Determination of Energetic Positions of Electronic States and the Exciton Dynamics in a pi-Expanded N-Heterotriangulene Derivative Adsorbed on Au(111)

Bridged triarylamines, so-called N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in optoelectronic devices. Thereby the electronic structure at organic/metal interfaces and within thin films as well as the electronically excited states dynamics after optical excitation is essential for the performance of organic-molecule-based devices. Here, we investigated the energy level alignment and the excited state dynamics of a N-HTA derivative adsorbed on Au(111) by means of energy- and time-resolved two-photon photoemission spectroscopy. We quantitatively determined the energetic positions of several occupied and unoccupied molecular (transport levels) and excitonic states (optical gap) in detail. A transport gap of 3.20 eV and an optical gap of 2.58 eV is determined, resulting in an exciton binding energy of 0.62 eV. With the first time-resolved investigation on a N-HTA compound we gained insights into the exciton dynamics and resolved processes on the femtosecond to picosecond timescale.

N-heteropolycyclic aromatic compounds represent a promising molecule class for applications in functional organic materials, since their electronic structure and the resulting individual molecular properties are efficiently tuneable by number and position of nitrogen atoms in the aromatic structural backbone.The isosteric replacement of a C-H unit by N leaves the geometric structure unchanged, while the electronic structure are altered.2][3][4][5] N-heterotriangulenes (N-HTAs), in which the originally propeller-shaped triphenylamine unit is locked into a planar configuration via bridging with appropriate molecular moieties (e.g.0][11][12] In electron donor/acceptor systems N-HTAs act as the donor compound. 13Recently, we analyzed the electronic and absorption properties of two N-HTA derivatives (N-HTA-550 and N-HTA-557, see Fig. 1) at the interface to Au(111) and within thin molecular films using vibrational and electronic high resolution electron energy loss spectroscopy and quantum chemical calcula-tions.Inter alia, we found that the additional -C=C-bridge forming the 7-membered ring in N-HTA-557 resulted in a pronounced reduction of the optical gap size by 0.9 eV from 3.4 eV in N-HTA-550 to 2.5 eV in N-HTA-557 due to an increase of the π-conjugated electron system. 14Thus, using structural extensions or substitution patterns opens the opportunity for fine-tuning the electronic properties. 7,10,15Here, we investigated the electronic structure and electronically excited state dynamics of a novel π-expanded N-heterotriangulene derivative (N-HTA-557-P, see Fig. 1), which consists of an electron-deficient pyrazine ring (electron accepting unit) 7 adsorbed on Au(111) utilizing two-photon photoemission (2PPE) spectroscopy.7][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In addition, femtosecond (fs) time-resolved 2PPE allows to gain insights into the electronically excited states dynamics after optical excitation such as exciton formation and decay dynamics.
Here we thoroughly elucidated the electronic structure of N-HTA-557-P/Au(111) in detail, including the determination of transport levels, the optical gap and accordingly the exciton binding energy.In particular for applications of this material in organic field effect transistors or solar cells knowledge about the energetic positions of transport levels (affinity levels and ionization potentials) are extremely important.Moreover, fs-time-resolved 2PPE, allowed to gain insights into the exciton dynamics.Our study represents the first time-resolved investigation on a N-HTA-derived compound, which is of great interest with respect to applications of this donor material in organic solar cells.
On the basis of their photon-energy-dependency they are assigned to occupied and unoccupied molecular orbitals as well as an excitonic state (see Fig. 2b).The energy axis reveals the final state (E F inal ) of photoemitted electrons with respect to the Fermi energy E F (E F inal − E F = E kin + Φ); thus, the low-energy cutoff corresponds to the work function (Φ) of the adsorbate/substrate system.b) Photon-energy-dependent peak position extracted to assign peaks observed in the 2PPE spectrum to occupied, unoccupied intermediate or final electronic states.A slope of 1 suggests that a peak originates from an unoccupied intermediate state, a slope of zero from an unoccupied final state (located above the vacuum level), while a slope of 2 is related to peaks originating from occupied states.The slopes from the fits are given next to the respective data.
