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\DOI{10.5802/crchim.427}
\datereceived{2025-09-28}
\daterevised{2025-10-02}
\dateaccepted{2025-10-10}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{Chiral phosphorescent derivatives from molecular engineering
between helicenic \textit{N},\textit{N}-type ligands and rhenium(I) metal centers}

\alttitle{D\'{e}riv\'{e}s phosphorescent chiraux issus  de
l'ing\'{e}nierie mol\'{e}culaire entre  ligands h\'{e}lico\"{i}daux de
type \textit{N},\textit{N} et centres m\'{e}talliques de rh\'{e}nium(I)}

\author{\firstname{Debsouri} \lastname{Kundu}}
\address{Institut des Sciences Chimiques de Rennes,
University of Rennes, CNRS, ISCR, UMR 6226, F-35000 Rennes, France}
\address{ICBMS, UMR 5246, Univ Lyon, Universit\'{e} Lyon 1, 
CNRS, INSA, CPE-Lyon, 1 rue Victor Grignard, 69622 Villeurbanne, France}

\author{\firstname{Jeanne} \lastname{Crassous}\CDRorcid{0000-0002-4037-6067}\IsCorresp}
\addressSameAs{1}{Institut des Sciences Chimiques de Rennes,
University of Rennes, CNRS, ISCR, UMR 6226, F-35000 Rennes, France}
\email[J. Crassous]{jeanne.crassous@univ-rennes.fr}

\keywords{\kwd{Helicenes}
\kwd{Rhenium}
\kwd{Stereochemistry}
\kwd{Circularly polarized luminescence}
\kwd{\textit{N},\textit{N} ligands}}

\altkeywords{\kwd{H\'{e}lic\`{e}nes}
\kwd{Rh\'{e}nium}
\kwd{St\'{e}r\'{e}ochimie}
\kwd{Luminescence polaris\'{e}e circulairement}
\kwd{Ligands \textit{N},\textit{N}}}

\dedicatory{Dedicated to the memory of Bertrand Carboni.}

\thanks{Minist\`{e}re de l'Education Nationale, de la Recherche et de la Technologie, 
Centre National de la Recherche Scientifique (CNRS), 
Rennes M\'{e}tropole, French National Agency (ANR, LumoMat-E project, 18-EURE-0012),
European Commission Research Executive Agency 
(Grant Agreement number: 859752 --
HEL4CHIROLED -- H2020-MSCA-ITN-2019).}

\begin{abstract}
The incorporation of rhenium atoms within an extended helical
\tralicstex{π}{$\rmpi$}-conjugated \textit{N},\textit{N}-type (bipyridine or phenanthroline) system
impacts the chiroptical and photophysical properties of the resulting
neutral or cationic complexes, leading to rhenium-based phosphorescent
derivatives that exhibit circularly polarized luminescence. In this
short review, we describe key results that have been obtained by our
group in this field in the past decade.
\end{abstract}

\begin{altabstract}
L'incorporation d'atomes de rh\'{e}nium au sein d'un  ligand \textit{N},\textit{N}
\tralicstex{π}{$\rmpi$}-conjugu\'{e} h\'{e}lico\"{i}dal \'{e}tendu (bipyridine ou
ph\'{e}nanthroline) influence les propri\'{e}t\'{e}s chiroptiques et
photophysiques des complexes neutres ou cationiques obtenus, conduisant
\`{a} des d\'{e}riv\'{e}s phosphorescents \`{a} base de rh\'{e}nium
pr\'{e}sentant une luminescence polaris\'{e}e circulairement. Dans
cette br\`{e}ve revue, nous d\'{e}crivons les principaux r\'{e}sultats
obtenus par notre groupe dans ce domaine au cours de la derni\`{e}re
d\'{e}cennie.
\end{altabstract}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\maketitle

\twocolumngrid

\end{noXML}

\dedication{\begin{quote}\textit{Dedicated to the memory of Bertrand
Carboni.}\end{quote}}

