Journal of the American Chemical Society ARTICLE Figure 9. Alternative molecules (names given in the Supporting Information) consisting of polar building blocks with equivalent numbers of heteroatoms along the backbone. The HSE06/6-311++G(d,p) calculated total molecular dipole moments are also listed, and terpyrimidine is included for the sake of comparison. bonded to the metal will diﬀer by several electron volts as a result of the collectively induced QCSE. This is a spectacular eﬀect, considering that the molecular ionization potentials of 3Nup and 3Ndown are essentially the same. To obtain a more complete picture, one needs to explicitly consider the bonding to the metal. This is a computationally formidable task when using hybrid functionals. We also note that it is still not fully quantitative: In the case of molecules adsorbed on a metallic substrate,82 describing the narrowing of the gap due to polarization of the metal will generally require both long-range exchange and long-range correlation, neither of which is present with HSE.84,119 Figure 8c shows the density of states projected onto the SAM region (PDOS) and the local density of states plotted for certain energy ranges117 for the 3Ndown and 3Nup metal-adsorbed SAMs. Also, in the presence of the metal the frontier states are localized on opposite “ends” of the monolayer in the 3Nup and 3Ndown SAMs, and indeed a completely diﬀerent level alignment is found in the two systems. While for 3Ndown the unoccupied states are closest to the Fermi-level (see Figure 8c), it is the occupied states for 3Nup.44,120 However, the thiolate Au bond also modiﬁes the SAM electronic structure. First, strongly dispersing metal-in- duced intragap states appear around 1.7 eV below the Fermi-level (overlapping with sharper peaks for 3Nup). They are strongly localized on the S atom (see inset for 3Ndown). This changes its hybridization with the π-electrons of the backbone upon adsorp- tion, and the π-states no longer have any weight on the S-atom unlike in the freestanding SAMs shown in Figure 8a. The highest occupied π-states are strongly stabilized and for both SAMs shift down to partly overlap with the next σ-states. This has next to no inﬂuence on the band gap of 3Nup (see Figure 8c). However, adsorption changes the symmetry of the 3Ndown HOMO, and the gap is again opened by 0.5 eV. Interestingly, this way a value close to the HSE-computed gas phase gap is reached (although with diﬀerent character of the HOMO). Having analyzed the electronic structure of the SAM bonded to the metal substrate one can now wonder how the eﬀects described in the paper would impact experimentally accessible physical observables. Beyond the potential of directly imaging the collectively induced electric ﬁeld in pyrimidine SAMs by high resolution XPS mentioned earlier, especially the QCSE-induced shifts of the eigenenergies should have drastic eﬀects. The diﬀerent positions of the molecular states relative to the Fermi- level will strongly impact charge-carrier injection into oligopyr- midine SAMs, for example, in scanning tunneling microscopy and spectroscopy experiments. For a given tunnel bias, it will also result in drastically diﬀerent tunneling microscopy images. In fact, the results in Figure 8c imply that for imaging 3Ndown SAMs a negative tip bias inducing the tunneling of electrons might be preferable, while the opposite (i.e., a preferred tunneling of holes) applies to 3Nup SAMs. The diﬀerent level alignment for the two terpyrimidine orientations should also be observable in UV photoelectron spectroscopy and will modify the eﬀective injection barrier when using SAMs to modify the work function of metal electrodes in organic devices. Charge transport through oligopyrimidine layers will be aﬀected also by orbital localization eﬀects, as they tend to deteriorate the transmittance of the transport channels. Moreover, excitation energies in oligopyrimi- dine layers will be reduced and two-photon photoemission provides a technique to study the relevant excited states.121 Whether or not certain experimental observations are indeed due to the collectively induced QCSE can then be checked in an (at least conceptually) straightforward manner, as they should all display a distinct dependence on the molecular packing density that might, for example, be varied by forming mixed monolayers with “inert” molecules. Relevance of the QCSE beyond Oligopyrimidines. As stressed repeatedly, all above effects are essentially caused by the collectively generated electric fields. Therefore, they should by no means be limited to oligopyrimidines, but occur in any SAMs made of molecules with a distributed dipole character, i.e., molecules consisting of a series of polar building blocks. A few examples for alternative (mostly heterocyclic) molecules are shown in Figure 9. A quick theoretical screening shows that most of them have appreci- able dipole moments, which for the methineimines is even larger than in the oligopyrimidines; i.e., while the above-described effects are expected to be weaker for the displayed oxazines, thiazoles, imidazoles, and oxazoles, they should be signifi- cantly stronger in the methineimines. While the latter have been treated theoretically,122 their synthesis will pose a considerable challenge. ’ SUMMARY AND CONCLUSIONS In summary, using a carefully selected theoretical approach that accounts for the infamous self-interaction error of density-functional theory, we have shown that the electronic structure of oligopyr- imidine SAMs is strongly inﬂuenced by the electric ﬁeld that results from a collective action of the intrinsic molecular dipole moments: Upon gradual transition from the isolated molecule to the densely packed monolayers, we observe (i) an increasing 18642 dx.doi.org/10.1021/ja203579c |J. Am. Chem. Soc. 2011, 133, 18634–18645