In Figure 3, we summarize the level alignments with respect to the vacuum level.Note that the work function of bare Au(111) (Φ Au(111) = 5.5 eV) decreases by 0.7 eV due to the adsorption of 1 ML N-HTA-557-P (see supporting information, Figs.S1 and S2) and stays constant for higher coverages.As discussed before, 24,25,30,36,37   populated via an electron transfer from the metal to the N-HTA-557-P molecule creating a negative ion resonance and occupied single-electron states are ionized, creating a positive ion resonance.Thus, we obtain the electron affinities (EA) and the ionization potentials (IP) of the investigated molecule.In addition, in 2PPE also intramolecular electronic excitation is possible, which corresponds to exciton generation, i.e. formation of an electron-hole pair, in which the molecule remains overall neutral.The minimal energy required for this process is the optical gap (E opt ), which corresponds to the excitation energy of the lowest excited singlet state.E opt is smaller than the difference between the lowest IP and the highest EA.The so-called transport gap is given as E transp = IP − EA.The difference between the transport gap and the optical gap corresponds to the exciton binding energy E transp − E opt = E B . 19,38r the N-HTA-557-P we obtain a transport gap of 3.20 eV (IP = 6.37 eV, EA = 3.17 eV), and an optical gap of 2.58 eV, the excitation energy of the lowest excited singlet state.
Hence, the exciton binding energy (E B ) of this lowest excitonic state amounts to 620 meV.
For comparison, N-HTA-557-P in solution (CH 2 Cl 2 ) possesses an optical gap of 2.61 eV (see supporting information, Fig. S3).The similar value indicates that the adsorbed molecules (5 ML) are decoupled from the metallic substrate and weak adsorbate-adsorbate (lateral) interactions.In addition, we identified two further unoccupied molecular states (unoccupied molecular orbital: UMO), an intermediate (at 1.17 eV) and a final state (0.11 eV above E vac ) as well as an occupied molecular state (occupied molecular orbital: OMO) at 8.74 eV.
All other 2PPE features are assigned to vibronic contributions related to the LUMO, and UMO 2 as well as to the HOMO.They exhibit an equidistance energy different of around 200 meV, which can be related to an excitation of ν(C-C) and δ(C-H) vibrational modes of the N-HTA-557 moiety of N-HTA-557-P located at 1636 cm −1 (see supporting information Fig. S4 and Tab.S1).Vibronic features in 2PPE data have recently been also proposed for another N-heterocyclic compound adsorbed on Au(111) supported by scanning tunneling spectroscopy results. 33For lower N-HTA-557-P coverages, i.e., around 1ML conributions from the Au(111) surface, the d -bands, the shifted surface state and the first image potential state are visible in the 2PPE data (see supporting information, Fig. S1).
We used femtosecond time-resolved 2PPE to study how the electronic coupling between the N-HTA-557-P molecules and the metal substrate influences the dynamics of optically excited states such as excitons in N-HTA-557-P.For low coverages (1-2 ML) none of the electronically excited states, e.g. the LUMO or excitonic states, exhibit a detectable lifetime, thus τ < 10 fs.The reason for these ultrashort lifetimes is a strong electron coupling enabling an efficient electron transfer from the molecule back to the metal.0][41] In contrast, for higher coverages longer lifetimes are usually observed.This is the case for our findings as can be seen Fig. 4a for 15 ML N-HTA-557-P on Au(111) in a two-dimensional false colored plot representation taken at a pump photon energy of  4c) clearly demonstrate that the lifetimes increase with rising coverage.40,42 An intrinsic channel due to the decay in the bulk material and a distance-dependent (external) channel resulting from a quenching process by the metallic substrate, i.e, transfer of the electrons to the metal.Note that, in literature no time-dependent measurements on the N-HTAs excited states dynamics are available.However, by comparing with our previous time-resolved 2PPE results obtained from other electron donating materials 24,25 we consider the following processes: Excitation with a photon energy of 3.08 eV leads to the creation of hot (E opt.= 2.58eV ), delocalized excitons.These excitons relax and localize on an ultrafast time-scale (τ 1 = 312±18 fs).[45][46] The longer lived component (τ 2 = 3.22±0.15ps) may be related to the decay of the exciton to the ground state.However, it could also be connected to the lifetime of polarons as well as electrons bound at defect sites.Note, that no energetic stabilization is observed, which would be expected for polaron formation or population of defect sites.
In summary, we employed energy-and time-resolved two-photon photoemission spectroscopy to quantitatively determine the energetic positions of unoccupied and occupied electronic states as well as an excitonic state and to resolve the exciton dynamics in a novel π-expanded N-heterotriangulene derivative adsorbed on Au(111).A transport gap of 3.20 eV and an optical gap of 2.58 eV have been identified, resulting in an exciton binding energy of 0.62 eV.Hot and delocalized excitons have found to localize on a femtosecond timescale followed by an exciton decay within picoseconds.The quantitative determination of the energetic positions of molecular electronic levels, the achieved detailed understanding of the electronic structure and insights into the exciton dynamics is an essential prerequisite for the design of new functional organic molecules for (opto)electronic devices.

