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\DOI{10.5802/crchim.299}
\datereceived{2023-09-29}
\daterevised{2024-01-10}
\dateaccepted{2024-02-08}
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\dateposted{2024-03-19}
\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{French/Nordic Special Issue on Materials and Coordination Chemistry}
%%\def\cdr@specialtitle@french{Num\'ero sp\'ecial franco-nordique sur les mat\'eriaux et la chimie de coordination}
%\makeatother

\CDRsetmeta{articletype}{preliminary-communication}

\title{Hypsochlorins}
\alttitle{Hypsochlorines}

\author{\firstname{Simon} \lastname{Larsen}\CDRorcid{0000-0002-6105-1552}}
\address{Department of Chemistry, UiT -- The Arctic University of
Norway, Troms{\o} N-9037, Norway}

\author{\firstname{Jeanet} \lastname{Conradie}\CDRorcid{0000-0002-8120-6830}}
\addressSameAs{1}{Department of Chemistry, UiT -- The Arctic University of Norway, Troms{\o} N-9037, Norway}
\address{Department of Chemistry, University of the Free State,
Bloemfontein 9300, Republic~of~South Africa}

\author{\firstname{Nicolas} \lastname{Desbois}\CDRorcid{0000-0002-1156-4608}}
\address{Institut de Chimie Mol\'{e}culaire de l'Universit\'{e} de
Bourgogne (ICMUB), UMR CNRS 6302, Universit\'{e} de
Bourgogne, 9 Avenue Alain Savary, BP 47870, 21078 Dijon
Cedex, France}

\author{\firstname{Claude P.} \lastname{Gros}\CDRorcid{0000-0002-6966-947X}\IsCorresp}
\addressSameAs{3}{Institut de Chimie Mol\'{e}culaire de
l'Universit\'{e} de Bourgogne (ICMUB), UMR CNRS 6302, Universit\'{e} de
Bourgogne, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France}
\email[C. P. Gros]{claude.gros@u-bourgogne.fr}

\author{\firstname{Abhik}\nobreakauthor \lastname{Ghosh}\CDRorcid{0000-0003-1161-6364}\IsCorresp}
\addressSameAs{1}{Department of Chemistry, UiT -- The Arctic University
of Norway, Troms{\o} N-9037, Norway}
\email[A. Ghosh]{abhik.ghosh@uit.no}

\shortrunauthors

\keywords{\kwd{Chlorin}\kwd{Hypsochlorin}\kwd{Hypso}\kwd{Hypsoporphyrin}\kwd{TDDFT}}

\altkeywords{\kwd{Chlorine}\kwd{Hypsochlorine}\kwd{Hypso}\kwd{Hypsoporphyrine}\kwd{TDDFT}}

\thanks{Research Council of Norway (Grant no.~262229), CNRS (Grant 
no.~UMR UBCNRS 6302), South African National Research Foundation (Grant
nos.~129270 and 132504)}

\begin{abstract} 
Both the Soret and Q~bands of chlorin e6 complexes blueshift on going
from Zn to Pd to Pt as the coordinated metal, establishing the latter
two complexes as hypsochlorins. DFT/TDDFT calculations indicate that
the blueshifts appear to be related to the presence of the
electronegative metal ion, which lowers the orbital energies of the
HOMO and HOMO-1, while leaving the LUMOs relatively unaffected.
\end{abstract} 

\begin{altabstract} 
La bande de Soret et les bandes~Q de complexes de chlorine e6
pr\'esentent un d\'eplacement vers le bleu lorsque l'on passe, au
niveau du m\'etal coordin\'e, de Zn \`a Pd puis \`a Pt, ce qui permet
d'\'etablir que ces deux derniers complexes correspondent \`a des
hypsochlorines. Les calculs DFT/TDDFT indiquent que les d\'ecalages
vers le bleu semblent \^etre li\'es \`a la pr\'esence de l'ion
m\'etallique \'electron\'egatif, ce qui abaisse les \'energies des
orbitales HOMO et HOMO-1, tout en laissant les orbitales LUMO
relativement inchang\'ees.
\end{altabstract} 

