Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV

Abstract

Thin spin-cast films of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared from samples whose weight-average molecular weight (M w ) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection ultraviolet-visible-nearinfrared (UV-vis-NIR) spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility (3) . Increasing molecular weight yields films with significantly larger (3) values, absorption coefficients, and refractive indices. The (3) values of films from the largest and lowest M w differ by a factor of 4, which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.

Second-harmonic generation in conjugated polymer films: A sensitive probe of how bulk polymer crystallinity changes with spin speed

“…One of the interesting features of conjugated polymer films is that their optical 35-51 and electrical 41,46,52,53 properties depend sensitively on the way in which the films were processed: casting from different solvents, 39,46,54-56 choosing different molecular weights 48,50,57 and different post-cast thermal 39,41,46,[58][59][60][61] and solvent 62 treatments all can have dramatic effects on the electronic and optical properties of the resulting films. This is because the morphology of the polymer chains in a thin film depends on the processing conditions, and the optical and electrical properties are in turn sensitive to interactions between the polymer chromophores and thus to the morphology.…”

Slab waveguides of poly(p-phenylenevinylene)s for all-optical switching: impact of side-chain substitution

“…Third-harmonic generation (THG) of thin films was measured with a similar setup as described recently. 22 We used a Nd:YAG laser (EKSPLA model PL 2143B), the second harmonic output of which pumped an optical parametric generator (OPG: EKS-PLA model PG 501), which gave laser pulses with a duration of 20 ps, repetition rate 10 Hz, and a wavelength tuning range between 680 nm and 2000 nm. The laser beam was focused on the sample, which was placed in an evacuated chamber and mounted on a rotation stage.…”

“…This effect is observed frequently in conjugated polymers and interpreted by the statistical distribution of effective p-conjugation lengths L c in a conjugated polymer. 22,48,49 Because of the super-linear increase of c (3) with L c , 4,9 the longer chain segments with larger L c dominate the THG response and cause the red-shift of the c (3) spectrum with respect to the linear absorption spectrum. The amount of red-shift depends mainly on the molecular weight distribution, which is rather similar for the polymers studied here.…”

“…Poly(p-phenylenevinylene) (PPV) and its derivatives are frequently studied because of their large third-order nonlinear optical susceptibilities and good film forming properties. 8,[13][14][15][16][17][18][19][20][21][22] A well-known model polymer is poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) whose chemical structure is shown in Scheme 1, together with structures of related polymers, which are substituted differently. Recently, we have reported the significant impact of the molecular weight of MEH-PPV on the optical properties of thin films, for example linear optical constants, refractive index anisotropy, chain packing morphology, waveguide propagation loss, and thirdharmonic generation.…”

“…Recently, we have reported the significant impact of the molecular weight of MEH-PPV on the optical properties of thin films, for example linear optical constants, refractive index anisotropy, chain packing morphology, waveguide propagation loss, and thirdharmonic generation. [22][23][24][25] The aim of this paper is to reveal the systematic trends in the structure-property relations of PPV derivatives, which result from a variation of the side-chain substitution. We present optical properties of the alkoxy-substituted PPVs shown in Scheme 1.…”