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\DOI{10.5802/crchim.282}
\datereceived{2023-05-31}
\daterevised{2023-10-03}
\dateaccepted{2023-12-13}
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\dateposted{2024-05-24}
\begin{document}

\begin{noXML}

%\makeatletter
%\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}{research-article}


\title{Derivatives of trigonal lanthanide complexes by reaction with
long aliphatic chain amines}

\alttitle{D\'{e}riv\'{e}s de complexes trigonaux de lanthanides par
r\'{e}action avec des amines \`{a} longue cha\^{i}ne aliphatique}

\author{\firstname{Yiwei} \lastname{Zhou}\IsEqualContrib}
\address{Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen \O{}, Denmark}

\author{\firstname{Christian Dirk} \lastname{Buch}\CDRorcid{0000-0001-8550-4168}\IsEqualContrib}
\addressSameAs{1}{Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen \O{}, Denmark}

\author{\firstname{Steen Hansgaard} \lastname{Hansen}}
\addressSameAs{1}{Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen \O{}, Denmark}

\author{\break\firstname{Stergios} \lastname{Piligkos}\IsCorresp}
\addressSameAs{1}{Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen \O{}, Denmark}
\email[S. Piligkos]{piligkos@chem.ku.dk}

\keywords{\kwd{Lanthanide complexes}
\kwd{Aliphatic chains}
\kwd{Magnetic properties}
\kwd{Crystal structure}
\kwd{Crystal field}}

\altkeywords{\kwd{Complexes de lanthanides}
\kwd{Cha\^{i}nes aliphatiques}
\kwd{Propri\'{e}t\'{e}s magn\'{e}tiques}
\kwd{Structure cristalline}
\kwd{Champ cristallin}}

\thanks{Novo Nordisk Foundation (Grant agreement no. NNF20OC0065610)}

\shortrunauthors

\begin{abstract} 
We present the synthesis and characterisation of a novel series of
lanthanide complexes bearing long aliphatic chains, obtained by
post-functionalisation of LnL (Ln = Tb--Tm and Y, H\textsubscript{3}L =
tris(((3-formyl-5-methylsalicylidene)amine)ethyl)amine) via Schiff-base
reaction with 1-octadecylamine. Crystal structures show that the first
coordination sphere is not radically perturbed by the
post-functionalisation although the trigonal symmetry is lost.
Post-derivatisation influences the static magnetic properties of the
Kramers and non-Kramers ions differently, with the non-Kramers ions
most sensitive to derivatisation. The observed differences likely stem
from changes in eigenvector composition as luminescence spectra showed
minor modifications of the energy spectra upon derivatisation.
\end{abstract} 

\begin{altabstract} 
Nous pr\'{e}sentons la synth\`{e}se et la caract\'{e}risation d'une
nouvelle s\'{e}rie de complexes de lanthanides portant de longues
cha\^{i}nes aliphatiques, obtenus par post-fonctionnalisation de LnL
(Ln = Tb-Tm et Y, H\textsubscript{3}L =
tris(((3-formyl-5-m\'{e}thylsalicylid\`{e}ne)amine)\'{e}thyl)amine) via
une r\'{e}action de Schiff avec la 1-octad\'{e}cylamine. Les structures
cristallines montrent que la premi\`{e}re sph\`{e}re de coordination
n'est pas radicalement perturb\'{e}e par la post-fonctionnalisation,
bien que la sym\'{e}trie trigonale soit perdue. La
post-d\'{e}rivatisation influence diff\'{e}remment les
propri\'{e}t\'{e}s magn\'{e}tiques statiques des ions Kramers et non
Kramers, les ions non Kramers \'{e}tant les plus sensibles \`{a} la
d\'{e}rivatisation. Les diff\'{e}rences observ\'{e}es proviennent
probablement de changements dans la composition des vecteurs propres,
car les spectres de luminescence ont montr\'{e} des modifications
mineures des spectres d'\'{e}nergie lors de la d\'{e}rivatisation.
\end{altabstract} 

\maketitle

\vspace*{.2pc}

\twocolumngrid

\end{noXML}

\section{Introduction} \label{sec1}

Lanthanide complexes coordinated to organic ligands have attracted
significant attention in many different fields due to the special
optical, electronic, and magnetic properties of the lanthanide ions.
These lanthanide-containing complexes exhibit various applications
ranging from luminescent thermometers~\cite{1,2,3,4}, and MRI contrast
agents~\cite{5,6,7} for medical imaging to quantum information 
processing~\cite{8,9,10,11,12,13,14}, and single molecule magnets
(SMMs)~\cite{15,16,17,18,19}. Understanding the electronic structure in
lanthanide complexes is important for optimising their performance and
designing new functional materials~\cite{20}. Lanthanide-containing
complexes that can be modified with various chemical groups to tune
their properties or give new features, such as functionalisation for
deposition on different surfaces or solubility in diverse media, offer
promising prospects. It is still challenging to synthesise lanthanide
complexes where the first coordination sphere remains constant upon
post-derivatisation. 

Recently, we studied the magnetic properties of a new family of
trigonal lanthanide complexes LnL\unskip\break (H$_{3}$L $=$
tris(((3-formyl-5-methylsalicylidene)amine){\ubreak}ethyl)amine) with Ln ranging
from Gd to Lu~\cite{21}. These complexes have three pendant carbonyl
groups that are non-coordinating to the lanthanide ion. Therefore,
these carbonyl groups can be used to react with primary amines via a
Schiff-base reaction to form imines. This post-derivatisation conserves
the number and nature of atoms in the first coordination sphere. We
have previously performed studies focusing on the derivatisation of
YbL, for example by reacting YbL with benzylamine to obtain complexes
suitable for deposition on graphene~\cite{21}, or by reacting YbL with
1-octadecylamine to form new complexes~\cite{22} that have potential in
making lanthanide-containing micelles~\cite{23,24,25,26,27,28} and
Langmuir--Blodgett film~\cite{29,30,31,32}.

Herein, we present the synthesis and characterisation of a novel series
of lanthanide complexes \textbf{LnL}$^{\mathbf{18}}$
(\textbf{H}$_3$\textbf{L}$^{\mathbf{18}} =$
tris(((3-(1-octadecylimine)-5-methylsalicylidene)amine)ethyl)amine)
with Ln $=$ Tb--Tm and Y. We investigate the crystal field splitting of
\textbf{LnL}$^{\mathbf{18}}$ by luminescence measurements and the static
magnetic properties by SQUID magnetometry. Additionally, the dynamic
magnetic properties were examined through alternating current SQUID
magnetometry. The results were compared to \textbf{YbL}$^{\mathbf{18}}$ and
(Tb--Tm)L to get an understanding of the impact of the
post-derivatisation on the electronic and magnetic properties of the Ln
ions. 