\maketitle
\vspace*{.72pc}

\twocolumngrid

\end{noXML}

\section{Introduction} \label{sec1}

Over a half-century ago, Martin Gouterman~\cite{1} devised his
eponymous four-orbital model of porphyrin spectra, and thereby was able
to classify them as belonging to three broad classes, normal, hyper and
hypso~\cite{2,3}. Briefly, hyper spectra exhibit redshifted features
and/or extra features relative to normal spectra, while hypso spectra
are blueshifted relative to normal spectra. Although the variety of
porphyrinoid compounds is vastly greater today, \mbox{Gouterman's}
classification remains useful. Thus, the hyperporphyrin concept has
emerged as a design principle for photosensitizers for photodynamic
therapy and related applications~\cite{4}. Many hypsoporphyrins are
phosphorescent and potentially useful as triplet photosensitizers, in
such applications as oxygen sensing and photodynamic and related
therapies~\cite{5,6,7}. The hypso effect had long been attributed to an
increase in the HOMO--LUMO gap in certain late transition porphyrins as
a result of antibonding interactions between occupied metal $d_{\uppi}$
orbitals and the porphyrin's $e_{\mathrm{g}}$ LUMOs. A recent DFT investigation,
however, has indicated a different origin for the hypso effect, namely,
a lowering of the $a_{2\mathrm{u}}$ HOMOs in metalloporphyrins with relatively
electronegative metal ions such as Pd(II) and Pt(II)~\cite{8}.

Many years ago, we showed that Gouterman's four-orbital model also
applies to corroles~\cite{9,10}. Unsurprisingly, many metallocorroles
exhibit hyper spectra~\cite{11,12,13} and quite a few also exhibit
hypso spectra~\cite{8}. Herein we show that the hypso effect also
applies to chlorins~\cite{14,15}, with both the Soret and Q~band maxima
of chlorins blueshifting, going from Zn to Pd to Pt as the coordinated
metal (Scheme~\ref{sch1}). The finding is surprising in that the HOMO
of closed-shell chlorins is invariably an $a_{1\mathrm{u}}$-type MO (in terms of
$D_{4h}$ irreps)~\cite{3,16,17,18,19} which has no amplitude on the
central nitrogen atoms and is, therefore, likely to be unaffected by
the electronegativity of the central metal. Gratifyingly, the spectral
data proved consistent with electrochemical cyclic voltammetry
measurements and DFT/TDDFT calculations, leading to a coherent picture
of hypsochlorins, as described \mbox{below}.

\begin{scheme}
\includegraphics{sc01}
{\vspace*{.6pc}}
\caption{\label{sch1}A chlorin e6 trimethyl ester metal complex with
partial atom numbering.}
\end{scheme}

\section{Results and discussion}

Table~\ref{tab1} lists the UV--vis absorption maxima and redox
potentials (V {versus} SCE) of the Zn, Pd, and Pt complexes of
chlorin e6 trimethyl ester, \mbox{M[Chl-e]}, where M ${=}$ Zn, Pd, and
Pt; the UV--vis spectra and cyclic voltammograms are depicted in
Figures~\ref{fig1} and~\ref{fig2},
respectively. From Zn to Pd and from Pd to Pt, the Q~band blueshifts by
14 and 10~nm, respectively, and the Soret by 8 and 10~nm, respectively.
The two minor features between the Soret and Q~bands also blueshift in
the same order. All complexes exhibit reversible 1st oxidations and 1st
reductions. While there are small variations in the 1st reduction
potentials there are greater variations in the 1st oxidation potentials
and a clear trend of increasing oxidation potentials in the order of Zn
${<}$ Pd ${<}$ Pt, which in turn results in an increasing
electrochemical HOMO--LUMO gap for the same series. The electrochemical
data thus suggests that the hypso effect in chlorins stems primarily
from a lowering of the HOMO energy in the presence of an
electronegative{\break} coordinated metal.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}UV--visible spectra of M[Chl-e], where M ${=}$ Zn,
Pd, and Pt, in $\text{CH}_2\text{Cl}_2$ at ambient temperature.}
{\vspace*{.7pc}}
\end{figure}

