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\DOI{10.5802/crchim.364}
\datereceived{2024-06-06}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Photophysical studies of Zn(II) tetra, \textit{tert-butyl}
phthalocyanine}

\alttitle{\'{E}tudes photophysiques de la
T\'{e}trakis(\textit{tert}-butyl)phthalocyanine de Zn(II)}

\author{\firstname{Sami} \lastname{Ayari}\CDRorcid{0000-0002-1505-1922}\IsCorresp}
\address{Universit\'{e} de Lorraine, CNRS, LRGP, F-54000 Nancy, France}
\address{Laboratoire Microorganismes et Biomol\'{e}cules Actives,
LR03ES03, Universit\'{e} de Tunis El Manar, Facult\'{e} des Science de
Tunis, 2092, Campus Universitaire, Tunis, Tunisie}
\email[S. Ayari]{sami.ayari@fst.utm.tn}

\author{\firstname{Philippe} \lastname{Arnoux}\CDRorcid{0000-0002-6834-9597}}
\addressSameAs{1}{Universit\'{e} de Lorraine, CNRS, LRGP, F-54000
Nancy, France}

\author{\firstname{Hadda Imene} \lastname{Ouzari}\CDRorcid{0000-0002-3237-554X}}
\addressSameAs{2}{Laboratoire Microorganismes et Biomol\'{e}cules
Actives, LR03ES03, Universit\'{e} de Tunis El Manar, Facult\'{e} des
Science de Tunis, 2092, Campus Universitaire, Tunis, Tunisie}

\author{\firstname{C\'eline} \lastname{Frochot}\CDRorcid{0000-0002-7659-3864}\IsCorresp}
\addressSameAs{1}{Universit\'{e} de Lorraine, CNRS, LRGP, F-54000
Nancy, France}
\email[C. Frochot]{celine.frochot@univ-lorraine.fr}

\shortrunauthors

\keywords{\kwd{UV--vis absorption} 
\kwd{\textit{tert}-ZnPc(II)} 
\kwd{Photophysical properties}}

\altkeywords{\kwd{Absorption UV--vis} 
\kwd{\textit{tert}-ZnPc(II)} 
\kwd{Propri\'{e}t\'{e}s photophysiques}}

\thanks{French Institute of Tunisia}

\begin{abstract}
Phthalocyanine derivatives have attracted significant attention due to
their unique chemical properties and applications in various fields,
including photodynamic therapy. However, the synthesis and
purification of phthalocyanines remain challenging. Among these
derivatives, zinc(II) tetra, \textit{tert}-\textit{butyl}
phthalocyanine (\textit{tert}-ZnPc(II)) is noteworthy for its
straightforward synthesis, making it a cost-effective option.
\textit{tert}-ZnPc(II) is often synthesized as a second-generation
byproduct in the synthesis of asymmetric phthalocyanines when
\textit{tert}-butyl moieties are introduced to enhance the solubility
of  derivatives in a wide range of organic solvents. The practical
applications of phthalocyanine derivatives depend mainly on their
photophysical properties, which in turn influence key parameters such
as absorption and fluorescence emission. This study investigates the
ultraviolet--visible (UV--vis) absorption and fluorescence emission spectroscopies of
\textit{tert}-ZnPc(II), focusing on the impact of solvent polarity and
the effect of different concentrations of water on UV--vis absorption
behavior.
\end{abstract}

