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Article

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

1
Univ Angers, CNRS, MOLTECH-ANJOU, SFR MATRIX, F-49000 Angers, France
2
École Nationale Supérieure d’Ingénieurs de Tunis (ENSIT), 13 Ave Taha Hussein, Tunis 1008, Tunisia
3
Department of Chemistry, University of Calgary, 731 Campus Place N.W., Calgary, AB T2N 1N4, Canada
4
Faculty of Chemistry and Pharmacy, University of Sofia, 1 James Bourchier blvd., 1164 Sofia, Bulgaria
5
Univ Rennes, CNRS UMR6226, F-3500 Rennes, France
6
Department of Chemistry, Kyung Hee University, 730-701 Seoul, Republic of Korea
7
IRL CNRS 2002, 2BFUEL, CNRS, Yonsei University, 03722 Seoul, Republic of Korea
*
Authors to whom correspondence should be addressed.
Colorants 2023, 2(1), 22-30; https://doi.org/10.3390/colorants2010002
Submission received: 1 December 2022 / Revised: 22 December 2022 / Accepted: 28 December 2022 / Published: 30 December 2022
(This article belongs to the Special Issue Recent Progress on Functional Dyes and Their Applications)

Abstract

:
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.

1. 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 new class of compounds was also successfully used as the active component in the above-mentioned 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 N-annulation 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].

2. 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).
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 (λmax from 548 nm to 589 nm) vs. ca. 28 nm for the DPP-BTI molecule (λmax 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 (λonset = 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.
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 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 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 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.

3. 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.

Supplementary Materials

The following supporting information, including synthetic procedures [24,29,43], spectroscopic data, photovoltaic fabrication details and computational data [44,45,46], can be downloaded at: https://www.mdpi.com/article/10.3390/colorants2010002/s1.

