Pyrene Molecular Orbital Shuffle—Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals

Abstract

We show that by judicious choice of substituents at the 2- and 7-positions of pyrene, the frontier orbital order of pyrene can be modified, giving enhanced control over the nature and properties of the photoexcited states and the redox potentials. Specifically, we introduced a julolidine-like moiety and Bmes (mes=2,4,6-Me C H ) as very strong donor (D) and acceptor (A), respectively, giving 2,7-D-π-D- and unsymmetric 2,7-D-π-A-pyrene derivatives, in which the donor destabilizes the HOMO-1 and the acceptor stabilizes the LUMO+1 of the pyrene core. Consequently, for 2,7-substituted pyrene derivatives, unusual properties are obtained. For example, very large bathochromic shifts were observed for all of our compounds, and unprecedented green light emission occurs for the D/D system. In addition, very high radiative rate constants in solution and in the solid state were recorded for the D-π-D- and D-π-A-substituted compounds. All compounds show reversible one-electron oxidations, and Jul Pyr exhibits a second oxidation, with the largest potential splitting (ΔE=440 mV) thus far reported for 2,7-substituted pyrenes. Spectroelectrochemical measurements confirm an unexpectedly strong coupling between the 2,7-substituents in our pyrene derivatives.

Synthesis, Photophysical and Electronic Properties of New Red‐to‐NIR Emitting Donor–Acceptor Pyrene Derivatives

“…The compound 1,1,7,7‐tetramethyl‐julolidine is known to be an even stronger π‐donor than diarylamines. The julolidine moiety has been thoroughly studied since its discovery in 1892 by Pinkus, and is used in a wide range of dyes . Its nitrogen lone pair is conformationally restricted to remain parallel to the aromatic system, in our case, the pyrene moiety, in both the ground and excited states, and our previous studies revealed a significantly enhanced electron donating effect on the pyrene core compared to diarylamines …”

“…The reaction was stirred at room temperature until monitoring by IR spectroscopy confirmed the complete disappearance of azide moieties (in the region 2000–2270 cm −1 ) and the appearance of primary amine bands at 3450, 3363 and 1620 cm −1 (Figure S26). The amine moieties were alkylated in MeCN with 1‐chloro‐3‐methylbutene in the presence of the base K 2 CO 3 , according to the approach used in our previously reported synthesis of I . The alkylation reaction was complete after 48 h and gave the desired product 6′ as a bright yellow solid (42 %).…”

“…To adjust the properties for a specific application, electron donors and/or acceptors are often introduced onto a PAH core as they strongly modulate the frontier orbitals. Substituting PAHs with donors and acceptors gives derivatives with properties such as a permanent dipole moment, charge transfer (CT) excited states, strong solvatochromism, environmentally influenced photophysics, the possibility for energy or electron transfer, and narrowed energy gaps . Furthermore, such chromophores can absorb and emit in the near‐infrared (NIR) region, which is in demand for applications such as bioimaging and cell recognition, as NIR light penetrates deeper into biological tissues, is less damaging than visible or UV light, and gives minimum interference from background autofluorescence by biomolecules .…”

“…Nevertheless, a significant drawback can be energy loss due to the rotational motion of the phenyl rings which can result in a reduction in luminescence efficiency . In addition, the possibility of rotation around the N−C(π) bond can lead to twisted intramolecular charge transfer (TICT) excited states . The compound 1,1,7,7‐tetramethyl‐julolidine is known to be an even stronger π‐donor than diarylamines.…”

Rational design of near‐infrared absorbing organic dyes: Controlling the HOMO–LUMO gap using quantitative molecular orbital theory

“…Near‐infrared (NIR) absorbing organic dyes (650–950 nm) are highly sought after for application as an emitter in tissue imaging and organic electronics, and as a photosensitizer in organic photovoltaics . Their tunability, synthetical accessibility, and low toxicity give them an advantage over alternate inorganic materials . However, advancing such organic dyes is hampered by complex molecular arrangements and a large π‐scaffold with poor excited state intramolecular charge transfer (ICT) nature .…”