\begin{altabstract}
Les d\'{e}riv\'{e}s de phtalocyanine ont suscit\'{e} un int\'{e}r\^{e}t
consid\'{e}rable en raison de leurs propri\'{e}t\'{e}s chimiques
uniques et de leurs applications dans divers domaines, notamment la
th\'{e}rapie photodynamique (PDT). Cependant, la synth\`{e}se et la
purification des phtalocyanines restent difficiles. Parmi ces
d\'{e}riv\'{e}s, la phtalocyanine de zinc(II)
t\'{e}tra-\textit{tert}-butyle (\textit{tert}-ZnPc(II)) se distingue
par sa synth\`{e}se simple, en faisant une option rentable. Le
\textit{tert}-ZnPc(II) est souvent synth\'{e}tis\'{e} comme
sous-produit de deuxi\`{e}me g\'{e}n\'{e}ration lors de la synth\`{e}se
de phtalocyanines asym\'{e}triques, lorsque des groupes
\textit{tert}-butyles sont introduits pour am\'{e}liorer la
solubilit\'{e} des d\'{e}riv\'{e}s dans une large gamme de solvants
organiques. Les applications pratiques des d\'{e}riv\'{e}s de
phtalocyanine d\'{e}pendent principalement de leurs propri\'{e}t\'{e}s
photophysiques, qui influencent \`{a} leur tour des param\`{e}tres
cl\'{e}s tels que l'absorption et l'\'{e}mission de fluorescence. Cette
\'{e}tude examine les spectroscopies d'absorption UV--visible et
d'\'{e}mission de fluorescence du \textit{tert}-ZnPc(II), en se
concentrant sur l'impact de la polarit\'{e} du solvant et sur l'effet
de diff\'{e}rentes concentrations d'eau sur le comportement
d'absorption UV--vis.
\end{altabstract}

\maketitle

\vspace*{11pt}

\twocolumngrid

\end{noXML}

\section{Introduction}   

Phthalocyanines (Pcs) and their derivatives represent aromatic
macrocycles with an 18-${\uppi}$-electron delocalization system
characterized by high chemical and thermal stability. These compounds
possess diverse coordination properties, exhibit low toxicity, and have
the ability to absorb within the visible region (600--800~nm).  The
versatility of Pcs is featured in a wide range of
applications~\cite{1,2,3,4,5},  such as  semiconductor devices~\cite{6},
liquid crystals~\cite{7,8,9},  sensors~\cite{10,11,12},
catalysts~\cite{13}, and dye-sensitized solar cells~\cite{14,15}.
Furthermore, Pcs have emerged as highly promising photosensitizers in the
context of photodynamic therapy (PDT)~\cite{16,17,18,19,20,21,22}. 

The photophysical characteristics of Pcs are notably influenced by the
central metal ion, with zinc (Zn) proving particularly valuable due to
its ability to confer Pcs with enhanced fluorescence and singlet oxygen
production properties. These attributes make them suitable for
applications in tumor detection and treatment~\cite{23}.

The solubility of Pcs is a pivotal factor for diverse applications.
Introducing functional groups such as alkyl, tertiary butyl groups,
crown ethers, alkoxy, alkylthio, and amide groups into  peripheral
benzene rings of the Pc structure enhances solubility in both protic
and non-protic solvents~\cite{24,25,26,27,28,29,30,31,32}. Notably,
water solubility assumes a crucial role in PDT
applications~\cite{33,34,35}. While sulfonates~\cite{36},\unskip\break
carboxylates~\cite{37,38}, and quaternized amino groups~\cite{39,40,41}
are commonly employed to achieve water solubility in dyes,
\textit{tert}-butyl groups confer Pcs with hydrophobic
properties~\cite{42}. 

An important consideration in the study of Pcs is their tendency to
form aggregates, especially in aqueous solutions~\cite{43}. This
property proves unfavorable as it reduces solubility and adversely
impacts photophysical characteristics, such as shortening the triplet
state lifetime and hindering the efficient generation of singlet
oxygen. In the context of in vivo activity as
photosensitizers, aggregation not only diminishes photoactivity but
also restricts access to neoplastic cells~\cite{44}. The degree of this
effect is significantly influenced by the molecular structure,
particularly the nature of the central metal ion and functional groups
present in the peripheral substituents~\cite{45,46}. Since
photochemical activity is important only for Pc monomers, assessing
aggregation parameters becomes essential for accurately determining the
photophysical properties of these compounds. The formation of
aggregates is primarily driven by ${\uppi}\ndash {\uppi}$ stacking
between the nearly planar Pc macrocycles and van der Waals forces.
Depending on their geometry, H-type (sandwich) and J-type
(head-to-tail) aggregates can be distinguished, exhibiting significant
differences in optical properties according to Kasha's exciton
model~\cite{47,48}. 

