\chapter[\uppercase{Towards efficient blue TADF emitter}]{\uppercase{Towards blue TADF emitter}}
\label{chap:chap_three}

This chapter summarizes the research published in papers~\citepapernumTHREE{PXAPYR}{DeepBlue}{Cbz} and presents several ways of optimizing emitter structures in order to produce fast and efficient blue TADF emission.


\FloatBarrier
\section*{Motivation}
\FloatBarrier

The recent advances in OLED technology allowed successful optimization of TADF compounds producing devices with external quantum yields approaching \SI{40}{\%} without applying any artificial light extraction techniques (only spontaneous molecular orientation) \cite{Lin2016,Cui2020,Wu2018}. 
However, large EQE values are usually displayed only at low brightness as their yield substantially reduces at high luminance  due to the pronounced  quenching by triplet-triplet, singlet-triplet and similar annihilation pathways \cite{Inoue2016,Murawski2013}. Shortening the TADF lifetime has been shown to be a successful strategy for lowering EQE roll-off \cite{Inoue2016, Tanaka2013, Im2017}.
While faster rISC rate can be achieved by spatially separating HOMO and LUMO orbitals, this also leads to reduction of \kr{r} which in turn reduces quantum yield of both prompt and delayed fluorescence. Low rate of singlet radiative decay is also harmful for lifespan of TADF devices, as the increased singlet exciton density in the light-emitting layer boosts the rate of unwanted singlet–singlet annihilation, increasing the molecular decomposition rate \cite{Liu2018a}. 
Moreover, TADF efficiencies are substantially lower in the deep-blue region (CIE\tb{y} < 0.1), desirable for colored OLED displays \cite{Cai2018}, as the EQE for such devices rarely exceeds \SI{20}{\percent} margin \cite{Lee2018,Ahn2019,Cui2017a}.
The high requirements of short conjugation length, high triplet energy and appropriate D--A spatial structure essential for deep-blue TADF are not easily satisfied. 
%Therefore controlling radiative and non-radiative rates is of crucial importance for TADF emitters.



In this chapter several optimization strategies are employed:
selectively adjusting lowest singlet energy, to achieve deep-blue emission,
control of radiative and non-radiative decays through linking pattern of donor units and fine-tuning of rISC rate by small methyl modifications. Emitters with electroluminance colors ranging from green (\SI{536}{\nano\meter}, CIE (0.35, 0.56)) to deep-blue (\SI{441}{\nano\meter}, CIE (0.16, 0.12)) were produced and peak efficiency of \SI{29.1}{\%} with low roll-off was reached.

\iffalse


\fi


\iffalse

Pananalizavom teorija, bla bla bal, yra poreikis, melynu, greitoss gyvavimos trukmes, reikia pritaikyti zinias.
INTRO



Although the highest external quantum yields are reported for green and sky-blue TADFs are reported, the high values of EQE are usually observed only at low luminance as efficiency drops considerably at higher luminance due to quenching by TTA and STA.
Reducing TADF lifetime proved to be a successful stratagy to lower EQE roll-off \cite{Im2017, Inoue2016, Tanaka2013}.

As established before, reducing the TADF lifetime is of extreme importance, due to several reasons




Parametrai daznai tik ties mazais lumimance, nes didelis rollofas

Norint sumazinti rolloffa is conformaciju itaka reikia sumazinti TADF lifetima

Sumazint lifetima reikia sumazinti energiju tarpus tarp singleto tripleto \cite{Gibson2016,DosSantos2018}.

Tam pasiekti reikia atitinkamo D ir A poros reikiamoje geometrijoje.



\fi

\FloatBarrier
\section[Achieving deep-blue emission]{Achieving deep-blue emission}
\label{sec:sec_chthree_one}
\FloatBarrier





\FloatBarrier
%\subsection*{Motivation}
\FloatBarrier


%High requirements of short conjugation length, high triplet energy and appropriate D-A spatial structure are not easily satisfied. 
%To retain  deep-blue emission of the compound the conjugation  must be small to ensure large enough HOMO--LUMO bandgap. 
In order to produce deep-blue emitters, usually dihydroacridine and carbazole donors are bound to sulphone and nitrogen containing acceptor moieties \cite{Cai2019}.
However, pyrimidine-carbazole compounds are rarely being explored as TADF emitters, despite being successfully applied as high triplet energy hosts \cite{Wu2018a,Li2018}.

\begin{figure}[ht]\centering
    \includegraphics[width=8cm]{figures_PDF/UV.pdf}
    \caption{Molecular strunctures of base \CbPy\ and modified version \tCbPy.}
    \label{fig:uv_mol}
\end{figure}

% Cia sita sakini kazkaip perrasyt reik
Seeing the potential of such compounds a carbazole-pyrimidine (\CbPy), displaying high energy triplet and the presence of RTP, which is usually a sign of favorable energy structure for TADF \cite{Huang2017}, was selected as a candidate for molecular modifications. 
Based on the knowledge acquired and described in the first chapter, the following modifications were performed in order to achieve TADF while maintaining deep-blue emission (Figure \ref{fig:uv_mol}). Firstly, the donor carbazole units were extended with 3,6-di-\textit{tert}-butyl in order to increase its donor strength. Secondly, a methyl group at the 5\ts{th} position of pyrimidine was introduced to increase steric hindrance  and lower the HOMO and LUMO overlap. Finally, a heavy sulphur atom was attached to acceptor unit to enhance the ISC and rISC rates and redshift the emission peak.



\FloatBarrier
\subsection{Evaluation of emitter properties}
\FloatBarrier

The initial DFT calculations revealed optimistic results (Figure \ref{fig:uv_dft}). 
The introduction of methyl group at 5\ts{th} position of pyrimidine unit increased the twist angle between the donor and acceptor units from \SI{37}{\degree} to \SI{48.8}{\degree}. 
This, in conjunction with enlarged donor strength by \textit{tert}-butyl, ensured the separation of HOMO and LUMO orbitals, confirmed by reduction of both \dEst\ and oscillator strength (from \SIlist[list-pair-separator={ to }]{540;289}{\milli\eV} and \numlist[list-pair-separator={ to }]{0.558;0.394}, respectively). The almost two times reduction of \dEst\ was achieved by selectively altering lowest singlet energy S\tb{1} (from \SIlist[list-pair-separator={ to }]{3.69;3.43}{\eV}), while keeping the lowest triplet energy T\tb{1} relatively the same (from \SIlist[list-pair-separator={ to }]{3.15;3.14}{\eV}).

