Rotationally resolved electronic spectroscopy of 5-methoxyindole

Abstract

Rotationally resolved electronic spectra of the vibrationless origin and of eight vibronic bands of 5-methoxyindole (5MOI) have been measured and analyzed using an evolutionary strategy approach. The experimental results are compared to the results of ab initio calculations. All vibronic bands can be explained by absorption of a single conformer, which unambiguously has been shown to be the anti-conformer from its rotational constants and excitation energy. For both anti- and syn-conformers, a (1)L(a)/(1)L(b) gap larger than 4000 cm(-1) is calculated, making the vibronic coupling between both states very small, thereby explaining why the spectrum of 5MOI is very different from that of the parent molecule, indole.

Position matters: High resolution spectroscopy of 6-methoxyindole

“…Taking the energetic order shown in Figure 3 as a reference, excitation to the 1 L a state is in general dominated by HOMO→LUMO excitation, while the 1 L b state involves equal contributions of HOMO-1→LUMO and HOMO→LUMO+4 (due to the insertion of Rydberg orbitals in 6MOI, LUMO+4 corresponds to LUMO+1 in the general case). 2,21 In Table I Table I. are listed. From the entries it is apparent that both states have one-electron excitations in common and hence are strongly mixed.…”

“…If one takes a closer look at the excited state lifetimes of other methoxyindole derivatives, it becomes apparent that they are below 7 ns. 2,8 So it seems that the methoxy group itself introduces a channel for radiationless deactivation to indole, which is independent of its position at the chromophore.…”

“…This is even more surprising as in 5-methoxyindole these problems do not occur. 2 The considerable shortening of the excited state lifetime compared to the parent molecule indole 29 is proposed to result from the methoxy group itself, introducing a new channel for radiationless deactivation to indole.…”

“…Figure 1). [1][2][3][4][5] Depending on whether electrons are donated to or withdrawn from the chromophore, the energetic position of the two lowest excited singlet states can be considerably altered, and even a change of their energetic order is observed. To elucidate the different effects systematically, one could also choose a substituent and change its position at the chromophore, like it was done, for instance, by Eftink et al, 6 and Albinsson and Nordén 7 for methyl-and methoxyindoles.…”

On the Additivity of Molecular Fragment Dipole Moments of 5‐Substituted Indole Derivatives

“…The rotationally resolved electronic spectra were fit by using evolutionary strategies (ES) described in detail in Refs. [40][41][42][43][44] For the fits of the Stark spectra, we used the CMA-ES algorithm. In this variant mutations were adapted by using ac ovariance matrix adaptation (CMA) mechanism.…”