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Cyclic voltammograms (V versus SCE)
of M[Chl-e], where M ${=}$ Zn,
Pd, and Pt, in $\text{CH}_2\text{Cl}_2$ containing 0.1~M
tetrabutylammonium perchlorate at ambient temperature; scan rate
$100~\text{mV}{\cdot}\text{s}^{-1}$.}
\end{figure}

\begin{table*} %tab1
\caption{\label{tab1}Redox potentials (V versus SCE) and UV--vis
absorption maxima of the compounds studied}
\begin{tabular}{ccccccccc}
\thead
Compound & $E_{1/2\mathrm{ox}2}$ & $E_{1/2\mathrm{ox}1}$ &
$E_{1/2\mathrm{red}1}$ & $E_{1/2\mathrm{red}2}$ & $E_{\mathrm{red}2}$ &
$\Delta E$ & Soret band (nm) & Q~band (nm)\\
\endthead
Zn[Chl-e] & 0.96 & 0.55 & ${-}$1.29 & -- & ${-}$1.79 & 1.84 & 411 & 637\\
Pd[Chl-e] & -- & 0.77 & ${-}$1.17 & ${-}$1.69 & -- & 1.94 & 403 & 623\\
Pt[Chl-e] & -- & 0.81 & ${-}$1.22 & ${-}$1.73 & -- & 2.03 & 393 & 613
\botline
\end{tabular}
\end{table*}

To obtain a qualitative understanding of the above findings, DFT and
TDDFT calculations were carried out using simple unsubstituted chlorin
models of the complexes studied. The
OLYP~\cite{20,21}-D3~\cite{22}/ZORA-STO-TZ2P optimized geometries of
M[Chl] (M ${=}$ Zn, Pd, Pt; Chl ${=}$ unsubstituted chlorin;
Figure~\ref{fig3}) revealed essentially planar (as opposed to ruffled)
geometries so differences in macrocycle conformation appear unlikely to
be a significant contributor to the observed spectral
differences~\cite{23}. Single-point CAMY-B3LYP~\cite{24} calculations
were then carried{\break} out on the OLYP-D3 geometries.
Figure~\ref{fig4} depicts selected CAMY-B3LYP frontier MOs along with
the corresponding orbital energies, while Figure~\ref{fig5} presents
the orbital energy levels in a graphical manner. 
Figure~\ref{fig6} depicts the simulated TD-CAMY-B3LYP-D3/Q4 STO-TZ2P-COSMO 
optical spectra in dichloromethane of the three M[Chl] derivatives.
The key result is
that, while the LUMO and LUMO${+}$1 energies are essentially constant
across all three M[Chl] derivatives, both the HOMO and \mbox{HOMO-1}
energies downshift going from Zn to Pd to Pt. \mbox{Understandably},
the downshifts for the \mbox{$a_{1\mathrm{u}}$-type} HOMO are smaller than those
for the $a_{2\mathrm{u}}$-type \mbox{HOMO-1}, given that only the latter has
substantial amplitudes on the central nitrogen atoms. In spite{\break}
of the simplicity of our models, these computational results appear to
nicely, if qualitatively, capture the essence of both the UV--vis and
electrochemical{\break} measurements.

\begin{figure*}
{\vspace*{6pt}}
\includegraphics{fig03}
{\vspace*{5pt}}
\caption{\label{fig3}Selected skeletal bond distances for M[Chl], where
M ${=}$ Zn, Pd, and Pt and Chl ${=}$ unsubstituted chlorin.}
{\vspace*{4pt}}
\end{figure*}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}CAMY-B3LYP frontier MOs of M[Chl] (M ${=}$ Zn, Pd,
Pt), with C2 irreps (a and~b) and orbital energies.}
{\vspace*{.5pt}}
\end{figure*}

\begin{figure}
\includegraphics{fig05}
{\vspace*{-.15pc}}
\caption{\label{fig5}\fontsize{9.8}{12}\selectfont{CAMY-B3LYP-D3/STO-TZ2P-COSMO} 
frontier MO energy levels along with $\text{C}_2$ irreps (a and~b).}
{\vspace*{.2pc}}
\end{figure}

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Simulated TD-CAMY-B3LYP-D3/{\ubreak}STO-TZ2P-COSMO optical
spectra in dichloromethane. The vertical lines represent calculated
transitions which have then been broadened with Gaussians to generate
the simulated spectra.}
{\vspace*{-.5pc}}
\end{figure}