Considering the cost-effectiveness and commercial availability of
\textit{tert}-ZnPc(II), it emerges as an intriguing compound with a vast
field of applications. In this study, we aim to explore the impact of
solvent polarity on the photophysical properties (absorption and
fluorescence emission spectroscopies) of \textit{tert}-ZnPc(II). This
work explores its photophysical properties, with special attention
given to the effect of different concentrations of water on absorption
behavior and aggregation in homogeneous\unskip\break solutions.

\section{Results and discussion} 

The most important properties of Pcs as materials lie in their
exceptional thermal, chemical, and photochemical stabilities.
Typically, unsubstituted Pc complexes exhibit low solubility across a wide
range of solvents. Solubility in organic solvents is crucial for
exploring the diverse applications of these molecules. Enhancing
solubility in water or common organic solvents can be effectively
achieved through peripheral modifications of Pcs, particularly by
introducing the right substitutions on benzene
rings\unskip\break \cite{49a,49b,49c}.

In contrast to unsubstituted Pc metal complexes, \textit{tert}-ZnPc(II),
despite existing as a mixture of four positional
isomers~\cite{50a,50b,51a,51b,51c,51d}, stands out due to the
incorporation of bulky \textit{tert}-butyl substituents. This
modification mitigates the aggregation tendencies of Pcs and enables an
increase in their solubility across various common organic solvents
without causing a significant shift in their ultraviolet--visible (UV--vis) spectra~\cite{52}.
The chemical structure of \textit{tert}-ZnPc(II) is shown in
Figure~\ref{fig1}.

\begin{figure}
\includegraphics{fig01}
\vspace*{-4pt}
\caption{\label{fig1}Chemical structure of \textit{tert}-ZnPc(II).}
\vspace*{-2pt}
\end{figure}


\vspace*{-2pt}

\subsection{Effect of the solvent}

\vspace*{-2pt}
\subsubsection{Ground-state electronic absorption spectra}

The photophysical characteristics of Pc macrocycles are primarily
influenced by their molecular structure and their interaction with the
surrounding environment~\cite{53}. The UV--vis absorption spectra of
\textit{tert}-ZnPc(II), as illustrated in Figure~\ref{fig2}, were
measured in various solvents. Solvent polarity is quantified by the
$\mathrm{E}_{\mathrm{T}}$(30) parameter, with higher
$\mathrm{E}_{\mathrm{T}}$(30) values indicating increased solvent
polarity ($\mathrm{E}_{\mathrm{T}}$(30) water ${=}$ 63.1,
$\mathrm{E}_{\mathrm{T}}$(30) glycerol ${=}$ 57,
$\mathrm{E}_{\mathrm{T}}$(30) methanol ${=}$ 55.4,
$\mathrm{E}_{\mathrm{T}}$(30) ethanol ${=}$ 51.9,
$\mathrm{E}_{\mathrm{T}}$(30) acetonitrile ${=}$ 45.6,
$\mathrm{E}_{\mathrm{T}}$(30) DMF ${=}$ 43.8,
$\mathrm{E}_{\mathrm{T}}$(30) chloroform ${=}$ 39.1,
$\mathrm{E}_{\mathrm{T}}$(30) ethyl acetate ${=}$ 38.1,
$\mathrm{E}_{\mathrm{T}}$(30) toluene ${=}$ 33,
$\mathrm{E}_{\mathrm{T}}$(30) FBS ${=}$ unknown)~\cite{54,55}.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Absorption spectra of \textit{tert}-ZnPc(II)
in 10 different solvents 
($7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$).}
\vspace*{-1pt}
\end{figure}

A DMSO stock solution ($\mathrm{m}_{\mathit{tert}\text{-ZnPc}
(\mathrm{II})} = 4.75~\mathrm{mg}$, $\mathrm{V}_{\mathrm{DMSO}} =
4.75~\mathrm{mL}$) was prepared, ensuring compound solubility. This
stock solution was further diluted in 10 different solvents to
investigate the specific impact of solvent polarity on photophysical
properties. Concentration standardization was achieved in toluene by
adjusting the peak absorbance at 610~nm to approximately 0.2,
equivalent to $6\times 10^{-3}~\mathrm{mg}{\cdot}\mathrm{mL}^{-1}$. The
solvents include toluene, ethyl acetate, ethanol, methanol, DMF,
acetonitrile, chloroform, glycerol, water, and a 10\% dilution of fetal
bovine serum (FBS) in a biological medium. The absorption spectra are
presented in Figure~\ref{fig2}.

