High-Resolution and Dispersed Fluorescence Examination of Vibronic Bands of Tryptamine: Spectroscopic Signatures for L_a/L_b Mixing near a Conical Intersection

Abstract

The vibronic spectrum of tryptamine has been studied in a molecular beam up to an energy of 930 cm(-1) above the S(0)-S(1) electronic origin. Rotationally resolved electronic spectra reveal a rotation of the transition dipole moment direction from (1)L(b) to (1)L(a) beginning about 400 cm(-1) above the (1)L(b) origin. In this region, vibronic bands which appear as single bands at low resolution contain rotational structure from more than one vibronic transition. The number of these transitions closely tracks the total vibrational state density in the (1)L(b) electronic state as a function of internal energy. Dispersed fluorescence spectra show distinct spectroscopic signatures attributable to the (1)L(b) and (1)L(a) character of the mixed excited-state wave functions. The data set is used to extrapolate to a (1)L(a) origin about 400 cm(-1) above the (1)L(b) origin. DFT-MRCI calculations locate a conical intersection between these two states at about 900 cm(-1) above the L(a) origin, whose structure is located along a tuning coordinate which is close to a linear interpolation between the two excited-state geometries. Along the branching coordinate, there is no barrier from (1)L(a) to (1)L(b). A two-tier model for the vibronic coupling is proposed.

Rotationally resolved electronic spectroscopy of 5-methoxyindole

“…3͒. This is very different from the cases of indole 17,18 and tryptamine, 39 where strong vibronic mixing significantly affects both the intensities and the band type of many vibronic bands. Also the band type, which reflects the orientation of the transition dipole moment, is practically the same in the FC and the FCHT approximation.…”

A Spectroscopic Survey of Substituted Indoles Reveals Consequences of a Stabilized 1Lb Transition

“…Both theoretical studies and measurements on indoles in van der Waals clusters have found evidence of 1 L a / 1 L b mixing. Gas phase geometrical parameters for the ground and 1 L a / 1 L b excited states of 5‐hydroxyindole have been calculated at the CASSCF and TDDFT levels of theory, respectively .…”

“…Emission from 1 L a is difficult to rule out because the energetic position of the 1 L a origin remains unknown, and is hidden by overlap with the 1 L b transition. Various spectroscopic techniques , theoretical calculations and tryptophan model compounds have been applied to resolve this conundrum.…”

Femtosecond Conical Intersection Dynamics of Tryptophan in Proteins and Validation of Slowdown of Hydration Layer Dynamics

“…The 1 L a (S 2 ) state in polar environments lies below the 1 L b (S 1 ) state due to its larger static dipole moment. 12 Ultrafast internal conversion through conical intersection (CI) was proposed from the higher 1 L b to lower 1 L a state 13-15 and observed to occur in ~40 fs in bulk water. 16,17 The internal conversion from 1 L a to 1 L b in gas phase (or in vacuum) has also been observed in 20-100 fs.…”

“…Note that fa/fba indicates the difference of emission coefficients of the initial excited 1 L a state and the transferred 1 L a state at the same emission wavelength (310 or 335 nm), reflecting that the emission at the same wavelength could be from the different vibronic 1 L a states and that the transferred 1 L a is not at the same energy level of the initial excited 1 L a state, consistent with the CI mechanism. 13-15 …”