Finally, the calculated absorption maxima from CAMY-B3LYP TDDFT
calculations on M[Chl] in dichloromethane (Figure~\ref{fig6}) were found to 
be in \mbox{surprisingly} good agreement with the experimental absorption maxima for
M[Chl-e]. Thus, for instance, the calculations correctly captured the
larger hypsochromic shifts for the Q~band relative to the Soret band.
In all three cases, the Q~band consists of approximately 95\% HOMO
${\rightarrow}$ LUMO character, with the remainder made up of HOMO-1
${\rightarrow}$ LUMO${+}$1 character (Table~\ref{tab2}). The Soret band,
by comparison, consists of a mixture of HOMO-1 ${\rightarrow}$ LUMO and
HOMO ${\rightarrow}$ LUMO${+}$1 character. Since both the HOMO and HOMO-1
downshift from Zn through Pd to Pt, it makes sense that both the Q and
Soret absorption maxima blueshift in the same order.

\begin{table*} %tab2
\caption{CAMY-B3LYP-D3/STO-TZ2P TDDFT results, including
transition energies ($E$) and wavelengths (${\lambda}$), oscillator
strengths ($f$), MO compositions, and symmetries}\label{tab2}
\begin{tabular}{ccccccccc}
\thead
\xmorerows{1}{Molecule} & \xmorerows{1}{Peak} &
\xmorerows{1}{$E$ (eV)} & \xmorerows{1}{$\lambda$ (nm)} & 
\xmorerows{1}{$f$} & \multicolumn{3}{c}{MO composition} &
\xmorerows{1}{State symmetry} \\\cline{6-8} 
& & & & & From & To & Weight (\%) & \\
\endthead
Zn[Chl] & Q & 2.12 & 584.6 & 0.45 & HOMO & LUMO & 92.6 & B\\
&&&&& HOMO-1 & LUMO${+}$1 & 6.3 & B\\
& Soret & 3.04 & 408.4 & 1.80 & HOMO & LUMO${+}$1 & 62.3 & A\\
&&&&& HOMO-1 & LUMO & 35.8 & A\\
&& 3.33 & 372.3 & 1.23 & HOMO-1 & LUMO${+}$1 & 91.4 & B\\
&&&&& HOMO & LUMO & 6.4 & B{\vspace*{6pt}}\\
Pd[Chl] & Q & 2.18 & 568.4 & 0.48 & HOMO & LUMO & 94.3 & B\\
&&&&& HOMO-1 & LUMO${+}$1 & 4.5 & B\\
& Soret & 3.11 & 398.7 & 1.60 & HOMO-1 & LUMO & 59.8 & A\\
&&&&& HOMO & LUMO${+}$1 & 38.2 & A\\
&& 3.47 & 356.9 & 1.10 & HOMO-1 & LUMO${+}$1 & 92.5 & B\\
&&&&& HOMO & LUMO & 4.5 & B{\vspace*{6pt}}\\
Pt[Chl] & Q & 2.23 & 556.1 & 0.50 & HOMO & LUMO & 94.8 & B\\
&&&&& HOMO-1 & LUMO${+}$1 & 3.9 & B\\
& Soret & 3.18 & 390.1 & 1.58 & HOMO & LUMO${+}$1 & 56.5 & A\\
&&&&& HOMO-1 & LUMO & 40.9 & A\\
&& 3.56 & 348.4 & 1.14 & HOMO-1 & LUMO${+}$1 & 92.6 & B\\
&&&&& HOMO & LUMO & 3.8 & B
\botline
\end{tabular}
\vspace*{2pt}
\end{table*}

In summary, a combined experimental and theoretical study indicates the
plausible occurrence of a hypso effect in chlorins. Thus, both the
Soret and Q~bands blueshift going from Zn to Pd to Pt as the
coordinated metal. As in the case of porphyrins, the blueshifts appear
to be largely attributable to the presence of an electronegative
coordinated atom, which lowers the orbital energies of the HOMO and
HOMO-1, while leaving the LUMOs relatively \mbox{unaffected}.

\section{Experimental section}
\subsection{Materials}

All reagents were purchased from Sigma-Aldrich and used as received
except chlorin e6 trimethyl \mbox{ester} (CAS 35038-32-5) which was
purchased from PorphyChem.\ Silica gel 60 (0.04--0.063~mm particle
size, \mbox{230--400} mesh, Merck) was employed for flash
chromatography. Silica gel 60 preparative thin-layer chromatographic
plates (20~cm ${\times}$ 20~cm ${\times}$ 0.5~mm, Merck) were used for
final purification of all\break compounds. 

