Ultrafast Relaxation and Reaction of Diiodide Anion after Photodissociation of Triiodide in Room-Temperature Ionic Liquids

Abstract

Vibrational dephasing, vibrational relaxation, and rotational relaxation of diiodide (I(2)(-)) after photodissociation of triiodide (I(3)(-)) in room-temperature ionic liquids (RTILs) were investigated by ultrafast transient absorption spectroscopy. The vibrational energy relaxation (VER) rate of I(2)(-) produced by the photodissociation reaction of I(3)(-) was determined from the spectral profile of the transient absorption. The rates in RTILs were slightly slower than those in conventional liquids. On the other hand, the coherent vibration of I(2)(-) was not observed in RTILs, and the vibrational dephasing of the photoproduced I(2)(-) was accelerated. This was explained by the interaction between I(2)(-) and I consisting of a caged contact pair in RTILs. The orientational relaxation time of I(2)(-) determined by the transient absorption anisotropy was much longer in RTILs than in conventional liquids due to their high viscosities although the relaxation time was shorter than the prediction from the Stokes-Einstein-Debye (SED) theory. The deviation from the SED prediction was interpreted by the frequency dependence of the shear stress acting on the molecule. The dynamics of I(2)(-) in 1-butyl-3-methylimidazolium iodide ([BMIm]I) were quite different from those in other conventional RTILs: the coherent vibration of I(2)(-) was observed for the time profile of the transient absorption and the initial value of the anisotropy was reduced to 0.31 from 0.36 in conventional RTILs. These results suggest that an ultrafast reaction between the photofragment I and the solvent I(-) may occur during the photodissociation process of I(3)(-). The anomaly in the ground state coherent vibration and steady state Raman spectrum of I(3)(-) also suggest the possibility that I(3)(-) and I(-) can be located in vicinity and interact strongly with each other in [BMIm]I.

Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation

“…2b is a clear manifestation of the gradual change of molecular character, since the symmetric stretching mode of I 3 − (110-112 cm -1 ) has slightly higher frequency than the stretching mode of I 2 −• (e.g. 102-106 cm -1 ), as established in previous solution-phase studies 9,12,22 and in our DFT calculations (see Supplementary Table 2-3).…”

“…The alternative two-body dissociation channel (I 3 − ℎ � � I 2 + I − ) contains a distinct monatomic fragment, the iodide anion (I − ), but its reactivity is low and the formation of the dianion I 4 2− through an analogous bimolecular association pathway, I − + I 3 − ⇌ I 4 2− , is thermodynamically discouraged 34,35 . Furthermore, the other photofragment, I 2 , will generate the well-characterized absorptive signal in the region of 500-600 nm from its major absorption bands 36 , and will follow different kinetics with respect to the spectral in condensed phases [9][10][11][12]14,22,38 . The use of thin single-crystal samples, broadband probe and significantly improved SNR allowed us to capture these reaction details.…”

“…1b). The conical-intersection-mediated 8 bond-breaking event takes place within a few hundred femtoseconds after photoexcitation, allowing the transfer of vibrational coherence from the reactant to the principal diatomic photoproduct, diiodide radical (I 2 −• ), in condensed phases [9][10][11][12] . The strength of the SBI is quantifiable by the vibrational dephasing time of 11 the recombination rate of I 2 −• , 13 and the recovery rate of the ground-state I 3 − .…”

Perspective: Two-dimensional resonance Raman spectroscopy

“…[58][59][60][61][62] The photodissociation process is extremely fast; reported time constants are comparable to 63,64 (or faster than) 58,60 the 300-fs period of the symmetric stretching vibration. Perhaps most surprising was the discovery that the diiodide product also exhibits coherent vibrational motions (i.e., a vibronic coherence transfer process takes place) despite strong interactions with the solvent environment.…”

Photo-dissociation dynamics of bis(p-dimethylaminophenyl) disulfide in ionic liquids studied by ultrafast transient absorption spectroscopy

“…In particular, the photo-dissociation reaction is an interesting reaction to probe solvent effects such as cage effect, diffusional effect, solvation dynamics etc. Previously, our group has studied photo-dissociation of triiodide in ILs by sub-picosecond transient absorption spectroscopy [3,4]. By the UV excitation, triiodide dissociates into diiodide and iodine, and the photo-fragments geminately recombine partially in conventional solvents.…”