Concentration dependence of the adsorption of metalfree tetra (4-carboxyphenyl) porphyrin (TCPP) on hexagonal boron nitride

The adsorption and self-assembly of metalfree tetra(4-carboxyphenyl) porphyrin (TCPP) from ethanol on the surface of hexagonal boron nitride (hBN) was studied to determine the influence of the concentration of the solution on the self-assembly/aggregation and spectroscopic properties of the adsorbed TCPP. The adsorption isotherm indicates that at saturation the TCPP molecules adsorb edge-on with the macrocycle nearly perpendicular parallel to the hBN surface. On the other hand, the atomic force (microscopy AFM) micrographs indicate that a monolayer of flat-on adsorbed TCPP molecules, observed for adsorption from a diluted solution transforms into edge-on adsorbed molecules when more concentrated solutions are used. While for adsorption from a dilute solution, the stationary emission spectra correspond to those reported in literature, they shift 15 nm to longer wavelengths when adsorption occurs from more concentrated solutions suggesting stronger interaction between neighboring chromophores for edge on adsorption. The latter is also suggested by a broadening of the red edge of the fluorescence excitation spectra. Fluorescence decays obtained for the different emission wavelengths can be analyzed globally as quadruple exponential decays linking the four decay times. The non-mono-exponential character of the decay is attributed mainly to energy transfer to non-fluorescent traps. The longest decay time decreases from 5.9 ns for the sample prepared by adsorption from a dilute solution to 4.6 and 4.0 ns from samples prepared from more concentrated solutions. These decay times are about 50 % shorter than the fluorescence decay time of TCPP in ethanol.