%----------DFT paveikslas----------------------
\begin{figure}[ht]\centering
    \includegraphics[width=10cm]{figures_PDF/UV_DFT.png}
    \caption{Electron densities of HOMO and LUMO, their energies, lowest singlet and lowest triplet energies, oscillator strengths and \dEst\ gaps and torsion angle of D-A of compounds \CbPy\ and \tCbPy.}
    \label{fig:uv_dft}
\end{figure}
%----------DFT paveikslas----------------------




The success of modifications were clearly visible in photophysical properties of the compounds.
The absorption of \tCbPy\ was two folds lower compared to \CbPy\ due to reduced oscillator strength, as a consequence of smaller HOMO and LUMO overlap, and peaked at wavelengths \SIlist{339;336}{\nano\meter}, respectively.
The fluorescence peaked at \SIlist{379;423}{\nano\meter} for compounds \CbPy\ and \tCbPy, respectively. The enlarged conjugation of donor unit together with sulphur atom redshifted the emission. In solid state, the emission peak of compound \tCbPy\ was blueshifted to \SI{415}{\nano\meter} when embedded in \SI{1}{wt\%} PMMA film, while in \SI{10}{wt\%} DPEPO host, the emission was redshifted to \SI{428}{\nano\meter} according to emission spectra in toluene. These shifts were mediated by different polarity of the hosts as well as conformational disorder.
\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/UV_fl.pdf}
    \caption{Absorbance, fluorescence emission and phosphorescence spectra of compounds \CbPy\ and \tCbPy\ in \SI{1e-5}{\Molar} toluene solution, \SI{1}{wt\%} PMMA films, \SI{10}{wt\%} DPEPO film.  }
    \label{fig:uv_fl}
\end{figure}
The phosphorescence shows vibronic resolution due to originating from donor unit and being of \ce{^3LE} nature. The peak of 0-0 transition remains relatively unchanged in both \CbPy\ and \tCbPy\  (\SIlist{416;417}{\nano\meter}, respectively), resulting in significantly lower \dEst\ for compound \tCbPy, especially when embedded in DPEPO host (\SI{290}{\milli\eV}).

Fluorescence quantum yield (\QY{F}) in dilute solutions were similar for both \CbPy\ and \tCbPy\ -- \numlist{0.1;0.07}, respectively, though minor decrease of yield was attributed to changes of radiative and non-radiative decay rates. The \kr{r} was reduced \num{2.6} times due to smaller \textit{f} of CT emission, however it was offset by a \num{1.8} time slower non-radiative decay due to increase steric hindrance of the compound \tCbPy.

No TADF was observed in, even oxygen-free, toluene solutions, typical for deep-blue TADF emitters \cite{Li2019a,Wong2018}.
However strong TADF emission was observed in \SI{1}{wt\%} PMMA and \SI{10}{wt\%} DPEPO films of \tCbPy\ where non-radiative decay is remarkably impeded. The TADF nature was implied due to observed activation by temperature and susceptibility to molecular oxygen.
Meanwhile, \CbPy\ displayed only weak RTP as the triplet states were unable to upconvert by virtue of high \dEst\ gap.
Fluorescence quantum yields were enhanced to \numlist{0.44;0.5} in oxygen free PMMA and DPEPO films, with \QY{TADF} amounting to \num{0.42} yield.

Though the TADF decay was moderately distorted by conformational disorder, a multi-exponential fit allowed to estimate the TADF lifetime of about \SI{143}{\micro\second} and rISC rate of $k\sub{rISC} \approx \SI{3e4}{\per\second}$ of compound \tCbPy\ in \SI{10}{wt\%} DPEPO film, comparable to to some of the other deep-blue TADF emitters \cite{Cui2017}.

\iffalse

\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/UV_decay.pdf}
    \caption{Decay transients of \tCbPy\ in oxygen free \SI{10}{wt\%} DPEPO film with various efficiencies and rates specified inset.}
    \label{fig:uv_decay}
\end{figure}

\fi


\FloatBarrier
\subsection{Deep-blue OLED device}
\FloatBarrier

The performance of \tCbPy\ was evaluated in the following device configuration: ITO/TAPC (\SI{30}{\nano\meter})/TcTa (\SI{5}{\nano\meter})/\SI{10}{wt\%} \tCbPy:DPEPO (\SI{20}{\nano\meter})/DPEPO (\SI{5}{\nano\meter})/TmPyPb (\SI{50}{\nano\meter})/LiF (\SI{0.8}{\nano\meter})/Al (\SI{100}{\nano\meter}).
The device showed deep-blue electroluminescence which peaked at \SI{441}{\nano\meter} with CIE coordinates (0.16, 0.12) at luminescence of \SI[per-mode=symbol]{100}{\candela\per\centi\meter\squared} (Figure \ref{fig:uv_oled}). The external quantum efficiency of \SI{8.7}{\%} was evaluated, comparable to other efficient deep-blue OLED emitters \cite{Zhang2018,Jurgensen2017,Rajamalli2019}.  However the efficiency suffered from pronounced roll-off at higher luminescence values, due to slow decay lifetime of TADF and low electroactivity of DPEPO host, though this problem is frequent with other deep-blue TADF OLEDs \cite{Chan2018}.





\begin{figure}[ht]\centering
    \includegraphics[width=11cm]{figures_PDF/UV_oled.pdf}
    \caption{a) External quantum efficiencies of OLED devices with \tCbPy\ as emitter at different luminance values. Inset the emission spectra of the same OLED device at luminance of \SI[per-mode=symbol]{100}{\candela\per\meter\squared}. b) Photo of manufactured device.}
    \label{fig:uv_oled}
\end{figure}

\FloatBarrier
\subsection*{Summary of the results}
\FloatBarrier

To summarize, an approach to design deep-blue TADF emitter was presented. Carbazole--pyrimidine--carbazole compound displaying high triplet energy and RTP was modified by increasing electron donating properties of donor fragment, increasing twist angle of D-A by methyl group and enhancing ISC and rISC via heavy sulphur atom. The modifications resulted in greatly reduced \dEst\ enabling efficient ($\Phi\sub{F} = 0.5$) TADF emission with an average lifetime of \SI{147}{\micro\second} in \SI{10}{wt\%} DPEPO film.
OLED device produced with \tCbPy\ emitter exhibited deep-blue emission with CIE coordinates (0.16, 0.12) and comparable high EQE of \SI{8.7}{\%}.
Modification of promising carbazole-pyrimidine compound proved to be a simple, yet successful strategy for designing efficient deep-blue TADF emitter.