\subsection{General instrumental methods}

UV--visible spectra were recorded on an HP 8454 spectrophotometer.
${}^1\text{H}$ NMR spectra were recorded on a 400~MHz Bruker Avance III
HD spectrometer equipped with a 5~mm BB/1H SmartProbe and
\mbox{referenced} to either residual $\text{C}_6\text{H}_6$ at 7.16~ppm
or residual $\text{CHCl}_3$ at 7.26~ppm. High-resolution
electrospray-ionization mass spectra were recorded on an Orbitrap
Exploris 120 spectrometer using methanolic solutions. Cyclic
voltammetry was carried out at ambient temperature with a Gamry
Reference 620 potentiostat equipped with a three-electrode system: a
3~mm disc glassy carbon working electrode, a platinum wire
counter-electrode, and a saturated calomel reference electrode (SCE).
Tetra($n$-butyl)ammonium perchlorate (\textit{CAUTION!}) was used as
the supporting electrolyte. Anhydrous $\text{CH}_2\text{Cl}_2$
(Sigma-Aldrich) was used as solvent. The electrolyte solution was
purged with argon for at least 2~min prior to all measurements, which
were carried out under an argon blanket. The glassy carbon working
electrode was polished using a polishing pad and 0.05 micrometer
polishing alumina from ALS Japan. All potentials were referenced\break
to the SCE.

\subsection{Synthesis}

The Zn complex was made by adding a solution of $\text{Zn(OAc)}_2
\bullet 2\text{H}_2\text{O}$ (29.5~mg, 4~eq.) in 1~mL MeOH to a
solution of chlorin e6 trimethyl ester (21.9~mg) in 5~mL chloroform.
The resulting solution was refluxed for 1~h. The Pd complex was made by
dissolving chlorin e6 trimethyl ester (17.3~mg) and $\text{Pd(OAc)}_2$
(19.7~mg, 3~eq.) in 5~mL pyridine and refluxing the resulting solution
for 4~h. The Pt complex was made by dissolving chlorin e6 trimethyl
ester (18.0~mg) and $\text{Pt(OAc)}_2$ (26.3~mg, 3~eq.) in 5~mL
benzonitrile and refluxing the resulting solution for 10~h. Following
\mbox{reflux}, each mixture was rotary-evaporated to dryness and the
residue dissolved in DCM and placed on a silica column.
Dichloromethane/ethyl acetate 20:1 eluted a blue fraction which was
evaporated to dryness and further purified by preparative TLC using
$n$-hexane/ethyl acetate 1:1 as solvent. Yields and analytical details
are as follows:

\subsubsection{Zn[Chl-e]}

Yield 16.8~mg, 70\%. UV--Vis ($\text{CH}_2\text{Cl}_2$)
$\lambda_{\mathrm{max}}$ (nm) [$\epsilon \times 10^{-4}\
(\text{M}^{-1}{\cdot}\text{cm}^{-1})$]: 411 (8.50), 505 (0.40), 593
(0.58), 637 (2.98). ${}^{1}\text{H}$ NMR (400~MHz,
$\text{C}_6\text{D}_6$, $\delta$ in ppm): 9.54 (s, 1H, \textbf{5}),
9.41 (s, 1H, \textbf{10}), 8.50 (s, 1H, \textbf{20}), 8.05 (dd, $J =
17.8$, 11.5~Hz, 1H, $\mathbf{3}^{\mathbf{1}}$), 6.18 (d, $J = 18.0$~Hz,
1H, $\mathbf{3}^{\mathbf{2}}$ \textbf{\textit{trans}}), 5.91 (d, $J =
1.8$~Hz, 1H, $\mathbf{3}^{\mathbf{2}}$ \textbf{\textit{cis}}), 5.45 (d,
$J = 19.0$~Hz, 1H, $\mathbf{15}^{\mathbf{1}}$), 5.28 (d, $J = 19.0$~Hz,
1H, $\mathbf{15}^{\mathbf{1}}$), 4.28 (d, $J = 10.2$~Hz, 1H,
\textbf{17}), 4.01 (t, $J = 7.1$~Hz, 1H, \textbf{18}), 3.95 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.59 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.47 (q, $J = 7.5$~Hz, 2H,
$\mathbf{8}^{\mathbf{1}}$), 3.38 (s, 3H, $\mathbf{OCH}_{\mathbf{3}}$),
3.26 (s, 3H, $\mathbf{12}^{\mathbf{1}}$), 3.25 (s, 3H,
$\mathbf{2}^{\mathbf{1}}$), 3.01 (s, 3H, $\mathbf{7}^{\mathbf{1}}$),
2.30 (dt, $J = 15.9$, 7.9~Hz, 1H, $\mathbf{17}^{\mathbf{1}}$), 2.07 (d,
$J = 10.0$~Hz, 1H, $\mathbf{17}^{\mathbf{1}}$), \mbox{2.00--1.91} (m, 1H,
$\mathbf{17}^{\mathbf{2}}$), 1.67 (d, $J = 7.2$~Hz, 3H,
$\mathbf{18}^{\mathbf{1}}$), 1.60 (t, $J = 7.6$~Hz, 3H,
$\mathbf{8}^{\mathbf{2}}$), $\sim$1.55 (1H, $\mathbf{17}^{\mathbf{2}}$
concealed by overlapping peaks). MS (ESI, positive mode): m/z calcd for
$\text{C}_{37}\text{H}_{40}\text{N}_{4}\text{O}_{5}\text{ZnH}$
701.2312; [$\text{M} + \text{H}^{+}$] found 701.2307.

