N -Annulation of the BTI Rylene Imide Organic Building Block: Impact on the Optoelectronic Properties of π -Extended Molecular Structures

: Benzothioxanthene imide ( BTI ) has recently emerged as an interesting and promising block for organic electronics. In this contribution, we report on the impact of the N -annulation of the latter dye on the optoelectronic of π -extended molecular structures. To do so, the thiophene-diketopyrrolopyrrole was selected, as central π -conjugated core, and either end-capped with two BTIs or its N -annulated version, namely the TCI . While almost similar band gaps were measured for individual rylene imide dyes, signiﬁcant differences were highlighted, and rationalized, on their π -extended counterparts.


Introduction
Over the last decades, rylene-imide-based dyes have attracted considerable research attention [1][2][3]. Among them, the naphthalene [4][5][6][7] and perylene [8][9][10][11], functionalized by either one or two imide groups, have quickly emerged as key players in the landscape of organic semiconducting materials. Thanks to the creativity of chemists, a myriad of site-selective functionalizations have been reported, leading to the preparation and characterization of a significant number of new and original π-extended molecular and macromolecular systems [12]. Characterized by remarkable optoelectronic properties, high chemical/thermal stability, and a certain ease of synthesis, a good number of these structures have been successfully used in several devices including organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic solar cells (OSCs) [1,[13][14][15][16][17][18].
On the fringe of this success story, we have recently focused our attention on another member of this rylene family, namely the benzothioxanthene imide (BTI, Figure 1A). Ignored or simply unknown by the organic electronic community, this sulfur-containing structure was first reported in the early 70 s [19,20] and was mainly used as a fiber dyeing agent and florescent probe for bio-imaging [21][22][23]. In these early reports, it was functionalized solely for grafting and/or solubility reasons on the imide nitrogen atom, however, we have demonstrated the first selective halogenation(s) of its π-conjugated core [24,25]. Beyond the preparation of original π-extended/conjugated BTI-based architectures, this mentioned devices, thus highlighting, for the first time, their promising potential of B in organic electronics [24,[26][27][28].
As a step forward in the chemical exploration of this dye, we lately achieved the annulation of its bay position, affording the thiochromenocarbazole imide, acronym TCI ( Figure 1A) [29]. Interestingly, incorporation of this fused, nitrogen-based 5-mem bered ring was found to induce a concomitant destabilization of both the highest occupi molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO), resulting in almost similar band gap (2.31 eV vs. 2.30 eV, Figure 1B In this study, thiophene-diketopyrrolopyrrole was selected as a π-bridge for its ease of synthesis [30], (ii) compatibility with direct (hetero)arylation cross coupling rea tions [31,32] and (iii) absorption in the visible that would, we hoped, shift those of t target compounds into the far red or near-infrared regions; an electromagnetic range interest in many optoelectronic and bio-related applications ( Figure 2) [33][34][35][36]. For i stance, this rylene imide-DPP-rylene imide scaffold has been used successfully to devel non-fullerene acceptors for organic solar cells [37][38][39]. As a step forward in the chemical exploration of this dye, we lately achieved the Nannulation of its bay position, affording the thiochromenocarbazole imide, acronymed TCI ( Figure 1A) [29]. Interestingly, incorporation of this fused, nitrogen-based 5-membered ring was found to induce a concomitant destabilization of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO), resulting in an almost similar band gap (2.31 eV vs. 2.30 eV, Figure 1B).