These results are presented in more detail in paper \citepapernumONE{DeepBlue}.


In the next section, the deep-blue emission will be sacrificed in order to gain higher efficiency.










\FloatBarrier
\section[Control of decay rates through linking patterns]{Control of decay rates through linking patterns}
\label{sec:sec_chthree_two}
\FloatBarrier
%Fig.~\ref{fig:exp_surf}



\FloatBarrier
%\subsection*{Motivation}
\FloatBarrier



Another strategy to reduce \dEst\ was deployed on \CbPy\ core. Firstly, the phenyl spacer was introduced between donor and acceptor unit to decrease HOMO--LUMO overlap (\CPa) (Figure \ref{fig:Cbz_mol}), however the redshift of emission, due to increased conjugation, is expected. A further reduction of \dEst\ was sought by altering linking pattern of carbazole donor, when two donor units were attached to acceptor core at \meta\ positions of phenyl spacer to produce \CPb. Finally, a third donor unit at \textit{para} position was crammed in order to increase steric hindrance due to crowding effect (similarly to \CzPN\ compound discussed in chapter \ref{chap:chap_one}). The increased rigidity should suppress non-radiative decay pathways therefore enhancing TADF emission properties and reducing unwanted effects of conformational disorder in solid-state.



\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/Cbz.pdf}
    \caption{Molecular structures of phenyl-pyrimidine acceptor decorated by one (\CPa), two (\CPb) or three (\CPc) carbazole donor units.}
    \label{fig:Cbz_mol}
\end{figure}



\FloatBarrier
\subsection{Quantum chemistry}
\FloatBarrier


The initial DFT simulations revealed that the phenyl-pyrimidine unit is rather flat in all compounds with dihedral twist angles in the range \SIrange{14.5}{16.5}{\degree}, with the molecules containing more bulky donor fragments showing slightly lower angles (Figure \ref{fig:cbz_dft}).
The angles between carbazole donor units and phenyl spacer were more perpendicular as they ranged from \SI{52}{\degree} for \CPa\ to  \SIrange{56.5}{67.8}{\degree} for \CPc, with the \meta\ carbazole units  having larger angle compared to \para\ units.
\begin{figure}[ht]\centering
    \includegraphics[width=11cm]{figures_PDF/Cbz_DFT.png}
    \caption{a) Optimized geometries of carbazole-pyrimidine TADF compounds and dihedral angles between phenyl-pyrimidine ($\alpha_1$) and carbazole-phenyl ($\alpha_2$); Energies and electron distributions of HOMO and LUMO orbitals as well as estimated oscillator strength and \dEst\ values are denoted.  }
    \label{fig:cbz_dft}
\end{figure}
As expected, introduction of more carbazole units reduced LUMO level (\SIlist{-1.99;-2.22;-2.37}{\eV}, respectively) while preserving same HOMO energy (from \SIlist[list-pair-separator={ to }]{-5.49;-5.52}{\eV}) therefore energy bandgap was reduced due to increased conjugation and promoted CT character. This should result in a redshift of emission for compounds with more donor untis.



The HOMO--LUMO overlap decreased due to the phenyl spacer and \dEst\ was reduced for compound \CPa\ compared to \CbPy\  (\SIlist{290;540}{\milli\eV}), yet not low enough for efficient TADF. The compound with two donor units showed most decoupled HOMO--LUMO orbitals and lowest \dEst\ of \SI{100}{\milli\eV} with smallest oscillator strength (\num{0.017}), while introduction of \para\ carbazole reinforced the HOMO--LUMO overlap leading to \dEst\ of \SI{140}{\milli\eV} and oscillator strength of \num{0.092}.


\FloatBarrier
\subsection{Evaluation of emission properties}
\FloatBarrier


The effects of increased conjugation were evident  in absorption and emission spectra of dilute toluene solutions, where direct CT absorption band peak shifted from \SIlist[list-pair-separator={ to }]{355;378}{\nano\meter} and the fluorescence peaked at \SIlist{408;442;464}{\nano\meter} for compounds \CPa--\CPc, respectively (Figure \ref{fig:cbz_fl}).
Similar emission peak wavelengths of \SIlist{408;440;446}{\nano\meter} were observed for compounds \CPa--\CPc\ in \SI{1}{wt\%} PMMA films, where the slight blueshift of compound \CPc\ was probably mediated by lower polarity of PMMA and small conformational disorder of more rigid molecular core.
On the other hand, the blueshift of compounds \CPa\ and \CPb\ in lower ambient polarity, might be compensated by redshift of emission caused by conformational disorder, which is more pronounced in labile molecules. The emission of \SI{10}{wt\%} DPEPO film for compound \CPc\ showed more CT character and larger redshift due to increased polarity of the host.