\section{Experimental}\label{sec2}

\subsection{Materials and physical measurements}\label{sec21}

All solvents and chemicals used for the syntheses of the complexes
herein were purchased from commercial sources and used as received. All
syntheses were made without any attempt to exclude moisture or oxygen.
No attempt was made to dry any of the solvents used. The
$\mathrm{Ln}(\mathrm{OTf})_3{\cdot}x\mathrm{H}_{2}\mathrm{O}$ salts,
2,6-diformyl-\textit{p}-cresol (dfmp) and LnL were prepared as
described in literature~\cite{21}. 

Positive-ion mode MALDI mass spectrometry was performed on a Bruker
Solarix XR 7T ESI/MALDI FT-ICR MS instrument at the Department of
Chemistry, University of Copenhagen. Infrared (IR) spectra were
measured on an Agilent Technologies Cary 630 FTIR spectrometer. Powder
X-ray diffraction (PXRD) was recorded on a BRUKER D8 ADVANCE powder
diffractometer using a Cu K${\upalpha}$ radiation (${\uplambda} =
1.5418$~\AA) source. $^{1}$H NMR was obtained using a Bruker 500 MHz
instrument equipped with a cryoprobe. For $^{1}$H NMR, calibration was
done against solvent signals from the deuterated solvent. Elemental (C,
H, and N) analyses were performed on a FlashEA 1112 instrument at The
Microanalytical Laboratory at the Department of Chemistry, University
of Copenhagen. Direct current (d.c.) and alternating current (a.c.)
magnetic susceptibility measurements were recorded using a
Quantum-Design MPMS-XL SQUID magnetometer. The measurements were done
on polycrystalline samples immobilised in a small amount of
${n}$-hexadecane to avoid orientation of the sample in the magnetic
field. The magnetic susceptibility measurements were corrected for the
diamagnetism of ${n}$-hexadecane
($186\times10^{-6}~\mathrm{cm}^{3}/\mathrm{mol}$)~\cite{33}
and of the sample using the approximation
($M_{\text{sample}}/2)\times10^{-6}~\mathrm{cm}^{3}/
\mathrm{mol}$~\cite{34}. Luminescence spectra of polycrystalline
samples of \textbf{TbL}$^{\mathbf{18}}$, \textbf{HoL}$^{\mathbf{18}}$ and
\textbf{ErL}$^{\mathbf{18}}$ were obtaining from a Horiba-Jobin Yvon Fluorolog
fluorimeter equipped with an InGaAs near-infrared (NIR) detector and a
photomultiplier detector for the UV/Vis range. Additionally, an Oxford
Instruments cryostat was used for the cryogenic measurements. UV/Vis
absorption spectroscopy was measured on a Lambda 2 UV/Vis spectrometer
manufactured by Perkin Elmer. The measurement was performed with a scan
rate of 120 nm/min and with a background correction to the pure
solvent.\looseness=1

\subsection{Synthesis of LnL$^{18}$ (H$_3$L$^{18} =$
Tris(((3-(1-octadecylimine)-5-methylsalicylidene)
amine)ethyl)amine))}\label{sec22}

\textbf{TbL}$^{\mathbf{18}}$. TbL (0.052 g; 0.070~mmol) and 1-octadecylamine
(0.189 g; 0.70~mmol) were dissolved in a MeOH:CHCl$_{3}$ 1:1
mixture (20 ml). The mixture was boiled until it became clear. Then the
solution was kept at 50~\textdegree C until a yellow precipitate formed.
The solution was left to slowly cool down to room temperature. The
precipitate was washed with MeCN and diethyl ether. 

Yield: 0.076 g (86\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Tb: C, 69.89; H, 9.71; N: 6.56. Found:
C, 69.86; H, 9.82; N: 6.58. MALDI mass spectrum: 1496.07 \textit{m/z}
[TbL$^{18}$H]$^{+}$  (Figure~S2). IR ${\upnu}$(C--H):
2917~cm$^{-1}$, 2849~cm$^{-1}$, ${\upnu}_{\text{C=N}}$: 1634~cm$^{-1}$,
1619~cm$^{-1}$ (Figure~S8).

The remaining \textbf{LnL}$^{\mathbf{18}}$ complexes (Ln $=$ Dy--Tm)
and \textbf{YL}$^{\mathbf{18}}$ were synthesised analogously to
\textbf{TbL}$^{\mathbf{18}}$.

\textbf{DyL}$^{\mathbf{18}}$. Yield: 0.071 g (73\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Dy: C, 69.73; H, 9.69; N: 6.54. Found:
C, 69.59; H, 9.76; N: 6.58. MALDI mass spectrum: 1499.07 \textit{m/z}
[DyL$^{18}$H]$^{+}$ (Figure~S3). IR \mbox{${\upnu}$(C--H):} 
2918~cm$^{-1}$, 2850~cm$^{-1}$, ${\nu}$(C${=}$N):
1633~cm$^{-1}$, 1619~cm$^{-1}$ (Figure~S9).

\textbf{HoL}$^{\mathbf{18}}$. Yield: 0.055 g (62\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Ho: C, 69.61; H, 9.67; N: 6.53. Found:
C, 69.12; H, 9.68; N: 6.59. MALDI mass spectrum: 1502.08 \textit{m/z}
[HoL$^{18}$H]$^{+}$ (Figure~S4). IR ${\upnu}$(C--H):
2919~cm$^{-1}$, 2851~cm$^{-1}$, ${\nu}$(C${=}$N): 1634~cm$^{-1}$,
1619~cm$^{-1}$ (Figure~S10).

\textbf{ErL}$^{\mathbf{18}}$. Yield: 0.069 g (80\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Er: C, 69.51; H, 9.65; N: 6.52. Found:
C, 69.04; H, 9.76; N: 6.88. MALDI mass spectrum: 1504.08 \textit{m/z}
[ErL$^{18}$H]$^{+}$ (Figure~S5). IR ${\upnu}$(C--H):
2919~cm$^{-1}$, 2851~cm$^{-1}$, ${\nu}$(C${=}$N): 1634~cm$^{-1}$,
1619~cm$^{-1}$ (Figure~S11).

\textbf{TmL}$^{\mathbf{18}}$. Yield: 0.064 g (73\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Tm: C, 69.43; H, 9.64; N: 6.51. Found:
C, 68.05; H, 9.45; N: 6.52. MALDI mass spectrum: 1506.08 \textit{m/z}
[TmL$^{18}$H]$^{+}$ (Figure~S6). IR ${\upnu}$(C--H):
2918~cm$^{-1}$, 2850~cm$^{-1}$, ${\nu}$(C${=}$N): 1634~cm$^{-1}$,
1620~cm$^{-1}$ (Figure~S12).

