We demonstrate the fabrication and characterization of an organic light-emitting diode (OLED) device based on self-assembled poly(4-vinyl-pyridine) (P4VPy) with poly(N-vinyl-carbazole) (PVK) and 2-(4-biphenylyl)-5-(4-tert-butyphenyl)-1,3,4-oxidiazole (PBD) as transport layers. The self-assembly is based on the electrostatic attraction of oppositely-charged polymers. We have shown here that we can use this self-assembled methodology to fabricate alternating multilayers, not only by the poly(phenyl-vinylene) (PPV) and derivatives, but also by partial protonation of P4VPy, by which the charge on P4VPy is generated by the protonation process. The multilayered structures are characterized by specular X-ray reflectivity (XRR), UV absorption, photoluminescence (PL) and electroluminescence (EL).

An FTIR study reveals the complex zwitterion (R₂C⁻–Py⁺–H) in a polymeric chain of poly(4-vinylpyridine), which results from self-protonation of the latter in a pyridine solution. Presumably this complex ion, together with the observed strongly hydrogen-bonded network, is responsible for the formation of a gel with unique tunable emission properties.

A solution of poly(4-vinyl-pyridine) in pyridine is converted into a gel over a 3 week period. The spontaneous self-organization is governed by interchain hydrogen bonding. These interactions resulted in a change of polymer's optical properties. Blue, green, and red emissions appear in the gel by appropriate excitation wavelength replacing the initial dark blue emission. Energy transfer between emitting centers is restricted because of the insulating nature of the polymer backbone.

A blue–red reversible optical switch has been created by combining EuIII, pyridine, and a fluorescent organic chromophore. The initial solution exhibits the expected blue emission of the chromophore until UV irradiation leads to red emission (see Figure), a process that is ascribed to protonation of the pyridine and formation of a charge-transfer complex with the chromophore in the excited state, allowing the red EuIII photoluminescence to be emitted.

In the present work we show a way of controlling photoluminescence (PL) properties through photoinduced quasi-crystal formation in a system based on poly(4-vinyl pyridine) (P4VPy). Under UV irradiation at 380 nm, concentrated solutions of P4VPy in pyridine turn into gel. This phase transition results in changes in the optical properties of this polymer. The position of the PL maximum can be changed continuously from 440 to 480 nm during irradiation. After several minutes of UV irradiation a new red-shifted PL at 492 nm appears upon excitation by light of a wavelength corresponding to that of the initial PL maximum, which is also red-shifted during irradiation. Solutions of P4VPy in pyrimidine show similar behavior, but those in pyridazine do not exhibit such behavior. We have found that the reason for the observed changes in the electronic properties is a photoinduced directional ordering of polymer molecules in a special quasi-crystal formation. The process originates from a structural change in the side chain of P4VPy, namely, protonation of the polymeric pyridine after solvation. During irradiation, the polymeric pyridinium ion interacts with neutral polymeric pyridine molecules. Interchain interaction through hydrogen bonds lead to an electronic property change. We observed that the process of photoinduced sol-gel transformation is reversible. Mechanical perturbation or heating can convert the gel back to a fluid solution. The red-shifted PL is not observed, and the initial PL is blue-shifted to 450 nm and stays there.