\section{Introduction}
Rhenium(I) tricarbonyl  d$^{6}$ complexes are well-known emissive
complexes, notably for their efficient visible-light and long-lived
luminescence~\cite{1}. Owing to the strong spin--orbit coupling of the
heavy rhenium atom, these complexes exhibit efficient room-temperature
(rt) phosphorescence. They have been investigated for diverse
applications, including photoredox chemistry~\cite{2,3}, chemo- and
electrochemiluminescence~\cite{4,5}, chemical and biological
sensing~\cite{6,7},  bioconjugation~\cite{8,9}, and as dopants in
organic light-emitting diodes (OLEDs)~\cite{10}. They can be easily
synthesized by refluxing in toluene a rhenium(I) source (commonly
ReX(CO)$_{5}$, where X ${=}$ Cl, Br) with a bidentate $N$,$N$ or  $C$,$N$
ligand (such as a 2,2$^{\prime}$-bipyridine, terpyridine, or
pyridyl-$N$-substituted heterocyclic carbene), yielding
complexes of general formula ReL$_{2}$X(CO)$_{3}$, with L$_{2}$
representing the bidentate ligand. An attractive research direction
involves the introduction of chirality into  L$_{2}$, for instance
helical chirality, and combination with chirality at the rhenium
center. Helicenes are helically chiral molecules obtained from the
\textit{ortho}-fusion of aromatic rings~\cite{11}. Over the past two
decades, our group has focused on synthesizing helicenic structures,
incorporating coordinating units to produce helically chiral ligands
and a broad range of chiral metal complexes upon subsequent
\mbox{coordination~\cite{12,13}.} This approach enables the combination of the
chirality-driven properties of the ligand with the intrinsic
photophysical, chiroptical, and magnetic characteristics of the chosen
metal ion, thereby generating unique functional molecular materials. In
this short account, the main results on rhenium complexes bearing
helicenic ligands functionalized with 2,2$^{\prime}$-bipyridine or
phenanthroline-type coordinating units are reviewed. These complexes
exhibited intense electronic circular dichroism, and circularly
polarized long-lived phosphorescence, appealing properties for the
development of chiral photoactive materials. The specific
stereochemical features that significantly influence the properties of
these chiral emitters are also highlighted.

\section{Results and discussion}

\subsection{Synthesis and characterization of  helicenic $N$,$N$
ligands} 
2,2$^{\prime}$-Bipyridine (bipy) units are widely used ligands in
coordination chemistry, enabling access to a broad range of luminescent
metal-based molecular materials~\cite{14}. Our group was the first to
prepare helical bipy ligands, i.e., bipy units fused with a helicenic
moiety, actually consisting of 4-aza[n]helicenes 3-substituted by a
2-pyridyl unit. As described in Scheme~\ref{sch1},  we developed in a
systematic fashion [4]helicene \textbf{3a}~\cite{15,16}, [6]helicene
\textbf{3b}~\cite{15,16}, with a single bipyridine unit at one end,
along with [6]helicene \textbf{3c}~\cite{17} containing a bipyridine
unit at both termini. We started our two-step synthesis from
2,2$^\prime$-bipyridine-6-carboxaldehyde \textbf{1}, by a Wittig reaction with
either 2-naphthyl phosphonium bromide \textbf{2a},
2-methylbenzophenanthrene phosphonium bromide \textbf{2b}, or
naphtyl-2,7-dimethyl-di-phosphonium bromide \textbf{2c}, respectively,
followed by a Mallory reaction (photocyclization under oxidative
conditions)~\cite{18}. Regarding
[4]helicene-pyrazino[2,3-$f$][1,10]phenanthroline (\textbf{6}), 
incorporating a dppz unit (dppz  ${=}$
benzo[$h$]dipyrido[3,2-$a$:2$^\prime$,3$^\prime$-$c$]phenazine)~\cite{19},
it was obtained from condensation of a 1,2-[4]helicene-diamine 
\textbf{4} with 1,10-phenanthroline-5,6-dione \textbf{6} 
(Scheme~\ref{sch1}) using catalytic para-toluene sulfonic 
acid~\cite{20}. A similar strategy was recently utilized by others to
generate  pyrazino-phenanthryl-based helicenic systems displaying high
photoconductive  properties~\cite{21}.

\begin{scheme*}
\includegraphics{sc01}
\vspace*{5pt}
\caption{\label{sch1}Synthetic procedures used to prepare 
helicenic $N$,$N$ ligands \textbf{3a}--\textbf{3c} and
\textbf{6}~\cite{15,17,20}.}
\vspace*{-5pt}
\end{scheme*}

\begin{scheme*}
\includegraphics{sc02}
\vspace*{5pt}
\caption{\label{sch2}Synthesis of neutral and charged rhenium 
complexes \textbf{7a}--\textbf{9a} and enantioenriched
$(M,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{1}}$, 
$(M,C_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$ and 
$(M,CA_{\mathrm{Re}})$-\textbf{8b}$^{\mathbf{1},\mathbf{2}}$ from,
respectively, [4]helicene-bipy \textbf{3a} and
[6]helicene-bipy $(M)$-\textbf{3b}. X-ray crystallographic structures of
racemic \textbf{7b}$^{\mathbf{2}}$, \textbf{8a}, and \textbf{9a} (only 
$(M,C_{\mathrm{Re}})$ stereoisomers are shown).  Adapted with
permission from Ref.~\cite{16}.}
\vspace*{-10pt}
\end{scheme*}