\textbf{YL}$^{\mathbf{18}}$. Yield: 0.05 g (56\%). Anal. Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Y: C, 73.33; H, 10.19; N: 6.88. Found:
C, 72.81; H, 10.29; N: 6.73. MALDI mass spectrum: 1426.03 \textit{m/z}
[YL$^{18}$H]$^{+}$ (Figure~S7). IR \mbox{${\upnu}$(C--H):}\break
2916~cm$^{-1}$, 2850~cm$^{-1}$, ${\nu}$(C${=}$N):
1634~cm$^{-1}$, 1620~cm$^{-1}$ (Figure~S13) $^1$H NMR (500 MHz,
CDCl$_{3}$) ${\delta}$/ppm 8.23 (s, 3 H), 8.17(s, 3 H), 7.76 (s, 3 H),
7.01 (s, 3 H), 4.30 (t, 3 H, $J = 13.3$~Hz), 3.40 (d, 3 H, ${J} =
14.1$~Hz), 3.15 (m, 6 H), 3.09 (m, 3 H), 2.84 (d, 3 H, ${J} =
12.7$~Hz), 2.21 (s, 9 H), 1.26 (m, 96 H), 0.88 (t, 9 H, ${J} = 6.9$~Hz)
(Figure~S1).\looseness=-1

The complexes diluted in a diamagnetic host (\textbf{Gd@YL}$^{\mathbf{18}}$,
\textbf{Dy@YL}$^{\mathbf{18}}$ and \textbf{Er@LuL}$^{\mathbf{18}}$) were synthesised
analogously to \textbf{TbL}$^{\mathbf{18}}$, but using a 5:95 mixture of either
GdL, DyL or ErL and YL or LuL as the diamagnetic host. 

\textbf{Gd@YL}$^{\mathbf{18}}$ at 5\% dilution. Yield: 0.09 g (62\%). Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Y$_{0.95}$Gd$_{0.05}$: C, 73.33; H,
10.19; N, 6.88. Found: C, 72.05; H, 10.13; N, 6.73. The PXRD was
checked for phase purity of the sample (Figure~S22). IR
${\upnu}$(C--H): 2917~cm$^{-1}$, 2850~cm$^{-1}$,
${\nu}$(C${=}$N): 1633~cm$^{-1}$, 1619~cm$^{-1}$ (Figure~S15). 

\textbf{Dy@YL}$^{\mathbf{18}}$ at 5\% dilution. Yield: 0.07 g (47\%). Calcd for
C$_{87}$H$_{144}$N$_{7}$O$_{3}$Y$_{0.95}$Dy$_{0.05}$: C, 73.33; H,
10.19; N, 6.88. Found: C, 72.23; H, 10.14; N, 6.51. The PXRD was
checked for phase purity of the sample (Figure~S22). IR
${\upnu}$(C--H): 2916~cm$^{-1}$, 2850~cm$^{-1}$,
${\nu}$(C${=}$N): 1633~cm$^{-1}$, 1619~cm$^{-1}$ (Figure~S16).

\textbf{Er@LuL}$^{\mathbf{18}}$ at 5\% dilution. Yield: 0.07 g (51\%). {Calcd
for C}$_{87}${H}$_{144}${N}$_{7}${O}$_{3}${Lu}$_{0.95}${Er}$_{0.05}${:
C, 69.15; H, 9.61; N, 6.49.} Found: C, 68.81; H, 9.64; N, 6.39. The
PXRD was checked for phase purity of the sample (Figure~S22). IR
${\upnu}$(C--H): 2917~cm$^{-1}$, 2850~cm$^{-1}$,
${\nu}$(C${=}$N): 1634~cm$^{-1}$, 1620~cm$^{-1}$ (Figure~S17).

{\vspace*{-2pt}}

\subsection{Crystal structure determination}\label{sec23}

{\vspace*{-2pt}}

Crystals suitable for single-crystal X-ray diffraction were obtained in
different manners for the complexes depending on the metal ion. For
\textbf{TbL}$^{\mathbf{18}}$ and \textbf{DyL}$^{\mathbf{18}}$, single crystals were
obtained through slow evaporation of a MeCN:chloroform 1:1 solution. 
For \textbf{HoL}$^{\mathbf{18}}$, single crystals were obtained by dissolving
the complex in chloroform and then adding the solution to acetone
(approximately five times the volume). Upon evaporation of the
solvents, single crystals of \textbf{HoL}$^{\mathbf{18}}$ formed.

Single-crystal X-ray diffraction on single crystals of
\textbf{TbL}$^{\mathbf{18}}$, \textbf{DyL}$^{\mathbf{18}}$, and \textbf{HoL}$^{\mathbf{18}}$ was
performed at 100~K using a Bruker D8 VENTURE diffractometer equipped
with a Mo K${\upalpha}$ high-brilliance I~${\upmu}$S S3 radiation source
(${\lambda}= 0.71073$~\AA), a PHOTON 100 CMOS detector, and an
Oxford Cryosystems cooling system. Data reduction was performed using
SAINT, and the absorption corrections was handled by SADABS using the
multi-scan method. The structures were recorded using APEX3 and solved
by SHELXT~\cite{35,36} using intrinsic phasing and refined using
SHELXL~\cite{37} (Least Squares). Visualisation of the crystallographic
data during the refinement was obtained using the OLEX2 program
package~\cite{38,39}. All atoms were refined anisotropically except
for hydrogen. Hydrogen atoms were placed at calculated positions in
OLEX2 using the ``Add H'' command. In the structure of
\textbf{TbL}$^{\mathbf{18}}$, additional electron density was found next to the
molecule. This electron density could not be modelled as solvent and a
solvent mask (SQUEEZE) in OLEX2 was thus used. The electron density
amounted to 9 electrons which could stem from a disordered water
molecule. 

\begin{scheme*}
\vspace*{-4pt}
\includegraphics{sc01}
\vspace*{4pt}
\caption{\label{sch1}Schiff-base reaction forming the
\textbf{LnL}$^{\mathbf{18}}$ complexes.}
\vspace*{-9pt}
\end{scheme*}

{\vspace*{-2pt}}

\section{Results and discussion}\label{sec3}

{\vspace*{-2pt}}

\subsection{Synthesis and characterisation}\label{sec31}

{\vspace*{-2pt}}

The complexes \textbf{LnL}$^{\mathbf{18}}$ (Ln $=$ Tb--Tm, Y) were synthesised
using a one-pot Schiff-base reaction between the corresponding LnL
complex and \mbox{1-octadecylamine} (Scheme~\ref{sch1}). In order to ensure that all
aldehyde groups of the complex were transformed to 1-octadecylimine
groups, a 1:10 ratio between LnL and 1-octadecylamine was used. MALDI
mass spectra show signals from all [LnL$^{18}$H]$^{+}$, confirming that
the complexes have been formed (Figure~S2--S7). The isotope pattern of
each complex matches with the experimental spectrum further
strengthening that the observed signals come from the intended product.
IR spectra of the \textbf{LnL}$^{\mathbf{18}}$ complexes show no trace of the
aldehyde stretch (1668--1669~cm$^{-1}$) found in the parent complex LnL
(Figure~S8--S14) confirming the complete reaction of all aldehyde
groups with 1-octadecylamine. Additionally, the C--H stretches between
2850--2919~cm$^{-1}$ for \textbf{LnL}$^{\mathbf{18}}$ are much more intense than
those for LnL (Figure~S14), as anticipated from the larger amount of
C--H units in \textbf{LnL}$^{\mathbf{18}}$ compared to LnL. 


