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Article Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX

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Designing of Push−Pull Chromophores with Tunable Electronic and Luminescent Properties Using Urea as the Electron Donor Arif Hassan Dar,† Vijayendran Gowri,† Arya Gopal,‡ Azhagumuthu Muthukrishnan,‡ Ashima Bajaj,† Shaifali Sartaliya,† Abdul Selim,† Md. Ehesan Ali,† and Govindasamy Jayamurugan*,† †

Institute of Nano Science and Technology (INST), Phase 10, Sector 64, Mohali, Punjab 160062, India School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram 695551, Kerala, India

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‡

S Supporting Information *

ABSTRACT: Urea-functionalized 4-ethynylbenzenes undergo facile formal [2 + 2] cycloaddition followed by retroelectrocyclization upon reaction with tetracyanoethylene, yielding 1,1,4,4-tetracyanobuta-1,3-dienes-based push−pull chromophores. Unlike the N,N′-dialkylamino group, urea functionalization provides easy access to further functionalization on these chromophores. The resulting chromophores exhibit unexpected white light emissions apart from various inherent properties like intramolecular charge-transfer band and redox behavior.

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INTRODUCTION The interactions between the UV/vis/NIR-radiation and push−pull chromophores are fascinating because of their potential applications in areas of optoelectronics,1a−d colorimetric, fluorescence sensing,1e,f bio-imaging,1g and so forth. The optoelectronic properties of push−pull chromophores could easily be tuned upon modulating the design of the donor and acceptor group. This is feasible primarily because of the control by the donor and acceptor groups over the intramolecular charge transfer (ICT) process. Thermal [2 + 2] cycloaddition (CA) of the strong electron acceptor tetracyanoethylene (TCNE) with alkynes containing the electron donating group (EDG: metal-ylides), followed by retroelectrocyclization (RE) to the obtained EDG-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) were first reported by Bruce and co-workers in 19812 and followed by others.3 In 2005, seminal work by Diederich and co-workers have shown that metal-free strong EDG based on the N,N′-dialkyanilino group can undergo clean transformation at ambient temperature.4 The products obtained are highly nonplanar and some of them showed a remarkable third-order nonlinear optical property.5 This strategy was greatly expanded to synthesize various molecular structures like dendritic and dendralene systems including cascade CA−RE reactions.6 Similarly, Shoji and co-workers have used azulene as EDG to obtain push−pull TCBDs.7 Interestingly, not only strong EDG, weaker EDGs like anisole and thiophene,8 N,N′-diphenyl aniline,9 and “electronically confused alkyne” having an EDG and an electron-withdrawing group such as −CN,10 and very recently ynamides11 were found tending to undergo CA−RE reaction but, in general, require more forcing conditions. Although the abovementioned EDGs have been investigated as push−pull © XXXX American Chemical Society

chromophores which exhibit intense ICT bands, redox behavior with high thermal stability; however, further functionalization on these EDGs are either difficult or not viable,7,12 and, in general, they do not exhibit fluorophore behavior.4b,8a Herein, we used urea as EDG for the alkyne part which we hypothesized (Figure 1) to undergo CA−RE

Figure 1. TCBDs syntheses from EDG-substituted alkyne and electron-acceptor alkene.

reaction with TCNE because the electron donating ability of the urea functional group is comparable to the dimethylamino (NMe2) group according to the field effect in contrast to the Hammett constant.13 The field effect is a collective effect that resulted because of combined effective interactions operating in a molecule involving noncovalent electrostatic and electronic delocalization effects through inductive/mesomeric/resonance effects. Moreover, urea offers easy access to functional groups, hydrogen bonding moiety, and unique Received: March 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acs.joc.9b00841 J. Org. Chem. XXXX, XXX, XXX−XXX

Article

The Journal of Organic Chemistry Scheme 1. Synthesis of Urea-Functionalized TCBD Derivativesa

Reagents conditions: (i) CH2Cl2, 25 °C, 90 (8), 93 (9), 92% (10); (ii) DMF, 0 °C; 60% (1), 25 °C; 72% (2), 0 °C; 70% (3); (iii) C6H6, 25 °C, 97% (4).4a

