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We report the electronic absorption spectroscopy of 7,7,8,8-tetracyanoquinodimethane ( TCNQ) and its fluorinated derivatives (F2TCNQ and F4TCNQ), well-known electron-accepting molecules in common organic solvents (toluene, chlorobenzene, acetonitrile, and ethanol) under controlled exposure to air (O ) and UV light. All compounds (FxTCNQ (x = 0, 2, 4)) were stable in a neutral state (FxTCNQ ) in toluene and chlorobenzene, even under both O and UV light. ·− On the other hand, in EtOH, the formation of FxTCNQ was monitored upon controlled exposure to O or UV light. Especially in air-equilibrated ethanol upon the UV-illumination, efficient α,α-dicyano-p-toluoylcyanide anion (DCTC ) and its fluorinated derivatives were generated evinced by the absorption peak near 480 nm, whereas the reaction was ·− shut off by removing O or blocking UV light, thereby keeping FxTCNQ stable. However, even in deaerated ethanol, ·− upon the UV-illumination, the anion formation of TCNQ and its fluorinated derivatives (FxTCNQ , x = 0, 2, 4) was inevi- table, showing the stability of FxTCNQ depends on the choice of solvent. Keywords Electron acceptor, Photochemistry, Radical anion, Tetracyanoquinodimethane comparable to metals (Saito and Yoshida 2007). These Introduction molecular electronics have motivated the synthesis of Organic semiconducting molecules have received atten- new functional molecules, and their optical/electrical/ tion for their applications in biosensors, nonlinear optics, photophysical properties have been examined. transistors, optoelectronics due to their nontoxicity, 7,7,8,8-tetracyanoquinodimethane (TCNQ) in Fig. 1 is bandgap tunability, facile fabrication, and flexibility (Kim a well-known electron-acceptor molecule and has been et al. 2022; Cha et al. 2020; Bronstein et al. 2020; Yuan utilized in various molecular electronics with its LUMO et al. 2019; Sun et al. 2019; Oh et al. 2019; Hiramoto energy level at − 4.23 eV (Kanai et al. 2009) that matches et al. 2019; Yu et al. 2018; Wang et al. 2018; Nayak et al. well with diverse electron donor molecules to produce 2016; Feier et al. 2016; Mishra and Bäuerle 2012; Hains functional CT complexes (Zhang et al. 2018). TCNQ et al. 2010; Coropceanu et al. 2007; Suchanski and Duyne undergoes one or two electron reduction reactions to 1976; Yanti et al. 2021). Despite an intrinsically large exci- ·− 2− form stable anion by-products, TCNQ and TCNQ ton binding energy (E > ~ 100 meV–1 eV ) in organic (Vishwanath et al. 2019). Previously, the spontane- semiconductors, organic charge transfer (CT) complexes 2 –1 ous formation of α,α-dicyano-p-toluoylcyanide anion have shown strikingly high conductivity (σ > 10 S cm ), (DCTC ) has been observed in TCNQ solution. Several potential mechanisms have been proposed (Ning et al. *Correspondence: 2019; Hertler et al. 1962; Mizoguchi et al. 1978; Krysze- JaeHong Park wski et al. 1981; Grossel et al. 2000; Le et al. 2011), one firstname.lastname@example.org 2− Department of Chemistry and Nanoscience, Ewha Womans University, of which includes the reaction of reduced T CNQ with Seoul 03760, Republic of Korea dissolved O (g) to produce DCTC in ambient solution © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Bang and Park Journal of Analytical Science and Technology (2023) 14:2 Page 2 of 6 absorption peak near 480 nm under UV, which was sup- pressed by removing O or blocking UV light. Experimental section Materials 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-Dif- luoro-7,7,8,8-tetracyanoquinodimethane (F2TCNQ), and tetrafluorotetracyanoquinodimethane (F4TCNQ, purified from sublimation) were purchased