The pronounced tendency of Pcs to form dimers and trimers in solution
leads to a reduction in its photoactivity as a photosensitizer. This is
attributed to the fact that the monomer structure is responsible for
optimal photochemical activity~\cite{44,45,46}. In
Figure~\ref{fig2}, the absorption spectra of \textit{tert}-ZnPc(II) in
toluene, ethyl acetate, ethanol, methanol, DMF, and acetonitrile are
depicted. The Soret and Q bands exhibit consistent shapes with minor
shifts, aligning with the characteristic UV--vis spectra of Pcs.

On the other hand, absorption spectra of \textit{tert}-ZnPc(II) in
chloroform, glycerol, water, and FBS show a significant alteration in
the shape, intensity, and split absorption peaks in comparison
to the spectra in toluene, indicating an aggregation effect.
\textit{tert}-ZnPc(II) exhibits two main absorption bands at 350~nm and
610~nm. These bands exhibit diminished intensity in aqueous medium,
glycerol, and chloroform, coupled with a red shift. Conversely, the
spectra appear quite similar in the other\break solvents.

The molar extinction coefficient (${\upvarepsilon}$) is an intrinsic
property of molecules depending mostly on their structure and
composition. It assesses how chemical species absorb photons at
specific wavelengths. The molar extinction coefficients of
\textit{tert}-ZnPc(II) are calculated and depicted in
Table~\ref{tab1}.

\begin{table*}
\caption{\label{tab1}Absorbance B and Q bands and molar extinction
coefficient of \textit{tert}-ZnPc(II) in different solvents\vspace*{-1pt}}
\begin{tabular}{cccccc} 
\thead 
Solvents & B band (nm) & Q band 1 (nm) & Q band 2 (nm) &
${\upvarepsilon}$350 & ${\upvarepsilon}$610 \\ 
\endthead 
Toluene & 350 & 610 & 676 & 56,802 & 29,403 \\ 
Ethyl acetate & 347 & 606 & 671 & 60,277 & 28,067 \\ 
Ethanol & 347 & 608 & 674 & 57,738 & 28,067 \\ 
Methanol & 346 & 608 & 673 & 55,064 & 27,933 \\ 
DMF & 349 & 609 & 675 & 61,079 & 31,675 \\ 
Acetonitrile & 346 & 606 & 672 & 56,535 & 27,933 \\ 
Chloroform & 357 & 679 & 718 & 30,740 & 11,494 \\ 
Glycerol & 349 & 633 & 685 & 17,776 & 12,162 \\ 
Water & 338 & 630 & 678 & 37,422 & 25,527 \\ 
FBS & 339 & 630 & 679 & 45,174 & 26,998 
\botline
\end{tabular}
\vspace*{-6pt}
\end{table*}

{\vspace*{-2pt}}

\subsubsection{Fluorescence quantum yields}

The fluorescence spectra in different solvents after excitation at
610~nm are shown in Figure~\ref{fig3}.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}Fluorescence spectrum of \textit{tert}-ZnPc(II)
in 10 solvents excited at 610~nm 
($7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$).}
\vspace*{-1pt}
\end{figure}

The fluorescence spectra of \textit{tert}-ZnPc(II) reveal two distinct
groups of solvents (Figure~\ref{fig3}). In FBS, chloroform, and
glycerol, the spectra exhibit two peaks of very low intensity at 690~nm
and 725~nm while no fluorescence is observed in water. Conversely, in
the remaining solvents, the emission spectra show the characteristic
patterns of Pc macrocycles. The first maxima spectra are situated
between 680~nm and 690~nm and the second between 740~nm and 750~nm,
with both peaks exhibiting higher intensity in alignment with the
absorption spectra of Pcs.

