Table 1. Summary of orbital energy levels and transition voltage of PPE, TTF–PPE, TTF²⁺–PPE, C12 and OPE3.

Entry	Eg* (eV)	UPS		Injection barrier (eV)		Vtrans/V	Exp. or Calc.
		HOMO† (eV)	LUMO‡ (eV)	Hole§	Electron||		HOMO (eV)	LUMO (eV)
C12	∼7 (ref. 40)	−6.30	—	2.16	—	—	—	—
OPE3	3.37 (ref. 34)	−5.48	−2.11	1.56	1.81	0.55±0.1	−5.31 (ref. 34)	−1.77 (ref. 34)
PPE	2.59	−5.24	−2.65	1.08	1.51	0.6±0.1	−6.30¶, (ref. 24)	−3.71‡
TTF–PPE	2.30	−5.14	−2.84	0.76	1.54	0.28±0.08	−4.84¶	−2.54‡
TTF2+–PPE	2.51	−5.68	−3.17	1.04	1.47	0.37±0.08	—	—

Calc., density functional theory (DFT) calculation; Exp., experiment; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; PPE, thioacetyl-end-functionalized poly(p-phenylene ethynylene); TTF, tetrathiafulvalene; UPS, ultraviolet photoelectron spectroscopy.

^*E_g refers to the optical energy gap.

^†E_HOMO is calculated from the UPS spectra of the self-assembled monolayers or ultrathin films on Au surface.

^‡E_LUMO=E_HOMO+E_g.

^§The energy barrier between E_HOMO,UPS and E_F.

^||The energy barrier between E_LUMO,UPS and E_F.

^¶E_HOMO is calculated from CVs and defined as −(eE^ox_onset+4.4 eV). Here E^ox_onset represents the onset potential for the oxidation.