In this study, thiophene-diketopyrrolopyrrole was selected as a π-bridge for its (i) ease of synthesis [30], (ii) compatibility with direct (hetero)arylation cross coupling reactions [31,32] and (iii) absorption in the visible that would, we hoped, shift those of the target compounds into the far red or near-infrared regions; an electromagnetic range of interest in many optoelectronic and bio-related applications ( Figure 2) [33][34][35][36]. For instance, this rylene imide-DPP-rylene imide scaffold has been used successfully to develop non-fullerene acceptors for organic solar cells [37][38][39].
structure was first reported in the early 70 s [19,20] and was mainly used as a fiber dyeing agent and florescent probe for bio-imaging [21][22][23]. In these early reports, it was functionalized solely for grafting and/or solubility reasons on the imide nitrogen atom, however, we have demonstrated the first selective halogenation(s) of its π-conjugated core [24,25]. Beyond the preparation of original π-extended/conjugated BTI-based architectures, this new class of compounds was also successfully used as the active component in the abovementioned devices, thus highlighting, for the first time, their promising potential of BTI in organic electronics [24,[26][27][28].
As a step forward in the chemical exploration of this dye, we lately achieved the Nannulation of its bay position, affording the thiochromenocarbazole imide, acronymed TCI ( Figure 1A) [29]. Interestingly, incorporation of this fused, nitrogen-based 5-membered ring was found to induce a concomitant destabilization of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO), resulting in an almost similar band gap (2.31 eV vs. 2.30 eV, Figure 1B). In this study, thiophene-diketopyrrolopyrrole was selected as a π-bridge for its (i) ease of synthesis [30], (ii) compatibility with direct (hetero)arylation cross coupling reactions [31,32] and (iii) absorption in the visible that would, we hoped, shift those of the target compounds into the far red or near-infrared regions; an electromagnetic range of interest in many optoelectronic and bio-related applications ( Figure 2) [33][34][35][36]. For instance, this rylene imide-DPP-rylene imide scaffold has been used successfully to develop non-fullerene acceptors for organic solar cells [37][38][39].

Results and Discussion
The synthetic route to the target compounds, namely DPP-BTI and DPP-TCI, is depicted in Scheme 1.
Colorants 2023, 2, FOR PEER REVIEW 3 Figure 2. Structure of the two DPP based compounds studies in this contribution, namely DPP-BTI and DPP-TCI.

Results and Discussion
The synthetic route to the target compounds, namely DPP-BTI and DPP-TCI, is depicted in Scheme 1. The materials were prepared following our previously reported procedure [24]; BTI-Br was selectively nitrated in its bay position prior to undergoing a Cadogan reductive cyclization reaction in presence of triphenylphosphine [29]. The resulting NH-TCI-Br was subsequently treated with 1-bromohexane under basic conditions to afford a N-alkylated and -annulated version of the BTI-Br, namely the TCI-Br. Both compounds were finally engaged in a heterogeneous palladium-catalyzed direct C-H arylation reaction with the DPP dye. Upon completion, both compounds were isolated from the crude by simple column chromatography on silica gel.
The molecules were found to be highly soluble in common organic solvents; their optical properties were first investigated in solution with comparison to their individual constituting building blocks, namely the BTI, TCI and DPP ( Figure 3).

Scheme 1. Synthetic routes to DPP-BTI and DPP-TCI.
The materials were prepared following our previously reported procedure [24]; BTI-Br was selectively nitrated in its bay position prior to undergoing a Cadogan reductive cyclization reaction in presence of triphenylphosphine [29]. The resulting NH-TCI-Br was subsequently treated with 1-bromohexane under basic conditions to afford a N-alkylated and -annulated version of the BTI-Br, namely the TCI-Br. Both compounds were finally engaged in a heterogeneous palladium-catalyzed direct C-H arylation reaction with the DPP dye. Upon completion, both compounds were isolated from the crude by simple column chromatography on silica gel.
The molecules were found to be highly soluble in common organic solvents; their optical properties were first investigated in solution with comparison to their individual constituting building blocks, namely the BTI, TCI and DPP ( Figure 3).