a

value of −OMe, the reactivity of the urea substituent is largely comparable with the NMe2 group. The high reactivity of urea may not be surprising because both NMe2 and urea exhibit similar field/inductive substituent parameter (F = 0.15 for NMe2 and F = 0.19 for NHCONHEt).13 Because out of three TCBDs (1, 2, and 3), 1 and 3 are attached with the alkyl (CH2) group; hence, the F value for −NHCONHEt is more appropriate to compare than with −NHCONHPh. To the best our knowledge, F values are not reported for aryl-substituted urea in the literature. However, the observed high reactivity for all three TCBDs (1, 2, and 3) implies that the F value of −NHCONHPh might be close enough with −NHCONHEt. The compound urea-TCBDs 1, 2, and 3 were fully characterized by IR and ESI-MS spectral data, whereas 1H NMR of di-TCBDs 2 and 3 (Figures S3 and S7 in the Supporting Information) showed additional peaks unlike mono-TCBD 1 (Figure S11 in Supporting Information) in DMSO-d6, even after several batches of samples showed similar behavior, thus ruling out the possibility of impurities. Furthermore, the 13C NMR spectra of bis-TCBDs 2 and 3 showed downfield shifted urea carbonyl peak at 198 ppm in comparison to mono-TCBD 1 which showed a peak at 168 ppm, indicating intra-/inter-molecular H-bonded network in the former case.14 To verify H-bonding interactions, compound 3 was examined by the concentration-dependent 1 H NMR studies in dimethyl sulfoxide (DMSO) and no significant change was observed. Hence, the H-bonding facilitating solvent such as acetone-d6 was used to measure 1 H NMR and was found to be similar to DMSO, unlike monoTCBD 1 and di-TCBDs 2 and 3 that showed more pronounced additional peaks corresponding to partially

electron-donating ability, which is expected to show new photophysical properties.

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RESULTS AND DISCUSSIONS Scheme 1a shows the synthetic methodology adopted for the synthesis of urea-substituted phenyl-TCBDs 1−3 (ureaTCBDs) in two-step reactions. The alkyne precursors 8, 9, and 10 were synthesized starting from the commercially available different isocyanates 5, 6, and 7, respectively, in reaction with 4-ethynylaniline (EA). Characterizations of the starting alkynes (8, 9, and 10) match well with NMR (1H and 13C), ESI-MS data (see Sections A1 and A2, Supporting Information). To test the versatility of urea-alkynes 8, 9, and 10 toward CA-RE reaction with TCNE, the spacer R group was chosen with different electron-donating ability of substituents like aryl and alkyl moieties, besides, to see the effect of mono- and di-TCBD units. DMF was chosen as the solvent because alkynes 8, 9, and 10 are not soluble in common organic solvents. It was found that alkyl spacers 8 and 10 afforded 1 (60%) and 3 (70%), respectively, in good yields even at low temperature (0 °C). Reaction performed at 25 °C yielded poor yield with more complex reaction mixture owing to the high reactivity of ureasubstituted alkynes. However, aryl-alkyne 9 afforded 2 in 72% isolated yield at 25 °C, indicating the aryl spacer is relatively less reactive than the alkyl spacer. Reactivity of EDG alkynes substituted with −OMe and −NMe2 in CA-RE reaction with TCNE usually obey Hammett constant values and the −OMe (σp = −0.27) group considered to be weak EDG in comparison with −NMe2 (NMe2 = −0.82).8a,13 Despite, the Hammett constant for urea (NHCONHEt, σp = −0.26)13 is close to the B

DOI: 10.1021/acs.joc.9b00841 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry

The density functional theory (DFT) calculations were performed to investigate the molecular and electronic structure properties to interpret observations in the optical and electrochemical studies. The molecular orbitals are obtained in DFT calculations adopting the B3LYP/def2-TZVP method (for details, see Section E, Supporting Information).16 The electronic spectra were computed applying time-dependent DFT16b calculations using the CAMB3LYP16c functional. Figure 3 and Table S3 show that the highest occupied

aggregated species (see Section A3, Figures S19−S21 in Supporting Information). To further verify the presence of Hbond-mediated partial aggregated species, D2O titration experiments were performed to ascertain the effect of D2O to disrupt the H-bonding in the acetone-d6 solvent. This resulted in changes in 1H NMR signals, indicating the disruption of H-bonding and the presence of additional species as the mixture (Figures S19−S21, Supporting Information).14b However, investigation of more detailed study about its self-assembly behavior and their properties are beyond the scope of this communication. For comparison of photophysical properties of urea-TCBDs with N,N′dimethylamino-substituted phenyl TCBD (DMA-TCBD) 4 was synthesized and characterized starting from 4-ethynylN,N′-dimethylaniline (EDA) as per the reported procedure (Scheme 1b).8a After the successful synthesis of chromophores 1, 2, and 3, the photophysical properties were studied. The urea-functionalized chromophores are maroon in color and high melting solids (262−269 °C), which are slightly soluble in CH3CN, whereas they are highly soluble in polar solvents like acetone, DMF, and DMSO because of the presence of the H-bonding facilitating urea group. Figure 2 shows the UV/vis spectra of

Figure 3. Combined HOMO (pink and green) and LUMO (orange and blue) as obtained from the B3LYP/def2-TZVP method are plotted for compounds 1 to 4 with the isosurface value of 0.04 a.u.

molecular orbital (HOMO) is delocalized over the spacerurea-phenyl except for the unsubstituted dicyanovinyl moiety, which is also an indication of facile electron delocalization that eventually leads to the ICT by the urea and spacers. Lowest unoccupied molecular orbital (LUMO) and LUMO + 1 are completely localized on the TCBD moiety. The lowest energy electronic transitions for 1 and 4: H → L, for 2: H → L, H − 1 → L, H − 2 → L, H − 3 → L, and for 3: H → L, H − 1 → L correspond with the CT bands (Tables S4−S7, Supporting Information). Electrochemical study was performed in order to analyze the electron-donating and -accepting ability of urea-TCBDs in 1, 2, and 3, in comparison with the DMA-analogue 4 (Table 1, for details see Table S2, Supporting Information).