% \SIlist[list-pair-separator={ to }]{408;464}{\nano\meter}


\begin{figure}[ht]\centering
    \includegraphics[width=8cm]{figures_PDF/Cbz_fl.pdf}
    \caption{Absorption, fluorescence and phosphorescence spectra of carbazole-pyrimidine compounds acquired in: \SI{1e-5}{\Molar} toluene solution (solid lines), \SI{1}{wt\%} PMMA films (dashed lines), \SI{10}{wt\%} DPEPO films (orange solid line for \CPc). Phosphorescence spectra (open dots) were acquired at \SI{10}{K} in PMMA films.}
    \label{fig:cbz_fl}
\end{figure}


Phosphorescence spectra acquired at \SI{10}{K} with \SI{100}{\micro\second} delay showed vibronic structure and was, therefore, attributed to \TLE\  state. 
The evaluated \dEst\ values in PMMA films  were \SIlist{480;270;320}{\milli\eV} for compounds \CPa--\CPc, respectively.  
The singlet-triplet gap  followed the same trend as predicted by DFT calculations: introduction of second donor unit lowered the \dEst{} gap, while, for compound with three donor units, \dEst{} increased due to somewhat boosted HOMO--LUMO coupling through \para-carbazole. 
The \dEst{} for compound \CPc\ was further lowered to \SI{110}{\milli\eV} when embedded in more polar \SI{10}{wt\%} DPEPO host.


To evaluate the effects of conformational disorder, time-resolved spectra of compounds \CPb\ and \CPc\ were recorded and are presented in figure \ref{fig:cbz_trs}. 
As expected a large temporal redshift of \SI{380}{\milli\eV} is observed for flexible compound \CPb, followed by a blueshift of \SI{310}{\milli\eV}, therefore the latest emissive states are quenched. Meanwhile the more sterically constrained compound \CPc\ showed remarkably smaller redshift of \SI{240}{\milli\eV} followed by a blueshift of \SI{190}{\milli\eV}. The smaller difference between shifts suggest lower quenching of the latest states. 
\CPa, on the other hand, showed no delayed emission due to large \dEst{}.



\begin{figure}[ht]\centering
    \includegraphics[width=8cm]{figures_PDF/Cbz_trs.pdf}
    \caption{Time-resolved spectra of compounds \CPb\ (a) and \CPc\ (b) in \SI{1}{wt\%} PMMA film. Red and blue horizontal arrows depicts redshift of prompt emission and blueshift of delayed emission, respectively.}
    \label{fig:cbz_trs}
\end{figure}


The decay transients for carbazole-pyrimidine compounds in \SI{1e-5}{\Molar} toluene solutions and \SI{1}{wt\%} PMMA films are presented in figure \ref{fig:cbz_decay}.
Both \CPb\ and \CPc\ showed bi-exponential decay of prompt and delayed emission while for compound with the largest \dEst\  \CPa\ only prompt emission was observed. The delayed emission was proven to be TADF due to susceptibility to molecular oxygen and activation with temperature.


\begin{figure}[ht]\centering
    \includegraphics[width=8cm]{figures_PDF/Cbz_decay.pdf}
    \caption{Decay transients of compounds \CPa--\CPc in \SI{1e-5}{\Molar} toluene solutions (a) and \SI{1}{wt\%} PMMA films (b). Decay kinetic of \CPc\ in \SI{10}{wt\%} DPEPO film is also presented as a solid green line.}
    \label{fig:cbz_decay}
\end{figure}


The decay lifetime of prompt emission was fastest for \CPa\ with strongest oscillator strength and LE type emission, while compounds with bulkier donor fragments showed remarkably slower decay in both solution and PMMA ambient.
Assessed rates of prompt and delayed emission in toluene solutions, together with efficiencies are presented in table \ref{tab:cbz_tadf}.

A remarkably low \kr{r} of \SI{0.7e7}{\per\second} was registered for \CPb, with largest HOMO--LUMO decoupling. This, in conjunction with largest non-radiative decay rate of \SI{11e7}{\per\second}, resulted in lowest \QY{PF} of 0.057. In contrast, rapid radiative decay rate (\SI{2.9e7}{\per\second}), one of the largest for TADF compounds \cite{Liu2019a}, together with moderate \kr{nr} (\SI{3.8e7}{\per\second}) ensured an exceptional prompt fluorescence yield of 0.43 for compound \CPc.
The \QY{PF} and \kr{fl} remained relatively the same in rigid PMMA film. This led to similar \kr{nr} values in both surroundings, meaning that non-radiative decay pathway occurring through vibrations of molecular core, could be neglected and the dominating non-radiative decay could be assigned to ISC \cite{Kreiza2019}. 

\begin{table}[]{\centering
\setlength{\tabcolsep}{5pt}
\caption{Fluorescence and TADF properties for carbazole-pyrimidine compounds in \SI{1e-5}{\Molar} toluene solutions.}
\label{tab:cbz_tadf}
\begin{tabular}{cccc}
\hline
\multicolumn{1}{l}{} & \kr{fl}/\kr{r}/\kr{nr}  & \kr{TADF}/\kr{ISC}/\kr{rISC}/$k\sub{nr}^{T}$ & \multirow{2}{*}{\QY{PF}/\QY{DF}/\QY{ISC}/\QY{rISC}} \\
\multicolumn{1}{l}{} & (\SI{1e7}{\per\second})  & (\SI{1e7}{\per\second})              &                                      \\
\hline
\textbf{1CbzPYR}     & 26.0/22.0/3.9        & --/3.9/--/--                         & 0.85/--/0.15/--                        \\
\textbf{2CbzPYR}     & 11.0/0.7/11.0        & $2.2^{a}$/11.0/$1.2^{b}$/$2.2^{a}$               & 0.057/0.003/0.94/0.05                \\
\textbf{3CbzPYR}     & 7/2.9/3.8            & $0.7^{a}$/3.8/$1.9^{b}$/$0.6^{a}$                & 0.43/0.07/0.57/0.25                 
\end{tabular}}


{\footnotesize $^{a} \times \SI{1e5}{\per\second}$.  $^{b} \times \SI{1e4}{\per\second}$.} 
\end{table}




Similar trends were observed for delayed emission, the compound with larger prompt fluorescence yield, \CPc, showed more efficient delayed fluorescence. Meanwhile \CPb, with low molecular rigidity, displayed high non-radiative triplet quenching ($k\sub{nr}^T$) which dominated, being about 18 times higher than \krISC, and resulted in negligible \QY{DF}. Moreover, low \kr{r} rate in conjunction with high \kr{ISC} rate (\SI{0.7e7}{\per\second} \vs \SI{11e7}{\per\second}, respectively) ensured that majority of upconverted excitons would cycle back to triplet states via ISC until they decay non-radiatively through $k\sub{nr}^T$.
For \CPc, however, the non-radiatively triplet pathway was suppressed by steric confinement of carbazole units. Together with high \kr{r}, this ensured delayed fluorescence yield up to 0.07.