from Tokyo Chemical Industry (TCI) and used without further puri- Fig. 1 Molecular structures of TCNQ, F2TCNQ, and F4TCNQ fication. Ethanol (HPLC/spectrophotometric grade, Sigma-Aldrich), acetonitrile (HPLC/spectrophotometric grade, J. T. Baker), toluene (HPLC grade, Wako Chemi- cals), and chlorobenzene (HPLC grade, Sigma-Aldrich) (Ning et al. 2019). On the other hand, several conflicting were used as received. mechanisms of the formation of DCTC , involving neu- ·− tral TCNQ or TCNQ also have been reported as well Instrumentation (Hertler et al. 1962; Mizoguchi et al. 1978; Kryszewski Electronic absorption spectra were acquired using a et al. 1981; Grossel et al. 2000). Hitachi U-3900 UV/visible/NIR spectrophotometry sys- For further energy level engineering, several fluori - tem in a quartz optical cell. Baseline corrections for the nated derivatives, tetrafluorinated/difluorinated TCNQ transmittance of an optical cell with solvent were made (F4TCNQ and F2TCNQ), were developed and successfully prior to each measurement. In addition, the evolution have shown the tunability of the first reduction potential of an absorption spectrum upon the UV-illumination ·− with respect to TCNQ: E (TCNQ /TCNQ) = − 54 mV; 1/2 (365 nm, 4 W, CW UV lamp) was monitored either with ·− ·− E (F2TCNQ /F2TCNQ) = + 165 mV; E (F4TCNQ / 1/2 1/2 molecular oxygen in an air-equilibrated solution or with- F4TCNQ) = + 365 mV versus Ag/Ag in CH CN (Le et al. out molecular oxygen in an N (99.999% purity)-purged 2011; Miyasaka et al. 2010). Furthermore, these fluorinated solution to examine the absorption behavior of TCNQ TCNQ derivatives have been shown to form radical ani- derivatives, dependent on the presence of O . ons and dianions in polar solvents such as acetonitrile and ethanol (Ma et al. 2014; Vo et al. 2018; Chae et al. 2014), Results and discussion while external conditions such as O or UV light for anion To fabricate OPV devices or to prepare solution samples species formation have not been well characterized. There - for photocatalyst characterization, electrical or photo- fore, to utilize the electron-accepting property of TCNQ physical studies involving TCNQ, solution processes and its fluorinated derivatives in many devices, it is cru - are commonly used. In this regard, electronic absorp- cial to understand the conditions to keep TCNQ deriva- tion spectra of TCNQ were explored in various common tives neutral (TCNQ ) and to prevent from producing organic solvents: toluene (Tol), chlorobenzene (CB), ace- by-products such as anions or DCTC . In this regard, the tonitrile (ACN), and ethanol (EtOH). Additional file 1: role of O and UV light on the spectroscopic properties of Figure S1 displays the electronic absorption spectra of TCNQ derivatives is worth investigating. Here, we report TCNQ in air-equilibrated solvents, and the spectro- the electronic absorption spectroscopy of 7,7,8,8-tetracy- scopic results are tabulated in Table 1. In all solvents, the anoquinodimethane (TCNQ) and its fluorinated deriva - absorption spectra of TCNQ revealed the characteristic tives (F2TCNQ and F4TCNQ) in various common organic S → S transition in the range of 394–403 nm, consist- 0 1 solvents (toluene-Tol, chlorobenzene-CB, acetonitrile- ent with the previous literature results (Suchanski and ACN, and ethanol-EtOH) either as air-equilibrated or Duyne 1976). However, in Additional file 1: Figure S1d as N -purged with controlled UV-illumination. TCNQ (black line), TCNQ in EtOH displays additional dual showed the production of DCTC upon the reaction peaks in 700–900 nm, which are typical peaks for TCNQ ·− of TCNQ with O in EtOH under UV-illumination, ·− anion radical (TCNQ ) (Melby et al. 1962). whereas in N -purged EtOH, no or minimal DCTC was The spectroscopic behavior of TCNQ was further observed. On the other hand, stable neutral form T CNQ tracked upon UV light illumination every 3-min to exam- was confirmed in Tol, CB, or ACN. Similarly, F2TCNQ ine its photostability for a total of 15 min, as shown in and F4TCNQ in air-equilibrated EtOH exhibited the effec - Fig. 2a, Additional file 1: Figures S1 and S3. The spectro - tive production of fluorinated DCTC derivatives with an scopic results of TCNQ, either in air-equilibrated Tol, CB, or ACN, showed a virtually consistent absorption Bang and P ark Journal of Analytical Science and Technology (2023) 14:2 Page 3 of 6 spectrum. In contrast, TCNQ in air-equilibrated EtOH evolution, contrasting to the previous studies in ACN (EtOH-air) exhibited a substantial drop of the inten- (Suchanski and Duyne 1976; Chae et al. 2014). sity for the absorption band at 396 nm even in 3 min Due to the potential role of molecular oxygen (O ) in in Fig. 2a. With the decrease of 396 nm band, multiple air-equilibrated EtOH in DCTC generation, the spec- absorption bands at 421, 743, and 841 nm, correspond- troscopic behavior of TCNQ was further examined in ·− ing to TCNQ increased, and a new band at 474 nm degassed EtOH (EtOH-N ) by pre-purging EtOH solvent emerged. At 6 min (green in Fig. 2a), even the absorp- and purging TCNQ solution in EtOH with high purity ·− tion bands of T CNQ at 421, 743, and 841 nm began to (99.999%) N gas. Figure 2b displays the spectral evolu- decrease, and the 474 nm band continuously gained its tion upon the same UV-illumination. Like the absorption intensity up to 15 min (magenta line in Fig. 2a), when spectrum of TCNQ in EtOH-air at 0 m in Fig. 2a (black the original absorption peak at 396 nm was wholly dis- line), TCNQ in EtOH-N also exhibited the characteristic appeared. The spectral change for TCNQ in EtOH-air in 396 nm band for TCNQ in addition to 743 and 841 nm ·− ·− Fig. 2a suggests that TCNQ reacted to generate T CNQ , bands corresponding to T CNQ in Fig. 2b (black line). At 0 ·− in contrast to stable TCNQ in Tol, CB, or ACN. Further- 3 m (red line), T CNQ peaks increased with the decrease more, the spectral evolution upon the UV-illumination of TCNQ peak, and the additional band at 421 nm is also ·− from 3 to 15 m, featuring the decrease of 743 and 841 nm the spectroscopic fingerprint of TCNQ , which was bur- ·− 0 band, implies that T CNQ was reacted to produce the ied with TCNQ band at 0 m due to the spectral proximity. species that shows an absorption peak at 474 nm. The From 6 min (green line), the spectral change in EtOH-N new species that shows the absorption band at 474 nm contrasts with that in EtOH-air, lacking the growth of the has been ascribed to α,α-dicyano-p-toluoylcyanide DCTC absorption band at 474 nm. Also, the absorption − ·− anion (DCTC , Additional file 1: Figure S2) previously peak intensity of TCNQ peaks in EtOH-N was kept con- ·− (Suchanski and Duyne 1976; Hertler et al. 1962; Mizo- stant, suggesting stable TCNQ and no further reactions ·− guchi et al. 1978; Kryszewski et al. 1981; Grossel et al. consuming TCNQ occurred. The formation of DCTC − − 2000; Xiulan et al. 2012), and the details of DCTC gen- in EtOH-N could be blocked because of the absence of 2− ·− eration are discussed below. No sign of TCNQ with a TCNQ even without O . Additional file 1: Figure S4 pre- peak at ~ 330 nm was observed during this spectroscopic sents the absorption spectra of TCNQ as a function of time passed since blocking UV light in the middle of UV-illumi- nation (exposed to UV light for 9 min), clearly showing the ·− presence of