Interestingly, their characteristic absorption bands can be specifically attributed to the spectral features of their constituent π-extended molecules (DPP-BTI and DPP-TCI), even if shifted, to a greater or lesser extent, toward the longer wavelengths. Correlated to an improved donating effect, induced by the carbazole moiety, this redshift was indeed found to be more pronounced in the case of the TCI-based compound (DPP-TCI). Regarding the optical signatures of the rylenes, a 13 nm shift was indeed observed for both characteristic bands of the N-annulated TCI (ca. 400 and 470 centered bands) when coupled to the DPP central core while only ca. 3 nm were measured for its BTI counterpart. Furthermore, the most drastic difference was observed for the band at lower energies, attributed to the DPP core, with a 41 nm shift for the DPP-TCI molecule (λ max from 548 nm to 589 nm) vs. ca. 28 nm for the DPP-BTI molecule (λ max from 548 nm to 576 nm). As a result, with an onset at ca. 640 nm, DPP-TCI exhibits a reduced band gap of ca. 0.06 Ev compared to its BTI-based parent compound (λ onset = 664 nm). On the other hand both molecule shows florescent properties in the far red-region (Table 1, Figure S10). With similar quantum yields (of ca. 60%), emission band of DPP-TCI also appears slightly redshifted (of ca. 6 nm) compared to that of DPP-BTI.  Interestingly, their characteristic absorption bands can be specifically attribute the spectral features of their constituent π-extended molecules (DPP-BTI and DPP-T even if shifted, to a greater or lesser extent, toward the longer wavelengths. Correlate an improved donating effect, induced by the carbazole moiety, this redshift was in found to be more pronounced in the case of the TCI-based compound (DPP-TCI) garding the optical signatures of the rylenes, a 13 nm shift was indeed observed for characteristic bands of the N-annulated TCI (ca. 400 and 470 centered bands) when pled to the DPP central core while only ca. 3 nm were measured for its BTI counter Furthermore, the most drastic difference was observed for the band at lower ener attributed to the DPP core, with a 41 nm shift for the DPP-TCI molecule (λmax from nm to 589 nm) vs. ca. 28 nm for the DPP-BTI molecule (λmax from 548 nm to 576 nm). result, with an onset at ca. 640 nm, DPP-TCI exhibits a reduced band gap of ca. 0.0 compared to its BTI-based parent compound (λonset = 664 nm). On the other hand molecule shows florescent properties in the far red-region (Table 1, Figure S10). With ilar quantum yields (of ca. 60%), emission band of DPP-TCI also appears slightly shifted (of ca. 6 nm) compared to that of DPP-BTI.
In the thin film state, absorption spectra broaden and redshift (due to solid stat gregation), thus leading to even more reduced band gaps while maintaining the s trends observed in solution (Figure 4a and Table 1).  In the thin film state, absorption spectra broaden and redshift (due to solid state aggregation), thus leading to even more reduced band gaps while maintaining the same trends observed in solution (Figure 4a and Table 1).