The fluorescence quantum yields in toluene, ethyl acetate, ethanol,
methanol, DMF, and acetonitrile exhibit close values,  ranging from
0.35 to 0.27 (Table~\ref{tab2}). In contrast, chloroform, glycerol, and
FBS show fluorescence quantum yields less than 0.1 while as expected,
the fluorescence quantum yield is zero for water. Notably, the
fluorescence quantum yields at 350~nm closely mirror those at 610~nm,
indicating that the quantum yield remains consistent
\mbox{irrespective} of the wavelength at which it is measured. This
emphasizes the wavelength-independent nature of fluorescence quantum
yield.

\begin{table*}
\caption{\label{tab2}Emission band, fluorescence quantum yield 
of \textit{tert}-ZnPc(II) in different solvents\vspace*{-2pt}}
\begin{tabular}{cccccc}
\thead 
Solvents & ${\rmLambda}_{\mathrm{ext}}$ (nm) &
${\rmLambda}_{\mathrm{em}}$ Band 1 (nm) &  ${\rmLambda}_{\mathrm{em}}$
Band 2 (nm) & ${\rmLambda}_{\mathrm{em}}$ Band 3 (nm) & $\rmPhi_f$ 610\\ 
\endthead 
Toluene & 610 & 689 & 749 & - & 0.35 \\ 
Ethyl acetate & 610 & 683 & 743 & - & 0.31 \\ 
Ethanol & 610 & 687 & 746 & - & 0.29 \\ 
Methanol & 610 & 686 & 745 & - & 0.27 \\ 
DMF & 610 & 689 & 746 & - & 0.29 \\ 
Acetonitrile & 610 & 687 & 741 & - & 0.28 \\ 
Chloroform & 610 & 687 & - & - & 0.03 \\ 
Glycerol & 610 & 692 & - & - & 0.02 \\ 
Water & 610 & 700 & 720 & 738 & 0 \\ 
FBS & 610 & 686 & - & - & ${<}$0.1 
\botline
\end{tabular}
\vspace*{-2pt}
\end{table*}



\subsection{Effect of pH medium}

The UV--vis spectrum of \textit{tert}-ZnPc(II) in water
reveals a broader absorption profile with split peaks, exhibiting an
extensive broadening of the Q band (Figure~\ref{fig4}). This is
accompanied by a red shift in the absorption maximum, compared to
toluene, and a blue shift in the Soret band from 350~nm to 336~nm
(Table~\ref{tab3}), indicative of H-aggregate formation. The
H-aggregate, characterized by a face-to-face arrangement, features a
large contact area and strong ${\uppi}\ndash {\uppi}$ forces. Previous
research has shown that nonaxial modified phthalocyanines, such as
tetra(4-\textit{tert}-butyl)phthalocyanine (abbreviated as TbPc), tend
to form H-aggregates during self-assembly~\cite{15}.

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}UV--vis absorption spectra of \textit{tert}-ZnPc(II)
in water at different pH ($7.5 \times 10^{-6}~\mathrm{mol}{\cdot}
\mathrm{L}^{-1}$).}
\end{figure}

\begin{table}
\caption{\label{tab3}Absorbance B and Q bands of \textit{tert}-ZnPc(II)
in water  at different pH ($7.5\times
10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$)
\vspace*{-2pt}}
\tabcolsep=2.5pt
\begin{tabular}{cccc}  
\thead
Water & B band (nm) & Q band 1 (nm) & Q band 2 (nm) \\
\endthead
pH4 & 336 & 630 & 677 \\ 
pH7 & 337 & 630 & 677 \\ 
pH10 & 337 & 630 & 677 
\botline
\end{tabular}
\vspace*{-2pt}
\end{table}

Moreover, pH is identified as a factor that could influence 
photophysical properties. UV--vis absorption and fluorescence
emission of \textit{tert}-ZnPc(II) in water at a concentration of
$7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$ were measured
under different pH conditions (pH  4, 7, and 10). The absorption spectra
are relatively similar for pH  7 and 10, whereas a significant decrease
in intensity is observed for pH 4. In water, $7.5\times
10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$ exhibits no fluorescence
across all pH ranges, indicating the presence of aggregation
(Figure~\ref{fig5}).