The photophysical properties of ligands \textbf{3b}, \textbf{3c}, and
\textbf{6} were studied in detail. These three compounds exhibited
absorption spectra in the UV--visible (UV--Vis) region between 270 and
500~nm. Furthermore, helicene derivatives \textbf{3b} and \textbf{3c}
were found to display vibronically structured blue fluorescence with
moderate quantum yields. For instance, in CH$_{2}$Cl$_{2}$ solution at
rt, ligand \textbf{3b} exhibited a structured blue fluorescence at 
421~nm, with a vibronic progression of 1400~cm$^{-1}$ (quantum yield
$\Phi = 8.4\%$)  (Table~\ref{tab1}). Ligand \textbf{6} was found to
emit redshifted fluorescence centered at 515~nm, which appeared broad
and unresolved as a result of the presence of charge transfer (CT), and
with a higher quantum yield of 29\%. Interestingly, long-lived
phosphorescence signals emerging around 530~nm were observed for
\textbf{3b},\textbf{c} and at 560~nm for \textbf{6} when cooling down to low
temperature (77~K) (Figure~\ref{fig3}d). 

\begin{table*}
\caption{\label{tab1}Absorption data, emission maxima, quantum yields,
lifetimes and emission dissymmetry factors of the compounds described.
In dichloromethane at  $298 \pm 3$~K, except otherwise indicated}
\begin{tabular}{cccccc}
\thead
Compound &
\parbox[t]{10.3pc}{\centering
Absorption\unskip\break
$\lambda_{\mathrm{max}}~(\mathrm{nm})~(\varepsilon~
({\times}10^3~\mathrm{M}^{-1}{\cdot}\mathrm{cm}^{-1}))$} & 
\parbox[t]{6pc}{\centering
Emission\unskip\break
$\lambda_{\mathrm{max}}~(\mathrm{nm})$} & 
$\Phi$ (\%) & $\tau$~(ns)$^{\mathrm{a}}$ & 
\parbox[t]{6pc}{\centering
CPL\unskip\break 
$g_{\mathrm{lum}}$}
\vspace*{2pt}\\
\endthead
\textbf{3b} & 
\parbox[t]{10.3pc}{\centering
240 (35.0), 266 (55.8), 322 (27.0), 353 (14.4), 372 (10.2), 
393 (2.63), 417 (1.80)} & 
421, 445, 473sh & 
8.4 & 6.6 &
\parbox[t]{6pc}{\centering
${+}3.4 \times 10^{-3}$\Lbreak
[$(P)$-\textbf{3b}]~\cite{15}}
\vspace*{2pt}\\

\textbf{3c} & 
\parbox[t]{10.3pc}{\centering
242 (35.1), 272 (60.3), 286 (65.3), 344~(26.5),
366sh (20.2), 398 (3.50), 420~(2.75)} & 
\parbox[t]{6pc}{\centering
422, 448, 476, 513sh} &
8.6 & 5.1 &
\parbox[t]{6pc}{\centering
${+}8.6 \times10^{-3}$\Lbreak
[$(P)$-\textbf{3c}]~\cite{15}}
\vspace*{2pt}\\
\textbf{6} & 
\parbox[t]{10.3pc}{\centering
260 (85), 288 (67), 350 (33), 435 (13), 410 (16)} & 
\parbox[t]{6pc}{\centering
515 (CH$_2$Cl$_2$)\Lbreak
560 (77~K)} & 29 & 5.8, 2.8 & n.d. 
\vspace*{2pt}\\ 
\textbf{7a} & 
\parbox[t]{10.3pc}{\centering
243 (34.5), 273 (30.3), 318 (30.3), 330 (43.2), 398 (12.7)} & 
678 & 0.11 & 25 & n.d. 
\vspace*{2pt}\\ 
\textbf{8a} & 
\parbox[t]{10.3pc}{\centering
251 (54.9), 273 (51.9), 326sh (35.9), 337 (46.6), 403 (14.6), 
422 (15.2)} & 
585, 618 & 16 & 67~000 & n.d. 
\vspace*{2pt}\\ 
\textbf{9a} & 
\parbox[t]{10.3pc}{\centering
248 (35.7), 280 (34.2), 327sh (27.3), 338 (37.7), 
408 (13.5), 422 (13.9)} & 
595, 623 & 8.3 & 11~500 & n.d. 
\vspace*{2pt}\\
\textbf{7b}$^{\mathbf{1}}$ & 
\parbox[t]{10.3pc}{\centering
236 (45.9), 277 (49.9), 307 (28.2), 339 (21.3), 420 (7.96), 
444 (7.30)} & 
680 & 0.13 & 27 & n.d. 
\vspace*{2pt}\\
\textbf{7b}$^{\mathbf{2}}$ & 
\parbox[t]{10.3pc}{\centering
237 (59.8), 278 (65.0), 305sh (36.3), 344 (26.3), 418 (11.0), 
445 (10.2)} & 
673 & 0.16 & 33 & 
\parbox[t]{8pc}{\centering
${+}3.1 \times 10^{-3}$\Lbreak
[$(P,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$]~\cite{16}} 
\vspace*{2pt}\\
\textbf{8b}$^{\mathbf{1},\mathbf{2}}$ & 
\parbox[t]{10.3pc}{\centering
272 (48.0), 339 (17.6), 444 (5.6)} &
598 & 6 &
79~000 & 
\parbox[t]{8pc}{\centering
${+}1.3 \times 10^{-3}$\Lbreak
[$(P,AC_{\mathrm{Re}})$-\textbf{8b}$^{\mathbf{1},\mathbf{2}}$]~\cite{16}}
\vspace*{2pt}\\
\textbf{10-1} & 
\parbox[t]{10.3pc}{\centering
283 (46.5), 299 (51.4), 361 (15.5), 387~(15.4), 450 (6.08)} &
664 & 0.20 & 38 & 
\parbox[t]{8pc}{\centering
${+}3.4 \times 10^{-3}$\Lbreak
[$(P,A_{\mathrm{Re}},A_{\mathrm{Re}})$-\textbf{10-1}]~\cite{17}}
\vspace*{2pt}\\
\textbf{10-2} & 
\parbox[t]{10.3pc}{\centering
283 (33.7), 299 (37.9), 359 (12.7), 389~(11.3), 450 (5.09)} & 
678 & 0.15 & 32 & n.d. 
\vspace*{2pt}\\
\textbf{11} & 
\parbox[t]{10.3pc}{\centering
267 (77), 304 (40), 353 (29), 460 (12)} & 
\parbox[t]{6pc}{\centering
518 (CH$_{2}$Cl$_{2}$)\Lbreak
560 (77~K)} & 
2.0 & 1.2, 7.6 & 
\parbox[t]{8pc}{\centering
${+}2.3 \times 10^{-3}$ at rt\Lbreak
${+}2.9 \times 10^{-2}$ at 77~K\Lbreak
[$(P,AC_{\mathrm{Re}})$-\textbf{11}]~\cite{20}}
\vspace*{2pt}
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$~In degassed solution. n.d. not
determined.}
\end{table*}

