Introduction

Photoinduced intramolecular charge transfer (ICT) is a fascinating fundamental research field and plays an important role to understand the process of photosynthesis and vision.^1–3  Experimental and theoretical studies of photoinduced ICT processes have been investigated significantly due to their biological importance.^4–6 The molecule with strong ICT nature displays large  Stokes’ shifts with instantaneous quantum yield changes and promote the charge separation.⁷ The photoinduced ICT is used for probing heterogeneous solution structures in various environments and its fluorescence characteristics are strongly depend upon local polarity.⁸ Fluorescence behaviour of ICT molecule is applied in chemical sensors, molecular electronic devices, nonlinear optical materials and solar energy conversion. These are also used for characterizing various host systems, including biomolecular systems, pH, bulk and confined liquids.^9–14 These applications stimulate the researchers to design and analyse the photophysical properties of ICT molecules in diverse environments. ^7,15–16 

Molecular rotors are ICT based fluorescent molecules, which show a twisting motion of donor group in the excited state while changing the external conditions like temperature, pressure, and solvent polarity. ^{17, 20} The twisted motion lead to a charge separation in the molecule and results in a red shift emission with narrow band gap.^18,19 These chromophores are often known as TICT based molecules, which help to observed conformational alterations in the excited states.²⁰ The twisted intramolecular charge transfer (TICT) hypothesis revealed that the molecule can show  interesting intramolecular twisting motion around a single bond and interpreted the anomalous dual emission band of 4-(dimethylamino)benzonitrile (DMABN).²¹ Due to the molecular rotation, dual emission bands are observed from locally excited (LE) and TICT states in organic solutions.²² TICT is strongly dependent on the solvent-fluorophore interaction like solvent polarity, hydrogen bonding interaction, and steric hindrance. In TICT state of quasi-planar fluorophore, the donor or acceptor moiety rotate towards a nearly perpendicular alignment upon photoexcitation. The rotation greatly enhances the ICT and the TICT state becomes stabilized in polar solvents due to strong solvent-solute interaction. The phenomena promote the formation of highly reactive charge separated non-emissive species.²³ Tuning the TICT state in organic fluorophores have found numerous applications in functional material field such as bulk viscosity measurement, fluorescent sensors of micro viscosity probe in living cells, probing dynamics of polymer formation and protein aggregation solvent free volume.^24–27

Donor-acceptor (D-A) organic compounds are the subject of many interests regarding ICT processes in solution. The photophysical study of D-A systems has drawn considerable attention in applied research fields. ^28,29 The phenomenon of charge transfer processes has been established  through theoretical and spectroscopic studies^30–32 We are interested to investigated the ground and excited state properties of cyano substituted aromatic compounds (Chart 1) depending upon excitation wavelength and concentration in solution. These compounds show interesting photophysical properties due to the presence of cyanide groups. These properties can be significantly explored in laser matrix for obtaining lesser band-width.

Chart 1: Structural Formula and Abbreviation for Investigated System Click here to view Figure

Experimental Section

Chemicals and Synthesis: All the chemicals (analytical grade) were procured from reputed company and used as received.

Procedure for Synthesis of DMDC and DMMC:

A mixture of 4-(Dimethylamino)benzaldehyde (1.0 mmol), manolonitrile derivative (1.01 mmol) and catalyst MP(DNP) (N-methylpiperidine-dinitrophenol MP(DNP): 0.05 mmol) in ethanol containing round bottom flask. The reaction mixture  was stirred at room temperature for 2 h. The reaction was monitored using TLC. After completion of the reaction, the solid product was obtained and collected by filtration. During filtration,  the solid ppt was washed with water and aqueous ethanol several time. The pure product was obtained and collected from recrystallization from ethanol. ³³

Characterization of DMMC:  State: Solid; Colour: Yellow; MP.: 122 °C; FTIR (KBr, cm^-1): 2945; 3059, 3036, 2938, 2220, 1600, 1542, 1550, 1490; ¹H NMR (500 MHz, CDCl₃): δ (ppm) 8.07 (s, 1H), 7.93 (d, J = 9.0 Hz, 2H), 6.69 (d, J = 9.5 Hz, 2H), 4.34 (q, J = 7.5 Hz, 2H), 3.10 (s, 6H), 1.37 (t, J = 7.0 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 164.3, 154.6, 153.6, 134.0, 119.5, 117.7, 111.6, 94.2, 62.1, 40.1, 14.3.