Further increase of \QY{DF} was achieved in PMMA films. Here, the suppression of $k\sub{nr}^T$ increased TADF yield to \numlist{0.16;0.33} and \QY{PL} to \numlist{0.22;0.81} for compounds \CPb\ and \CPc, respectively. Despite the suppressed non-radiative decay pathway \CPb\ still suffered from low \kr{r} and, therefore, the performance of this compound was still poor.

% Uzsiminti, kad T non-r mazesnis kur disorderis mazesnis



\iffalse

\begin{table}[]\centering
\caption{Fluorescence, radiative and non-radiative decay rates and quantum yields of prompt and TADF emission for carbazole-pyrimidine compounds in \SI{1}{wt\%} PMMA films.}
\label{tab:cbz_tadf}
\begin{tabular}{cccccc}
\hline
                 & \kr{fl}    & \kr{r}     & \kr{nr}   & \multirow{2}{*}{\QY{PF}} & \multirow{2}{*}{\QY{TADF}} \\
                 & \multicolumn{3}{c}{(\SI{1e7}{\per\second})} &                                                               &                                                                 \\
                 \hline
\textbf{1CbzPYR} & 24.0     & 20.0     & 4.9   & 0.8                                                           & -                                                               \\
\textbf{2CbzPYR} & 14.0     & 0.8    & 13.0    & 0.06                                                          & 0.16                                                            \\
\textbf{3CbzPYR} & 7.9    & 3.8    & 4.1   & 0.48                                                          & 0.33                                                           
\end{tabular}
\end{table}


\fi





\FloatBarrier
\subsection{Device performace}
\FloatBarrier




Due to efficient TADF and fast radiative decay, compound \CPc\ was  exploited as emitter.
The OLED device was constructed with the following structure: ITO/TAPC (\SI{30}{\nano\meter})/TCTA (\SI{5}{\nano\meter})/\SI{10}{wt\%} \CPc:DPEPO (\SI{15}{\nano\meter})/TmPyPB (\SI{65}{\nano\meter})/LiF (\SI{0.8}{\nano\meter})/Al (\SI{100}{\nano\meter}).
Blue electroluminescence peaking at \SI{473}{\nano\meter} with CIE coordinates of (0.16, 0.23) was observed from fabricated device.
The OLED demonstrated exceptional peak external quantum efficiency of \SI{19.7}{\percent} which is inline with large \QY{F} assuming \SIrange{20}{30}{\percent} outcoupling efficiency.
Moderate EQE roll-off was observed at larger luminance due to the average solid-state TADF lifetime of \CPc\ and the weak electroactivity of the DPEPO host, though the latter issue could be solved by further optimization of the OLED architecture.




\begin{figure}[ht]\centering
    \includegraphics[width=8cm]{figures_PDF/Cbz_oled.pdf}
    \caption{External quantum efficiency at different luminance values of OLED device with \CPc\ emitter. Inset: electroluminance spectrum of device at \SI[per-mode=symbol]{850}{\candela\per\meter\squared}.}
    \label{fig:cbz_oled}
\end{figure}


\FloatBarrier
\subsection*{Summary of the results}
\FloatBarrier


To summarize, altering number and linking pattern of donor units in carbazole-phenylpyrimidine compounds proved to be an efficient way to optimize fluorescence decay rates and achieve efficient TADF emission. 

The singlet energies where tuned by introducing more carbazole units without altering the triplet energies much. This enabled TADF emission in compounds with two or three carbazoles, which was absent in compound with one \para-carbazole.
Prompt and delayed fluorescence properties, on the other hand, were drastically different for both compounds. 
Low overlap of HOMO--LUMO orbitals, in double \meta-carbazole substituted \CPb, drastically lowered radiative decay rate, which was outcompeted by rapid intersystem crossing and resulted in low emission yield.
Even with pronounced ISC rate, \CPb\ showed low TADF yield as the majority of triplets were quenched by the rapid nonradiative decay, promoted by rather flexible molecular structure. 
The non-radiative singlet and triplet decays were suppressed by introducing another \para-carbazole unit, boosting prompt and TADF emissions to 0.43 and 0.33, respectively. The stiffened core also reduced redshift induced by conformational disorder.
The compound \CPc\ with optimized donor-acceptor architecture was used as blue emitter in OLED device. Highly efficient electroluminescence with peak EQE of \SI{19.7}{\%} and colour coordinates of (0.16, 0.23) was achieved. 
The results are presented in more detail in paper \citepapernumONE{Cbz}.

In the next section, a different molecular structure with a known fast TADF lifetime  will be further improved.










\FloatBarrier
\section[Fine-tuning rISC rate by small methyl modifications]{Fine-tuning rISC rate by small methyl modifications}
\label{sec:sec_chthree_three}
\FloatBarrier

\FloatBarrier
%\subsection*{Motivation}
\FloatBarrier







Another popular donor fragment phenoxazine (PXZ), known for its strong electron donating properties, is commonly used to construct efficient emitters in sky-blue -- yellow color region \cite{Guo2019,Liu2019,Liu2016a,Lu2015,Tanaka2012a,Komatsu2017a,Xiang2017,Jiang2016}.
It was shown that PXZ, in conjunction with diphenyl-pyrimidine acceptor  unit (\PXZPYR), can exhibit fast ($\tau\sub{TADF} = \SI{2.56}{\micro\second}$) which can be utilized in devices with EQE of \SI{19.9}{\%} \cite{Jiang2016}.
Further modifications with aryl and phenyl groups on pyrimidine acceptor lowered the TADF lifetime to \SI{1.99}{\micro\second} in together enhancing the EQE up to \SI{24.6}{\%} \cite{Jiang2016,Komatsu2017a}.
\begin{figure}[ht]\centering
    \includegraphics[width=10cm]{figures_PDF/PXAPYR.pdf}
    \caption{Molecular structures of compounds \PXZPYR, \PXZu, \PXZd\ and \PXZm.}
    \label{fig:pxa_mol}
\end{figure}
Another strategy of incorporating a heavy Cl or Br atom to accelerate ISC and rISC resulted in enhanced \tu{TADF} of \SI{1.32}{\micro\second} and EQE of \SI{25.3}{\%} \cite{Xiang2017}, however the inclusion of heavy atom results in redshift of emission -- an unwanted effect when aiming for blue emitters \cite{Xiang2017,Kretzschmar2015}.
% Gal ikalt, kad su metilais galima panasiai padaryti tik su blueshiftu citata reiktu.

