1124
J. Phys. Chem. B 1999, 103, 1124-1133
Molecular Orientation, Aggregation, and Order in Rhodamine Films at the Fused Silica/Air Interface Tanya Kikteva, Dmitry Star, Zhihong Zhao, Tammy L. Baisley, and Gary W. Leach* Department of Chemistry, Simon Fraser UniVersity, Burnaby, British Columbia V5A 1S6, Canada ReceiVed: September 1, 1998; In Final Form: NoVember 25, 1998
We report optical second harmonic generation studies of the organic dye molecule rhodamine 6G spin cast on fused silica surfaces. The concentration dependence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer surface coverage. This behavior reflects the formation of ordered molecular adlayers which persist for approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orientation within adjacent layers to be determined. Optical absorbance measurements of the films indicate the onset of rhodamine 6G aggregate formation at surface coverages of approximately one monolayer. However, the onset of dimer or aggregate fluorescence is observed to occur only at much higher surface coverages, consistent with the loss of orientational order within the adlayers. Our results indicate strong adsorbate-substrate interaction which gives rise to orientational anisotropy within the first molecular layer. Well-defined order within subsequent layers is determined by interlayer adsorbate-adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfacial ordering at the solid/air interface.
I. Introduction It is well-known that the optical characteristics of molecules are affected by their physical and chemical environments. The photophysics and photochemistry of a vast array of species examined in the gas phase, solution phase, and matrix environments have provided both a qualitative and quantitative understanding of these phenomena. While the level of understanding of these interactions in the bulk phases is relatively mature, the situation is much less clear for interface and surface environments. In part, this results from the complex nature of surfaces (defined by an inherent asymmetry, many uncharacterized surface defects, and inhomogeneities) and also from a paucity of tools capable of probing the surface and its interactions specifically, without contamination from the adjacent bulk phases. At a surface, the strength and nature of adsorption processes and adsorbate intermolecular interactions will be important factors determining molecular characteristics. Modifications of a molecule’s properties can be large if they result from chemical bonding to the surface or a neighboring adsorbate molecule, or much more modest in the case of a physically adsorbed species interacting relatively weakly with its environment. Of particular interest in interface chemistry is the question of interfacial ordering of molecules and its effect on physical and chemical properties. The asymmetric forces defining the interface are well-known to result in orientational anisotropy of adsorbates even when the adsorbate-substrate interaction is physical in nature. This anisotropy can lead to large changes in macroscopic properties of the surface such as the surface hydrophilicity or hydrophobicity. From the chemical perspective, an alignment of molecules at the surface may also be expected to lead to rather large differences in surface reaction rate and * Author to whom correspondence should be addressed. E-mail:
[email protected].
extent in comparison to their occurrence in the bulk phases. Such surface-aligned chemistry would result from the much smaller distribution of reactant configurations defining the reactive system. While the anisotropy of adsorbate geometries imparted by the surface is relatively well understood, it is not clear to what extent this “surface order” extends away from the interface and into the bulk, nor what contribution the surface plays in defining it. The question of interfacial ordering is of considerable practical importance as well. There is currently a great deal of interest in developing organic-based optical, optoelectronic, and electroluminescent devices which take advantage of the unique conducting, luminescent, and processibilty properties of organic systems. Excellent progress has been made in the development of devices such as polymer light-emitting diodes1 (LEDs) and light-emitting electrochemical cells2 (LECs), for example. One important aspect of these devices is the degree of order or crystallinity within the photoactive or electroactive polymer system, since the degree of molecular order in these organic thin films is crucial in determining their luminescent3 and conducting4 efficiencies and therefore, their efficacy as device materials. While film crystallinity is determined