Characterization of DMDC:  State: Solid; Colour: Yellow; MP.: 180 °C; FTIR (KBr, cm^-1): 3050, 3030, 2939, 2225, 1545, 1555, 1499; ¹H-NMR (300MHz, CDCl₃): δ (ppm) 7.80 (d, J = 9.0 Hz, 2H), 7.45 (s, 1H), 6.69 (d, J = 8.4 Hz, 2H), 3.16 (s, 6H); ¹³C-NMR (75MHz, CDCl₃): δ(ppm) 158, 133.7, 131.9, 111.5, 110.9, 40.0.

Instrumentation

UV-Vis and Fluorescence Measurement: The steady state absorption spectra  were recorded on Hitachi UV-vis U-3501 spectrophotometer. The fluorescence spectra were recorded on a Perkin-Elmer LS55 fluorescence spectrophotometer equipped with a 10 mm quartz cell and a thermostat bath.

Results and discussion

UV-Visible studies in different solvents

Initially the UV-Vis spectra of DMMC and DMDC is carried out in different solvents. The normalized spectra of DMMC and DMDC is shown in Figure 1 and 2 respectively. Presence one strong electron donor (NMe₂) and two electron acceptor (CN and CO₂Et) group led to the formation of ICT band  in DMMC (Figure 1). The spectral shift is found to be ~28 nm from cyclohexane (405 nm) to DMSO (433 nm). The π →π* electron transition for core benzene ring of the D-A system is found to be very strong compared to weak ground state ICT of DMMC molecule and appeared at 326 nm in cyclohexane and 354 nm in H₂O respectively.

Figure 1: Normalized UV-Vis spectra of DMMC in solvents (Conc:1.2x10-5M). Click here to view Figure

Further, strong ICT band is observed in DMDC due to presence of one strong electron donor (-NMe₂) and two electron acceptor (-CN ) groups. The relatively strong ICT band of DMDC is shown in Figure 2 and spectral shift is found to be 26 nm from cyclohexane (414 nm) to DMSO (440 nm).  The π→π* electron transition for core of the donor-acceptor system is found to be very weak compared to strong ICT band and appeared at 340 nm.

Figure 2: Normalized UV-Vis spectra of DMDC in solvents (Conc:1.2x10-5M). Click here to view Figure

The strong π→π* electron transition of DMMC is to be high electron rich character of benzene ring (Figure 1) and the weak π→π* electron transition of DMDC is due to quinoide character of the benzene ring (Figure 2).

Fluorescence Studies in different solvents at different excitation wavelength

The fluorescent (FL) spectra of DMMC and DMDC are measured in different solvent and shown in figure 3 and 4 respectively at different excitation wavelength. The FL spectra of DMMC in different solvents revealed that an emission band is observed for solvents (Figure 3a-3b) except H₂O (at 330 nm and 420 nm excitation wavelength). The emission band is strong and observed at lower energy (445 nm-500 nm) for DMSO (493 nm), ACN (487 nm), CHCl₃ (473 nm), and hexane (448 nm) (figure 3a-3b) at 330 nm and 420 nm excitation wavelength. In case of H₂O, a strong emission is observed at 542 nm in both excitation wavelength (Figure 3a-3b at 330 nm and 420 nm excitation wavelength). Interestingly, solvent polarity affects the emission spectra significantly. Form nonpolar solvent to polar solvents, emission band is shifted from 449 nm (Hexane) to 493 nm (DMSO) (fig. 3a-3b) except emission band in H₂O. In H₂O, a single emission band is observed at 542 nm (at 420 nm excitation) due to the presence of aggregates. The emission band is observed at 360 nm due to emission from higher excited energy state (S₂) at 330 nm excitation (Fig 3b).

Figure 3: Fluorescence Spectra of DMMC in different solvents (@420 nm (a) and @330 nm (b)) (Conc:1.2x10-5M) Click here to view Figure

The FL spectra DMDC displays almost similar fluorescence behaviour in solution under identical condition. Emission spectra is observed at range of 350-500 nm depending upon the nature of solvents (Figure 4a-4b, ESI-Table S4) except H₂O (at 330 nm and 420 nm excitation wavelength). The strong emission bands are appeared at lower energy (465-490 nm) for DMSO (495 nm), ACN (490 nm), CHCl₃ (477 nm), and hexane (454 nm) (figure 4a-4b) at 330 nm and 420 nm excitation wavelength respectively. In case of H₂O, a strong emission band is observed at 488 and 547 nm in both excitation wavelength (Figure 4a-4b at 330 nm and 420 nm excitation wavelength). The overlapped two emission bands are observed at 488 nm and 555 nm with different intensity (at 420 nm and 320 nm excitation, after deconvolution). At 420 nm excitation, the intensity of in 547 nm emission band is slightly lower compare to higher energy (488 nm) emission band in H₂O (Figure 4). Interestingly, the solvent polarity influences the emission spectra significantly. Form nonpolar solvent to polar solvents, emission band is shifted from 454 nm (Hexane) to 495 nm (DMSO) (fig. 4a-4b) except emission band in H₂O. In H₂O, two emission band is observed at 488 nm and 547 nm (at 420 nm and 310 nm excitation) due to the formation of aggregates. The intensity of these two emission band in H2O is altered depending upon excitation wavelength (Fig4a-4b). The emission band at 345 nm is observed due to emission from higher excited energy state (S₂) at 310 nm excitation (Fig 3b).

