2O (left) and HCl (right). (c) 1H NMR (500 MHz, D2O, 25
̊C) spectrum
of the preassembled Porp-2H ⊂ XCage8+ in D2O
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–
4Cl–
8CF3CO2
N
N
N
N
N
N
N
N
N
N
B
F
N
A
N
C
D
E
ExBox•4Cl
XCage•8CF3CO2
N N
Zn
N HN
NH N
R
R
R
O
R
N N
4
3
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2
1
R
H
8
mPorp-Zn
Porp-Zn
mPorp-2H
10
Me
7
N
N
O
6
O
Porp-2H
9
44
N
Figure 1
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TOC
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