Figure 2. UV/vis spectra of chromophores 1, 2, 3, and 4 in MeCN (5 × 10−5 M).

Table 1. Redox Potentials of Chromophores 1−4.a

chromophores 1, 2, 3, and 4 in MeCN (5 × 10−5 M). Chromophores 1 and 3 with alkyl spacers exhibit absorption bands centered at ∼262, 394, and 417 nm corresponding to π−π* and n−π* transitions, whereas 2 with the aryl spacer showed bathochromic-shifted bands at 272 and 300 nm instead of 262 nm band and no bands observed at 394 and 417 nm. However, all chromophores exhibit longer wavelengths absorption bands at 482, 550, and 591 nm because of ICT, whereas DMA-TCBD 4 exhibited λmax around 340 nm in addition to broad longer wavelength CT bands between 410 and 700 nm indicating the urea group is responsible for lower wavelength bands which is also tunable significantly by aryl or alkyl substitutions. The CT nature was corroborated by acidbase titration experiments (Figures S22−25, Supporting Information). All ICT bands of 1, 2, and 3 have diminished upon addition of trifluoroacetic acid, indicating the absence of ICT because of protonation on the urea moiety which is similar to DMA-TCBD.15,8a However, a new peak arises in the range of ∼360−480 nm which partially overlaps in the ICT bands region. This peak is presumably because of the new CT band that occurs because of the protonation of the urea moiety. Interestingly, upon neutralization with Et3N, it regained ICT bands because of the comeback of its original state.

TCBDs

method

Ered,1 [V]c

Ered,2 [V]c

Eox,1 [V]

H−L gap [eV]

1

CV (DPV)b CV (DPV)b CV (DPV)b CV

−0.911 −0.932 −0.898 −0.858 −0.916 −0.913 −0.580

−1.780 −1.752

0.638

1.549

0.643

1.541

0.850

1.766

0.863

1.443

2 3 4

−1.784 −1.760 −1.741 −1.199

a

All potentials are measured against the ferrocene/ferrocenium (Fc/ Fc+) ion internal reference electrode in DMF containing 0.1 M Bu4NClO4 (at the scan rate of 0.1 V s−1). bDPV peak potentials are measured to support the peaks which is not well-resolved compound 2. cThe formal reduction potentials of the reversible peaks was estimated from peak potentials, i.e., E0 = (Epc + Epa)/2.

All urea-TCBDs have exhibited two reversible well-resolved reduction peaks at E0 ≈ −0.9 and −1.7 V vs Fc/Fc+, which corresponds to the formation of the anionic radical and dianionic species which oxidize back by two stepwise 1electron oxidation as shown by its reversibility, except 2 in which the peak was not resolved. However, differential pulse voltammetric (DPV) experiments were carried out to predict the position of the reduction waves. The TCBD moiety is C

DOI: 10.1021/acs.joc.9b00841 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry

observation is quite remarkable considering that the previously reported TCBDs do not show significant fluorescence, for example, DMA analogue 4 does not show any emission (Figure 4).8a Different batches of samples were tested and consistent results were found in order to rule out the possibility of fluorescence originating from impurities. The quantum yield was determined by the comparative method with quinine sulfate (ΦF = 0.54 in 0.1 M H2SO4)17 as a standard. The estimated ΦF value for urea-TCBDs chromophores 1, 2, and 3 was found to be 0.034, 0.033, and 0.043, respectively, at λex = 375 nm. Although the fluorescence quantum yields of ureaTCBDs were found to be 3.3−4.3%, such emission from the DMA analogue 4 with significant fluorescence has not been observed presumably because of the PET from EDG (NMe2).8a