Figure 4: Fluorescence Spectra of DMDC in different solvents (@420 nm (a) and @310 nm (b)) (Conc:1.2x10-5M). Click here to view Figure

Concentration dependent Fluorescence Spectra

Further, the concentration dependent and excitation wavelength dependent fluorescence spectra of DMMC and DMDC are measured in different solvents (Fig. 5a-5b and 6a-6b and ESI-Fig. 1-36).  Concentration effect on fluorescence spectra of DMMC is observed at different excitation (at 420 nm and 330 nm excitation). At 420 nm excitation, fluorescence quenching is observed in all solvents except H₂O (Fig. 5 and ESI-Fig.1-9). There is no change of fluorescence spectral pattern at 420 nm excitation for DMMC in solution (ESI Fig.1-9). However, change of spectral pattern is observed at 330 nm excitation in solution (ESI-Fig. 10-18). Interestingly, concentration dependent fluorescence quenching is not observed with the variation of concentration in polar solvents (ESI-Fig.10-14), but observed in non-polar solvents at 330 nm excitation (ESI-Fig. 15-18). Hence, FL spectra DMMC exhibits different emission behaviour depending upon concentration and excitation wavelength in solution.

Figure 5: Concentration dependent Fluorescence Spectra of DMMC in H2O (@330 nm (left) and in ACN@420 nm(right)). Click here to view Figure

Concentration effect on fluorescence spectra of DMDC is also investigated at different excitation (at 420 nm and 310 nm excitation). Excitation at 420 nm resulted in fluorescence quenching in all solvents except H₂O (Fig. 6a and ESI-Fig.19-36). There is no change of fluorescence spectral patter at 420 nm excitation for DMDC in solution (ESI Fig.18-27). However, in H₂O, two emission bands are observed and altered depending upon concentration (fig 5a). At 310 nm excitation, fluorescence intensity increases with concentration in polar solvents, whereas solvent dependent fluorescence quenching or enhancement are observed for DMMC at 310 nm excitation. The change of spectral pattern is witnessed at 310 nm excitation in nonpolar solvents (ESI-Fig. 23-27). Remarkably, the fluorescence intensity increases with the variation of concentration in polar solvents (ESI-Fig.18-23), but decreases in non-polar solvents at 330 nm excitation (ESI-Fig. 24-27). From the detailed analysis of FL spectra DMDC, it shows different emission behaviour depending upon concentration and excitation wavelength in solution.

Figure 6: Concentration dependent Fluorescence Spectra of DMDC in H2O (@310 nm (left) and THF @310 nm(right)). Click here to view Figure

Conclusion

In conclusion, we have demonstrated the TICT and ICT process of dipolar molecule (DMMC and DMDC) in solution at ground state. Excitation wavelength and concertation dependent emission behavior of dipolar molecule (DMMC and DMDC) have been investigated in solution. DMMC shows TICT process but DMDC shows ICT process at ground state in solution. Both molecule show the ICT emission process at excited state depending upon excitation wavelength and concentration. During concentration dependent study, concentration dependent quenching is observed in all solvents except H₂O at 420 nm excitation  for DMDC and DMMC in solution. Concentration dependent quenching is not observed in polar solvent but observed in nonpolar solvent at 330 nm excitation for DMDC and DMMC in solution. Further, interesting factor is DMMC shows single emission band is observed at 542 nm due to the presence of aggregates. But in case of DMDC in H₂O, two emission band is observed at 488 nm and 547 nm due to the presence of ICT emission and aggregates. Interestingly, solvent polarity effects the emission spectra significantly. Form nonpolar solvent to polar solvents, emission band is shifted from ~449 nm (Hexane) to ~ 493 nm (DMSO) except emission band in H₂O. These different ground and excited state of DMMC and DMDC are obtained due to slight modification of structural and dipolar state.