Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching

Abstract

Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (—COOD) and amide (—ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S1 ← S0 origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states. From the best-fit parameters, the positions of the H-atoms in the principal axis frames of each conformer have been determined and serve to unambiguously identify the syn forms (i.e., COH⋯O) of the cis and trans rotamers. The experimental S0 and S1 inertial parameters, hydrogen atom positions, and transition dipole moment (TDM) orientations are compared with the results of theoretical calculations. The TDM orientation indicates that the S1 state is the 1La state in contrast to most substituted indoles. The molecular orbital properties and natural charges are investigated to better understand the 1La/1Lb state reversal and the extent of photoinduced intramolecular charge transfer that impacts the rotamer-dependent fluorescence lifetimes.

Tryptophan as a Template for Development of Visible Fluorescent Amino Acids

“…Similarly, Schmitt and co-workers , have studied the electronic transition properties of 4CN-indole, using both experimental and theoretical methods, and concluded that unlike most other indole derivatives, the lowest excited singlet (S 1 ) state of 4CN-indole in the gas phase is mainly 1 L a -character and the corresponding transition dipole moment runs through positions 1 and 4 (Figure ). A similar conclusion was also reached for indole-4-carboxylic acid (4CO 2 H-indole) by Plusquellic and co-workers . In comparison, however, for both 4-methoxyindole and 4-fluoroindole, the S 1 state was found to be representative of the 1 L b character. , These results are interesting in that the absorption spectra of 4CN-indole and 4CO 2 H-indole are red-shifted from that of indole and that the CN and COOH groups are strong EWGs, whereas F is a weak EWG and the absorption spectrum of 4-fluoroindole is blue-shifted .…”

“…A similar conclusion was also reached for indole-4-carboxylic acid (4CO 2 H-indole) by Plusquellic and co-workers. 58 In comparison, however, for both 4-methoxyindole and 4-fluoroindole, the S 1 state was found to be representative of the 1 L b character. 59,60 These results are interesting in that the absorption spectra of 4CN-indole and 4CO 2 H-indole are red-shifted from that of indole and that the CN and COOH groups are strong EWGs, whereas F is a weak EWG and the absorption spectrum of 4-fluoroindole is blueshifted.…”

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm

“…Efforts are also on-going to improve sensitivity by using dual-drive Mach-Zehnder modulators, to expand coverage through non-linear processes after amplification using EDFAs and to adapt the methods for multiheterodyne IPDA and DIAL studies of GHGs. The enhanced bandwidth and rapid scan rate of dual comb methods can be advantageous for pump-probe molecular dynamics experiments and for quantum coherence studies in cold molecular beams [35,37]. Finally, simple modifications to this method open the way to improve sensitivity using background-free detection methods [21,55].…”

“…For example, standoff methods for remote sensing of greenhouse gases (GHGs) in the atmosphere including differential absorption LIDAR (DIAL) [27,28] and natural target integrated path differential absorption LIDAR (IPDA) [29,30] typically require photomultipliers or avalanche photodiodes [31,32] to detect faint backscattered returns and therefore, require high power per comb tooth over bandwidths of (2-5) cm -1 and at comb resolutions from 100 MHz to 250 MHz [33]. Single quantum level studies of molecules prepared in cold environments also require (2-5) cm -1 of spectral coverage to investigate a single rovibrational [34] or rovibronic band [35,36,37] but with sub-MHz resolution. Under these conditions, achieving the highest signal-to-noise ratio for a given source power and measurement time requires limiting the bandwidth coverage.…”

High-resolution UV spectroscopy of 1-indanol

“…13,14 Rotationally resolved laser-induced fluorescence (LIF) spectra can probe the geometrical structures of interrogated molecules in both ground and electronically excited states. [15][16][17][18][19] In the present work, we report electronic spectra recorded in an apparatus that can perform both low-resolution REMPI and high-resolution LIF measurements under identical conditions. Fits to the fully rotationally-resolved UV spectra of the electronic origin bands of the two lowest energy conformers (eq1 and ax2, Figure 1.a&b) provide further structural insight to the conformational geometries.…”