Vibrational Sum Frequency Generation Study of the Interference Effect on a Thin Film of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and Its Interfacial Orientation

Abstract

Molecular organization of vapor-deposited organic molecules in the active layer of organic light-emitting diodes (OLEDs) has been a matter of great interest as it directly influences various optoelectronic properties and the overall performance of the devices. Contrary to the general assumption of isotropic molecular orientation in vacuum-deposited thin-film OLEDs, it is possible to achieve an anisotropic molecular distribution at or near the surface under controlled experimental conditions. In this study, we have used interface-specific vibrational sum frequency generation (VSFG) spectroscopy to determine the orientation of a low-molecular weight OLED material, 4,4′-bis­(N-carbazolyl)-1,1′-biphenyl (CBP), at free (air) and buried (CaF2) interfaces. VSFG spectra were measured at four different polarization combinations for five different thicknesses of the CBP film. The spectral shift and VSFG intensity changes with the film thickness can be accurately modeled by considering the optical interference effect of the signals coming from the CBP/air and CBP/CaF2 interfaces. A global fitting of the experimental spectra for all thicknesses along with theoretical simulations reveal that the long molecular axis of CBP is oriented at an angle of ∼58° (47–70°) from the surface normal at the air/CBP interface, whereas at the CBP/CaF2 interface, the angle is ∼48° (43–52°). Such a change in the angle (∼10°) suggests that the CBP molecule tends to orient more vertically (edge-on) at the buried CaF2 interface, which may be attributed to the intermolecular π–π stacking interaction between adjacent CBP molecules.

Vibrational Stark shift spectroscopy of catalysts under the influence of electric fields at electrode–solution interfaces

“…different normal modes. [52][53][54][55][56] Equation 3 represents the intensity of the VSFG signal that contains both real and imaginary parts of the resonant molecular response ( ), the non-resonant response…”

“…The orientation of the molecular chromophores at the interface can be inferred from direct comparisons to the calculated relative intensities of VSFG signals I VSFG , as obtained from ab initio calculations of χ (2) , for different polarization combinations of the incoming and outgoing pulses and/or for different normal modes. 52–56 Eqn (3) represents the intensity of the VSFG signal that contains both real and imaginary parts of the resonant molecular response ( χ (2) R ), the non-resonant response ( χ (2) NR ) from the substrate and the cross terms that are generated because of the interference of these resonant and non-resonant terms. One must rely on complicated spectral fitting procedures to separate out each contribution from such homodyne detected VSFG spectrum, which sometimes can be difficult because of the interference of nearby resonances and in presence of huge non-resonant signal that also interferes with resonant signals.…”

Controlling Structure and Properties of Vapor-Deposited Glasses of Organic Semiconductors: Recent Advances and Challenges

“…Presently, even the length scale over which the substrate can influence the structure of a vapor-deposited glass is poorly understood, with the first studies on this topic just recently appearing. 68,69 While dichroism studies by Yokoyama et al suggest the substrate can influence glass structure in films as thick as 100 nm, 51 X-ray scattering measurements and molecular dynamics simulations suggest the substrate influences PVD glass structure for less than 10 nm. 70,68 The structure at buried interfaces plays an important role in organic electronic devices.…”

“…While well-established strategies exist to control the bulk structure of vapor-deposited glasses, little is known about the structure of vapor-deposited glasses at buried interfaces. Presently, even the length scale over which the substrate can influence the structure of a vapor-deposited glass is poorly understood, with the first studies on this topic just recently appearing. , While dichroism studies by Yokoyama et al suggest the substrate can influence glass structure in films as thick as 100 nm, X-ray scattering measurements and molecular dynamics simulations suggest the substrate influences PVD glass structure for less than 10 nm. , …”

Why Are Water Droplets Highly Mobile on Nanostructured Oil-Impregnated Surfaces?

“…We used sum frequency generation (SFG) vibrational spectroscopy to characterize the liquid/liquid interface when a water droplet is deposited on a nanostructured lubricated surface. SFG is a second-order nonlinear optical characterization technique that has been widely used to study interfaces (solid/air, liquid/air, liquid/liquid, solid/solid, or solid/liquid). − In contrast to other vibrational spectroscopic techniques, it is intrinsically surface-sensitive, detecting only signals from surfaces and interfaces under the electric dipole approximation . The intensity of the SFG signal for a specific functional group is dependent on the number and orientation of the molecules at the interface.…”