responsible for the reversible reduction waves found in the cathodic region as observed for other reported TCBDs.4,8a The first (0.34 V) and second (0.56 V) reduction potentials of urea TCBDs are cathodically shifted than DMA-TCBD 4 regardless of aryl and alkyl spacers. The reduction potentials of the acceptors are tuned by properly choosing the donor moiety. The higher electron-donating ability of the EDG increases the reduction potential of the acceptors by increasing the electron density around the acceptor side. The 200 mV increase in the first reduction potential of 1 and 2 (because of the reduction of TCBD) compared with 4 indicates that urea could be treated as strong EDG similar to DMA. The irreversible oxidation peak may be attributed to the oxidation of the donor, that is, urea followed by the chemical reaction which leads to the decomposition of the compounds. The chromophores 1 and 2 exhibit low HOMO−LUMO (H−L) band gap of ∼1.54 eV, which is close to DMA-TCBD 4 (1.44 eV). It occurs because of the elevation of HOMO energy level, whereas the complete alkyl spacer 3 shows increased an H−L gap of 1.77 eV that exhibits oxidation potential only at 0.85 V. These results clearly indicate that the spacer itself plays a fundamental role in the ICT characteristics, and there is even no change on the donor and acceptor moieties. This observation is quite significant for tuning the opto-electronic properties of the urea-based chromophores. Given the unique electron donating ability of the urea group led us to investigate the photoluminescence (PL) properties of these chromophores. It is well known that previously synthesized TCBDs with other organic donors such as N(alkyl)2, N(aryl)2, O(alkyl), thiophene, azulene, and ynamide does not exhibit luminescence because of quenching by photoinduced electron transfer (PET)/twisted ICT processes,8a,4b whereas urea TCBDs 1, 2, and 3, upon exciting at λex = 380 nm, exhibit broad emission covering the entire visible range (400−750 nm) with two λmax centered around 430−472 and at 633 nm (Figure 4). However, 2 and 3 showed longer emission

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CONCLUSIONS

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EXPERIMENTAL SECTION

We have designed a versatile and effective CT chromophores encompassing the inherent properties, that is, PL and strong ICT bands, using formal [2 + 2] CA−RE between ureasubstituted alkynes as new EDG and acceptor alkene at ambient conditions. Interestingly, these chromophores exhibit white light emission when excited at λex = 420 nm with relatively good quantum yield. For the first time, we show that the unusual PL of TCBD chromophore is due to unique electron donating ability of the urea donor through the field effect. Easy access to differently functionalized precursors18 makes it promising for the synthesis of functionalized urea push−pull chromophores which may open up the possibility of several avenues like hydrogen bond-mediated organocatalysis, as inhibitors, sensors, and above all, optoelectronic materials. Currently, we are exploring versatility of this approach to synthesize various TCBDs from mono- and bis-substituted urea-functionalized alkynes in order to tune their optoelectronic and photoluminescent properties.

Experimental Details. General Procedures and Materials. Compounds EA, EDA, 5, 6, 7, and chemicals were purchased from Aldrich and TCI-India and used as received. Compound 4 was synthesized according to the literature procedure.4a,8a Dimethylformamide (DMF), CH3CN, CH2Cl2 were freshly distilled from CaH2 under the nitrogen atmosphere. THF was dried in sodium. Column chromatography (CC) was carried out with neutral Al2O3 (particle size 60−325 m). Thin-layer chromatography was performed on precoated plastic sheets of silica gel G/UV-254 of 0.2 mm thickness (Macherey-Nagel, Germany) using appropriate solvents and visualized with UV light (λ = 254 nm). Melting points (mp) were measured in open capillaries with a Stuart (automatic melting point SMP50) apparatus and are uncorrected. “Decomp.” refers to decomposition. 1 H NMR and 13C NMR spectra were measured on a Bruker AVANCE II 400 MHz instrument at 25 °C in CD3CN or DMSO-d6. Residual solvent signals in the 1H and 13C NMR spectra were used as the internal reference. Chemical shifts (δ) are reported in ppm downfield from SiMe4, with the residual solvent signal. Coupling constants (J) are given in Hz. The apparent resonance multiplicity is described as s (singlet), d (doublet), t (triplet), and m (multiplet). Fourier transform infrared were recorded on a Cary Agilent 660 IR spectrophotometer; signal designations: s (strong), m (medium), and w (weak). Selected absorption bands are reported in wavenumbers (cm−1). UV/vis spectra were recorded on a Shimadzu UV/vis spectrophotometer. The spectra were measured in a quartz cuvette of 1 cm at 298 K. The absorption maxima (λmax) are reported in nm with the extinction coefficient (ε) in dm3 mol−1 cm−1 in brackets. Shoulders are indicated as sh. Fluorescence spectra were measured on

Figure 4. Fluorescence spectra of TCBDs based chromophores 1, 2, 3, and 4 in MeCN (5 × 10−5 M). The inset shows the images of chromophores taken under the UV lamp (365 nm).