by the nature of intermolecular interactions within the film, one potentially important contribution to the extent of molecular ordering in these systems is that which arises naturally from interface formation. Material discontinuity can induce molecular order and ultimately lead to film crystallization, even in situations where crystallinity is to be avoided. Despite their importance, the structural properties of organic-based thin films have remained difficult to characterize and control, and represent an area of active investigation. Optical second harmonic generation (SHG) has proven itself to be a particularly valuable method for investigating surface and interface structure,5-8 providing information regarding adsorbate orientation in a variety of systems, ranging in nature
10.1021/jp9835824 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/28/1999
Rhodamine Films at the Fused Silica/Air Interface
J. Phys. Chem. B, Vol. 103, No. 7, 1999 1125
from the solid/air9,10 to solid/liquid,10,11 liquid/vapor,10,12 and liquid/liquid13,-15 interfaces. The technique takes advantage of the inherently asymmetric environment of the interface formed between two bulk phases of matter. When the two bulk phases are each centrosymmetric, the noncentrosymmetric interface between them possesses a second-order nonlinearity (χs(2)) which can give rise to interface-specific, second-order, nonlinear optical effects. An intense laser beam of frequency ω and polarization eˆ ω incident on a surface or interface at an angle φ with respect to the surface normal will produce second harmonic light at frequency 2ω and polarization eˆ 2ω with an intensity given by16
()
I2ω ) 32π3
ω2 2 s φ|eˆ 2ω‚χ(2) ˆ ωeˆ ω|2Iω2 s (2ω):e c3
(1)
In situations where the macroscopic second-order surface nonlinear susceptibility χs(2) is large due to the presence of adsorbed molecules, contributions from the substrate and local field effects can be ignored and the susceptibility written as an averaged sum of the second-order nonlinear molecular polarizabilities (β(2)) of the adsorbed species (2) χ(2) s ) Ns〈β 〉
(2)
where Ns is the number of adsorbate molecules on the surface and the brackets denote an average taken over all adsorbate orientations. Thus, given information regarding the molecular polarizabilities, measurement of the surface susceptibility can provide an indication of the averaged molecular orientation (see below). To help characterize the degree and extent of ordering in organic thin films, we have conducted linear and nonlinear optical measurements of an organic probe molecule deposited at the solid/air interface. We have conducted optical second harmonic generation, absorbance, and fluorescence studies of rhodamine 6G (R6G) spin coated on fused silica. In the next section we describe the method of film deposition and the experimental apparatus used to carry out these studies. Following this, we present the results of our surface second harmonic generation experiments which show that there exists welldefined order within several surface layers before orientational anisotropy is lost. Before discussing the results of absorbance and fluorescence measurements carried out on these films, a brief discussion of molecular aggregation and exciton band splitting is presented. Finally, our work is summarized. II. Experiment Optical second harmonic generation measurements of organic molecules adsorbed at the fused silica/air interface were carried out in a total internal reflection geometry using the regeneratively amplified output of a mode-locked titanium:sapphire oscillator. A schematic representation of the apparatus is presented in Figure 1. Specifically, we employ 8 W from a 10 W argon ion laser (Spectra Physics, Beamlok 2060-10SA) to pump a mode-locked Ti:sapphire oscillator (Spectra Physics, Tsunami 3960) which produces an 82 MHz pulse train, tunable from approximately 750 to 850 nm and with a nominal pulse duration of 80 fs. A portion of this pulse train is then amplified in a Ti:Sapphire chirped pulse regenerative amplifier (Positive Light, Spitfire) which is pumped by an intracavity frequencydoubled, Q-switched Nd:YLF laser (Positive Light, Merlin) operated at a 1 Khz repetition rate. This system is capable of producing 100 fs pulses with an energy of 1 mJ, tunable from 750 to 850 nm. Since pulses of such high peak intensity can
Figure 1. Schematic diagram of the apparatus employed for SHG studies. λ/2, halfwave plate; IF, interference filter; STF, spatial filter; SCF, spectral filter; Pol, polarizer; M, Monochromator; PMT, photomultiplier tube; GE, gated electronics; PC, personal computer.
lead to optical damage of the interfaces under investigation, only a fraction (typically