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Fluorescence spectrum of \textit{tert}-ZnPc(II)
in water excited at 610~nm 
($7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$) at different
pH.}
\end{figure}

\subsection{Effect of water concentration on UV--vis absorption
properties of \textit{tert}-ZnPc(II)}

The observation of a notable change in the Q band shape within the
UV--vis absorption spectrum of \textit{tert}-ZnPc(II) in a water
environment motivated us to examine the impact of water on the
absorption intensity for detecting trace amounts of water in diverse
organic solvents, including ethanol, methanol, and acetonitrile.

Indeed, water stands as an important and indispensable substance,
holding significance not only in the realm of everyday human existence
but also within the domain of chemistry. It serves as a \mbox{critical}
impurity requiring elimination from numerous chemical and industrial
production processes.

Water possesses distinctive characteristics that engender specific
interactions with excited molecules owing to its substantial
dielectric constant and ability to donate and accept protons. This
makes it a potential sensor for detecting moisture in organic solvents.
Another important factor that needs to be considered is the strong
tendency of  \textit{tert}-ZnPc(II) to form
aggregates.

The probe solution of \textit{tert}-ZnPc(II) ($7.5\times
10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$) is dissolved in each of the dry
organic solvents ethanol, methanol, and acetonitrile, and various
concentrations of water (0--70\%, v/v) were added. All spectra were
obtained immediately after the addition of water to each solvent at
25~\textdegree C.

Various concentrations of water in ethanol, methanol, and acetonitrile
solutions containing \textit{tert}-ZnPc(II) did not lead to
significant wavelength shifts or notable changes in spectral shape; the
only difference lay in the absorption coefficient magnitudes. Dicelio
and colleagues conducted a study on the aggregation of tetra,
\textit{tert}-butyl phthalocyanine, revealing that both the monomer and
dimer of tetra, \textit{tert}-butyl phthalocyanine exhibited nearly
identical UV--vis absorption spectra~\cite{56}. Consequently, the
addition of water to  \textit{tert}-ZnPc(II) induced the formation
of \textit{tert}-ZnPc(II) aggregates without entirely altering the
UV--vis absorption spectra. Explaining the shapes and intensities of the
dimer spectra accurately would necessitate a comprehensive theoretical
investigation, extending Kasha's exciton theory to degenerate excited
states and incorporating vibronic coupling into the model as proposed
by previous research~\cite{47,57}.

The UV--vis absorption spectra of \textit{tert}-ZnPc(II) in ethanol,
methanol, and acetonitrile (Figures~\ref{fig6}--\ref{fig8}) 
exhibited a nearly identical absorption shape even
after the addition of varying concentrations of water. However, the
notable alteration was observed in the absorption intensity.
Specifically, the introduction of small aliquots of water (0.1\%) into
ethanol, methanol, and acetonitrile resulted in a decrease in
absorption intensity from 1.228 to 0.593, 1.115 to 0.415, and 1.270 to
0.632, respectively. These values were 2, 2.60, and 2 times lower than
those observed in pure ethanol, methanol, and acetonitrile, respectively
(Tables~4--6 in Supplementary Material).

