\chapter[\uppercase{Bounding donor and acceptor by hyperconjugated bridge}]{\uppercase{Bounding donor and acceptor by hyperconjugated bridge}}
\label{chap:chap_two}


\newcommand*{\CT}[2]{\ce{^{#1}CT_{#2}}}
\newcommand*{\bl}[1]{\textbf{#1}}


This chapter summarizes the research published in paper~\citepapernumONE{Sigma} and presents a deeper insight in intricate photophysical properties of several D-\textsigma-A compounds initially studied by Geng \etal \cite{Geng2017}.



\FloatBarrier
\section*{Motivation}
\FloatBarrier


While decoupling of HOMO and LUMO is commonly accomplished by twisting donor and acceptor fragments in single molecule, it can also be realized by breaking the bond between D an A moieties and relying on intermolecular charge transfer in exciplex \cite{Sarma2018}. The TADF properties, in this case, is strongly dependant on distance between D and A, which can hardly be controlled \cite{Park2013,Liu2016,Chen2016}.
Recently, Geng \etal \cite{Geng2017} suggested a novel strategy to utilize hyperconjugated bridge to bound donor and acceptor in a non-conjugated controlled manner.
Hyperconjugation, first introduced by Mulliken \etal{} \cite{Mulliken1941}, describes interaction of electrons in \textsigma\ orbital with an adjacent \textpi--orbital providing somewhat weak electron delocalization between them  \cite{Xie2018,Mullins2012}.
It was utilized, when several donor units were bound to triazine acceptor fragment by hexafluoroisopropylidene spacer unit \cite{Geng2017}. The compounds displayed intermolecular charge transfer emission with negligible \dEst.
DFT-estimated oscillator strengths were very low (in the range of \numrange{0.0008}{0.0054}), typical for weakly interacting donor and acceptor systems.
Two TADF decays were observed, one of which was very rapid with lifetime of only \SIrange{200}{400}{\nano\second} which is one of the lowest reported \cite{DosSantos2018}. 

In this chapter, time-resolved spectroscopy is exploited, to gain deeper insight into intriguing properties of D--\textsigma--A compounds. Analysis of these molecules in various concentrations, temperatures and solution viscosities revealed the origin of dual emission.




\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/Sigma_h.pdf}
    \caption{Molecular structures of D-\textsigma-A compounds presented in \cite{Geng2017} and investigated in this work.}
    \label{fig:sigma_mol}
\end{figure}


Three TADF donor-\textsigma-acceptor compounds were studied (Figure \ref{fig:sigma_mol}). All molecules contain
planar triphenyltriazine acceptor
bounded to donors of different strength: 1 - dihydroacridine, 2- tetramethylcarbazole, 3 - triphenylamine by hexafluoroisopropylidine  \textsigma\ bridge.



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\section[Excited state relaxation of D--\textsigma--A in diluted solutions]{Excited state relaxation of D--\textsigma--A in diluted solutions}
\label{sec:sec_chtwo_one}
\FloatBarrier
%Fig.~\ref{fig:exp_surf}





The decay transients in oxygen deficient toluene followed bi-exponential decay profiles (Figure \ref{fig:sigma_decay}). The first decay component had a lifetime in the range of \SIrange{37}{470}{\nano\second} ($\tau\sub{FL1}$) while the lifetime of second component was \SIrange{0.45}{10}{\micro\second} ($\tau\sub{FL2}$).
Both of these decays were quenched by oxygen. Initially \cite{Geng2017}, this, along with activation by temperature, was used to evidence that both states are thermally activated delayed fluorescence. 
\begin{figure}[ht]\centering
    \includegraphics[width=6cm]{figures_PDF/sigma_decays.pdf}
    \caption{Decay transients of molecules \bl{1}--\bl{3} in \SI{1.2e-5}{\Molar} toluene solution in the presence and absence of oxygen.}
    \label{fig:sigma_decay}
\end{figure}
The TADF emission lifetime should become more rapid with reduced \dEst\ as the rISC rate is directly dependant on the gap. However, an opposite trend can be seen from the table \ref{tab:sigma_tol}  for \tu{FL1}. The lifetime of first fluorescence is decreasing as the \dEst\ increases, but is smaller for larger oscillator strength (\textit{f}). This is expected for prompt fluorescence - the molecules with the strongest donor and largest HOMO--LUMO decoupling has the longest decay lifetime and vice-versa.   
On the other hand, the decay lifetime of the second fluorescence \tu{FL2} (\SIlist{4.5;10;0.45}{\micro\second}) increases with increasing \dEst\ (\SIlist{60;70;180}{\milli\eV} for compounds \textbf{1} and \textbf{2}, which is inline with TADF emission. The small lifetime \tu{FL2} of compound \textbf{3} can be explained by vastly larger \dEst, which makes TADF emission very weak. Moreover the DF/PF ratios are also inline with the energy gap \dEst, if we consider FL1 as prompt fluorescence and FL2 as delayed emission. 


