Optical properties of irradiated imidazolium based room temperature ionic liquids : new microscopic insights into the radiation induced mutations †

Considering the future perspectives of room temperature ionic liquids (RTILs) in areas involving high radiation fields (such as the nuclear fuel cycle and space applications), it is essential to probe and have a microscopic understanding of the radiation induced perturbations in the molecular structures and the intrinsic bonding interactions existing in the ILs. Herein, a focused investigation concerning the photophysical behavior of postirradiated FAP (fluoroalkyl phosphate) imidazolium ILs revealed considerable rearrangements and bonding realignments of the ionic moieties in the ILs on irradiation, however, their physicochemical properties do not change significantly even at high absorbed doses. Most interestingly, the well-established excitation wavelength dependent fluorescence (FL) behavior of the ILs was considerably perturbed on irradiation and this is attributed to the radiation induced decoupling of pre-existing different associated structures of ions, and the subsequent formation of oligomers and other species containing multiple bond order groups. This was further substantiated by vibrational studies, where peaks appearing in the range 1600–1800 cm 1 indicated the formation of double bonded products. Furthermore, for the hydroxyl functionalized (in the alkyl side chain of the imidazolium cation) IL, a blue shift in the O–H stretching frequency was observed for the –OH group H-bonded to the FAP anion (nOH [FAP] ), while a red shift was observed for the H-bonded –OH groups in the cationic clusters. The FL lifetime values were found to increase with irradiation, which clearly indicates the enhancement in the rigidity level in the vicinity of the ions, thereby hindering the non-radiative decay processes. Such studies could contribute to the fundamental understanding of the radiation driven perturbations in the structure–property relationships, which eventually affect the radiolytic degradation pathways and the product distribution in RTILs.


FTIR spectral analysis of pre-and post-irradiated FAP1 IL
Up to a dose of 100 kGy, no significant changes were observed in the FTIR spectra of irradiated ILs, which reflects their radiation stability.2][3] At the same time, some perturbations in the imidazolium ring and possible dissociation (or dislocations) of the alkyl groups attached to it were noticed from the decrease in the intensity of peaks located in the region 1400-1480 cm -1 on irradiation of IL.These peaks (at ~ 1433 cm -1 , ~1457 cm -1 and ~ 1463 cm -1 ) originates due to the ring in-plane asymmetric stretching, (N)CH3 CN stretching, (N)CH3 HCH symmetric bending. 3,4 Siilar variations in the region 1400-1480 cm -1 were also observed in FAP2, when irradiated to high radiation doses.To precisely determine the changes in the C-H bond stretching frequencies of the ethyl and the terminal methyl group attached to the side chain of the imidazolium cation, a Gaussian peak fitting was carried out in the region 2900-3000 cm -1 and has been shown in the blue color in Fig. S6b.The peaks at ~ 2996 cm -1 accompanied by a shoulder peak at ~ 2972 cm -1 (assigned to Ethyl HCH symmetric stretching) 4 were found to be broadened and red shifted by 2-3 cm -1 in post-irradiated FAP1 (@ 400 kGy).

FTIR spectral analysis of pre-and post-irradiated FAP2 IL
The FTIR spectra of FAP2 irradiated at different radiation doses (same as in case of FAP1) has been shown in Fig. S8.On careful evaluation of the spectra, some perturbations were noticed in the bonding interactions as well as their strengths, which have been discussed as follows.A blue shift in the peak frequency originated due to C-N stretching (~ 615 cm -1 ) 5 was observed (see Fig. S9).Analogous to FAP1, a broad peak in the range 1700-1750 cm -1 was observed in IR spectra of irradiated FAP2 IL (not shown), which indicates the formation of radiolytic products with multiple bonding functional groups i.e.C=O, C=C, -CF=CF-, -CF=CF 2 . 6Peak at ~ 2976 cm -1 representing the HCH stretching vibrations in the ethoxy group (attached to ring N) 3,4 of FAP2 red shifted to ~ 2972 cm -1 on irradiation.Also, the peak at ~ 3127 cm -1 for the ring NC(H)NCH (or C 2 -H) 7 was found to be red shifted to 3123 cm -1 .The introduction of conjugation by the attachment of alkenyl groups (as mentioned in the main script) could also be one of the probable reasons behind the as observed red shift.In fact, the peaks corresponding to the formation of conjugated C=C groups were noticed in the Raman spectra of irradiated ILs (discussed in the manuscript).