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}UV--vis absorption spectra as a function 
of water content of \textit{tert}-ZnPc(II) in ethanol at 
$7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$.}
\vspace*{-2pt}
\end{figure}

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}UV--vis absorption spectra as a function 
of water content of \textit{tert}-ZnPc(II) in methanol at 
$7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$.}
\end{figure}

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig8}UV--vis absorption spectra as a function 
of water content of \textit{tert}-ZnPc(II) in acetonitrile at 
$7.5\times 10^{-6}~\mathrm{mol}{\cdot}\mathrm{L}^{-1}$.}
\end{figure}

This diminished intensity, attributed to the addition of a minimal
quantity of water, emphasizes the high sensitivity of
\textit{tert}-ZnPc(II) to water content. The absorption intensity
at 674~nm, 673~nm, and 672~nm in ethanol, methanol, and acetonitrile
progressively decreased with increasing water \mbox{content} from 0.1\%
to (30\%, 10\%, and 70\%), reaching a plateau at each respective
concentration. At the 30\%, 10\%, and 70\% water content levels, the
absorption intensity at 674~nm, 673~nm, and 672~nm were approximately
7-, 4.62-, and 12-fold lower than in pure ethanol, methanol, and
acetonitrile, respectively (Figures~\ref{fig6}--\ref{fig8}). 
Evidently, a linear decrease in absorption intensity was observed
across all wavelengths with  increasing percentage of
water\unskip\break content.

It is noteworthy that despite the likely occurrence of aggregation, the
spectra maintained their consistent absorption shapes even when
incorporating 30\%, 10\%, and 70\% water into homogeneous solvents of
ethanol, methanol, and acetonitrile, respectively.

\section{Conclusion}

This study focuses on the exploration of \textit{tert}-ZnPc(II)
photophysical properties. The investigation highlights the influence of
solvent polarity on ground-state electronic absorption spectra and
fluorescence capability. It is noteworthy that \textit{tert}-ZnPc(II)
exhibits typical UV--vis spectra in all organic solvents examined
except chloroform, water, glycerol, and FBS. In these specific
solvents, a significant alteration in the shape of absorption spectra
is observed coupled with a notable absence of fluorescence, likely
attributed to well-known aggregation phenomena inherent in
phthalocyanines. Additionally, the UV--vis spectra of \textit{tert}-ZnPc(II)
in water demonstrate high sensitivity to water content,
maintaining consistent absorption shapes even with the incorporation of
a substantial amount of water into the solvents.

\section{Materials and methods} 

All solvents and reagents were of reagent grade quality and obtained
commercially from Aldrich, TCI, Fluka, and Merck. 

\subsection{Photophysical properties}

Absorption spectra were recorded on a UV-3600 UV--visible double beam
spectrophotometer (Shimadzu, Marne la Vall\'{e}e, France). Fluorescence
spectra were recorded on a Fluorolog FL3-222 spectrofluorometer (HORIBA
Jobin Yvon, Longjumeau, France) equipped with a 450~W xenon lamp, a
thermostated cell compartment (25~\textdegree C), a UV--visible
photomultiplier R928 (HAMAMATSU, Japan), and an InGaAs infrared detector
(DSS-16A020L, Electro-Optical Systems Inc, PA, USA).

The excitation beam and the emission beam were diffracted by a
double-ruled grating SPEX monochromator with  1200 grooves/mm blazed at
330~nm  and 1200 grooves/mm blazed at 500~nm, respectively. All spectra
were measured in four-face quartz cuvettes. All the emission spectra
have been displayed with the same absorbance (less than 0.2) with the
lamp and photomultiplier correction.

\subsection{Parameters for fluorescence quantum yields}

The fluorescence quantum yields of \textit{tert}-ZnPc(II) ($\Phi_{f}$)
 were calculated in different
solvents by Equation~(\ref{eq1})~\cite{58}:
{\begin{eqnarray}\label{eq1}
\Phi_f = \Phi_{f_o} \cdot \frac{I_f}{I_{f_o}} \cdot \frac{DO_o}{DO} \cdot
\left(\frac{n}{n_o}\right)^2,
\end{eqnarray}}\unskip
where ${\Phi}_{f}$ and $\Phi_{f_o}$, $I_{f}$ and $I_{f_o}$, $DO$ and
$DO_{o}$, and $n$ and $n_{o}$ are the quantum yields, fluorescence
intensities, optical densities, and refractive indices
respectively of the sample and the standard.

\section*{Funding}

The authors thank the French Institute of Tunisia for providing
financial support.

\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*{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-364-suppl.pdf}}

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