Inspired by good efficiency and rapid TADF we upgraded this compound by modifying \PXZPYR\ with either \ortm\ (\PXZd) or \metm\ (\PXZu), or both \meta{} and \orto{} methyl modifications (\PXZm) (Figure \ref{fig:pxa_mol}),  to increase \krISC\ while maintaining sky-blue -- green emission.










\FloatBarrier
\subsection{Tuning of decay rates}
\FloatBarrier


\iffalse

\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/PXA_DFT.png}
    \caption{.}
    \label{fig:pxa_dft}
\end{figure}

(Figure \ref{fig:pxa_dft})


\fi


The initial DFT calculations revealed that the angle between the donor fragment and phenyl spacer was almost orthogonal  with some slight variations (\SIrange{84.05}{89.62}{\degree}), for all compounds.  
On the other hand, the phenyl rings were only slightly twisted (\SI{19}{\degree}) in respect to pyrimidine unit for both \PXZPYR\ and \PXZu\ while introduction of \ortm\ unit increased the twist angle to nearly \SI{43}{\degree} due to enhanced steric hindrance. 
This also led to increase of LUMO energies from \SIlist{-2.04;-2.10}{\eV} for \PXZPYR\ and \PXZu, respectively, to  \SIlist{-1.82;-1.88}{\eV} for \PXZd\ and \PXZm, respectively, weakening the electron-accepting ability and enlarging bandgap, while maintaining HOMO energies relatively same (from \SIlist[list-pair-separator={ to }]{-4.79;-4.76}{\eV}). 
The enlarged badgap should result in blueshift of emission for compounds with methyl moiety.
The larger twist in \ortm\ modified compounds also led to reduced HOMO-LUMO overlap and reduction of oscillator strength from \numlist{0.0016;0.0013} for compounds \PXZPYR\ and \PXZu, respectively, to  \numlist{0.0002;0.0003} for \PXZd\ and \PXZm, respectively.


The absorption and emission spectra in \SI{1e-5}{\Molar} toluene solution are presented in figure \ref{fig:pxa_fl}a. 
Two absorption regions can be discerned: first at about \SIrange{280}{350}{\nano\meter}, which is comprised of individual absorption of donor and acceptor fragments, and second at about \SIrange{360}{450}{\nano\meter}, attributed to charge transfer state.
Fluorescence spectra of unmodified \PXZPYR\ peaked at \SI{543}{\nano\meter}, while the emission was blueshifted to \SIlist{530;528;519}{\nano\meter} for methyl modified compounds \PXZu, \PXZd, \PXZm, respectively, as predicted by DFT calculations. Therefore the introduction of methyl groups allowed us to tune CT emission wavelength for \SI{100}{\milli\eV}.


\iffalse

Quantum yield measurements revealed that the compounds without \metm\ fragment exhibit higher quantum yield (\num{0.1} \vs \num{0.15}). To understand the reasons behind that 
Quantum yields and variations of radiative and non-radiative decay rates are depicted in Table \ref{tab:pxz_tol_air}. It is apparent that compounds with the largest oscillator strength \PXZPYR\ and \PXZu displayed the most rapid radiative decay, however the \kr{r} was almost twice lower for \PXZu, resulting in decrease of emission yield. Simultaneously, borh \PXZPYR\ and \PXZu\ showed fastest non-radiative decays, due to most flexible structure, though introduction of \metm\ fragment lowered the non-radiative rate by about \SI{17}{\percent}.
Despite having lower \kr{r} due to smaller HOMO--LUMO overlap, the compounds \PXZd\ and \PXZm\ benefited from more rigid molecular core, decreasing the non-radiative decay rate by about \SI{60}{\%}. The benefits of rigid core counteracted the effects of reduced oscillator strength, allowing for \PXZd\ to achieve similar quantum yield as \PXZPYR.

\begin{table}[] \centering
\caption{Quantum yield, fluorescence, radiative and non-radiative decay rates of compounds: \PXZPYR, \PXZu, \PXZd, \PXZm\ in toluene solution.}
\label{tab:pxz_tol_air}
%\renewcommand{\arraystretch}{1.5}
\begin{tabular}{cccccc}
\hline
 \upstrut{} & \QY{F} & $k\sub{FL} (\SI{1e7}{\per\second})$ & $k\sub{r} (\SI{1e6}{\per\second})$ & $k\sub{nr} (\SI{1e7}{\per\second})$ \\ [0.5ex]
 \hline
\textbf{PXZ-PYR}      & 0.15                    & 7.5                                   & 11.3                                 & 6.4                                   \\
\textbf{PXZ-muPYR}    & 0.1                     & 6.1                                   & 6.1                                  & 5.5                                   \\
\textbf{PXZ-mdPYR}    & 0.14                    & 4                                     & 5.6                                  & 3.4                                   \\
\textbf{PXZ-2mPYR}    & 0.1                     & 4.2                                   & 4.2                                  & 3.8                                  
\end{tabular}
\end{table}


Embedding into solid PMMA matrix increased \QY{F} to \numlist{0.26;0.16;0.19;0.13}. As expected, the more flexible compounds benefited more from rigid surrounding due to reduced non-radiative quenching pathways.