TCNQ . Over the 10 min since blocking UV light, no DCTC generation was noticed, highlighting the Table 1 Electronic absorption spectroscopic data ·− role of both O and TCNQ . Solvent Samples Since the initial report of TCNQ, several DCTC gen- TCNQ (nm) F2TCNQ (nm) F4TCNQ (nm) eration mechanisms from TCNQ have been proposed, 0 ·− whether the reaction starts either from T CNQ, TCNQ , Tol 396 390 385 2− or TCNQ (Hertler et al. 1962; Mizoguchi et al. 1978; CB 403 398 389 Kryszewski et al. 1981; Grossel et al. 2000; Xiulan et al. ACN 394 393 387 2012). To identify the origin of DCTC formation, we EtOH 396 394 390 Fig. 2 a–c Comparative electronic absorption spectral evolution of TCNQ in a air-equilibrated, b degassed EtOH as a function of UV-illumination time, and in c air-equilibrated EtOH as a function of time without UV-illumination Bang and Park Journal of Analytical Science and Technology (2023) 14:2 Page 4 of 6 e, no appreciable spectral change was probed in Tol or plotted the absorbance (A bs ) of DCTC product, CB even under UV-illumination, while in ACN, F4TCNQ monitored at 474 nm against the absorbance (A bs ) of ·− − showed a noticeable decrease in 387 nm band, contrast- TCNQ at 841 nm using the data in Fig. 2a. If DCTC is ·− ·− ing to F2TCNQ that displayed no spectral change (Figure produced from T CNQ , as the stoichiometry of T CNQ S5c, f ). This is likely due to the higher electron affinity of and DCTC is 1:1 based upon the potential mechanism F4TCNQ, resulting in the reduction of F4TCNQ. On the shown in Additional file 1: Figure S2, the concentration of − − other hand, in EtOH-air, both F2TCNQ and F4TCNQ the produced DCTC ([DCTC ] ) at a given t should be p,t ·− ·− in Fig. 4a, b, d, e displayed a dramatic change as in equivalent to that of the consumed TCNQ ([TCNQ ] ) c,t TCNQ. As the formation of an oxidized product, DCTC at the same t. − ·− from TCNQ was evident from a new absorption − ·− DCTC = TCNQ (1) band at ~ 480 nm in Fig. 2a, F2TCNQ and F4TCNQ in p,t c,t EtOH-air similarly show the rise of ~ 480 nm band under From the Beer’s law: UV light. Congruent to TCNQ, in EtOH-N , the effec - tive growth of the 480 nm band was not observed, again − − DCTC = Abs /ε DCTC 474 474 (2) p,t suggesting that fluorinated DCTC derivatives were not produced without O . Additionally, even in EtOH-air, ·− ·− ·− without UV light, FxTCNQ (x = 0, 2, 4) was not effec - TCNQ =−Abs /ε TCNQ 841 841 (3) c,t tively generated in Fig. 4c, f, evincing the stable neutral 0 0 where ε (A) is the molar absorption coefficient at x nm TCNQ and F2TCNQ , contrary to unstable F4TCNQ 2− for A species. Therefore, from the equivalence in Eq. ( 1) that underwent F4TCNQ formation (Melby et al. as well as Eqs. (2) and (3), the Abs should be linearly 1962). proportional to Abs as in Eq. (4), and the propor- tionality constant would be the ratio of ε (DCTC ) to ·− ε (TCNQ ): Conclusions In summary, the electronic absorption spectroscopy of − ·− Abs =− ε DCTC /ε TCNQ Abs 474 474 841 841 FxTCNQ (x = 0, 2, 4) probed the stability of FxTCNQ (4) (x = 0, 2, 4) in common organic solvents upon controlled Figure 3a displays the Abs (DCTC ) against exposure to O or UV light. In general, all FxTCNQ ·− Abs (TCNQ ), and the linear fit deter - compounds were stable in Tol and CB even under both ·− mined the proportionality constant to be −0.814 O and UV light, whereas in EtOH, the FxTCNQ for- (± 0.03). From the literature, ε (DCTC ) and mation was monitored. Furthermore, in air-equilibrated ·− ε (TCNQ ) have been reported in various sol- EtOH upon UV-illumination, efficient α,α-dicyano- p- − −1 −1 vents. Using ε (DCTC ) = 38,800 cm M and toluoylcyanide anion (DCTC ) and its