(633 nm) with low intensity, which appear to increase when excited at 420 nm (Figures S28−29, Supporting Information). A detailed study on determining the exact nature of this broad emission band is beyond the scope of this communication and is the subject of a follow-up paper. The observed large Stokes shift (λem,1 = 62−92 nm; λem,2 = 253 nm) is quite interesting as this leads to white light emission. The CIE chromaticity graph matches nearly with the white light (0.33,0.33) emission coordinates of 1 (0.34,0.38), 2 (0.36,0.41), and 3 (0.35,0.38) upon excitation at 420 nm (Figure S32, Supporting Information). The inset in Figure 4 shows bright white light emission from the product excited by the UV lamp. This D

DOI: 10.1021/acs.joc.9b00841 J. Org. Chem. XXXX, XXX, XXX−XXX

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The Journal of Organic Chemistry

H NMR (400 MHz, DMSO-d6, 298 K): δ 1.24−1.36 (m, 4H), 1.37− 1.47 (m, 4H), 3.04−3.09 (m, 4H), 3.97 (s, 2H), 6.18−6.21 (m, 2H), 7.27−7.43 (m, 8H), 8.62 ppm (s, 2H); 13C{1H} NMR (100 MHz, DMSO-d6, 298 K): δ 26.1, 29.9, 39.0, 79.0, 84.2, 113.6, 117.1, 132.2, 141.2, 155.0 ppm; IR (KBr) ν̃: 3297 (m), 3039 (m), 2904 (m), 2355 (w), 2104 (w), 1900 (w), 1636 (s), 1595 (s), 1541 (s), 1514 (s), 1399 (s), 1304 (s), 1229 (m), 829 (m), 768 (w), 646 (w), 544 cm−1 (w); HRMS (ESI): m/z (%) calcd for C24H27N4O2+ [M + H]+, 403.2134; found, 403.2130. Synthetic Procedures and Characterization for UreaFunctionalized Push−Pull Chromophores 1−3. 1-Benzyl-3-(4(1,1,4,4-tetracyanobuta-1,3-dien-2-yl)phenyl)urea (1). TCNE (76.6 mg, 0.6 mmol) was added to a solution of 1-benzyl-3-(4ethynylphenyl)urea 8 (150 mg, 0.6 mmol) in dry DMF (1.5 mL) in a round-bottomed flask under a N2 atmosphere and stirred at 0 °C for 12 h. After completion of the reaction, the reaction mixture was directly passed through alumina CC eluted with CH2Cl2/MeOH 90:10. The solid red purple colored compound was isolated and dried in vacuo. The compound 1 (136 mg, 60%) obtained is well soluble in acetone, DMF, DMSO, and MeOH and less soluble in MeCN, Rf = 0.3 (SiO2; CH2Cl2/MeOH 90:10); mp 268−270 °C; (decomp.); 1H NMR (400 MHz, DMSO-d6, 298 K): δ 4.32 (d, J = 5.8 Hz, 2H), 6.87 (t, J = 6.2 Hz, 1H), 7.23−7.36 (m, 5H), 7.43 (d, J = 8.4 Hz, 2H), 7.55 (d, J = 8.5 Hz, 2H) 7.94 (s, 1H), 9.08 ppm (s, 1H); 13C{1H} NMR (100 MHz, CD3CN, 298 K): δ 44.2, 114.1, 114.6, 114.8, 115.9, 116.3, 116.8, 117.5, 117.7, 118.6, 118.8, 123.0, 127.9, 128.1, 129.4, 131.1, 141.1, 144.0, 156.1, 161.4, 168.7, 173.1 ppm; IR (KBr) ν̃: 3303 (m), 2873 (m), 2871 (s), 2356 (s), 2210 (s), 1627 (s), 1583 (m), 1401 cm−1 (m); HRMS (ESI): m/z (%) 379.1301 ([M + H]+ calcd for C22H15N6O+, 379.1302). 