\fi


\iffalse

\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/PXA_fl.pdf}
    \caption{Molecular structures of .}
    \label{fig:pxa_fl}
\end{figure}


\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/PXA_ph.pdf}
    \caption{Molecular structures of .}
    \label{fig:pxa_ph}
\end{figure}

\fi




\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/PXA_flph.pdf}
    \caption{a) Absorption and emission spectra of compounds \PXZPYR, \PXZu, \PXZd\ and \PXZm\ in \SI{1e-5}{\Molar} toluene solutions together with absorption of separate pyrimidine (A) and phenoxazine (D) fragments; b) phosphorescence spectra of same compounds in \SI{1}{wt\%} PMMA films at \SI{10}{K} temperature, measured at delay of \SI{100}{\micro\second}.  }
    \label{fig:pxa_fl}
\end{figure}




The low temperature phosphorescence spectra in \SI{1}{wt\%} PMMA film, displayed clear vibronic progression, typical for locally excited triplet (\TLE) emission 
(Figure \ref{fig:pxa_fl}b).
Vibronic bands of more rigid compounds \PXZd, \PXZu\ and \PXZm\ peaked at the same wavelengths as that of PXZ donor. 
%The 0\ts{th} vibronic band was largest for the most rigid \PXZm, indicating negligible reorganization of molecular structure in excited state.
On the other hand, the least sterically constrained \PXZPYR\ exhibited strongly overlapped and broadened phosphorescence emission, yet weak 0\ts{th} vibronic band was still visible.
Therefore phosphorescence of all compounds originated from the same donor fragment with the same T\tb{1} energy of \SI{2.72}{\eV}. 
The \dEst\ was estimated to be \SIlist{10;70;150;130}{\eV} for \PXZPYR, \PXZu, \PXZd and \PXZm, respectively. 
Surprisingly, the \dEst\ increased for compounds with smaller HOMO and LUMO overlap, this was made possible due to the \TLE\ nature of the lowest triplet state.





All compounds produced bi-exponential decays in diluted toluene solutions and similar, yet multi-exponential due to conformational disorder, decay profiles in solid-state films (Figure \ref{fig:pxa_decay}). The origin of delayed fluorescence was attributed to TADF as it was quenched by oxygen and showed activation with temperature.
The TADF lifetimes in toluene solutions were rather fast as the values ranged from \num{1.6} to \SI{4.0}{\micro\second} for compounds \PXZPYR, \PXZd\ and \PXZm. The \metm\ modified compounds \PXZu\ displayed \tu{TADF} of \SI{0.8}{\micro\second} which is among the lowest values reported for solutions \cite{Geng2017, Kretzschmar2015,Chen2018e,Tanaka2013,Kim2018}.


\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/PXA_decay.pdf}
    \caption{Decay transients of compounds \PXZPYR, \PXZu, \PXZd\ and \PXZm\ in (a) \SI{1e-5}{\Molar} toluene solutions and (b) \SI{10}{wt\%} DPEPO films. }
    \label{fig:pxa_decay}
\end{figure}


TADF properties of all phenoxazine-pyrimidine are presented in table \ref{tab:pxz_tadf}. Here, several important properties should be noted:

\begin{itemize}
    \item The decreased TADF lifetime is related to reduced rotational flexibility of PXZ unit and the adjustment of \SCT-\TLE\ energy gap by changing HOMO--LUMO overlap. The \TCT-\TLE\ gap may also be altered, while spin-orbit coupling should remain relatively the same for similar molecules \cite{Kohler2009}. 
    
    \item The ISC and rISC rates suffered from increased \SCT-\TLE\ energy gap for molecules with same flexibility of PXZ \ie the \krISC\ decreased from \SIlist[list-pair-separator={ to }]{1.8e6; 0.7e6}{\per\second} for compounds \PXZPYR\ and \PXZd\ with more labile donor unit and from \SIlist[list-pair-separator={ to }]{6.5e6;2.5e6}{\per\second} for compounds \PXZu{} and \PXZm\ with more constrained donor unit. 
    
    \item Surprisingly, the increase in \krISC\ was found for compounds with more sterically hindered PXZ units, lowered IC rates, and, supposedly, weakened vibronic coupling between \TLE\ and \TCT\ states (\PXZPYR\ \vs \PXZu\ and \PXZd\ \vs \PXZm) \cite{Ward2016}. The reduction of \TLE\ and \TCT\ energy gap was attributed to the cause of enhanced ISC and rISC rates for compounds with \metm\ modifications (\PXZu\ and \PXZm), though the mechanism is still debatable.
    
    \item Large \QY{TADF} (\num{0.42}) of compound \PXZu\ was determined by the fastest ISC and rISC rates. However similar TADF quantum yield (\num{0.43}) was achieved by \PXZm\ with lower rISC rate, due to reduced IC rate mediated by more rigid molecular structure.
\end{itemize}











\begin{table}[] \centering
\caption{TADF properties of compounds: \PXZPYR, \PXZu, \PXZd, \PXZm\ in oxygen-free toluene solution.}
\label{tab:pxz_tadf}
\begin{tabular}{ccccccc}
\hline
\multicolumn{1}{l}{\upstrut{}} & \multirow{2}{*}{\QY{TADF}} & \dEst{}  & \tu{TADF} & \kr{IC}       & \kr{ISC}      & \krISC{}     \\
\multicolumn{1}{l}{} &                        & (\si{\milli\eV}) & (\si{\micro\second})  & (\SI{1e6}{\per\second}) & (\SI{1e7}{\per\second}) & (\SI{1e6}{\per\second}) \\ [0.5ex]
\hline
\textbf{PXZ-PYR}              & 0.27                   & 10    & 1.6   & 16        & 4.8       & 1.8       \\
\textbf{PXZ-muPYR}            & 0.42                   & 70    & 0.8   & 5.6       & 4.9       & 6.5       \\
\textbf{PXZ-mdPYR}            & 0.24                   & 150   & 4     & 9.2       & 2.5       & 0.7       \\
\textbf{PXZ-2mPYR}            & 0.43                   & 130   & 2.1   & 3.7       & 3.4       & 2.5      
\end{tabular}
\end{table}






The non-radiative pathways were suppressed even more in rigid surrounding of \SI{1}{wt\%} PMMA film. 
Here, the quantum yield was boosted to near unity for compounds \PXZPYR, \PXZd\ and \PXZu\ (\numlist{0.92; 0.94; 0.95}, respectively), while for compound \PXZm\ it remained lower at \num{0.72}.  
Comparable \QY{F} were estimated in \SI{10}{wt\%} DPEPO host, where increased ambient polarity redshifted emission peaks to \SIrange{515}{542}{\nano\meter}.
Decay transients in DPEPO hosts were similar to those in toluene solution (Figure \ref{fig:pxa_decay}), however the TADF decays were prolonged and multiexponential due to conformational disorder.
Only the \PXZu\ with the shortest decay transient was successfully approximated with three-exponential fit. Average TADF lifetime of \SI{860}{\nano\second} was estimated, being one of the lowest among the ever reported various TADF emitters \cite{Konidena2018, Im2017,Liu2018,Kim2018}.