Raman spectral analysis of pre-and post-irradiated FAP ILs
As can be seen, the Raman spectra of the ILs did not exhibit significant changes on irradiation up to a radiation dose of 100 kGy (see Fig. S10 & S11).However, at a radiation dose ≥ 200 kGy, the spectra appeared to vary considerably which in actual case is due to fluorescence (from the radiolytic products).The perturbations observed in the networking structure of FAP1 and FAP2 on irradiation has been explained as follows.A red shift was observed in the symmetric bending modes of -CF 3 (at ~ 742 cm -1 ) for FAP2 on irradiation (see Fig. S13a).A broad peak centered at ~ 985 cm -1 emerges at a radiation dose ≥ 50 kGy (for both FAP1 (Fig. S12a) and FAP2 (Fig. S13b)), which indicates the formation of radiolytic products with vinyl C-H groups. 8While, broad peak marked as [2] (1050-1070 cm -1 ) in Fig. S12a is most plausibly attributed to the formation of fluoroalkanes. 8Besides, in-plane bending C-H vibrational frequency (in FAP1) at ~ 1088 cm -1 red shifted to 1085 cm -1 on irradiation at a radiation dose of 100 kGy. 2 Furthermore, red shifts in the frequencies representing ring in-plane asymmetric stretching, CC stretching, CH 3 (N) CN stretching (from 1421 cm -1 to 1419 cm -1 ) and broadening in the CH 2 (N) CN stretching vibrations (~ 1570 cm -1 ) 3 was observed in Raman spectra of FAP1 on irradiation.Similar distortions in the corresponding wavenumber regions were observed for post-irradiated FAP2 (see Fig. S13).There have been additional strong evidences which indicate perturbations in the bonding interactions amongst the cationic and the anionic moieties and their surrounding environments.This could be observed primarily from the variations in the C-H stretching frequencies of the alkyl side chains in the region 2800-3200 cm -1 .For instance, in case of FAP1, peak most probably representing the ring CH 3 HCH symmetric stretching at ~ 2932 cm -1 red shifted to ~ 2929 cm -1 at high radiation dose of 100 kGy. 4 Peak at ~2953 cm -1 could be assigned to CH 2 HCH stretching frequency 4 was found to broaden at high radiation dose (see Fig. S12b), and most possibly be attributed to the presence of radiolytic products with variations in the C-H bond strengths.Further, a red shift (from 2976 cm -1 to 2970 cm -1 @ 100 kGy) in the asymmetric stretching vibrational frequency of ethyl HCH was observed (in FAP1), which signifies the weakening of the respective bond strength. 4Similar red shift was observed (asymmetric stretching vibrational frequency of ethyl HCH) for irradiated FAP2 (@ 100 kGy).Peak at ~ 3180 cm -1 representing the ring HCCH symmetric stretching 4 broadened at higher radiation doses in both the FAP ILs, which is a clear indication of the presence of radiolytic products derived from the imidazolium cation with varying bonding strengths.Table S3.Lifetime values for post-irradiated FAP1 and FAP2 at various absorbed dose values.
The values provided in the parentheses are the respective amplitudes for each times constants (T1, T2, and T3).

Fig. S2 .
Fig.S2.Plots of FL intensity vs. dose (a) and  em vs. dose (b) for FAP1 at different excitation

Fig. S4 .
Fig.S4.Plots of FL intensity vs. dose (a) and  em vs. dose (b) for FAP2 at different excitation

Fig. S5 .
Fig.S5.FL spectra of neat FAP2 irradiated with an absorbed dose of 400kGy.Inset: Plot of  exc.vs. em showing the shifts in the maximum intensity peak positions.

Fig. S10 .
Fig.S10.Raman spectra of neat unirradiated and post irradiated FAP1 at various radiation doses

Fig. S11 .
Fig.S11.Raman spectra of neat unirradiated and post irradiated FAP2 at various radiation doses

Fig. S12 .
Fig.S12.Raman spectra of neat unirradiated and post-irradiated FAP1 in different wave number

Fig. S13 .
Fig.S13.Raman spectra of neat unirradiated and post-irradiated FAP2 in different wave number