The polycrystalline samples of \textbf{LnL}$^{\mathbf{18}}$ (Ln $=$ Tb--Tm, Y)
are phase-pure and of the same structure as the one determined for the
\textbf{LnL}$^{\mathbf{18}}$ single crystals (\textit{vide infra}). This is
confirmed by a PXRD simulation from a single crystal studied by X-ray
diffraction at room temperature (Figure~S21).

In \textbf{LnL}$^{\mathbf{18}}$ all Ln(III) ions except for Y(III) are
paramagnetic. The paramagnetism of these ions complicates the recording
of NMR spectra and hence only $^1$H NMR on \textbf{YL}$^{\mathbf{18}}$ has been
performed. The $^1$H NMR spectrum of \textbf{YL}$^{\mathbf{18}}$ shows the
expected signals and integrals of these signals (Figure~S1). As
anticipated by the structure of the complexes, most of the protons in
the octadecyl chains have almost identical chemical shifts with only
the two protons next to the imine group and the protons on the terminal
methyl group of each chain showing markedly different chemical shifts
than the rest of the chain. The NMR spectrum shows no signs of the
starting materials confirming that the complexes are formed purely as
also indicated by the above analyses. 

The CHN elemental analyses also agree with the expected values for the
\textbf{LnL}$^{\mathbf{18}}$ complexes indicating that the complexes have been
obtained pure.

The complexes containing the larger Ln ions (La(III), Pr(III), Nd(III),
Sm(III), Eu(III) and Gd(III)) were also synthesised following a similar
procedure as for \textbf{(Tb--Tm, Y)L}$^{\mathbf{18}}$. However, the PXRD
revealed that they were difficult to get phase-pure or in the same
phase as the complexes containing the smaller Ln ions \textbf{(Tb--Tm,
Y)L}$^{\mathbf{18}}$. This was also seen for (La--Eu)L and is likely the result
of solvent coordination to the Ln ion. It is common that the greater
size of the early Ln ions leads to higher coordination numbers through
solvent coordination~\cite{40}. By doping \textbf{GdL}$^{\mathbf{18}}$ into the
\textbf{YL}$^{\mathbf{18}}$ diamagnetic host it was possible to obtain 
\textbf{GdL}$^{\mathbf{18}}$ in the same phase as the complexes
\textbf{LnL}$^{\mathbf{18}}$ (Ln $=$ Tb--Tm, Y, Figure~S22).

Last, it should be mentioned that introduction of long aliphatic chains
makes the post-derivatised \textbf{LnL}$^{\mathbf{18}}$ much more soluble in
organic solvents than their parent complexes. Hence, these complexes
are readily soluble in, for example chloroform and dichloromethane, but
remain  insoluble in alkanes.

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig01}
\vspace*{-4pt}
\caption{\label{fig1} Solid state structure (upper left) and unit cell
(upper right) of \textbf{TbL}$^{\mathbf{18}}$. Colour code: Tb, green; N, blue;
O, red; C, grey. All hydrogen atoms have been omitted for clarity.
Thermal ellipsoids are set to 50\% probability. Structural overlay of
the solid state structures of \textbf{TbL}$^{\mathbf{18}}$ (blue) and TbL (red)
viewed from the side (bottom).} 
\vspace*{-4pt}
\end{figure*}

\subsection{Crystal structure}\label{sec32}

Single crystals large enough for single-crystal X-ray diffraction were
obtained for \textbf{TbL}$^{\mathbf{18}}$, \textbf{DyL}$^{\mathbf{18}}$ and
\textbf{HoL}$^{\mathbf{18}}$. The complexes crystallise in the triclinic space
group \textit{P}$\overline{1}$ with two complexes contained in the
 unit cell. The complexes contain three octadecyl chains
which extend from the metal-containing part of the complex (Figure~\ref{fig1}).
The alkyl chains do not extend in a straight line away from the metal
centre but instead form a bend thereby lowering the symmetry of the
complex from \textit{C}$_{3}$, as in the parent complex LnL, to
\textit{C}$_{1}$. This bend is likely a consequence of both
crystal-packing effects and van der Waals interactions between
neighbouring complexes (Figure~\ref{fig1}). In fact, extended networks
of van der Waals interactions are found throughout the crystal
structure, with these networks forming large arrays of hydrocarbon
stackings (Figure~S26). Similar extended networks have been found for
\textbf{YbL}$^{\mathbf{18}}$~\cite{22}. The large amount of van der Waals
interactions in the crystal structure is also reflected in the unit
cell dimensions. The side lengths of the unit cell are for instance
12.5~\AA{} $\times$ 12.5~\AA{} $\times$ 30.0~\AA{} 
in \textbf{TbL}$^{\mathbf{18}}$
with the \textit{c} directions clearly being much longer than the two
other dimensions which are equivalent in size (Table~S1). 

As in \textbf{YbL}$^{\mathbf{18}}$, the newly formed imine groups point away
from the Ln ion and are therefore non-coordinating. 

The post-derivatisation of LnL with 1-octadecylamine leads to small
structural changes to the first coordination sphere of the complexes.
For instance Tb--N$_{\text{imine}}$ in \textbf{TbL}$^{\mathbf{18}}$ is on
average 2.474(17)~\AA{}~compared to 2.490(1)~\AA{}~in TbL, and
Tb--N$_{\text{apical}}$ in \textbf{TbL}$^{\mathbf{18}}$ is 2.6346(17) \AA{}
compared to 2.647(2)~\AA{}~in TbL. The other bond lengths also remain
very similar to the parent complexes, as do the bond angles (Table~\ref{tab1}).
Still the bond angles do vary more than the bond lengths as is clearly
visible when the two structures are overlaid (Figure~\ref{fig1}).