While benzophenanthryl system \textbf{3a} was found to be
configurationally unstable, hexahelicenic enantiopure $(M)$- and
$(P)$-\textbf{3b}, and  $(M)$- and $(P)$-\textbf{3c} could successfully
be obtained by HPLC over chiral stationary phases. Unfortunately, due
to very poor solubility, attempts to separate the enantiomers of
\textbf{6} proved unsuccessful. $(M)$ and $(P)$ enantiomers of
\textbf{3b} and \textbf{3c} displayed the typically strong electronic
circular dichroism (ECD) responses, which are very typical of helicenic
organic derivatives~\cite{22}. For instance, enantiomer
$(P)$-\textbf{3b} exhibited a very strong negative band at 265~nm and a
very strong positive band at 335~nm along with weaker positive bands
between 380 and 430~nm (Figure~\ref{fig2}a). 

\begin{figure*}
\includegraphics{fig01}
%\vspace*{-4pt}
\caption{\label{fig1}(a)~Experimental  
ECD spectra of enantiopure  $(M)$ and  $(P)$-\textbf{3b} and
their corresponding enantiopure  Re$^{\mathrm{I}}$ complexes 
$(M,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{1}}$,
$(P,C_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{1}}$, 
$(M,C_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$, and
$(P,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$.  Inset: ECD spectra of
\textbf{7b}$^{\mathbf{1}}$ enantiomers between 450 and 550~nm.  (b)~CPL
(upper curves within each panel) and total luminescence (lower curves
within each panel) spectra of  $(M)$-\textbf{3b}, $(P)$-\textbf{3b},
$(M,C_{\mathrm{Re}})$-7\textbf{b}$^{\mathbf{2}}$, 
$(P,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$, 
$(M,AC_{\mathrm{Re}})$-\textbf{8b}$^{\mathbf{1,2}}$, 
$(P,AC_{\mathrm{Re}})$-\textbf{8b}$^{\mathbf{1,2}}$ in degassed
CH$_{2}$Cl$_{2}$ at rt. (c)~Calculated ECD spectra of 
$(P,C_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{1}}$, and
$(P,A_{\mathrm{Re}})$-\textbf{7b}$^{\mathbf{2}}$ and Boltzmann-averaged
spectrum for  $(P)$-\textbf{8b}$^{\mathbf{1,2}}$ conformers. View of
HOMO and LUMO of  \textbf{7b}$^{\mathbf{1}}$ and 
\textbf{8b}$^{\mathbf{1}}$. First excitation energies indicated by
dots on the abscissa.  Adapted with permission from
Ref.~\cite{16}.}
\end{figure*}

\begin{scheme*}
\includegraphics{sc03}
\vspace*{7pt}
\caption{\label{sch3}(a)~Synthesis of dinuclear rhenium(I) complexes
(\textit{rac})-\textbf{10-1} and (\textit{rac})-\textbf{10-2} from
racemic diaza [6]helicene-bis-bipyridine ligand \textbf{3c}. 
(b)~X-ray crystallographic molecular structures of \textbf{3c},
\textbf{10-1}, and \textbf{10-2} (racemic structures, only one
enantiomer shown). Adapted with permission from Ref.~\cite{17}.}
\vspace*{-6pt}
\end{scheme*}