Experimental Methods
2PPE measurements were conducted in an ultra-high vacuum chamber.A clean Au(111) substrate was prepared by a standard procedure including Ar + sputtering and subsequent annealing to 800 K under ultra-high vacuum conditions.The N-HTA-557-P molecules were synthesized according to Ref., 7 purified by column chromatography and degassed in vacuum at 400 K for several hours.They were deposited from an effusion cell held at a temperature of 453 K while the surface was kept at 100 K.The experiments were performed at a sample temperature of around 90 K.In 2PPE, a femtosecond pump laser pulse induces electronic transitions from occupied electronic states of the substrate or the adsorbate into unoccupied electronic or virtual states.These excited states are then probed with a second laser pulse by lifting the excited electron above the vacuum level.The kinetic energy of the emitted electrons is measured with a time-of-flight spectrometer.Photoelectron signals may arise from the occupied or unoccupied electronic states; therefore, photon energy-dependent measurements are needed for the assignment (for details, see refs see 24,25,47 ).A temporal delay between pump and probe pulse was introduced to obtain population dynamics of excited states on a femtosecond time scale. 24,42

Figure 2 :
Figure2: a) 2PPE spectrum of 5 monolayer (ML) N-HTA-557-P adsorbed on Au(111).The data are fitted with an exponential background and Gaussian-shaped peaks (red curves).The energy axis reveals the final state (E F inal ) of photoemitted electrons with respect to the Fermi energy E F (E F inal − E F = E kin + Φ); thus, the low-energy cutoff corresponds to the work function (Φ) of the adsorbate/substrate system.b) Photon-energy-dependent peak position extracted to assign peaks observed in the 2PPE spectrum to occupied, unoccupied intermediate or final electronic states.A slope of 1 suggests that a peak originates from an unoccupied intermediate state, a slope of zero from an unoccupied final state (located above the vacuum level), while a slope of 2 is related to peaks originating from occupied states.The slopes from the fits are given next to the respective data.

Figure 3 :
Figure 3: Energy level diagram of N-HTA-557-P adsorbed on Au(111) for N-HTA-557-P coverages above 2 ML with respect to the vacuum level.The blue levels are the ionization potentials and the red ones are the electron affinities, which are in the molecular orbital picture represented by the energies of the HOMO, LUMO, UMOs (unoccupied molecular orbital) or OMO (occupied molecular orbital), respectively.The dashed lines indicate vibronic transition.E F is the Au(111) Fermi level.

Figure 4 :
Figure 4: a) 2D-plot of the 2PPE intensity as a function of final-state energy and the pumpprobe delay for 15 ML N-HTA-557-P on Au(111).b)The cross-correlation (XC) curves fitted by a sech 2 function convoluted by a double exponential decay (XC energy range in a)).c) Coverage dependent lifetimes.
depending on the excitation mechanism 2PPE probes transport levels.Unoccupied single-electron states can be