Lightening up a Dark State of a Pentacene Derivative via N-Introduction

Abstract

N-Heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field effect transistors. Here, we studied the structural and the electronic properties of an arrow-shaped N-heteropentacene derivative (triisopropylsilyl-dibenzodiazapentacene, TIPS-BAP) adsorbed on Au(111) in the monolayer and thin films using temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, and two-photon photoemission spectroscopy. In addition, we performed state-of-the-art quantum chemical calculations to further elucidate electronic properties. TIPS-BAP adopted an adsorption geometry in which the molecular backbone is oriented parallel to the Au(111) surface. We quantitatively determined the energetic position of several unoccupied as well as occupied molecular electronic states (transport states) with respect to the Fermi level of the gold substrate and resolved the optical gap (S0 → S1 transition) to be 1.9 eV. Compared to the corresponding polycyclic aromatic hydrocarbon TIPS-dibenzodipentacene (TIPS-BP), the optical gap is reduced by 0.2 eV due to nitrogen substitution. Based on our quantum chemical calculations, we attributed this effect to a stabilization of the first excited singlet state (S1) in the polar environment. Furthermore, an intense α-band (S0 → S2) in TIPS-BAP compared to TIPS-BP is observed due to an enhanced oscillator strength in the N-heteropolycyclic aromatic compound.

Influence of N-introduction in pentacene on the electronic structure and excited electronic states

“…In particular, the α-band in the experimental electronic spectra increases in N-heteropentacenes compared to pentacene depending on the substitution pattern. 11–13 From a theoretical perspective, it has been noticed that, in addition, Platt's classification scheme for the energetically lowest ππ* states of PAHs as L a and L b states is not generally applicable to aromatic N-heteropolycycles. 14 Hence, an alternative, generally applicable characterization scheme of these states has been developed, which is based on the physical properties of the excited states.…”

“…15–28 High-resolution electron energy loss spectroscopy (HREELS) provides the required insight into both adsorption and electronic properties of molecular compounds at interfaces to (semi)conducting surfaces and within thin films. 11,29–39 Depending on the primary energy of the incident electrons molecular vibrations or electronic transitions can be excited, including optically forbidden transitions (singlet to triplet states) as already successfully demonstrated for some organic compounds 40–42 including also the N-heteropolycycle 6,13-diazapentacene (6,13-DAP). 12 While the adsorption and electronic properties of pentacene (PEN) and its derivatives adsorbed on various metallic substrates have been investigated in great detail (see for example ref.…”

“…43–47 and references therein), so far only a limited number of studies are available addressing N-substituted pentacene derivatives. 11,12…”

Deciphering excited state properties utilizing algebraic diagrammatic construction schemes of decreasing order

“…In the absorption spectrum of anthracene, for example, the 1 L w (1B 2 u ) state is visible at 3.67 eV, whereas the 1 L s (1B 3 u ) state has practically no oscillator strength at an excitation energy of 3.89 eV at the theoretical level of ADC(2) (Figure 2). However, when acenes are chemically modified by substitution or, as is the case here, by heteroatom introduction, the intensity of the 1 L s state increases 37,38 . Focusing only on the 1 L s (1B 3 u ) state, this effect is here demonstrated for (1,4,5,8)‐tetraazaanthracene (Figure 1), for which the computed absorption spectrum shows an increase of the intensity of the 1 B 3 u ( 1 L s ) and a decrease of the intensity of the corresponding energetically higher‐lying 2 B 3 u state compared to anthracene (Figure 2).…”

Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation

“… 17 The latter can be determined in surface-adsorbed molecules with 2PPE, 10 − 13 , 18 differential reflectance spectroscopy, 19 − 22 or high-resolution electron energy loss spectroscopy. 18 , 23 − 29 …”