\section{Coordination to rhenium(I)}

\subsection{Monometallic helicene--bipy--Re(I)\newline complexes} 
The coordination of \textbf{3a} and \textbf{3b} to rhenium(I) was then
performed under classical conditions, i.e., by reacting them with
Re(CO)$_{5}$Cl in refluxing toluene and isolating the complexes by
simple filtration thanks to their low solubility in toluene (Scheme~\ref{sch2}).
Specifically, the neutral Re(I) complexes \textbf{7a} and \textbf{7b}
were synthesized from 4-(2-pyridyl)-5-aza[4]helicene \textbf{3a} and
4-(2-pyridyl)-5-aza[6]helicene \textbf{3b}, respectively. In contrast
to the starting ligands, Re(I) complexes \textbf{7a} and \textbf{7b}
displayed red phosphorescence at  ${\sim}$673--678~nm in
CH$_{2}$Cl$_{2}$ at rt, but with very low quantum yields  ($\Phi =
0.11\ndash0.13\%$, Table~\ref{tab1}).
In order to improve efficiency,
charged complexes \textbf{8a}/\textbf{8b}$^{\mathbf{1},\mathbf{2}}$ and
\textbf{9a}, were readily prepared by direct substitution of the
chloride ligand with pyridine or isocyanide. Satisfyingly, these were
found to exhibit red phosphorescence with markedly higher quantum
yields \mbox{(6--16\%)} \mbox{(Table~\ref{tab1})} 
and much longer lifetimes. From the
stereochemical viewpoint, ligand \textbf{3a} is in average planar and
achiral, while ligand \textbf{3b} adopts helical $(P)$ or 
$(M)$ chirality. Due to the dissymmetric nature of the bipy ligands,
the slightly distorted octahedral rhenium center also becomes chiral,
with its \mbox{configuration} described as \mbox{anticlockwise} $(A_{\mathrm{Re}})$
or clockwise $(C_{\mathrm{Re}})$~\cite{23,24}. Consequently, complex
\textbf{7a} can exist as $(A_{\mathrm{Re}})/(C_{\mathrm{Re}})$
enantiomers, whereas \textbf{7b} forms two diastereomeric pairs of
enantiomers: $(P,A_{\mathrm{Re}})/(M,C_{\mathrm{Re}})$ and
$(P,C_{\mathrm{Re}})/(M,A_{\mathrm{Re}})$.  In the charged complexes,
the Re center readily isomerized, thus yielding 
\mbox{inseparable} racemic
mixtures (\textbf{8a}) or epimeric mixtures
(\textbf{8b}$^{\mathbf{1},\mathbf{2}}$). In contrast,  enantiomerically
and diastereomerically pure samples of  \textbf{7a} and
\textbf{7b}$^{\mathbf{1},\mathbf{2}}$ were successfully isolated by
chiral HPLC.  Having these species in hand allowed a detailed
investigation of their  chiroptical properties. For instance, 
Figure~\ref{fig1}a represents the comparison of  the ECD spectra of
pure stereoisomers of \textbf{3b}  with those of
\textbf{7b}$^{\mathbf{1}}$ and \textbf{7b}$^{\mathbf{2}}$, while 
Figure~\ref{fig1}c shows the 
\mbox{comparison} of the calculated ECD spectra of 
\textbf{7b}$^{\mathbf{1}}$ and \textbf{7b}$^{\mathbf{2}}$ with the
epimeric mixture of  \textbf{8b}$^{\mathbf{1},\mathbf{2}}$,  and
\mbox{Figure~\ref{fig1}b} presents their CPL  (circularly polarized
luminescence) responses. The ECD spectra of 
\textbf{7b}$^{\mathbf{1},\mathbf{2}}$ and
\textbf{8b}$^{\mathbf{1},\mathbf{2}}$ feature a new low-energy band
which was absent in \textbf{3b}.  The calculated ECD spectra revealed
that this transition mainly  corresponded to the
HOMO${\rightarrow}$LUMO excitation and arose  from the $\rmpi$-helical
ligand system in the HOMO and  the bipy--rhenium fragment, with partial
$\rmpi$-extension  into the helicenic core in the LUMO. Importantly,
ligand \textbf{3b},  complex \textbf{7b}$^{\mathbf{2}}$, and complex 
\textbf{8b}$^{\mathbf{1},\mathbf{2}}$ were found CPL-active, with
emission dissymmetry factors  $(g_{\mathrm{lum}})$ on the order of
10$^{-3}$  (Table~\ref{tab1}). Theoretical calculations indicated that
the emission process  involves Re orbitals, enabling the formally
spin-forbidden 
\mbox{$\mathrm{T}_{1}\rightarrow\mathrm{S}_{0}$}
phosphorescence transitions via spin--orbit coupling. The reduced 
metal-orbital participation (lower metal-to-ligand CT [MLCT] character)
in the T$_{1}$ state of  \textbf{8b}$^{\mathbf{1},\mathbf{2}}$ compared
with \textbf{7b}$^{\mathbf{1},\mathbf{2}}$ explained the~higher
emission efficiency of the charged 
\mbox{complexes.}