\begin{table}[]
\centering
\caption{Emission properties of D-\textsigma-A compounds in oxygen-free \SI{1.2e-5}{\Molar} toluene solution.}
\label{tab:sigma_tol}
\begin{tabular}{ccccccc}
\hline
\upstrut{}  & $\tau\sub{FL}$ (\si{\nano\second}) & $\tau\sub{TADF}$ (\si{\micro\second}) & DF/PF & \textit{f}      & \dEst\ (\si{\milli\eV})  \\ [0.5ex]
  \hline
1 & 470      & 4.5        & 0.03  & 0.0008 & 60 \\
2 & 250      & 10         & 0.015 & 0.0012 & 70 \\
3 & 37       & 0.45       & 0.002 & 0.0054 & 180 \\ 
\end{tabular}
\end{table}


The increase of \tu{FL1} in low temperatures was used as a proof of TADF nature  for initial decay \cite{Geng2017}. However, the low temperature measurements were carried out in toluene solutions, above toluene freezing point, and the variation of non-radiative recombination rate was not neglected. 
%where the viscosity of solution is influenced by temperature.
Contrary, after performing temperature dependence measurements in rigid PMMA matrix where non-radiative pathways are mostly impeded -- no temperature activation was observed.

All the evidence are implying that the first emission \tu{FL1} is prompt fluorescence and the second emission \tu{FL2} is of TADF nature.
The long decay times of \tu{FL1} and susceptibility to quenching by oxygen are not typical for prompt emission. However the minuscule oscillator strengths prolongs the fluorescence lifetime enough to be quenched by oxygen \cite{Qu2019,Zhang2016c}.


\FloatBarrier
\section[Dual emission in solutions]{Dual emission in solutions}
\label{sec:sec_chtwo_two}
\FloatBarrier




Even though a single emission band was observed in time-integrated spectra, time-resolved analyses revealed a more complex picture. 

\begin{figure}[ht]\centering
    \includegraphics[width=6cm]{figures_PDF/sigma_trs_tol.pdf}
    \caption{Time resolved spectra of compounds \bl{1}--\bl{3} in oxygen free toluene at concentraion of \SI{1.2e-5}{\Molar}. The numbers denote to initial and last delay times.}
    \label{fig:sigma_trs}
\end{figure}

Two different CT bands can be depicted at different time scales in figure \ref{fig:sigma_trs}. The initial CT fluorescence band \CT{1}{1} is observed at about \SIrange{445}{465}{\nano\meter} up to about \SIlist{4;2;0.5}{\micro\second} for compounds \bl{1}--\bl{3}, respectively. Then later, a second \CT{1}{2} band emerges at \SIrange{475}{505}{\nano\meter}. 
Though a clear second band is observed only at later times, a long-wavelength shoulder is visible at all delay times, suggesting that the \CT{1}{2} is present from the beginning, however it is overwhelmed by a stronger \CT{1}{1} emission.

While both bands are clearly CT emission, as the peak wavelength depends on the strength of donor (the emission is redshifted with stronger donors), the nature of second band is not clear. Several possibilities can be considered: dynamic molecular conformer states, emerging after reorganization of molecular structure \cite{Woon2019}, or diffusion led, intermolecular interaction of donor and acceptor units \cite{Sarma2018}.




\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/sigma_kl_h.pdf}
    \caption{Decay transients of compound \bl{1} in oxygen-free: a) toluene solutions of different viscosity (the viscosity was increased by introducing PMMA polymer at different concentrations); b) toluene solutions at different concentrations of \SIlist{1.2e-5;1e-4}{\Molar}.}
    \label{fig:sigma_visc}
\end{figure}




The nature of the second \CT{1}{2} component was investigated by obtaining fluorescence decay transients in toluene solutions with varied viscosity (Figure \ref{fig:sigma_visc}a). The increased ambient viscosity reduced the \tu{FL} slightly from \SIlist[list-pair-separator={ to }]{470;280}{\nano\second}, which can be attributed to impeded non-radiative decay. However, the TADF emission was almost completely suppressed in viscous solution, reducing DF/PF ratio from 0.15 to 0.003 (measured at peak wavelength of \CT{1}{2} band).