\begin{table*}[t!]
\caption{\label{tab1}Average bond lengths and angles in
\textbf{TbL}$^{\mathbf{18}}$, TbL, \textbf{DyL}$^{\mathbf{18}}$, DyL,
\textbf{HoL}$^{\mathbf{18}}$, HoL, and \textbf{YbL}$^{\mathbf{18}}$}
\tabcolsep=1pt
\begin{tabular}{cccccccc}
\thead
 & \textbf{TbL}$^{\mathbf{18}}$ & \textbf{TbL} & \textbf{DyL}$^{\mathbf{18}}$ & \textbf{DyL} & 
 \textbf{HoL}$^{\mathbf{18}}$ & \textbf{HoL} & \textbf{YbL}$^{\mathbf{18}}$ \\
\endthead
\multicolumn{8}{c}{{Bond length (\AA)}}\vspace*{5pt} \\
Ln--O & 2.200(1) & 2.208(1) & 2.192(2) & 2.196(2) & 2.184(2) & 2.191(2) & 2.156(1) \\
Ln--N$_{\text{imine}}$ & 2.474(2) & 2.490(1) & 2.462(1) & 2.473(2) & 2.453(1) & 2.462(2) & 2.418(2) \\
Ln--N$_{\text{apical}}$ & 2.635(2) & 2.647(2) & 2.631(2) & 2.628(3) & 2.631(2) & 2.626(3) & 2.612(2)\vspace*{5pt} \\
\multicolumn{8}{c}{{Bond angle (\textdegree)}\vspace*{5pt}} \\
${\angle}$ N$_{\text{imines}}$ & 57.26(4)--61.44(4) & 60 & 57.35(5)--61.44(5) & 60 & 57.48(5)--61.40(5) & 60 & 57.62(5)--61.34(5) \\
${\angle}$ O$_{\text{phenoxides}}$ & 59.22(4)--60.5(4) & 60 & 59.19(5)--61.07(5) & 60 & 59.07(5)--61.02(5) & 60 & 59.15(5)--61.11(3)\vspace*{2pt}
\botline
\end{tabular}
\tabnote{The data for TbL, DyL, HoL and \textbf{YbL}$^{\mathbf{18}}$
were taken from literature~\cite{21,22}.}
\vspace*{-5pt}
\end{table*}


Although the trigonal symmetry of the complexes is lost upon
post-derivatisation, the first coordination sphere around the Ln centre
remains approximately trigonal. The Ln--O bond varies between
2.195(1)--2.203(1)~\AA{}~for \textbf{TbL}$^{\mathbf{18}}$,
2.187(2)--2.199(2)~\AA{}~for \textbf{DyL}$^{\mathbf{18}}$, and
2.179(2)--2.192(2) for \textbf{HoL}$^{\mathbf{18}}$, while
Ln--N$_{\text{imine}}$ bond varies between
2.468(2)--2.479(2)~\AA{}~for \textbf{TbL}$^{\mathbf{18}}$,
2.461(2)--2.462(2)~\AA{}~for \textbf{DyL}$^{\mathbf{18}}$, and
2.453(2)--2.454(2) for \textbf{HoL}$^{\mathbf{18}}$ (Table~S3). Additionally,
the three angles in the triangle defined by the three phenoxides remain
close to the expected 60\textdegree~for trigonal symmetry, while the
angles involving the three coordinating imines deviate more
(Table~\ref{tab1}). 

Moving from \textbf{TbL}$^{\mathbf{18}}$ to \textbf{HoL}$^{\mathbf{18}}$, a slight
contraction in the bond lengths between the Ln ion and the \mbox{ligand} is
observed (Tables 1 and S3). This is to be expected as the Ln(III) ions
shrink in size through the series. The contraction in the bond lengths
is even more visible when comparing the aforementioned complexes to
\textbf{YbL}$^{\mathbf{18}}$. As the bond lengths shrink, the previously
mentioned angles between the coordinating imine groups come closer to
60\textdegree, and the complexes thereby become closer to trigonal with
smaller Ln ions.

Like for the previously studied \textbf{YbL}$^{\mathbf{18}}$~\cite{22}, the
differences in the first coordination sphere between the
post-derivatised \textbf{LnL}$^{\mathbf{18}}$ and their parent LnL
remain minor. The biggest difference between the post-derivatised complexes
\textbf{LnL}$^{\mathbf{18}}$ and their parent complexes LnL is seen in the
length of the complexes and the distances to neighbouring Ln centres.
The addition of the three octadecyl chains extends the longest distance
within one complex (\textit{r}$_{1}$) from 13.2~\AA{}~to
32.8~\AA{}~(Figures~S24 and S28). Consequently, the distance between
two Ln(III) ions within a unit cell (\textit{r}$_{2}$) also
increases from 7.8~\AA{}~to 30.0~\AA{}~(Figures~S24 and S29). Because
the aliphatic chains stack inside a unit cell (Figure~\ref{fig1}),
\textit{r}$_{2}$ gets elongated quite significantly;
however, the distances to the nearest neighbours or the next nearest
neighbours remain very similar between \textbf{LnL}$^{\mathbf{18}}$ and LnL
(Table~S2 and Figures~S25--S26 and S30--S31).
\vspace*{-5pt}

\subsection{Influence of the post-derivatisation on the crystal field
splitting}\label{sec33}

In an effort to understand the influence of the octadecyl chains on the
electronic structure of the ground multiplets of
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$, the complexes were studied
using variable-temperature-variable-field (VTVB) measurements,
luminescence spectroscopy and d.c. magnetic \mbox{susceptibility}
\mbox{measurements,} and compared to those of their parent complexes. 

\subsubsection{Luminescence spectroscopy}\label{sec331}

Luminescence spectroscopy was measured on polycrystalline samples of
\textbf{TbL}$^{\mathbf{18}}$, \textbf{HoL}$^{\mathbf{18}}$, and \textbf{ErL}$^{\mathbf{18}}$ as
well as TbL, HoL, and ErL (Figure~\ref{fig2}). To eliminate possible hot bands
and decrease the bandwidth, the measurements were carried out at low
temperature (4~K). All the compounds share an intense absorption around
400 nm from the phenyl rings as seen in the UV/Vis absorption spectrum
(Figure~S66). All the luminescence spectra were measured by exciting
the ligand in this absorption region (400--425 nm). Following
absorption by the ligand, energy is then transfered to the lanthanide.
{The excitation wavelength was adjusted to avoid higher order
diffractions from the excitation monochromator in the emission
spectrum.} 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Luminescence spectra of: \textbf{TbL}$^{\mathbf{18}}$ and
TbL (left) measured at 4~K, with excitation at 400 nm;
\textbf{HoL}$^{\mathbf{18}}$ and HoL (middle) measured at 4~K, with excitation
at 400 nm; \textbf{ErL}$^{\mathbf{18}}$ and ErL (right) measured at 4~K, with
excitation at 425 nm.}
\end{figure*}


The emission spectra of \textbf{LnL}$^{\mathbf{18}}$ and of the parent complex
LnL are very similar, as expected from the close structural resemblance
of the first coordination spheres (Figure~\ref{fig2}). For \textbf{TbL}$^{\mathbf{18}}$
the signal to noise ratio is much lower than seen for the parent
complex TbL. We suspect this to be due to the additional Schiff-base of
the post-functionalisation which could lead to a fast quenching of the
emitting $^{5}$D$_{4}$ state of Tb due to the very large amount of
additional high-energy C--H stretches being introduced, which is
clearly seen from IR spectroscopy (Figure~S8).  

