Since optical trapping and manipulation of a micro-meter size particle were achieved by A. Ashkin and co-workers in 1970–80s [1
], laser trapping has been applied not only to biological cells [3
], but also to smaller particles such as biomolecules [5
] and nanoparticles [6
]. Although there have also been reports on the possibility of single molecular trapping at room temperature [10
] and a manipulation method of single molecules with resonant laser irradiation has been theoretically proposed [12
], stable trapping and manipulation of 1-nm-sized molecules at room temperature via light-gradient force is challenging. This is because the optical trapping potential (
) of molecules due to the light-induced force (gradient force) is much smaller than the kinetic energy of the thermal motion of the molecules (KBT
) at room temperature due to the tiny polarizability volume for
of the molecules.
In general, for stable trapping of particles or other moieties at room temperature, the trapping potential has to be deeper than the thermal energy at the temperature. Although the use of a sufficiently high-power laser is a possible choice to approach this condition, it can cause thermal effects and disturbs optical trapping. Therefore, recent research has focused on enhancing the laser-induced force by exploiting the surface plasmonic effects to overcome the thermal disturbance and limited focus size due to diffraction [15
]. In many cases, plasmon optical tweezers employ metal nano structures to enhance the optical force between metal nano-gaps [17
]. Another experimental method exploits the opto-thermoelectric field generated from opto-thermal fluidics [19
Photophysical crystallization or aggregation with non-resonant laser irradiation has also been studied for decades [20
] and is applied for preparing protein single crystals for X-ray diffraction analysis of the atomic structure of proteins [22
]. The optical trapping force is also applied for laser crystallization and aggregation [26
]. In most of these studies, supersaturated solutions of solute molecules that form crystals or aggregates in equilibrium states were irradiated with laser light. In these experiments, the laser works as either a trigger for nucleation or an accelerator of crystal growth [28
]. Recently, there have been interesting reports on the crystallization or aggregation of organic molecules “on air-liquid surfaces” utilizing light-induced force as a direct trapping force instead of a mere trigger [30
For molecular J-aggregates [33
], some reports have demonstrated the formation of J-aggregates in a saturated solution via laser irradiation [26
]. The “bottom-up” approach exploits the self-assembly nature of J-aggregates to form nanostructures, e.g., layer structures [35
]. It is well known that the concentration of solutes and electrolytes, pH [38
], temperature [41
], and solution viscosity [42
] contribute to formation of J-aggregates. Moreover, there is a collaborative effect on aggregation formation among the molecular hydrophobic effect and molecular-solvent and intermolecular hydrogen bonding [43
], making the solvent effect on the aggregation phenomenon diverse. Additionally, it is difficult to control the size of micro-aggregates that configure macro-aggregates [46
] because the generation of micro-aggregates occurs during the rapid change of the first-order phase transition and is highly dependent on the surrounding environment [41
]. Intermediate states in the formation process of micro-aggregates are considered to have a very short transient lifetime and are difficult to clearly identify.
In our preceding paper [48
], we proposed to employ an absorption spectrum to observe the changes in molecular aggregation states introduced by light-induced force. In previous research, the processes of crystal growth or aggregation of amino acids and proteins by light-induced force have been observed primarily via fluorescence imaging, light-scattering imaging, or fluorescence spectrum measurement [26
]. However, fluorescence signals generally reflect only the lowest excited electronic states, and light-scattering signal intensity is too weak to provide the properties of micro-aggregates. Thus, it is difficult to obtain detailed information about aggregation states such as aggregation numbers from the spectroscopic methods listed above.
We have successfully identified in the previous report [48
] that the aggregation states of porphyrin molecules, 4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid (TPPS), are formed in an unsaturated solution with continuous-wave laser irradiation, off-resonant from main molecular absorption peaks. We have observed absorbance change,
, in the resonance region of the optical transition to the aggregation state. Surprisingly, this absorbance change is approximately 104
times larger than the ratio of the optical trapping potential energy to the thermal energy (10−7
), and its physical mechanism is unknown. Differing from previous studies [10
], our study employs neither a metal nanostructure to enhance the optical force via a plasmonic effect [15
] nor other surface effects [30
]. In this paper, to determine how to elucidate the mechanism of the phenomenon, we investigate the solvent dependence of the light-induced absorbance change.
In this paper, we observed light-induced changes in the molecular aggregation states in the absorption spectra and performed comparative experiments with different solvents. Moreover, the irradiation intensity dependence of the absorption change spectra was compared between an aqueous solution and ethanol solution.
In the aqueous solution, the absorbance change per TPPS concentration grew larger by one order of magnitude than in the alcohol solutions. The protonation of TPPS could have been related to the solvent effect because of the larger protonation constant in the water compared with that in the alcohol solutions [57
]. However, it has been argued that intermolecular hydrogen bonds in the molecular J-aggregates are disturbed by interactions with surrounding solvent molecules [44
], so further experiments are needed regarding the role of hydrogen bonds in the aggregate formation of porphyrin molecules.
The irradiation intensity dependence of the absorbance change was nonlinear and showed saturation behavior for the aqueous solution; however, it was linear without saturation for the ethanol solution. The absorption change tended to be saturated with intensity only in the aqueous solution, and its saturated value determined from the fitting was two orders of magnitude smaller than the sample absorbance. This means that there was a limit to the change induced by the aggregate formation, even if the irradiated light intensity increased to infinity.
These results and the increase in absorbance in higher energy regions than the F monomer are qualitatively consistent with a model wherein light-induced interactions work only between molecules that are accidentally closely located among those randomly distributed and oriented within the light-irradiated volume as a mechanism of light-induced molecular aggregation. However, the Keesom interaction and thermal convection effect are not enough to explain our results, although the thermophoresis effect has not been considered. To consider the contribution of the thermophoresis effect, the Soret coefficient of TPPS has to be estimated. However, no literature on the Soret coefficient of TPPS or porphyrin has been found yet, and no experimental or computational estimation of the coefficient was performed herein.
Although the mechanism is still unknown, the difference in the efficiency of light-induced aggregation between each solvent observed in this paper is an interesting result that contributes to determining the mechanism of highly efficient molecular aggregation via light-induced force for further improvement of efficiency.