\FloatBarrier
\subsection{Device performace}
\FloatBarrier



As pyrimidine--phenoxazine compounds showed promising TADF properties, OLED devices were prepared. Typical OLED structure used for similar devices  \cite{Jiang2016,Komatsu2017a, Xiang2017} was employed, however the usual CBP host was exchanged for higher triplet energy host DPEPO, to efficiently confine triplet excitons in the emissive layer. The following structure was used: ITO/TAPC (\SI{30}{nm})/TCTA (\SI{5}{nm})/\SI{10}{wt\%} TADF emitter:DPEPO (\SI{15}{nm})/TmPyPB (\SI{65}{nm})/LiF (\SI{0.8}{nm})/Al (\SI{100}{nm}).


\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/PXA_oledf.pdf}
    \caption{a) Prepared OLD devices with \PXZu\ used as emitter; b) External quantum efficiencies of OLED devices at different luminance values. Inset: electroluminance spectra of devices at driving voltage of \SI{9}{\volt}.}
    \label{fig:pxa_oled}
\end{figure}

Photo of the final device as well its EQE and electroluminance spectra are presented in figure \ref{fig:pxa_oled}. The EL emission color was successfully tuned from green, peaking at \SI{536}{\nano\meter} for \PXZPYR\ (CIE coordinates of (0.35.0.56)) to cyan, peaking at \SI{502}{\nano\meter} for \PXZm\ (CIE coordinates of (0.23, 0.42)).
Exceptionally high EQE values of \SIlist{27.9;29.1;27.5;26.3}{\percent} were obtained for compounds \PXZPYR, \PXZu, \PXZd\ and \PXZm, respectively. These values are among the highest values reported for pyrimidine compounds without using any additional outcoupling technique (see \href{https://pubs.acs.org/doi/suppl/10.1021/acsami.9b21394/suppl_file/am9b21394_si_001.pdf}{Table S3 in the Supporting Information of \citepapernumONE{PXAPYR} for a comparison}). 
The compound \PXZu\ not only showed the highest EQE of \SI{29.1}{\%}, but also distinguished with very low EQE roll-off as efficiency dropped to only \SI{20.5}{\%} at \SI[per-mode=symbol]{1000}{\candela\per\meter\squared}. A low EQE roll-off was achieved due to the short TADF lifetime of only \SI{860}{\nano\second}. The combination of submicrosecond TADF lifetime in solid-state along with EQE of nearly \SI{30}{\percent} puts compound \PXZu\ among the best TADF emitters.
Though other emitters displayed similarly high peak EQE values, the EQE roll-off was substantially worse due to longer TADF lifetimes and enhanced quenching by TTA or STA.



\iffalse

\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/PXA_oledV.pdf}
    \caption{Molecular structures of .}
    \label{fig:pxa_oledv}
\end{figure}


\begin{figure}[ht]\centering
    \includegraphics[width=12cm]{figures_PDF/PXA_oled3.pdf}
    \caption{a) Voltage -- current density and voltage -- luminance curves of OLED devices ; b) external quantum efficiencies at different luminance values. Inset electroluminance spectra of devices at driving voltage of \SI{9}{\volt}.}
    \label{fig:pxa_oled}
\end{figure}


\begin{figure}[ht]\centering
    \includegraphics[width=7cm]{figures_PDF/PXA_oled.pdf}
    \caption{External quantum efficiency of OLED devices with compounds \PXZPYR, \PXZu, \PXZd\ and \PXZm\ used as emitter at different luminance values. Inset: electroluminance spectra of devices at driving voltage of \SI{9}{\volt}.}
    \label{fig:pxa_oled}
\end{figure}


\fi



\FloatBarrier
\subsection*{Summary of the results}
\FloatBarrier

To summarize, introduction of methyl group at phenyl spacer in pyrimidine-phenylphenoxazine compounds proved to be a powerful strategy for improving TADF properties.  
First, the introduction of \ortm\ units were found to reduce the acceptor strength, resulting in enlarged \SCT-\TLE\ gap, lowered rISC rate and TADF efficiency. 
Modification with \metm\ fragments, on the other hand, increased the rISC rate up to \SI{6.5e6}{\per\second} producing one of shortest TADF lifetimes of \SI{800}{\nano\second} in dilute solutions. 
Lowering of \TCT-\TLE\ gap was attributed to TADF enhancement. 
Introduction of both \metm\ and \ortm\ groups enlarged the \SCT-\TLE\ energy gap, resulting in reduced rISC rate ant slowest TADF lifetime in solid-state. 
However, the weakened IC rate promoted radiative decay ensuring high TADF quantum yield.

The suppression of non-radiative pathways in solid-state enhanced emission quantum yield to near unity in PMMA films.
Presence of conformational disorder was minimized by fast rISC rate in compound with \metm\ which allowed to estimate (by three-exponential fit) one of the fastest reported TADF decay lifetime of \SI{860}{\nano\second} in \SI{10}{wt\%} DPEPO film.
Such fast TADF resulted in high external quantum efficiency  (\SI{29.1}{\%}) device with very low EQE roll-off as efficiency dropped to only \SI{20.5}{\%} at \SI[per-mode=symbol]{1000}{\candela\per\meter\squared}.

Unmodified and \ortm\ modified phenoxazine-pyrimidine produced efficient green OLEDs with EQE in rage of \SIrange{27.5}{27.9}{\percent}.
Cyan EL with peak EQE of \SI{26.3}{\percent} was obtained for compound with both \metm\ and \ortm\ modifications, though it suffered from large EQE roll-off due to prolonged TADF lifetime.


These results are presented in more detail in article \citepapernumONE{PXAPYR}.