The number of observed bands is very similar between
\textbf{TbL}$^{\mathbf{18}}$ and TbL, \textbf{HoL}$^{\mathbf{18}}$ and
HoL, and \textbf{ErL}$^{\mathbf{18}}$ and ErL.
\textbf{TbL}$^{\mathbf{18}}$ and TbL have 10 and 11 observable bands,
respectively, while \textbf{HoL}$^{\mathbf{18}}$ and HoL have 8 and 7,
respectively, and \textbf{ErL}$^{\mathbf{18}}$ and ErL have both 5
bands. In the case of the Ho and Er complexes, the bands stem from the
$^{5}$F$_{5}\rightarrow{}^{5}$I$_{8}$ and
$^{4}$I$_{13/2}\rightarrow{}^{4}$I$_{15/2}$ transitions, respectively.
Thus, these luminescence spectra only give information about the
splitting of the ground multiplet. For the Tb complexes the situation
is different with luminescence transitions observed for $^{5}$D$_{4}
\rightarrow{}^{7}$F$_{6}$, $^{5}$D$_{4}\rightarrow{}^{7}$F$_{5}$ and
$^{5}$D$_{4}\rightarrow{}^{7}$F$_{4}$. Here the 3 observed bands
between 480--500 nm ($^{5}$D$_{4}\rightarrow {}^{7}$F$_{6}$) provide
information on the ground multiplet splitting. The available
experimental data (Figure~\ref{fig2} and Table~S9) do not allow to
unambiguously assign the observed bands as of electronic or vibrational
origin. Therefore, for the sake of simplicity, we assume for all
complexes that all the observed transitions are of electronic origin.
In each spectrum, the highest energy band is assigned as the
zero-phonon line; these are at 20492~cm$^{-1}$, 15423~cm$^{-1}$,
6578~cm$^{-1}$ for \textbf{TbL}$^{\mathbf{18}}$,
\textbf{HoL}$^{\mathbf{18}}$, and \textbf{ErL}$^{\mathbf{18}}$,
respectively, and 20500~cm$^{-1}$, 15442~cm$^{-1}$ and 6587~cm$^{-1}$
for TbL, HoL and ErL, respectively. The energies of the various ground
term sublevels can be found in Table~\ref{tab2}. We note there are
fewer emission bands observed in the luminescence spectra of
\textbf{LnL}$^{\mathbf{18}}$ and LnL than what is expected based on the
number of energy levels in the ground multiplet. The remaining energy
levels may be \mbox{hidden} in the linewidth of the peaks. For
instance, the three bands originating from $^{5}$D$_{4}\rightarrow
{}^{7}$F$_{6}$ in TbL have linewidths of 91--186~cm$^{-1}$.

\begin{table}[t!]
\caption{\label{tab2}The observed energies (cm$^{-1}$) of the ground
term sublevels of \textbf{TbL}$^{\mathbf{18}}$, TbL;
\textbf{HoL}$^{\mathbf{18}}$, HoL; and \textbf{ErL}$^{\mathbf{18}}$,
ErL}
\begin{tabular}{cccccc}
\thead
\textbf{TbL}$^{\mathbf{18}}$ & \textbf{TbL} & \textbf{HoL}$^{\mathbf{18}}$ & \textbf{HoL} & \textbf{ErL}$^{\mathbf{18}}$ & \textbf{ErL} \\
\endthead
\0\00 & \0\00 & \0\00 & \0\00 & \0\00 & \0\00 \\
138 & 142 & \038 & \043 & \039 & \045 \\
383 & 395 & \090 & \086 & \070  & \073 \\
 &  & 211 & 212 & 102 & 104 \\
 &  & 394 & 400 & 333 & 341 \\
 &  & 448   & 440  &  &  \\
 &  & 484 & -- &  &  \\
 &  & 524 & 521 &  & 
\botline
\end{tabular}
\end{table}

 
The energy level splitting of the ground multiplet is essentially
unchanged upon post-derivatisation of TbL, HoL or ErL to
\textbf{TbL}$^{\mathbf{18}}$, \textbf{HoL}$^{\mathbf{18}}$ or \textbf{ErL}$^{\mathbf{18}}$. The
energies of the ground term sublevels (Table~\ref{tab2}) indicate small changes
of 2--13~cm$^{-1}$, however, considering the step size of 0.2--0.3 nm
used in the measurements, the uncertainties of the energies are
comparable to these numbers, being ${\pm}$25~cm$^{-1}$ for
\textbf{TbL}$^{\mathbf{18}}$, ${\pm}$9~cm$^{-1}$ \textbf{HoL}$^{\mathbf{18}}$ and
${\pm}$2~cm$^{-1}$ for \textbf{ErL}$^{\mathbf{18}}$. This is also expected from
the close structural resemblance of the first coordination spheres. In
our previous study on \textbf{YbL}$^{\mathbf{18}}$ we also saw very small
changes in the energy level structure of the ground
multiplet~\cite{22}. However, when the derivatisation is made on the
phenyl rings, changes in the energy level splitting of up to 45\% have
been observed between Er(trensal) derivatives (H$_{3}$trensal $=$
2,2$'$,2$''$-tris(salicylideneimino)trimethylamine)~\cite{41}. 
The~larger shifts in the energy levels moving between different Ln(trensal)
derivatives are likely due to the substitution groups being placed
directly on the phenyl rings rather than an aldehyde being changed into
an imine. 


\subsubsection{Static magnetic properties}\label{sec332}

The static magnetic properties of
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ were studied by d.c. magnetic
susceptibility measurements and VTVB measurements. The d.c. magnetic
susceptibility measurements (Figure~\ref{fig3}) were recorded in the
temperature range 2--270~K in a static magnetic field \textit{B} of
1000 Oe. 

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Temperature dependence of the
{${\upchi}$}\textit{T} product of
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ and TbL--TmL. The
{${\upchi}$}\textit{T} data of LnL are obtained
from literature~\cite{21}. Curie constants
(\textit{C}$_{\mathrm{Ln}}$) of Tb(III), Dy(III), Ho(III), Er(III),
and Tm(III) are shown as dashed lines.}
\end{figure}

The d.c. susceptibility data are plotted as the $\upchi$T
product, where ${\upchi} = {M/B}$, with ${\upchi}$ being the molar magnetic
susceptibility and ${M}$ being the magnetisation, and ${B}$ being the
magnetic field. 

\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$_{18}$ reach their lowest $\upchi$T
value at 2~K being 2.80, 6.59, 4.91, 5.33, 
$0.60~\mathrm{cm}^{3}{\cdot}\mathrm{K}{\cdot}\mathrm{mol}^{-1}$,
respectively. Upon increasing the temperature, the $\upchi T$ {products
continuously increase, which is due to the} population of excited
levels in the ground multiplets of the complexes as there are
negligible magnetic couplings to neighbouring complexes (the nearest
paramagnetic centre is about 7~\AA{}~away). At 270~K, the $\upchi T$
products of \textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ reach their
highest values at 10.41 (${C}_{\mathrm{Tb}} = 11.82$), 13.23
(${C}_{\mathrm{Dy}} = 14.17$), 12.28 (${C}_{\mathrm{Ho}} = 14.07$),
10.90 (${C}_{\mathrm{Er}} = 11.48$), and 5.70 (${C}_{\mathrm{Tm}} =
7.15$) cm$^{3}{\cdot}\mathrm{K}{\cdot}\mathrm{mol}^{-1}$, respectively.
These $\upchi T$ {products} are smaller than the values expected for the
isolated Ln(III), known as the Curie constants ($C_{\mathrm{Ln}}$).
This suggests that the total energetic splitting of their ground states
is larger than the thermal energy available at 270~K. This is
corroborated by the luminescence spectra and has also been observed for
other complexes with a similar coordination sphere such as the
Ln(trensal)~\cite{42, 43}, Ln(trenovan) (H$_{3}$ trenovan $=$
tris(((3-methoxysaliclidene)amino){\ubreak}ethyl)amine)~\cite{44}, and
LnL~\cite{21}. 

