Resonance Raman intensity analysis of a dicyanovinyl-azaadamantane: Mode-specific reorganization energies for charge-transfer and locally-excited states

Abstract

A resonance Raman intensity analysis is performed on the intramolecular charge-transfer molecule 1-aza-adamantane-4-ylidenemalononitrile in acetonitrile solution. We explore the extent to which changes in molecular structure upon charge transfer can be obtained from resonance Raman intensity analysis, and extend the analysis method for charge-transfer excitation to take into account the possible influence of nearby locally excited states. Absolute scattering cross sections are measured at five excitation wavelengths spanning both the charge-transfer band at 324 nm and the lowest locally excited band at 231 nm, and the absorption spectra and resonance Raman intensities are modeled self-consistently to obtain the mode-specific reorganization energies accompanying electronic excitation to both states. Interference effects between the two states are considered but are found to be of minimal importance for this particular charge-transfer molecule. The reorganization parameters in terms of dimensionless normal coordinates are converted to actual bond length and bond angle changes by making use of a previously developed ground-state normal mode analysis and by comparing with electronic structure calculations on models for the donor and acceptor ends to reduce the indeterminacy in the signs of the dimensionless displacements. The geometry changes upon excitation to the LE state are dominated by lengthening of the ethylenic C=C bond, while for CT excitation the distortions are distributed over the donor, acceptor, and adamantane bridge, with a smaller C=C bond length change.

Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane

“…We have used ab initio calculations to help choose the appropriate sign of the normal mode displacements. [105][106][107][108][109][110] Table III compares the results of CIS/3-21G*//Becke3P86/3-21G*, and DT͑RPA͒/3-21G*// Becke3P86/3-21G* computations for the normal mode displacements with the absolute value of those determined from resonance Raman intensity analysis of the experimental spectra. There is generally reasonable agreement between the calculated and experimental values for the lower frequency modes ͑below 1000 cm Ϫ1 ͒ associated mainly with the carbon ring and iodine atom as well as the axial v 11 mode at 1190 cm Ϫ1 and the equatorial v 8 mode at 1308 cm Ϫ1 .…”

Chromophore-specific theoretical spectroscopy: From subsystem density functional theory to mode-specific vibrational spectroscopy

“…(205) and (208); see also Refs. [526,527]. The experimentally determined excitation energies of 3.61 and 4.17 eV, (and a laser wavelength of 388 nm) were assumed in these simulations in order to avoid problems caused by the wrong calculated energy gap between the two states.…”

Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

“…[29][30][31] In order to elucidate the natures of the different electronic transitions we use 457 and 785 nm excitations, which selectively probe the high and low energy absorption bands of the polymers in solution, (see Figure 2b). [29][30][31] In order to elucidate the natures of the different electronic transitions we use 457 and 785 nm excitations, which selectively probe the high and low energy absorption bands of the polymers in solution, (see Figure 2b).…”

“…[29][30][31] The different distortions resulting from excitation at different wavelengths can be readily probed using resonant Raman spectroscopy. [29][30][31] The different distortions resulting from excitation at different wavelengths can be readily probed using resonant Raman spectroscopy.…”