Perhaps one of these solutions will be more or less what you want: I defined a hardspace
caption label format that ensures a normal space between caption name and caption number. A fixed space between label and first word can be achieved with the RaggedRight
option (which enables some hyphenation). However, as the whole caption is centred with respect to text width, its placement may look odd, depending on the effective width of the figure, and it may be better to have caption width equal to figure width and have caption on the left side rather than centred. This can be achieved with the measuredfigure
environment from the threeparttable
package.
So I propose 4 variants, according to justification and use of this environment or not:
\documentclass[11pt, a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{mathtools}
\usepackage{caption}
\usepackage{threeparttable}
\DeclareCaptionLabelFormat{hardspace}{\mbox{#1~#2}}
\DeclareCaptionLabelSeparator{hardspace}{.~}
\captionsetup{labelformat = hardspace, labelfont = bf,justification = RaggedRight}
\usepackage[demo]{graphicx}
\usepackage{chngcntr}
\renewcommand\thesection{\Alph{section}}
\counterwithin{figure}{section}
\setcounter{section}{1}
\setcounter{figure}{21}
\begin{document}
\noindent\texttt{Justification = RaggedRight: }
\begin{figure}[!h]
\centering
\begin{measuredfigure}
{\includegraphics[width =0.9\linewidth]{myfigure}}
\caption{The $\prescript{1}{}{\text{H-NMR}}$ spectrum of \textbf{12},4-bis(2,2’-bithienyl-(4-hydroxypenyl)methane thymine-1-acetate, in deuterated chloroform.}
\end{measuredfigure}
\end{figure}
\begin{figure}[!h]
\centering
\includegraphics[width =0.9\linewidth]{myfigure}
\caption{The $\prescript{1}{}{\text{H-NMR}}$ spectrum of \textbf{12},4-bis(2,2’-bithienyl-(4-hydroxypenyl)methane thymine-1-acetate, in deuterated chloroform.}
\end{figure}
\clearpage
\noindent\texttt{Justification = justified: }
\captionsetup{justification = justified}
\begin{figure}[!h]
\centering
\begin{measuredfigure}
{\includegraphics[width =0.9\linewidth]{myfigure}}
\caption{The $\prescript{1}{}{\text{H-NMR}}$ spectrum of \textbf{12},4-bis(2,2’-bithienyl-(4-hydroxypenyl)methane thymine-1-acetate, in deuterated chloroform.}
\end{measuredfigure}
\end{figure}
\begin{figure}[!h]
\centering
\includegraphics[width =0.9\linewidth]{myfigure}
\caption{The $\prescript{1}{}{\text{H-NMR}}$ spectrum of \textbf{12},4-bis(2,2’-bithienyl-(4-hydroxypenyl)methane thymine-1-acetate, in deuterated chloroform.}
\end{figure}
\end{document}


justification=raggedright
to the options list of thecaption
package.bpchem
package provides very fine grained control over it, so any ambiguity might be avoided as the OP sees fit.