1,1′-(Methylenebis(4,1-phenylene))bis(3-(4-(1,1,4,4-tetracyanobuta-1,3-dien-2-yl)phenyl)urea) (2). TCNE (158.6 mg, 1.238 mmol) was added to a solution of 1,1′-(methylenebis(4,1-phenylene))bis(3(4-ethynylphenyl)urea) 9 (300 mg, 0.6191 mmol) in dry DMF (5 mL) in a round-bottomed flask under a N2 atmosphere and stirred at 25 °C for 12 h. After completion of the reaction, the reaction mixture was directly passed through alumina CC eluted with CH2Cl2/MeOH 90:10. The solid red purple colored compound was isolated and dried in vacuo. The compound 2 (216 mg, 72%) obtained is well soluble in Me2CO, DMF, DMSO, and MeOH and less soluble in MeCN. Rf = 0.25 (SiO2; CH2Cl2/MeOH 90:10); mp 269−271 °C (decomp.); 1H NMR (400 MHz, DMSO-d6, 298 K): δ 3.80 (s, 2H), 7.06−7.40 (m, 10H), 7.50−7.90 (m, 8H), 8.73 (s, 2H), 9.05 ppm (s, 2H); 13C{1H} NMR (100 MHz, CD3CN, 298 K): δ 41.1, 118.6, 119.2, 120.6, 123.5, 130.1, 130.6, 131.1, 132.2, 137.5, 138.0, 143.4, 145.2, 153.4, 161.3, 168.7, 173.0, 197.7 ppm; IR (KBr) ν̃: 3359 (w), 2925 (w), 2362 (w), 2215 (w), 1660 (m), 1589 (m), 1509 (m), 1411 cm−1 (w); HRMS (ESI, negative mode): m/z (%) calcd for C43H23N12O2 [M − H + H2O]−, 757.2178; found, 757.2371. 1,1′-(Hexane-1,6-diyl)bis(3-(4-(1,1,4,4-tetracyanobuta-1,3-dien2-yl)phenyl)urea) (3). TCNE (190.94 mg, 1.490 mmol) was added to a solution of 1,1′-(hexane-1,6-diyl)bis(3-(4-ethynylphenyl)urea) 10 (300 mg, 0.745 mmol) in dry DMF (5 mL) in a round-bottomed flask under a N2 atmosphere and stirred at 0 °C for 12 h. After completion of the reaction, the reaction mixture was directly passed through alumina CC eluted with CH2Cl2/MeOH 90:10. The solid red purple colored compound was isolated and dried in vacuo. The compound 3 (344 mg, 70%) obtained is well soluble in Me2CO, DMF, DMSO, and MeOH and less soluble in MeCN. Rf = 0.3 (SiO2; CH2Cl2/MeOH 90:10); mp 262−265 °C; (decomp.); 1H NMR (400 MHz, DMSOd6): δ 1.28−1.35 (m, 4H), 1.39−1.49 (m, 4H), 3.05−3.14 (m, 4H), 6.32−6.42 (m, 2H), 7.35−7.55 (m, 8H), 7.81−7.86 (m, 2H), 8.90− 8.98 ppm (m, 2H); 13C{1H} NMR (100 MHz, CD3CN): δ 27.0, 30.6, 40.4, 118.7, 119.0, 121.0, 122.8, 128.1, 130.6, 131.1, 133.6, 142.2, 144.2, 146.0, 156.0, 156.2, 161.4, 168.7, 173.1, 197.6 ppm; IR (KBr) ν̃: 3359 (w), 2933 (w), 2852 (w), 2364 (m), 2206 (w), 1671 (m), 1590 (m), 1527 (m), 1410 cm−1 (w); HRMS (ESI): m/z (%) calcd for C36H27N12O2+ (M + H)+, 659.2380; found, 659.2382. 1