\subsubsection{Pd[Chl-e]}

Yield 1.9~mg, 9.4\%. UV--Vis ($\text{CH}_2\text{Cl}_2$)
$\lambda_{\mathrm{max}}$ (nm) [$\epsilon \times 10^{-4}\
(\text{M}^{-1}{\cdot}\text{cm}^{-1})$]: 403 (5.14), 486 (0.35), 582
(0.62), 623 (3.22). ${}^1\text{H}$ NMR (400~MHz, $\text{CDCl}_3$,
$\delta$ in ppm): 9.57 (s, 1H, \textbf{5/10}), 9.56 (s, 1H,
\textbf{5/10}), 8.62{\break} (s, 1H, \textbf{20}), 7.98 (dd, $J =
17.8$, 11.5~Hz, 1H, $\mathbf{3}^{\mathbf{1}}$), 6.16 (dd, $J = 17.8$,
1.7~Hz, 1H, $\mathbf{3}^{\mathbf{2}}$ \textbf{\textit{trans}}), 6.03
(dd, $J = 11.5$, 1.7~Hz, 1H, $\mathbf{3}^{\mathbf{2}}$
\textbf{\textit{cis}}), 5.14 (d, $J = 19.1$~Hz, 1H,
$\mathbf{15}^{\mathbf{1}}$), 4.93 (d, $J = 19.0$~Hz, 1H,
$\mathbf{15}^{\mathbf{1}}$), 4.48--4.35 (m, 2H, overlapping
\textbf{17} and \textbf{18}), 4.19 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.86 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.75 (q, $J = 7.6$~Hz, 2H,
$\mathbf{8}^{\mathbf{1}}$), 3.62 (s, 3H, $\mathbf{OCH}_{\mathbf{3}}$),
3.44 (s, 3H, $\mathbf{12}^{\mathbf{1}}$), 3.33 (s, 3H,
$\mathbf{2}^{\mathbf{1}}$), 3.29 (s, 3H, $\mathbf{7}^{\mathbf{1}}$),
2.58--2.46 (m, 1H, $\mathbf{17}^{\mathbf{1}}$), 2.24--2.11 (m, 1H,
$\mathbf{17}^{\mathbf{1}}$),{\break} 2.07--1.95 (m, 1H,
$\mathbf{17}^{\mathbf{2}}$), 1.75 (d, $J = 7.1$~Hz, 3H,
$\mathbf{18}^{\mathbf{1}}$), 1.69 (t, $J = 7.6$~Hz, 3H,
$\mathbf{8}^{\mathbf{2}}$) $\sim$1.69 (1H, $\mathbf{17}^{\mathbf{2}}$
concealed by overlapping peaks). MS (APCI, positive mode): m/z calcd
for $\text{C}_{37}\text{H}_{40}\text{N}_{4}\text{O}_{5}\text{PdH}$
743.2055; [$\text{M} + \text{H}^+$] found 743.2065.