fluorinated deriva - ·− −1 −1 ε (TCNQ ) = 43,500 cm M in acetone (Hertler tives were generated, whereas the reaction was shut off by et al. 1962; Grossel et al. 2000; Melby et al. 1962), removing O or blocking UV light. With the significance − ·− ε (DCTC )/ε (TCNQ ) can be calculated to be of TCNQ and its derivatives (F2TCNQ and F4TCNQ) 474 841 0.892, which matches reasonably well despite the solvent not only in molecular electronics but also in electro- ·− difference. In addition, Abs (TCNQ ) is plotted against chemistry, this work will provide an understanding of the UV-illumination time in Fig. 3b, showing the pseudo- ·− first-order reaction for T CNQ behavior is observed, ·− again supporting that the reaction of T CNQ is not mediated by the self-collisions, consistent with the pro- posed mechanism by Hipps et al. (Qi et al. 2012) and the −1 resulting fit determined the rate constant to be 0.13 m . Structurally related fluorinated TCNQ molecules (F2TCNQ and F4TCNQ: FxTCNQ, x = 2 and 4) have been widely utilized as comparative and tunable func- tional molecules to TCNQ, as their spectroscopic sig- natures are similar in spite of the modified reduction − ·− energy levels. The absorption spectra of F xTCNQ, either Fig. 3 a Plot of Abs (DCTC ) against Abs ( TCNQ ) and its 474 841 ·− linear fit (blue line). b Plot of the natural log (ln(Abs ( TCNQ )) for EtOH-air or EtOH-N , in Fig. 4a, b, d, e showed substan- 841 ·− Abs ( TCNQ ) as a function of UV-illumination time and its linear fit tial change over time, like TCNQ in EtOH. On the other (blue line) hand, as presented in Additional file 1: Figure S5a, b, d, Bang and P ark Journal of Analytical Science and Technology (2023) 14:2 Page 5 of 6 Fig. 4 a–f Comparative electronic absorption spectral evolution of a–c F2TCNQ and d–f F4TCNQ in a, c air-equilibrated, b, d degassed EtOH as a function of UV-illumination time, and those in c, f air-equilibrated EtOH as a function of time without UV-illumination Availability of data and materials the impact of external stimuli such as O and UV light on The datasets used and/or analyzed during the current study are available from TCNQ compounds. the corresponding author on reasonable request. Supplementary Information is available. Supplementary Information The online version contains supplementary material available at https:// doi. Declarations org/ 10. 1186/ s40543- 022- 00364-z. Competing interests The authors declare that they have no competing interests. Additional file 1. Figure S1: Electronic absorption spectra of TCNQ in a various solvent as a function of UV-illumination time in air-equilibrated condition: toluene ( Tol), chlorobenzene (CB), acetonitrile (ACN), ethanol Received: 16 November 2022 Accepted: 23 December 2022 (EtOH). Figure S2: The formation of DCTC from TCNQ radical anion upon the exposure to O and UV light. Figure S3: Normalized electronic absorp- tion spectra of TCNQ in a various solvent as a function of UV-illumination time in air-equilibrated condition: toluene ( Tol), chlorobenzene (CB), and acetonitrile (ACN). Figure S4: Electronic absorption spectra of TCNQ as References a function of time passed since blocking UV light with the initial 9 mins Bronstein H, Nielsen CB, Schroeder BC, McCulloch I. The role of chemical of UV-illumination. Figure S5: Electronic absorption spectra of (a,b,c) design in the performance of organic semiconductors. Nat Rev Chem. 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Journal of Analytical Science and Technology – Springer Journals
Published: Jan 9, 2023
Keywords: Electron acceptor; Photochemistry; Radical anion; Tetracyanoquinodimethane
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