On the intrinsic photophysics of indigo: a time-resolved photoelectron spectroscopy study of the indigo carmine dianion

Abstract

The intrinsic photophysics of indigo has been studied using gas-phase time-resolved photoelectron imaging of the indigo carmine dianion (InC 2À ). The action spectrum reveals that the gas-phase absorption spectrum arising from the S 1 ' S 0 transition in InC 2À has a similar solvent shift to that of neutral indigo. Femtosecond spectroscopy shows that the S 1 state decays on a 1.4 ps timescale. Through isotopic substitution, the primary mechanism on the S 1 excited state can be assigned to an intra-molecular proton transfer, which is the same as that which has been observed in solution. However, the excited state lifetime is significantly shorter in vacuum. These similarities and differences are discussed in terms of recent theoretical investigations of the S 1 excited state of indigo.

Effects of resonant excitation, pulse duration and intensity on photoelectron imaging of a dianion

“…We had previously also measured an action spectrum of InC 2-by monitoring only the electron loss channel and this indicated a 90 broad absorption with a maximum at 2.2 eV (560 nm). 28 An overview of the energy level diagram of InC 2-is shown in Fig. 3 and this compares well with previous DFT calculations computed with the Gaussian09 package 38 and using the B3LYP functional 39 with a 6-311++G** basis set.…”

“…In our previous study on InC 2-, we showed that the S 1 state is short lived, with a lifetime of 1.2 ps. 28 Hence, the S 1 lifetime is an 15 order of magnitude longer than our femtosecond pulses, but at least 3 orders of magnitude shorter than the nanosecond pulses. The consequence of this is that multiple photons can be absorbed using the same S 1 ← S 0 transition, leading to large amounts of energy into the internal modes of InC 2-(2.30 eV per photon 20 cycle).…”

“…We have recently studied the excited state photophysics of InC 2-in the gas-phase using time-resolved PE imaging and showed that the mechanism for decay involves an excited state intramolecular proton transfer reaction followed by rapid internal conversion. 27,28 Our current focus is on the PE 10 imaging of InC 2-.…”

“…Previous density functional theory (DFT) calculations showed that the highest occupied orbital (HOMO) of InC 2-is the π-system on the 15 chromophore. 28 Hence, electron emission from this leaves a positively charged hole in the chromophore, while the two SO 3 groups remain negatively charged and provide a repulsive force on the emitted PE. 20,31 Because the RCB is highly anisotropic, one might intuitively anticipate that the PAD will also be highly 20 anisotropic, as long as there exists some correlation between the laboratory and molecular frames of reference.…”

Ultrafast dynamics of temporary anions probed through the prism of photodetachment

“…Our choice of ESI was primarily driven by our desire to study large complexes that could be probed by time-resolved PE imaging, 57-60 including dianions. [61][62][63][64] The focus here is on radical anions and actually, ESI is not the most ideal source as it tends to deprotonate molecules and form closed-shell anions. Nevertheless, the versatility of ESI is very appealing and, with care, radical anions can be readily generated in many cases.…”