Understanding UV–Vis Spectra of Halogenated Tetraazaperopyrenes (TAPPs): A Computational Study

Abstract

The UV–vis absorption and emission spectra of halogenated tetraazaperopyrenes (TAPPs) have been investigated employing second-order approximate coupled cluster (CC2) and (time-dependent) density functional theory (DFT). We have found that the qualitative estimates of (vertical) absorption and excitation energies are possible within a single particle picture based on frontier orbitals, but the single particle picture is not sufficient to achieve quantitative accuracy. Going from the single-particle picture to the many-particle picture improves the agreement with experimental results, but still no satisfying correlation of theory and experiment is obtained. The comparison of CC2- and DFT-based methods reveals that deviations from the experimental results cannot be explained by deficiencies of the electronic-structure methods but rather stem from neglecting vibrational effects. An agreement of theoretical results and experimental spectra is found for adiabatic excitation energies, which are given as energy differences of vibronic states, which are directly accessible using both theoretical and experimental methods. The most pronounced vibronic influence is found for the Stokes shifts, which are significantly overestimated by computing the vertical electronic transitions only. Based on the vibronic contributions, the small Stokes shift of the TAPP compounds can be explained by the temperature dependence of the vibrationally resolved UV–vis spectra.

Twisting the TAPPs: Bay‐Substituted Non‐planar Tetraazapero‐pyrenes and their Reduced Anions

“…Accurate geometries for the Franck-Condon simulations were obtained with tightened thresholds of 10 À9 and employing D 2 symmetry. [48] The calculations of the vibronic (Franck-Condon) spectra were conducted using the HOTFCHT program. [73][74][75] Interconversion energies:T he geometries and their relative energies have been computed using the Turbomole program V7.2.…”

“…Since computed vertical electronic transition energies cannotb ed irectly related to the experimental results, we also simulated vibrationally resolved adiabatic transitions. [48] Simulated absorption and emission spectra of 5d are displayed in Figure 5. At zero temperature, the simulated spectra show distinct transitions for individual vibronic transitions.…”

“…A class of molecular dyes which has displayed photophysical and redox properties similar to the ubiquitous PDIs are the tetraazaperopyrenes (TAPPs) which have been investigated for more than a decade . Whereas the electron accepting character of the PDIs is due to carboximide substituents at both ends of the perylene core, TAPPs contain two pyrimidine rings fused with the central perylene unit.…”

“…[34,35] Ac lass of molecular dyes which has displayed photophysical and redox properties similart ot he ubiquitous PDIs are the tet-raazaperopyrenes (TAPPs) whichh ave been investigated for more than ad ecade. [36][37][38][39][40][41][42][43][44][45][46][47][48] Whereas the electron accepting character of the PDIs is due to carboximide substituents at both ends of the perylene core, TAPPs contain two pyrimidine rings fused with the centralp erylene unit. The electron accepting character of the resultingN -heteropolycyclesi st hus effectively built into the aromatic core itself rather than substituent-induced.…”

The KOALA program: Wavefunction frozen‐density embedding

“…A comparison with the vertical absorption energy of 3.50 eV yields a shift of 0.28 eV, which is in agreement with earlier CC2 results. [ 144 ] Note that this difference does not correspond to the experimental Stokes shift in this case as, for these compounds, the 0‐0 transition is allowed, leading to a vanishing Stokes shift at zero temperature. Using the KOALA program, it is also possible to seminumerically compute second derivatives from which zero‐point vibrational energies and vibrations can be obtained to model vibrationally resolved UV/vis spectra.…”