The greater impact on the HOMO level can be attributed to the improved donor character of the TCI blocks induced by the alkylated nitrogen atom constituting the carbazole ring while a reduced gap can be attributed to better conjugation along the backbone. The shallower HOMO level of DPP-TCI was also highlighted by the photovoltaic parameters observed in organic solar cells prepared from both the DPP based compounds, used as molecular donors. To do so, direct solar cells of architecture: ITO/PEDOT:PSS/active layer/LiF/Al were fabricated and tested under AM 1.5 G conditions. Once blended with (6,6)-Phenyl C 71 butyric acid methyl ester (PC 71 BM) best power conversion efficiencies were achieved in an optimal 1 to 3 weight to weight donor: acceptor ratio. As depicted in the respective current-to-tension (J-V) curves, plotted in Figure 5, higher open circuit voltage (V oc ) values and therefore efficienceies were systematically achieved with the DPP-BTI than DPP-TCI. (Table 2).   The greater impact on the HOMO level can be attributed to the improved donor character of the TCI blocks induced by the alkylated nitrogen atom constituting the carbazole ring while a reduced gap can be attributed to better conjugation along the backbone. The shallower HOMO level of DPP-TCI was also highlighted by the photovoltaic parameters observed in organic solar cells prepared from both the DPP based compounds, used as molecular donors. To do so, direct solar cells of architecture: ITO/PEDOT:PSS/active layer/LiF/Al were fabricated and tested under AM 1.5 G conditions. Once blended with (6,6)-Phenyl C71 butyric acid methyl ester (PC71BM) best power conversion efficiencies were achieved in an optimal 1 to 3 weight to weight donor: acceptor ratio. As depicted in the respective current-to-tension (J-V) curves, plotted in Figure 5, higher open circuit voltage (Voc) values and therefore efficienceies were systematically achieved with the DPP-BTI than DPP-TCI. (Table 2).  It is indeed usually generally accepted that the latter parameter (Voc) is proportional to the difference between the LUMO of the acceptor (fullerene) and the HOMO of the donor (rylene imide DPP based compound), consistent with the observed trend in energy levels [40]. In order to understand the electronic structure of the molecules, density functional theory (DFT) calculations were performed. Optimized geometries are shown in Figure 6a, while the torsion angle between DPP and BTI or TCI units (measured as the dihedral angle in the bonds between atoms 1, 2, 3 and 4, highlighted in green in Figure 6a was  It is indeed usually generally accepted that the latter parameter (V oc ) is proportional to the difference between the LUMO of the acceptor (fullerene) and the HOMO of the donor (rylene imide DPP based compound), consistent with the observed trend in energy levels [40].
In order to understand the electronic structure of the molecules, density functional theory (DFT) calculations were performed. Optimized geometries are shown in Figure 6a, while the torsion angle between DPP and BTI or TCI units (measured as the dihedral angle in the bonds between atoms 1, 2, 3 and 4, highlighted in green in Figure 6a was calculated and revealed a larger torsion angle for DPP-BTI (61.85 • ) than for DPP-TCI (50.14 • ), as depicted in Figure 6b. DFT results show that adding the N-annulated 5-membered ring in DPP-TCI increases (i) the energy of both the HOMO and LUMO orbitals of the molecule relative to DPP-BTI, consistent with HOMO and LUMO energy levels determined experimentally by PESA and UV-vis, and (ii) the HOMO energy to a greater extent than the LUMO, resulting in the reduction of the band gap. This decreased band gap in DPP-TCI (2.260 eV) compared to 2.309 eV for DPP-BTI, corresponds to HOMO-LUMO transitions occurring at 537 nm or 549 nm for DPP-BTI or DPP-TCI, respectively (Figure 6c). These calculated values are somewhat blueshifted compared to the measured values in solution (577 and 588 nm, respectively) however, gas-phase DFT calculations are expected to be blue-shifted compared to condensed phases (solution or film) and the difference in absorption onset and optical band gap between the two chromophores is in excellent agreement with the experimentally observed value (12 nm calculated red-shift vs. 11 nm observed red-shift).
These differences can be attributed to the relatively stronger electron-donating character of the annulated N-atom compared to the two C-H bonds that it replaces in DPP-BTI. N-alkyl groups are indeed known to be strongly electron-donating, additionally, the N-containing 5 membered ring incorporates N as a subunit of larger pyrrole, indole or carbazole structures, all of which are known as relatively electron-rich/electron-donating sub-structures. The measured frontier orbital energies and decrease in bandgap confirm the electron-donating character of the appended N-annular ring.