Moreover, the decay kinetics of compound \bl{1} in different concentrations of \SIlist{1.2e-5;1e-4}{\Molar} of toluene solution were compared (Figure \ref{fig:sigma_visc}b). 
While the prompt fluorescence remained relatively the same, the TADF part was clearly enhanced as the DF/PF ratio increased from \num{0.18} in lower concentration to \num{0.49} in higher concentration.
The emission wavelength was also shifted from \SIlist[list-pair-separator={ to }]{475;485}{\nano\meter} for increased concentration as the fraction of \CT{1}{2} increased. 


Strong dependence on the concentration and viscosity suggest that the process in question is diffusion determined, therefore the nature of the second band was attributed to delayed fluorescence from exciplex. Formation of dimers was neglected as were no changes present in absorption spectra.


\FloatBarrier
\section[Emission origin in solid state]{Emission origin in solid state}
\label{sec:sec_chtwo_three}
\FloatBarrier




To fully quench the exciplex emission the compounds were incorporated into rigid PMMA polymer matrix, where diffusion is suppressed. Emission from isolated molecules were ensured by low concentration of \SI{1}{wt\%}. 

\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/sigma_pmma.pdf}
    \caption{a) Integrated fluorescence spectra; b) decay transients; c) time-resolved spectra of compounds \bl{1}--\bl{3} in oxygen deficient \SI{1}{wt\%} PMMA films.}
    \label{fig:sigma_pmma}
\end{figure}


The fluorescence spectra, as in dilute solutions, exhibited intramolecular CT emission which peaked at \SIlist{460;455;445}{\nano\second} for compounds \bl{1}--\bl{3}, respectively (Figure \ref{fig:sigma_pmma}a).
The decay transients in thin films were also similar to those in dilute solutions with bi-exponential decay profile, however they were greatly disfigured due to the presence of conformational disorder (Figure \ref{fig:sigma_pmma}b). Though approximating the decay lifetimes was obstructed by multiexponential profile, the same pattern as for diluted solutions were observed for prompt fluorescence -- its lifetime increased with increasing donor strength. The prompt fluorescence was also unaffected by low temperatures as expected for prompt emission.
The second component was quenched by oxygen and was activated by temperature therefore was attributed to TADF. 
The DF/PF ratios of \numlist{1.08;0.92;0.6} for compounds \bl{1}--\bl{3}, respectively, showed increased intensity of TADF emission, due to impeded non-radiative decay in rigid surrounding.

Large temporal shifts, discussed in the previous chapter, are observed in time-resolved spectra of PMMA films (Figure \ref{fig:sigma_pmma}c). The smallest redshift was observed for the \bl{2} compound as it possess the most sterically hindered tetramethyl-carbazole donor unit. 
As expected, the \CT{1}{2} band was absent in thin films -- confirming  their exciplex nature.
As the non-radiative decay and diffusion are suppressed  in rigid polymer matrix, both prompt and delayed emissions originate from the same intramolecular charge transfer state.


To thoroughly investigate the exciplex emission, neat films of compounds \bl{1}--\bl{3} were analyzed.
The fluorescence spectra were of charge transfer nature, but redshifted by about \SIrange{140}{200}{\milli\eV} compared to both thin films and dilute solutions, peaking at \SIlist{495;482;470}{\nano\meter} for compounds \bl{1}--\bl{3}, respectively. The redshift is not unusual in neat films and is mainly caused by intermolecular interactions \cite{Zhang2019a,Cotts2017,Serevicius2017,Zhang2015e}.
In our case, the integrated fluorescence spectra of neat film almost perfectly coincides with \CT{1}{2} band observed in toluene solution and, therefore, is attributed to exciplex formation (Figure \ref{fig:sigma_neat} a,b and c).
Unsurprisingly, the exciplex emission dominates in tightly-packed molecular environment of in neat film.


\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/sigma_neat.pdf}
    \caption{Integrated emission spectra (a-c) and decay transients (d-f) of compounds \bl{1}--\bl{3} in neat film, \SI{1.2e-5}{\Molar} toluene solution and \SI{1}{wt\%} PMMA film. Time resolved spectra of \CT{1}{2} state in toluene solution is also presented in (a-c).}
    \label{fig:sigma_neat}
\end{figure}