Comparing the temperature dependence of the $\upchi T$ products of
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ {to TbL--TmL the Kramers ion
(Dy and Er)-containing complexes show almost no difference between the
post-derivatised and parent complexes, as expected from the very subtle
changes in the first coordination sphere and the similar energy level
splitting of the ground multiplet found between}
{\textbf{ErL}}$^{\mathbf{18}}$ {and ErL. This was also observed for
the} $\upchi T$ {products of the Kramers ion complexes}
{\textbf{YbL}}$^{\mathbf{18}}$ {and YbL}~\cite{22}. In terms of the
non-Kramers ion (Tb, Ho and Tm) complexes, larger deviations between
the post-derivatised complexes and the parent complexes are found.
Emission spectroscopy showed very similar energy level splittings of
the ground multiplets of \textbf{TbL}$^{\mathbf{18}}$ and TbL as well
as for \textbf{HoL}$^{\mathbf{18}}$ and HoL, leading us to believe that
the change in the temperature dependence of the $\upchi T$ products
between post-derivatised complexes and their parent complexes likely
stems from different eigenvector compositions. This also explains why
the $\upchi T$ {product at 2~K for} {\textbf{TbL}}$^{\mathbf{18}}$ {is
2.80} cm$^{3}{\cdot}$K$\cdot$mol$^{-1}$ while it is
1.20~cm$^{3}{\cdot}$K$\cdot$mol$^{-1}$ for TbL. The large difference in
the eigenvector compositions is also corroborated by VTVB measurements
(\textit{vide infra}). 

VTVB measurements were conducted with applied magnetic fields of 500,
5000, 10,000, 20,000, 30,000, 40,000, and 50,000~Oe and with
temperatures in the range 2--10~K (Figures~S32--S36). 

For all complexes the maximum magnetisation value is reached at 2~K and
50,000~Oe.  The reduced magnetisation of
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ does not
superimpose, suggesting that the ground state is not thermally isolated
from excited states. This is similar to the parent complexes LnL, where
only the GdL and YbL complexes show superimposable reduced
magnetisation curves~\cite{21}. Comparing the VTVB \mbox{measurements}
of the post-derivatised complexes
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ with the
parent complexes \mbox{TbL--TmL,} the difference between the Kramers and
non-Kramers ion-containing complexes becomes more apparent. The VTVB
measurements of the Kramers ion-containing complexes
\textbf{DyL}$^{\mathbf{18}}$ and DyL as well as
\textbf{ErL}$^{\mathbf{18}}$ and ErL are almost identical (Figures~S33
and S35), suggesting very similar eigenvector compositions of the
ground energy levels for the post-derivatised and parent complexes. On
the contrary, the non-Kramers ion-containing complexes show different
VTVB magnetisation for \textbf{LnL}$^{\mathbf{18}}$ and LnL
(Figures~S32, S34 and S36). The difference between magnetisation curves
for the Kramers and non-Kramers ion complexes upon derivatisation is
likely due to the difference in the degeneracy of the energy levels of
the Kramers and non-Kramers ions and the loss of crystallographic
trigonal symmetry upon derivatisation, with the latter leading to
additional off-diagonal terms in the crystal field Hamiltonian, causing
more mixings of the states. Unlike in the case of Kramers ions, the
energy levels of non-Kramers ions are not forced to be degenerate in
pairs at zero magnetic field. Previous measurements on TbL, HoL and TmL
indicated that the ground state is likely a singlet but with an excited
doublet very close to the ground state for TbL and HoL~\cite{21}. If
the degeneracy of this doublet is lifted due to the loss of trigonal
symmetry, this may lead to much different magnetisation curves. 

\subsubsection{Influence of the post-derivatisation on the dynamic
magnetic properties}\label{sec333}

A.c. susceptibility measurements were performed on
\textbf{TbL}$^{\mathbf{18}}$--\textbf{TmL}$^{\mathbf{18}}$ 
within the frequency range
1--1500~Hz of an oscillating magnetic field of 3.5 Oe, and with an
applied static magnetic field $H$ of 0 to 5000 Oe
(Figures~S37--S46). When no static magnetic field is applied,
none of the complexes show any out-of-phase a.c. signal
(${\upchi}''$) with respect to the available frequency range. This
suggests that a large degree of quantum tunnelling of magnetisation
(QTM) is present in the complexes, similar to the parent complexes and
other complexes with the Ln(trensal) motif~\cite{21,44,45}. In an
attempt to quench the QTM, the a.c.~susceptibility was measured in
static magnetic fields. When applying a static magnetic field, an
out-of-phase signal emerged for the complexes containing Kramers ions
(Dy and Er). This was not observed for the complexes containing
non-Kramers ions. The difference between the Kramers and non-Kramers
ion complexes is likely due to the non-Kramers complexes having a
singlet instead of a doublet as the ground state. In three-fold
symmetry, only Kramers ions are required to have a degenerate ground
state in zero magnetic field. This difference between the Kramers and
non-Kramers ions has also been observed in the parent complexes LnL
and for the similar Ln(trensal) complexes~\cite{21, 44}.

The field dependence of the ${\upchi}''$ for
\textbf{DyL}$^{\mathbf{18}}$ and \textbf{ErL}$^{\mathbf{18}}$ show
similar features (Figures~S40 and S44). At low magnetic fields a peak
in the ${\upchi}''$ is found at high frequencies. Upon increasing the
magnetic field this process disappears from the measurement window, and
instead a very broad signal emerges at low frequencies. The same
behaviour was observed in the parent complexes DyL and ErL, where the
broad relaxation process was found to be
temperature-independent~\cite{21}. Unfortunately the relaxation process
at high frequencies has its maximum outside our measurement window
preventing an investigation of its temperature dependence. 