an Edinburgh FS5 spectrophotometer in a 1 cm quartz cuvette. ESIMS spectra were measured on a Bruker maXis ESI-Q-TOF spectrometer. The most important signals are reported in m/z units with M+ as the molecular ion. Quantum yields for 1, 2, and 3 were determined by comparing the integrated PL intensities (excited at 375 nm) and the absorbance values (at 375 nm) of the urea chromophores using quinine sulphate (ΦF = 0.54) as shown in ref 17. Electrochemistry. The redox properties were measured by cyclic voltammetry. N,N′-Dimethylformamide (DMF) (HPLC Grade, Spectrochem, India) was vacuum-distilled over anhydrous calcium hydride protected by 4 Å molecular sieves under the argon atmosphere before each experiments. Tetrabutylammonium perchlorate ([Bu4NClO4], TBAP) (Sigma-Aldrich, electrochemical grade) was used as the supporting electrolyte without further purification. Solutions (1 mM) of the compounds in distilled DMF was used for all studies. The voltammetric experiments were carried out in a single compartment electrochemical cell using the three-electrode potentiostat (CH Instruments 660A).The working electrode (Glassy Carbon electrode of 3 mm dia, CH Instruments, USA) was polished with 0.05 μm alumina slurry to mirror finish and sonicated to remove the abrasive particles before each experiment. The Ag/Ag+ (0.01 M) electrode is used as the quasi-reference electrode prepared from the same supporting electrolyte in acetonitrile (caution: preparation of the Ag/Ag+ reference electrode in DMF/TBAP leads to the formation of Ag nanoparticles). Platinum wire was used as a counter electrode. Later, the potential scale is corrected with ferrocene/ferrocenium-ion using ferrocene as an internal standard. Cyclic voltammetry experiments were performed with a CH Instruments three-electrode potentiostat. Synthetic Procedures and Characterization Data for UreaFunctionalized Alkynes 8−10. 1-Benzyl-3-(4-ethynylphenyl)urea (8). A solution of EA (0.44 g, 3.755 mmol) in CH2Cl2 (5 mL) under N2 was treated with benzyl isocyanate 5 (0.5 g, 3.755 mmol) dissolved in 5 mL of CH2Cl2 and stirred at 25 °C for 24 h. The white precipitate formed was collected by filtration, washed with CH2Cl2 and Et2O, and dried in vacuo. The white solid 8 (0.85 g, 90%) obtained is air-stable and well soluble in DMF and DMSO. mp 212− 216 °C (decomp.); 1H NMR (400 MHz, DMSO-d6, 298 K): δ 4.00 (s, 1H), 4.31 (d, J = 5.9 Hz, 2H), 6.70 (t, J = 5.9 Hz, 1H), 7.20−7.27 (m, 1H), 7.27−7.37 (m, 6H), 7.40−7.47 (m, 2H), 8.80 ppm (s, 1H); 13 C{1H} NMR (100 MHz, DMSO-d6, 298 K): δ 42.8, 79.1, 84.0, 113.9, 117.5, 126.8, 127.2, 128.4, 132.4, 140.2, 141.2, 155.0 ppm; IR (KBr) ν̃: 3331 (m), 2931 (m), 2856 (m), 2108 (w), 1907 (w), 1629 (s), 1589 (s), 1562 (s), 1304 (m), 1236 (m), 842 (m), 768 (m), 632 (w), 544 cm−1 (m); HRMS (ESI): m/z (%) calcd for C16H15N2O+ [M + H]+, 251.1184; found, 251.1191. 1,1′-(Methylenebis(4,1-phenylene))bis(3-(4-ethynylphenyl)urea) (9). A solution of EA (0.468 g, 4 mmol) in dry CH2Cl2 (7 mL) under N2 was treated with bis(4-isocyanatophenyl)methane 6 (0.5 g, 2 mmol) dissolved in 8 mL of CH2Cl2 and stirred at 25 °C for 24 h. The white precipitate formed was collected by filtration, washed with CH2Cl2 and Et2O, and dried in vacuo. The white solid 9 (0.9 g, 93%) obtained is air-stable and well soluble in DMF and DMSO. mp 338− 344 °C (decomp.); 1H NMR (400 MHz, DMSO-d6, 298 K): δ 3.81 (s, 2H), 4.02 (s, 2H), 7.11−7.48 (m, 16H), 8.66 (s, 2H), 8.84 ppm (s, 2H); 13C{1H} NMR (100 MHz, DMSO-d6, 298 K): δ 39.9, 79.4, 83.8, 114.5, 117.9, 118.6, 129.0, 132.5, 135.3, 137.4, 140.5, 152.3 ppm; IR (KBr) ν̃: 3304 (w), 2870 (w), 3032 (w), 2101 (w), 1635 (m), 1574 (w), 1304 (w), 1236 (w), 829 (w), 747 (w), 704 (w), 649 (w), 530 cm−1 (w); HRMS (ESI): m/z (%) calcd for C31H25N4O2+ [M + H]+, 485.1978; found, 485.1976. 1,1′-(Hexane-1,6-diyl)bis(3-(4-ethynylphenyl)urea) (10). A solution of EA (0.696 g, 5.94 mmol) in dry CH2Cl2 (7 mL) under N2 was treated with 1,6-diisocyanatohexane 7 (0.5 g, 2.97 mmol) dissolved in 8 mL of CH2Cl2 and stirred at 25 °C for 24 h. The white precipitate formed was collected by filtration, washed with CH2Cl2 and Et2O, and dried in vacuo. The white solid 10 (1.1 g, 92%) obtained is air-stable and well soluble in DMF and DMSO. mp 344−348 °C; (decomp.); E