The decay transients of compounds \bl{1}--\bl{3} in neat films are depicted in figures \ref{fig:sigma_neat} d, e, and f along the decay kinetics in \SI{1.2e-5}{\Molar} toluene solutions and \SI{1}{wt\%} PMMA film. Interestingly, the initial prompt fluorescence closely resembles the decay kinetic in thin film (except compound \bl{3}) -- lowered \tu{FL} perturbed by conformational disorder is observed.
Meanwhile, the second (TADF) component is very similar to that of the dilute solution. It is not disturbed by conformational disorder and even shows comparable decay lifetime, supporting the assumption that they both arise from exciplexes. 
There was no TADF emission present in compound \bl{3}, as the weak delayed fluorescence, observed in toluene solutions, was probably quenched at defect sites.
The TADF emission in neat films was greatly enhanced compared to 
toluene solution due to large concentration of TADF-active exciplex states with DF/PF ratios of \numlist{1.63;1.44} for compounds \bl{1} and \bl{2}, respectively. The TADF nature of the second component was also confirmed by activation with temperature.



\FloatBarrier
\section[Optimization of intra/intermolecular TADF]{Optimization of intra/intermolecular TADF}
\label{sec:sec_chtwo_four}
\FloatBarrier




As intramolecular TADF emission is dominant in diluted films and intermolecular TADF fluorescence is prevailing in neat films -- a gradual interplay of these processes was observed by tuning the doping concentration (in the range \SIrange{0.01}{100}{wt\%}) of PMMA films.
Figure \ref{fig:sigma_dope}a shows how the peak wavelength shifts as the balance of intra-intermolecular TADF is shifted. 
At low doping concentrations (\SIrange{0.01}{2}{wt\%}) (when no intermolecular interactions can occur) the emission peaked at \SI{460}{\nano\meter}, similarly to \CT{1}{1} emission. The wavelength is redshifted in toluene solution due to the larger ambient dipole moment. The emission wavelength peak gradually increased up to \SI{490}{\nano\meter} at \SI{30}{wt\%}, as the portion of exciplex state rises. In the range \SIrange{30}{100}{wt\%} the emission wavelength remains stable as the exciplex emission is dominant.


\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/sigma_dope2.pdf}
    \caption{a) Integrated emission wavelength peak of compound \bl{1} at different doping concentrations in PMMA film. Horizontal lines denote to \CT{1}{1} and \CT{1}{2} peak wavelengths. b) Normalized fluorescence decay transients of compound \bl{1} in PMMA at different doping concentrations. Inset depicts the DF/PF ratio at different doping loads.}
    \label{fig:sigma_dope}
\end{figure}

%a) Integrated emission wavelength peak of compound \bl{1} at different doping concentrations in PMMA film. Horizontal lines denote to \CT{1}{1} and \CT{1}{2} peak wavelengths. b) Normalized fluorescence decay transients of compound \bl{1} in PMMA at different doping concentrations. Inset depicts the DF/PF ratio at different doping loads.

Similar trend was observed in fluorescence decay transients. Intramolecular TADF, highly disturbed by conformation disorder, is dominant in concentrations \SIrange{1}{5}{wt\%} (Figure \ref{fig:sigma_dope}b). However increasing concentration give rise to undistorted and more efficient exciplex delayed fluorescence. The DF/PF ratio peaks at about \SI{20}{wt\%} concentration, as further increase of doping load enhance excitation mobility leading to quenching of the latest delayed emission at defect sites. The TADF lifetime also decreases at larger doping concentrations due increased quenching of the latest states. 
Therefore the DF/PF ratio can be tuned by selecting appropriate doping concentration in order to achieve the most efficient delayed fluorescence.











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\section*{Summary of the results}
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A series of novel design donor-\textsigma-acceptor TADF compounds with different donor units were analyzed. Minimization of HOMO-LUMO orbital overlap, in order achieve negligible \dEst\ values, was realized through hyperconjugated hexafluoroisopropylidene spacer.
This resulted in weak donor-acceptor coupling leading to several unusual effects. 
First, the prompt CT fluorescence outlined with a remarkably prolonged decay lifetime, which was susceptible to molecular oxygen. Due to unique characteristics it was initial misidentified as TADF emission. 
Second, coexisting intra- and intermolecular fluorescence of different wavelengths were observed at different timescales even in dilute solutions. The second emission was proven to originate from exciplex states.
In dilute polymer films with suppressed non-radiative recombination, however, both prompt and delayed emission were of intramolecular nature.
Lastly, the exciplex TADF emission dominated in neat films, however the optimal doping concentration of \SI{20}{wt\%} was established as further increase of doping load enabled excitation migration towards the defect sites subsequently quenching TADF emission.

The work presented in this chapter emphasises that the weak donor-acceptor coupling is not favorable for efficient intramolecular TADF and \textsigma-bridged compounds should be designed with great care. 