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig02}
\vspace*{2pt}
\caption{\label{fig2}(a)~Experimental ECD spectra of $(P)$- and
$(M)$-\textbf{3c} and $(P)$- and $(M)$-\textbf{10-1} in 
CH$_{2}$Cl$_{2}$ at rt. (b)~Simulated ECD spectra of 
$(P)$-\textbf{3c}, $(P,A_{\mathrm{Re}},A_{\mathrm{Re}})$-\textbf{10-1},
and  $(P,A_{\mathrm{Re}},C_{\mathrm{Re}})$-\textbf{10-2} with  selected
calculated excitation energies and rotatory strengths indicated  as
``stick'' spectra.  (c)~Isosurfaces of selected molecular orbitals for
\textbf{3c}, \textbf{10-1}, and \textbf{10-2}.  Adapted with permission
from Ref.~\cite{17}.}
\vspace*{1pt}
\end{figure*}

\subsection{Bimetallic helicene--bis-bipy--Re(I)\newline complexes}
Using the same strategy, the dinuclear Re(I) complexes \textbf{10-1}
and \textbf{10-2} were synthesized from
3,14-bis(2-pyridyl)-4,13-diaza[6]helicene  \textbf{3c} incorporating
two bipy units  (Scheme~\ref{sch3}). Similarly to mono-Re complexes,
each Re center in \textbf{10-1} and \textbf{10-2} adopts a distorted
octahedral geometry with \textit{fac}-oriented carbonyl groups. In
\textbf{10-1}, both chlorides point toward the helicene core, giving
$C_2$ symmetry with identical $C$ or  $A$ configurations at the two Re
centers and a helical angle of  39.13\textdegree. In \textbf{10-2}, one
chloride points inward  $(C)$ and the other outward $(A)$, breaking the
symmetry $(C_1)$ and increasing the helical angle to 46.39\textdegree. 
The isomer with both chlorides pointing outward was not observed. X-ray
diffraction established the absolute configurations as
$(M,C_{\mathrm{Re}},C_{\mathrm{Re}})/(P,A_{\mathrm{Re}},
A_{\mathrm{Re}})$ for \textbf{10-1} and
$(M,C_{\mathrm{Re}},A_{\mathrm{Re}})/
(P,A_{\mathrm{Re}},C_{\mathrm{Re}})$ for \textbf{10-2}, consistent with
their respective $C_2$ and  $C_1$ symmetry. Incorporation of two Re
centers into the helicenic ligand induced  a marked redshift of the
lowest-energy bands and increased molar  absorptivity beyond 375~nm 
(Table~\ref{tab1}). The nature of these excitations in \textbf{10-1}
and \textbf{10-2} was investigated in detail by TDDFT calculations.
Bands around 299~nm are assigned to intraligand $\rmpi$--$\rmpi^*$
transitions, while the lower-energy bands corresponded to intra-ligand
CT (ILCT) excitations with MLCT contributions. In CH$_{2}$Cl$_{2}$ at
rt, \textbf{10-1} and \textbf{10-2} displayed weak, broad red
phosphorescence ($\lambda_{\mathrm{max}} = 673$ and 687~nm;  $\Phi=
0.20\%$ and 0.15\%;  $\tau = 38$ and 32~ns), typical of
$\mbox{}^{3}$MLCT emission in Re(CO)$_{3}$($N$,$N$)Cl complexes. At
77~K, the complexes exhibited structured phosphorescence blueshifted by
over 100~nm 
\mbox{relative} to rt, with $\tau$ over  40~$\rmmu$s. This
\mbox{low-temperature} emission closely resembled that of ligand \textbf{3c}
suggesting that a ligand-centered $\mbox{}^{3}\rmpi$--$\rmpi^*$ state
lies below the $\mbox{}^{3}$MLCT state under these conditions, with
metal 
\mbox{coordination} slightly stabilizing the $\rmpi^*$ orbitals and
accelerating the  $\mathrm{T}_{1}\rightarrow \mathrm{S}_{0}$\break
transition. 

