Low‐Energy Electronic Excitations of N‐Substituted Heteroacene Molecules: Matrix Isolation Spectroscopy in Concert with Quantum‐Chemical Calculations

Abstract

N‐Heteropolycycles are attractive as materials in organic electronic devices. However, a detailed understanding of the low‐energy electronic excitation characteristics of these species is still lacking. In this work, the matrix isolation technique is applied to obtain high‐resolution absorbance spectra for a series of tetracene and core‐substituted N‐analogues. The experimental electronic excitation spectra obtained for matrix‐isolated molecules are then analysed with the help of quantum‐chemical calculations. Additional lower energy excitation bands in the spectrum of the core‐substituted N‐derivatives of tetracene could be explained in terms of intensity borrowing from dipole‐forbidden transitions due to Herzberg–Teller vibronic coupling. In the case of tetracene, evidence for the additional formation of London dimers (J aggregates) is found at higher tetracene concentrations in the matrix.

High‐Resolution Electronic Excitation and Emission Spectra of Pentacene and 6,13‐Diazapentacene Monomers and Weakly Bound Dimers by Matrix‐Isolation Spectroscopy

“…Following established procedures, [44] we varied the concentration of the analyte by regulating the flow rate of the matrix gas. An increasing neon flow rate leads to a decreasing concentration of the analyte in the resulting matrix.…”

“…This technique is powerful in terms of the simulation of gas phase analytics and the deconvolution of the vibronic states in the absorbance spectra and was hitherto appliedt oa series of higher acenes, including pentacene, in solid argon by Bettinger et al [43] as well as tetracene anda zatetracenes in neon. [44] Here, we extend our studies to the pentacene analogues trapped in solid neon (Scheme 1).…”

“…As imilar occurrence in the matrix spectra was previously observed in the case of tetracene and assigned to van der Waals dimers. [44] We also assign herein the additional bands to PEN dimers, possibly in the form of Ha ggregation due to the blue shift in the absorbance, for which we are however not yet able to provide am atching structural proposal due to the computational demands.…”

How to Predict Excited State Geometry by Using Empirical Parameters Obtained from Franck‐Condon Analysis of Optical Spectrum

“…Therefore, in order to access such data, in general, the matrix isolation technique is exploited to record the high resolution absorption spectra at very low temperatures and in gaseous matrices. [70][71][72] The limited availability of such experimental spectra may hamper the wide scope applicability of the presented method, but this is only true for relatively large molecules with many vibrational degrees of freedom. The excited state geometry of small molecules with fewer vibrational modes can always be predicted as long as there is some discernible vibronic progression present in the experimental absorption spectra.…”

“…Some experimental spectra clearly show the presence of hot bands [73] or vibrational progressions belonging to dimeric species. [70,71] Therefore, the applied theoretical model should account for those features in the optical spectra in order to obtain satisfactory fitting results.…”