\subsubsection{Pt[Chl-e]}

Yield 5.8~mg, 24.7\%. UV--Vis ($\text{CH}_2\text{Cl}_2$)
$\lambda_{\mathrm{max}}$ (nm) [$\epsilon \times 10^{-4}\
(\text{M}^{-1}{\cdot}\text{cm}^{-1})$]: 393 (6.49), 478 (0.36), 613
(3.49). ${}^1\text{H}$ NMR (400~MHz, $\text{CDCl}_3$, $\delta$ in ppm):
9.56 (s,~1H, \textbf{5}), 9.52 (s, 1H, \textbf{10}), 8.70 (s, 1H,
\textbf{20}), 7.98 (dd, $J = 17.8$, 11.5~Hz, 1H,
$\mathbf{3}^{\mathbf{1}}$), 6.14 (dd, $J = 17.8$, 1.7~Hz, 1H,
$\mathbf{3}^{\mathbf{2}}$ \textbf{\textit{trans}}), 6.02 (dd, $J =
11.4$, 1.6~Hz, 1H, $\mathbf{3}^{\mathbf{2}}$ \textbf{\textit{cis}}),
5.13 (d, $J = 19.1$~Hz, 1H, $\mathbf{15}^{\mathbf{1}}$), 4.93 (d, $J =
19.1$~Hz, 1H, $\mathbf{15}^{\mathbf{1}}$), 4.46--4.34 (m, 2H,
overlapping \textbf{17} and \textbf{18}), 4.20 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.87 (s, 3H,
$\mathbf{OCH}_{\mathbf{3}}$), 3.71 (q, $J = 7.6$~Hz, 2H,
$\mathbf{8}^{\mathbf{1}}$), 3.63 (s, 3H, $\mathbf{OCH}_{\mathbf{3}}$),
3.50 (s, 3H, $\mathbf{12}^{\mathbf{1}}$), 3.31 (s, 3H,
$\mathbf{2}^{\mathbf{1}}$), 3.24 (s, 3H, $\mathbf{7}^{\mathbf{1}}$),
2.54 (ddd, $J = 16.1$, 9.0, 7.0~Hz, 1H, $\mathbf{17}^{\mathbf{1}}$),
2.20 (ddd, $J = 16.2$, 7.2, 4.8~Hz, 1H, $\mathbf{17}^{\mathbf{1}}$),
2.09--1.97 (m, 1H, $\mathbf{17}^{\mathbf{2}}$), 1.76 (d, $J =
7.1$~Hz, 3H, $\mathbf{18}^{\mathbf{1}}$), 1.68 (t, $J = 7.6$~Hz, 3H,
$\mathbf{8}^{\mathbf{2}}$), $\sim$1.71 (1H, $\mathbf{17}^{\mathbf{2}}$
concealed by overlapping peaks). MS (APCI, positive mode): m/z calcd
for $\text{C}_{37}\text{H}_{40}\text{N}_{4}\text{O}_{5}\text{PtH}$
832.2668; [$\text{M} + \text{H}^+$] found 832.2670.

\subsection{Computational method}

The three M[Chl] (M ${=}$ Zn, Pd, Pt] complexes were optimized with
scalar-relativistic DFT calculations, with the OLYP~\cite{20,21}
functional augmented with Grimme's D3~\cite{22} dispersion correction,
all-electron ZORA STO-TZ2P basis sets, fine integration grids, and
tight criteria for both SCF cycles and geometry optimizations, as
implemented in the ADF 2019 program system~\cite{25}. All the
optimizations were carried out with a $\text{C}_2$ symmetry constraint
and the COSMO (Conductor like Screening Model)~\cite{26} solvation
model with dichloromethane as solvent. TDDFT calculations were
performed on the OLYP-D3 optimized geometries using the
CAMY-B3LYP~\cite{24} range-separated functional (which has been
extensively calibrated for calculations of porphyrin
spectra~\cite{4,7,27}), with basis set and other parameters the same as
in the ground-state calculations).

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and
have declared no affiliations other than their research organizations.

\section*{Funding}

This research was supported by the Research Council of Norway (grant
no. 262229 to AG), the CNRS (grant UMR UBCNRS 6302), and the South
African National Research Foundation (grant nos. 129270 and 132504 to
JC). 

\section*{Acknowledgment}

The authors also warmly thank Mrs.~Sandrine Pacquelet for technical
assistance.

\CDRGrant[RCN]{262229}
\CDRGrant[CNRS]{UMR UBCNRS 6302}
\CDRGrant[NRF]{129270}
\CDRGrant[NRF]{132504}

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\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-299-suppl.pdf}}

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