Highly efficient photothermal nanoagent achieved by harvesting energy via excited-state intramolecular motion within nanoparticles

The exciting applications of molecular motion are still limited and are in urgent pursuit, although some fascinating concepts such as molecular motors and molecular machines have been proposed for years. Utilizing molecular motion in a nanoplatform for practical application has been scarcely explored due to some unconquered challenges such as how to achieve effective molecular motion in the aggregate state within nanoparticles. Here, we introduce a class of near infrared-absorbing organic molecules with intramolecular motion-induced photothermy inside nanoparticles, which enables most absorbed light energy to dissipate as heat. Such a property makes the nanoparticles a superior photoacoustic imaging agent compared to widely used methylene blue and semiconducting polymer nanoparticles and allow them for high-contrast photoacoustic imaging of tumours in live mice. This study not only provides a strategy for developing advanced photothermal/photoacoustic imaging nanoagents, but also enables molecular motion in a nanoplatform to find a way for practical application.


Methods
General. All chemicals were commercially available and used as supplied without further purification. Deuterated solvents were purchased from J&K. TEP-B(OH)2 was purchased from AIEgen Biotech Co., Ltd. Tetrahydrofuran (THF) was dried by distillation using sodium as drying agent and benzophenone as indicator. Compounds 2, 3, 4, 2TPE-PDI-C8 and 2TPE-PDI-C16 were synthesized according to the published procedures. 1 H and 13 C NMR spectra were recorded on a Bruker ARX 400 NMR spectrometer using tetramethylsilane (TMS; δ = 0) as internal reference. High-resolution mass spectra (HRMS) were obtained on a Finnigan MAT TSQ 7000 Mass Spectrometer operated in a MALDI-TOF mode. Absorption spectra were measured on a JASCO V-570 UV-vis-NIR spectrophotometer. Steady-state photoluminescence (PL) spectra were recorded on an Edinburgh FLS980 fluorescence spectrophotometer.
Quantum yield was determined by a Quanta- integrating sphere. Particle size analyses were implemented using a ZetaPlus Potential Analyzer (Brookhaven, ZETAPLUS). Transmission electron microscopy (TEM) investigations were carried out on a JEOL-6390 instrument.

PA equation.
Under low-intensity irradiation, the PA response exhibits a linear dependence with respect to the incident light intensity as described by Supplementary Equation (1) 1 : where εg is the ground-state molar extinction coefficient of the contrast agent at the incident wavelength, Cg is the ground-state concentration of dye molecules, Γ is the Grüneisen coefficient, I is the incident photon fluence, and Φnr is the quantum yield for nonradiative decay.
The Grüneisen coefficient, Γ, is a constant that quantifies a medium's ability to conduct sound efficiently that is defined by Supplementary Equation (2): where Vs is the velocity of sound,  is the thermal expansion coefficient of the medium, and Cp is the specific heat of the medium at constant pressure.
Photothermal conversion efficiency calculation. Photothermal conversion efficiency (η) represents the efficiency of transducing incident absorbance to thermal energy, which could be calculated as follows, according to the literature 2−4 .
From an energy balance on a system, the total energy balance is: Where the i terms miCp,i are products of mass and heat capacity of system components. T is system temperature, and t is time.
Qin,np is the photothermal energy input from the agents, which can be described as: Where I is the laser power in the photothermal experiment. Aλ is the absorbance at 808 nm.
Qin,surr is the heat input due to light absorption by the solvent and container, which can be described as: Where hSbuff is the parameter relevant with container and solvent (h and S represent heat transfer coefficient and surface area of the container, respectively). Tmax,buff is the maximum steady-state temperature of solvent (without agents). Tsurr is the ambient surrounding temperature.
Qout is the heat lost to the surrounding, which can be described as: The hS and hSbuff can be determined by measuring the rate of temperature decrease after removing the light source.
At the maximum steady-state temperature, equation (3) equals to 0 and we can obtain: Where Tmax is the maximum steady-state temperature of nanoparticles. As a consequence, Synthesis. then purified by silica-gel column chromatography, affording compounds 2 and 3 with 73 mg and 46 mg, respectively. yield: 55% for 2 and 37% for 3. found, 2990.2.