On the other hand, the decrease in torsion angle between the DPP and TCI groups can be rationalized as being caused by an increase in bond order between DPP and TCI relative to DPP and BTI; in other words, more double-bond character, better p-orbital overlap, and increased π-conjugation exists in the DPP-TCI bond, which manifests as a smaller torsion angle. It has been shown by Marder et al., in push-pull type chromophores, the bond length alternation (BLA) is decreased when stronger electron donating or electron withdrawing groups are used, leading to improved quinoid contribution to the ground state and corresponding increase in bond order between coupled aromatic rings [41,42]. Both DPP-BTI and DPP-TCI can be considered push-pull chromophores, DPP being a relatively electron-withdrawing moiety, while TCI shows a stronger electron-donating character compared to BTI. Hence, the increased electron-donating character of TCI results in more double bond character between DPP and TCI than between DPP and This decreased band gap in DPP-TCI (2.260 eV) compared to 2.309 eV for DPP-BTI, corresponds to HOMO-LUMO transitions occurring at 537 nm or 549 nm for DPP-BTI or DPP-TCI, respectively (Figure 6c). These calculated values are somewhat blueshifted compared to the measured values in solution (577 and 588 nm, respectively) however, gas-phase DFT calculations are expected to be blue-shifted compared to condensed phases (solution or film) and the difference in absorption onset and optical band gap between the two chromophores is in excellent agreement with the experimentally observed value (12 nm calculated red-shift vs. 11 nm observed red-shift).
These differences can be attributed to the relatively stronger electron-donating character of the annulated N-atom compared to the two C-H bonds that it replaces in DPP-BTI. N-alkyl groups are indeed known to be strongly electron-donating, additionally, the Ncontaining 5 membered ring incorporates N as a subunit of larger pyrrole, indole or carbazole structures, all of which are known as relatively electron-rich/electron-donating sub-structures. The measured frontier orbital energies and decrease in bandgap confirm the electron-donating character of the appended N-annular ring.
On the other hand, the decrease in torsion angle between the DPP and TCI groups can be rationalized as being caused by an increase in bond order between DPP and TCI relative to DPP and BTI; in other words, more double-bond character, better p-orbital overlap, and increased π-conjugation exists in the DPP-TCI bond, which manifests as a smaller torsion angle. It has been shown by Marder et al., in push-pull type chromophores, the bond length alternation (BLA) is decreased when stronger electron donating or electron withdrawing groups are used, leading to improved quinoid contribution to the ground state and corresponding increase in bond order between coupled aromatic rings [41,42]. Both DPP-BTI and DPP-TCI can be considered push-pull chromophores, DPP being a relatively electron-withdrawing moiety, while TCI shows a stronger electron-donating character compared to BTI. Hence, the increased electron-donating character of TCI results in more double bond character between DPP and TCI than between DPP and BTI, despite identical steric interactions near the bond linking the two groups. This interpretation is consistent with the observed shortening of the bond length in DPP-TCI (1.468 Å) compared to DPP-BTI (1.473 Å), as seen in the computed optimized geometries.

Conclusions
As part of our endeavor to fully explore the potential of BTI-based derivatives, we have thus investigated the impact of N-annulation on the properties of π-extended structures, since similar band gaps were observed for both rylene parent structures (BTI and its N-annulated version, TCI). Thus, two π-conjugated compounds, based on a common thiophene-diketopyrrolopyrrole central core and end-capped with each rylene imide, were prepared by direct arylation. We observed that incorporation of the nitrogen-containing 5-membered ring in the TCI structure resulted in increased donor character and improved electronic conjugation along the backbone, resulting in shallower frontier orbital energy levels and a concomitant reduction of the band gap (approaching the near-infrared region in this case). The reduced band gap and increased π-conjugation in the DPP-TCI structure were attributed to a decrease in bond length alternation and an increase in the order of the bond linking DPP to TCI relative to the DPP-BTI bond. Hence, we show that a simple Cadogan cyclization turns out to be an easy and accessible strategy to fine-tune the energetics of BTI-based extended molecules, while at the same time providing additional orthogonal reactive sites at the carbazole N-atom available for functionalization with a variety of lateral side chains/groups. This work opens the door for further customization of BTI-based molecules, which can be conveniently optimized as absorbers, emitters and transport materials for a variety of optical and optoelectronic applications.