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(b) Onitsuka, K.; Ose, N.; Ozawa, F.; Takahashi, S. Reactions of Acetylene-bridged Diplatinum Complexes with Tetracyanoethylene. J. Organomet. Chem. 1999, 578, 169−177. (c) Yamamoto, Y.; Satoh, R.; Tanase, T. Preparation of [Ru(6-C6Me6)Cl(C≡CPh)(RNC)](R=C8H9 or C6H2Me3-2,4,6) and its Reaction with Tetracyanoethylene. Crystal Structures of [Ru(6-C6Me6)Cl{C[=C(CN)2]CPh=C(CN)2}(C8H9NC)] and cis-[RuCl2(C8H9NC)2]. J. Chem. Soc., Dalton Trans. 1995, 307−311. (d) Mochida, T.; Yamazaki, S. Mono- and Diferrocenyl Complexes with Electron-Accepting Moieties formed by the Reaction of Ferrocenylalkynes with Tetracyanoethylene. J. Chem. Soc., Dalton Trans. 2002, 3559−3564. (e) Cordiner, R. L.; Smith, M. E.; Batsanov, A. S.; Albesa-Jové, D.; Hartl, F.; Howard, J. A. K.; Low, P. J. The Synthesis, Structure, Reactivity and Electrochemical Properties of Ruthenium Complexes Featuring Cyanoacetylide Ligands. Inorg. Chim. Acta 2006, 359, 946− 961. (f) Hussain, M.; Albert, D.; Wartchow, R.; Butenschön, H. The First Cyclopentadienyl alkylphosphane Nickel Chelates: Synthesis, Structures, and Reactions. Chem.Asian J. 2007, 2, 782−793. (4) (a) Michinobu, T.; May, J. C.; Lim, J. H.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.; Biaggio, I.; Diederich, F. A new class of organic donor-acceptor molecules with large third-order optical nonlinearities. Chem. Commun. 2005, 737−739. (b) For a review, see: Michinobu, T.; Diederich, F. The [2+2] CycloadditionRetroelectrocyclization (CA-RE) Click Reaction: Facile Access to Molecular and Polymeric Push-Pull Chromophores. Angew. Chem., Int. Ed. 2018, 57, 3552−3577. (5) (a) Koos, C.; Vorreau, P.; Vallaitis, T.; Dumon, P.; Bogaerts, W.; Baets, R.; Esembeson, B.; Biaggio, I.; Michinobu, T.; Diederich, F.; Freude, W.; Leuthold, J. All-optical high-speed signal processing with silicon-organic hybrid slot waveguides. Nat. Photonics 2009, 3, 216− 219. (b) Vallaitis, T.; Bogatscher, S.; Alloatti, L.; Dumon, P.; Baets, R.; Scimeca, M. L.; Biaggio, I.; Diederich, F.; Koos, C.; Freude, W.; Leuthold, J. Optical Properties of Highly Nonlinear Silicon-Organic Hybrid (SOH) Waveguide Geometries. Opt. Express 2009, 17, 17357−17368. (6) Kivala, M.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.; Diederich, F. Charge-Transfer Chromophores by CycloadditionRetro-electrocyclization: Multivalent Systems and Cascade Reactions. Angew. Chem., Int. Ed. 2007, 46, 6357−6360. (7) (a) Shoji, T.; Ito, S.; Toyota, K.; Iwamoto, T.; Yasunami, M.; Morita, N. Reactions between 1-Ethynylazulenes and 7,7,8,8Tetracyanoquinodimethane (TCNQ): Preparation, Properties, and Redox Behavior of Novel Azulene-Substituted Redox-Active Chromophores. Eur. J. Org. Chem. 2009, 4316−4324. (b) Shoji, T.; Maruyama, M.; Maruyama, A.; Ito, S.; Okujima, T.; Toyota, K. Synthesis of 1,3-Bis(tetracyano-2-azulenyl-3-butadienyl)azulenes by the [2+2] Cycloaddition-Retroelectrocyclization of 1,3-Bis(azulenylethynyl)azulenes with Tetracyanoethylene. Chem.Eur. J. 2014, 20, 11903−11912. (c) Shoji, T.; Ito, S. Azulene-Based DonorAcceptor Systems: Synthesis, Optical, and Electrochemical Properties. Chem.Eur. J. 2017, 23, 16696−16709. (8) (a) Michinobu, T.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Frank, B.; Moonen, N. N. P.; Gross, M.; Diederich, F. DonorSubstituted 1,1,4,4-Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge-Transfer Interactions by Atom-Economic Synthesis. Chem.Eur. J. 2006, 12, 1889−1905. (b) Pappenfus, T. M.; Schneiderman, D. K.; Casado, J.; Navarrete, J. T. L.; Delgado, M. C. R.; Zotti, G.; Vercelli, B.; Lovander, M. D.; Hinkle, L. M.; Bohnsack, J. N.; Mann, K. R. Oligothiophene Tetracyanobutadienes: Alternative Donor−Acceptor Architectures for Molecular and Polymeric Materials†. Chem. Mater. 2011, 23, 823−831. (9) (a) Tang, X.; Liu, W.; Wu, J.; Lee, C.-S.; You, J.; Wang, P. Synthesis, Crystal Structures, and Photophysical Properties of Triphenylamine-Based Multicyano Derivatives. J. Org. Chem. 2010, 75, 7273−7278. (b) Leliège, A.; Blanchard, P.; Rousseau, T.; Roncali, J. Triphenylamine/Tetracyanobutadiene-Based D-A-D π-Conjugated Systems as Molecular Donors for Organic Solar Cells. Org. Lett. 2011, 13, 3098−3101. (c) Misra, R.; Maragani, R.; Gautam, P.; Mobin, S.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.9b00841. NMR spectra, spectroscopic, electrochemistry, and ab initio calculations data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ashima Bajaj: 0000-0002-1179-063X Md. Ehesan Ali: 0000-0001-6607-5484 Govindasamy Jayamurugan: 0000-0001-9870-5209 Author Contributions

A.H.D. and V.G. contributed equally. The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by SERB, Department of Science and Technology (DST), grant nos. ECR/2016/000441 and SB/ S2/RJN-047/2015. G.J. thanks DST-SERB for Ramanujan Fellowship. The authors thank Panjab University (SAIF facility) for NMR and IIT-Ropar for mass facility. M.E.A. thanks CDAC-Pune for providing computational resources.

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REFERENCES

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