In an effort to slow the relaxation dynamics of the process found at
high frequencies the complexes \textbf{DyL}$^{\mathbf{18}}$ and
\textbf{ErL}$^{\mathbf{18}}$ were diluted at 5\% into the diamagnetic
hosts \textbf{YL}$^{\mathbf{18}}$ and \textbf{LuL}$^{\mathbf{18}}$,
respectively. The dilution into the diamagnetic host limits the dipolar
interactions between neighbouring magnetic centres, thereby limiting
the effect of dipolar couplings on the spin-lattice relaxation. The
field dependence of the {${\upchi}$}{'' of}
\textbf{Dy@YL}$^{\mathbf{18}}$ and \textbf{Er@LuL}$^{\mathbf{18}}$ show
a single peak with a maximum around 100 Hz (Figures~S54 and S60). At
1000 Oe the {${\upchi''}$} signal is most intense and at the lowest
frequency for both \textbf{Dy@YL}$^{\mathbf{18}}$ and
\textbf{Er@LuL}$^{\mathbf{18}}$, presenting the optimal field to study
the temperature dependence. Upon increasing the temperature, the signal
quickly moves outside our measurement window with the maximum of the
signal moving past the measurement window already at 3~K (Figures~S56
and S62). Using CC-FIT2~\cite{46}, the temperature dependence of the 
a.c.~susceptibility data was fitted to a generalised Debye model. The
limited temperature interval that could be extracted from the
measurements prevents a detailed analysis of the results. To compare
the relaxation dynamics of the post-derivatised complexes to those of
the parent complexes, DyL and ErL were diluted into their diamagnetic
hosts YL and LuL at 5\%, respectively. \mbox{However,} the dilution
into a diamagnetic host did not result to observation of a maximum
within the available \mbox{frequency}
range \mbox{(Figures~S58 and S64).} This shows
that the spin-lattice relaxation of the Dy and Er ions is slowed down
upon the derivatisation, a feature which was also observed for
\textbf{YbL}$^{\mathbf{18}}$~\cite{22}.\looseness=1

As previously mentioned, \textbf{GdL}$^{\mathbf{18}}$ could not be
obtained phase-pure. However, when \textbf{GdL}$^{\mathbf{18}}$ is
diluted at 5\% into the diamagnetic host \textbf{YL}$^{\mathbf{18}}$,
it adopts the same phase as the other members of the
\textbf{LnL}$^{\mathbf{18}}$ family presented herein. Having obtained
the Gd complex phase-pure we decided to compare its relaxation dynamics
to those of the parent complex GdL. For this, GdL was diluted at 5\%
into the diamagnetic host YL. Field dependence measurements of the a.c.
susceptibility for \textbf{Gd@YL}$^{\mathbf{18}}$ showed an optimal
relaxation at 3000 Oe (Figure~S48). Temperature dependence was
subsequently measured with a static magnetic field of 3000 Oe for both
Gd complexes (Figures~S49--S52). Using CC-FIT2~\cite{46}, the
relaxation times were obtained for each complex. Contrary to the Dy and
Er containing complexes, relaxation times for
\textbf{Gd@YL}$^{\mathbf{18}}$ and Gd@YL could be obtained over a large
temperature interval (Figure~\ref{fig4} and Tables~S5 and S6). Both
complexes show very similar relaxation times up to 5~K. Going to higher
temperatures, \textbf{Gd@YL}$^{\mathbf{18}}$ starts to relax faster,
which may be attributed to the increased amount of vibrations from the
octadecyl chains. In a double logarithmic plot the temperature
dependence of the relaxation time of Gd@YL is fully linear with a slope
of 1.5, which indicates that the relaxation could be governed by a
direct process in this temperature regime (Figure~S65). In the case of
\textbf{Gd@YL}$^{\mathbf{18}}$, the temperature dependence of the
relaxation shows a slight curvature, suggesting that at least two
different relaxation processes are present within the studied
temperature interval. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}Temperatue dependence of the relaxation time
(${\uptau}$) of \textbf{Gd@YL}$^{\mathbf{18}}$ at 5\% dilution, \textbf{Gd@YL} at
5\% dilution, \textbf{Dy@YL}$^{\mathbf{18}}$ at 5\% dilution, and
\textbf{Er@LuL}$^{\mathbf{18}}$ at 5\% dilution. ${\uptau}$ was obtained by
fitting the generalised Debye model using CC-FIT2~\cite{46} to the
temperature dependence of the a.c. magnetic susceptibility of these
complexes at a fixed applied static magnetic field.}
\end{figure}

\section{Conclusion}\label{sec4}

A novel series of aliphatic chain containing lanthanide complexes
\textbf{(Tb--Tm, Y)L}$^{\mathbf{18}}$ have been obtained through Schiff-base
reaction post-derivatisation of LnL with 1-octadecylimine chains.
Single-crystal X-ray diffraction revealed that upon post-derivatisation
the first coordination sphere of \textbf{LnL}$^{\mathbf{18}}$ shows great
similarities to the one of the parent complex. However, due to van der
Waals interactions between the octadecyl chains in the solid state, the
trigonal symmetry of the parent complex (LnL) was lost upon
post-functionalisation. \mbox{Emission} \mbox{spectra} of \textbf{(Tb, Ho,
Er)L}$^{\mathbf{18}}$ and (Tb, Ho, Er)L showed little change in the energetic
splitting of the ground multiplet upon post-derivatisation. The changes
in the energy level splitting were much smaller than what has been
observed between different Er(trensal) derivatives, suggesting that
changes in the second coordination sphere that do not change the
electron density on the phenyl rings of the complexes have very little
effect on the energy levels of the Ln ions in this type of complexes.
Although the energy level splitting seems to be little perturbed by the
post-derivatisation, the static magnetic properties were found to
change significantly upon post-derivatisation for the non-Kramers ion
complexes. The effect was largest for the magnetisation measurements at
low temperature and is likely the result of a large change in the
eigenvector compositions because of non-diagonal terms induced by the
low symmetry. This large difference in the static magnetic properties
of the non-Kramers ions upon post-derivatisation was not observed for
the Kramers ions. It is surprising that post-derivatisation influences
the static magnetic properties of the LnL \mbox{complexes} differently
depending on whether they contain Kramers or non-Kramers ions. This is
\mbox{important} to the future integration of Ln complexes into devices, where
bulk properties are wished to be retained upon device integration. If
LnL complexes are to be integrated into such devices the focus should
be on integrating the Kramers ion-containing complexes which seem less
susceptible to small perturbations. \looseness=1

Further studies will investigate how different chain lengths of the
amines affect the electronic and magnetic properties of the Ln ions to
get a better understanding of the relationship between the static and
dynamic properties of the systems as well as between the Kramers and
non-Kramers ion complexes. Moreover, Langmuir--Blodgett films formed
with these complexes will be studied, to see how the static magnetic
and electronic properties change upon organisation in a 2D structure. 

\section*{Declaration of interests}

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

\section*{Funding}

We thank the Novo Nordisk Foundation for research grant NNF20OC0065610.

\CDRGrant[NNF]{NNF20OC0065610}

\section*{Accession codes}

CCDC 2222267, 2222268 and 2263962 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data\_request/cif, or by emailing
data\_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.

\back{}

\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-282-suppl.pdf}}

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