Enantiopure dinuclear Re(I) complexes were prepared from enantiopure
ligands. Interestingly, only one diastereomer was formed from each
ligand, namely $(M,C_{\mathrm{Re}},C_{\mathrm{Re}})$ and
$(P,A_{\mathrm{Re}},A_{\mathrm{Re}})$-\textbf{10-1} from $(M)$- and
$(P)$-\textbf{3c}, respectively, while enantiopure \textbf{10-2}  was
not detected. This selectivity most likely originated from  the lower
stability of enantiopure \textbf{10-2}, which under  refluxing toluene
isomerized to the thermodynamically more  stable \textbf{10-1}. This
interpretation was supported by DFT  calculations.  Figure~\ref{fig2}a
compares the experimental ECD spectra of the ligand and its complexes.
The enantiomers of \textbf{10-1} exhibited mirror-image spectra
featuring several bands between 272 and 459~nm. The MOs involved in the
low-energy intense excitations of \textbf{3c} and \textbf{10-1} are
shown in  Figure~\ref{fig2}c. Transitions of the ligands are dominated
by $\rmpi$--$\rmpi^*$ ones, with extended 
\mbox{conjugation} through the whole
system. In complex \textbf{10-1}, the strong ECD bands mainly arise
from metal and halogen to helicene--bis-bipyridine charge transfers
(MLCT and halogen-to-ligand CT [XLCT]), with additional helicene ILCT
and $\rmpi$--$\rmpi^*$ transitions. Finally, nearly mirror-image CPL
spectra were recorded in degassed CH$_{2}$Cl$_{2}$ solutions at rt,
with $g_{\mathrm{lum}}$ values of ${+}8.6 \times10^{-3}$ at 
${\sim}$450~nm for $(P)$-\textbf{3c} and  ${+}3.4 \times 10^{-3}$ at
${\sim}$630~nm for $(P,A_{\mathrm{Re}},A_{\mathrm{Re}})$-\textbf{10-1}.
The latter value is similar to the one of complex 
\textbf{7b}$^{\mathbf{2}}$ (see  Table~\ref{tab1}). It is worth
mentioning that complex \textbf{10-1} displayed a CPL sign which was
opposite to that of the lowest-energy ECD band ($g_{\mathrm{abs}} =
-3.4 \times 10^{-3}$ for
$(P,A_{\mathrm{Re}},A_{\mathrm{Re}})$-\textbf{10-1}), suggesting that
absorption and emission involve different electronic states  and/or the
transition is not Franck--Condon-allowed~\cite{25}.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}(a)~Optimized structures of $(P)$-\textbf{6},
$(P,C_{\mathrm{Re}})$-\textbf{11-1}, and
$(P,A_{\mathrm{Re}})$-\textbf{11-2} with relative electronic energy
$\Delta E$ values (in kcal/mol) and respective Boltzmann populations at
298~K. (b)~FMOs of $(P)$-\textbf{6} and
$(P,C_{\mathrm{Re}})$-\textbf{11} obtained based on
LC-PBE0*/def2-TZVP/PCM(CH$_{2}$Cl$_{2}$) calculations. Values listed
are the corresponding orbital energies (in eV). (c)~Experimental ECD
spectra of \textbf{11} enantiomers in CH$_{2}$Cl$_{2}$ at rt, along
with the corresponding simulated
(TDDFT-LC-PBE0*/def2-TZVP/PCM(CH$_2$Cl$_{2}$)) spectral envelopes
obtained for $(P,C_{\mathrm{Re}})$-\textbf{11-1} and
$(P,A_{\mathrm{Re}})$-\textbf{11-2}. Selected numbered excitation
energies and the corresponding rotatory strengths obtained for
$(P,C_{\mathrm{Re}})$-\textbf{11-1} indicated as ``stick''.
(d)~Experimental photoluminescence spectra of \textbf{11} in
CH$_{2}$Cl$_{2}$ at rt and in 2-MeTHF at 77~K. Inset: Enlarged spectra.
Adapted with permission from Ref.~\cite{20}.}
\end{figure*}