Author Contributions

Organic synthesis: T.G., J.M.A.C., P.J., J.B. and Y.Z. Characterization and device fabrication: S.A. and G.C.W., computational chemistry: B.W., supervision of S.A., S.T., P.B., ideas and writing: P.J., B.W. and C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ANR-20-CE05-0029 (BTXI-APOGEE), Marie Sklodowska Curie Grant No.722651 (SEPOMO), ANR-18-EURE-0012 (EUR LUMOMAT, project AZA-BTX) and IRP CNRS MAPLE.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The ANR is acknowledged for the BTXI-APOGEE grant (ANR-20-CE05-0029). J.M.A.C. thanks the European Union’s Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement No.722651 (SEPOMO). S.A. thanks the University of Tunis for her PhD grant. This research received no external funding. This work also received financial support under the EUR LUMOMAT project and the Investments for the Future program ANR-18-EURE-0012 (P.J.). C.C. and G.C.W. are also grateful to the CNRS for financial support provided through the IRP MAPLE.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Structures and solid state UV-visible spectra of BTI and TCI. (B) Frontier orbital energy diagram of both dyes in solid.
Figure 1. (A) Structures and solid state UV-visible spectra of BTI and TCI. (B) Frontier orbital energy diagram of both dyes in solid.
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Figure 2. Structure of the two DPP based compounds studies in this contribution, namely DPP-BTI and DPP-TCI.
Figure 2. Structure of the two DPP based compounds studies in this contribution, namely DPP-BTI and DPP-TCI.
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Scheme 1. Synthetic routes to DPP-BTI and DPP-TCI. (i) HNO3, CH2Cl2, r.t. (ii) triphenylphosphine, DMF, reflux. (iii) K2CO3, 1-bromohexane, DMF, 120 °C. (iv) SiliaCat® DPP-Pd, K2CO3, PivOH, DMAc, 80 °C.
Scheme 1. Synthetic routes to DPP-BTI and DPP-TCI. (i) HNO3, CH2Cl2, r.t. (ii) triphenylphosphine, DMF, reflux. (iii) K2CO3, 1-bromohexane, DMF, 120 °C. (iv) SiliaCat® DPP-Pd, K2CO3, PivOH, DMAc, 80 °C.
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Figure 3. Comparison of the UV-visible spectra of DPP-BTI (black, full line) and DPP-TCI (red, full line) with their respective building blocks, i.e., the BTI (black, dash line), the TCI (red dash line) and the DPP (blue, full line) in CHCl3 solution.
Figure 3. Comparison of the UV-visible spectra of DPP-BTI (black, full line) and DPP-TCI (red, full line) with their respective building blocks, i.e., the BTI (black, dash line), the TCI (red dash line) and the DPP (blue, full line) in CHCl3 solution.
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Figure 4. (a) Comparison of absorption spectra in solution (full line) and solid state (dash line) of DPP-BTI (black) and DPP-TCI (red). (b) Frontier orbital energy levels were estimated and compared, as shown in (b). The N-annulation of the BTI results in a concomitant destabilization (upward shift in energy) of both the HOMO (∆HOMO = 0.23 eV) and LUMO (∆LUMO = 0.17eV) of the π-conjugated DPP-TCI, along with a reduction of the band gap (∆Eg = 0.06 eV).
Figure 4. (a) Comparison of absorption spectra in solution (full line) and solid state (dash line) of DPP-BTI (black) and DPP-TCI (red). (b) Frontier orbital energy levels were estimated and compared, as shown in (b). The N-annulation of the BTI results in a concomitant destabilization (upward shift in energy) of both the HOMO (∆HOMO = 0.23 eV) and LUMO (∆LUMO = 0.17eV) of the π-conjugated DPP-TCI, along with a reduction of the band gap (∆Eg = 0.06 eV).
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Figure 5. (a) Architecture of the organic solar cells used to evaluate the photovoltaic properties of DPP-BTI and DPP-TCI. (b) J-V curves of the best devices prepared either with DPP-BTI (black) and DPP-TCI (red) once blended with PC71BM (in a 1:3 w:w donor:acceptor ratio).
Figure 5. (a) Architecture of the organic solar cells used to evaluate the photovoltaic properties of DPP-BTI and DPP-TCI. (b) J-V curves of the best devices prepared either with DPP-BTI (black) and DPP-TCI (red) once blended with PC71BM (in a 1:3 w:w donor:acceptor ratio).
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Figure 6. Optimized geometries of both DPP-BTI and DPP-TCI computed using the B3LYP density-functional treatment in the standard 6-311G basis set and values of the torsion angles measured between the rylenes and the thiophene connector.
Figure 6. Optimized geometries of both DPP-BTI and DPP-TCI computed using the B3LYP density-functional treatment in the standard 6-311G basis set and values of the torsion angles measured between the rylenes and the thiophene connector.
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Table 1. Summary and comparison of the optical data of DPP-BTI and DPP-TCI.
Table 1. Summary and comparison of the optical data of DPP-BTI and DPP-TCI.
Compoundλmax Solution
(nm)
ε
(L·mol−1·cm−1)
λem Solution
(nm)
Qf aλmax Film
(nm)
Onset Film
(nm)
Egopt Film
(eV)
HOMO
(eV) b
LUMO (eV) c
DPP-BTI577
480
411
308
48,000
50,000
27,000
43,000
6550.62601
488
426
337
7081.75−5.68−3.93
DPP-TCI588
499
416
324
48,000
43,000
32,000
35,500
6610.60616
505
425
340
7331.69−5.45−3.76
a Measured using Coumarin-153 as reference (Φf = 0.45 in MeOH); b Determined by photoemission spectroscopy in air (PESA); c LUMO = (HOMO) − Egopt(film).
Table 2. Best photovoltaic parameters (average values from 6 devices in brackets).
Table 2. Best photovoltaic parameters (average values from 6 devices in brackets).
CompoundVOC
(V)
JSC
(mA·cm−2)
FF
(%)
PCE
(%)
DPP-BTI1.02
(1.01 ± 0.02)
8.55
(8.49 ± 0.07)
31
(29 ± 2)
2.70
(2.48 ± 0.18)
DPP-TCI0.88
(0.87 ± 0.01)
8.19
(8.15 ± 0.08)
28
(27 ± 1)
2.01
(1.93 ± 0.07)
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MDPI and ACS Style

Andrés Castán, J.M.; Abidi, S.; Ghanem, T.; Touihri, S.; Blanchard, P.; Welch, G.C.; Zagranyarski, Y.; Boixel, J.; Walker, B.; Josse, P.; et al. N-Annulation of the BTI Rylene Imide Organic Building Block: Impact on the Optoelectronic Properties of π-Extended Molecular Structures. Colorants 2023, 2, 22-30. https://doi.org/10.3390/colorants2010002

AMA Style

Andrés Castán JM, Abidi S, Ghanem T, Touihri S, Blanchard P, Welch GC, Zagranyarski Y, Boixel J, Walker B, Josse P, et al. N-Annulation of the BTI Rylene Imide Organic Building Block: Impact on the Optoelectronic Properties of π-Extended Molecular Structures. Colorants. 2023; 2(1):22-30. https://doi.org/10.3390/colorants2010002

Chicago/Turabian Style

Andrés Castán, José María, Sana Abidi, Tatiana Ghanem, Saad Touihri, Philippe Blanchard, Gregory C. Welch, Yulian Zagranyarski, Julien Boixel, Bright Walker, Pierre Josse, and et al. 2023. "N-Annulation of the BTI Rylene Imide Organic Building Block: Impact on the Optoelectronic Properties of π-Extended Molecular Structures" Colorants 2, no. 1: 22-30. https://doi.org/10.3390/colorants2010002

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