\subsection{Monometallic helicene--DPPZ--Re(I)\newline complexes}
Coordination of Re(I) to helical DPPZ-type ligand \textbf{6}, using
Re(CO)$_{5}$Cl in refluxing toluene, yielded complex \textbf{11}.
Contrary to ligand \textbf{6}, it was possible to obtain
enantiomerically enriched samples of \textbf{11} on an analytical scale
using the chiral HPLC separation technique~\cite{20}. The $(P)$ and
$(M)$ helical configurations were confirmed by (TD)DFT calculations.
However, these systems may exist in two octahedral diastereomeric forms
differing in rhenium-centered chirality
($(P,C_{\mathrm{Re}})$-\textbf{11-1} and
$(P,A_{\mathrm{Re}})$-\textbf{11-2} and their mirror images), but they
could not be clearly distinguished in this particular case. This
outcome suggested either a weak differentiation between both
diastereomers, a potential dynamic equilibrium in solution, or a
preferential formation of one stereoisomer under the synthetic
conditions, as already seen in the complexes described above. Complex
\textbf{11} exhibited very similar UV--Vis absorption in the
higher-energy region as for the ligand  (Figure~\ref{fig3} and 
Table~\ref{tab1}) but with an additional tail of absorption going down
to 575~nm due to the presence of the rhenium atom. TDDFT calculations
and MO analysis of \textbf{6} highlighted the presence of strong
charge-transfer transitions
([4]helicene${\rightarrow}$pyrazino-phenanthroline) in addition to the
classical $\rmpi$--$\rmpi^*$ transitions  (Figure~\ref{fig3}b), while
for \textbf{11} they revealed similar electronic characteristics
corresponding to almost pure HOMO${\rightarrow}$LUMO ILCT transition
with some helicene-centered $\rmpi$--$\rmpi^*$ signature but with
additional Metal-to-Ligand CT (MLCT) component due to the involvement
of the metal orbital. The emission spectra of \textbf{11} recorded in
CH$_{2}$Cl$_{2}$ at rt and in 2-MeTHF at 77~K (Figure~\ref{fig3}d) were
nevertheless found to be mainly influenced by the ligand scaffold with
rather minimal impact of the metal center on the photophysical
properties but with a quantum yield  $(\Phi = 2\%)$ which was found one
order of magnitude higher than for the previously reported neutral
mononuclear rhenium--bipyridine--helicene complexes (0.1--0.3\%, see
above) although lower than for the charged systems (8.3--16\%, see
above) and the rhenium--NHC--helicene complexes also described by our
group (5--13\%)~\cite{26,27}. Similarly to \textbf{6}, emission decay
of \textbf{11} was found bi-exponential, with a faster lifetime
component at 1.2 ns (87\%) and a more extended lifetime component of
7.6 ns (13\%). The observed lifetime was much shorter than that
reported for rhenium--bipyridine--helicene complexes (25--38~ns) 
(Table~\ref{tab1}) indicating again that the emission predominantly
originated from the ligand itself with small contribution of the
rhenium. Regarding the chiroptical properties, complex \textbf{11}
exhibited mirror-image ECD and CPL spectra  
\mbox{(Figures~\ref{fig3}c,~d).} 
In ECD,  $(P)$-\textbf{11} enantiomer exhibited
mainly four bands, a negative one around 240~nm, and three positive
ones between 311 and 540~nm. The  $(P)$ and $(M)$ assignments were
based on theoretical calculations. However, calculations showed similar
ECD spectra for $(P,C_{\mathrm{Re}})$-\textbf{11-1} and
$(P,A_{\mathrm{Re}})$-\textbf{11-2} preventing any further assignment
about the rhenium stereochemistry. CPL signals of \textbf{11} were
examined both at rt in CH$_{2}$Cl$_{2}$ and at 77~K in 2-MeTHF. At rt,
the CPL signal correlated with the ECD signals, and $(P)$-\textbf{11}
showed a positive CPL signal and a $g_{\mathrm{lum}}$ value of
${+}2.3\times10^{-3}$ at 507~nm. Notably, the CPL signal was
significantly increased at 77~K, with a $g_{\mathrm{lum}}$ value of
${+}2.9\times10^{-2}$ at 560~nm and displayed a clear vibrational
progression. Noteworthy, the  $g_{\mathrm{lum}}$ was found to be stable
throughout the band suggesting that the emission at 77~K originated
from a single excited state, which was not the case at rt, as also
indicated by the presence of bi-exponential decay. 

\section{Conclusion}
In this review, we have described helicenic organic bipy- or phen-type
fluorophores and their use as efficient  $N$,$N$ ligands for
coordination to rhenium(I) heavy transition metal centers. This
strategy enabled us to generate enantiopure or enantioenriched
\textit{fac}-ReX(bpy)(CO)$_3$-type complexes for which the structure
and stereochemical features were carefully analyzed. Furthermore, this
gave access to helically chiral absorbers and phosphorescent
derivatives with strong experimental ECD responses, substantial CPL
activity, and long-lived emission. Their photophysics and chiroptics
were studied in detail by theoretical calculations, allowing
characterization of  CT/$\rmpi$--$\rmpi^*$ transitions in the ligands,
and additional MLCT, XLCT, and ligand-to-ligand CT (LLCT) in the
complexes. Further molecular engineering enabled access to optimized
chiral phosphorescent compounds. Indeed, going from neutral to charged
complexes led to significantly improved photophysical characteristics,
especially improved quantum yields and much longer-lived emission,
while lowering the temperature significantly improved the CPL response,
with emission dissymmetry factors increasing by one order of magnitude
(from ${\sim}$10$^{-3}$ to ${\sim}$10$^{-2}$). Overall, we think that
such specific features in these unique helically shaped heavy-metal
\mbox{complexes~provide} potentially \mbox{important} \mbox{future} 
\mbox{applications,} notably in
the domain of photoactive catalysts such as in the use of rhenium
tricarbonyl bipy complexes for CO$_{2}$-to-CO
reduction~\cite{28,29,30}. This work is currently ongoing in our group.

\section*{Acknowledgements}
All collaborators and students involved in this work reviewed in this
article are warmly thanked for their precious contributions.

\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 acknowledge  the Minist\`{e}re de l'Education Nationale,  de la
Recherche et de la Technologie,  the Centre National de la Recherche
Scientifique (CNRS),  Rennes M\'{e}tropole and the French National
Agency (ANR, LumoMat-E project, 18-EURE-0012).  The European Commission
Research Executive Agency  (Grant Agreement number: 859752 --
HEL4CHIROLED -- H2020-MSCA-ITN-2019) is thanked for financial support. 

\CDRGrant[ANR]{18-EURE-0012}
\CDRGrant[REA]{859752 -- HEL4CHIROLED -- H2020-MSCA-ITN-2019}

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