Supramolecular free radicals: near-infrared organic materials with enhanced photothermal conversion† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c5sc01167a

A novel kind of supramolecular free radical with significantly improved free radical yield and enhanced near-infrared photothermal conversion has been fabricated.


Synthesis of BPDI.
The synthetic route is shown in Scheme S1. 3,4,9,10-perylenetetracarboxylic dianhydride (0.5 g) was dissolved in N,N-Dimethyl-1,2-ethanediamine (15 mL). The solution was refluxed with stirring overnight. The solution was allowed to cool and 50-100 mL methanol was added to the mixture, which was then boiled for 30 min. The dark purple solid precipitate (Compound 1) was filtered and washed with methanol then dried under vacuum. The yield was 90 %. 1 Figure S2. Full UV-Vis Spectra of BPDI and BPDI radical anion in aqueous solution. The characteristic absorption bands peaked at 500 nm and 540 nm decreased accompanied with the increase of absorbances at 732 nm and 819 nm, indicating the consuming of BPDI to produce BPDI radical anions.

Quantitative calculation for the concentration of BPDI radical anions and BPDI/(CB[7]) 2 radical anions.
By further analysing the EPR signals, the integration of BPDI radical anions and BPDI/(CB[7]) 2 radical anions could be determined. Then we could calculate the radical anion concentrations from the integration on the basis of the standard curve ( Figure S4).   Improvement ratio is defined as follow:

Stability of supramolecular free radicals.
The stability of the supramolecular free radical was measured at 25°C and 60°C, respectively. The BPDI/(CB[7]) 2 supramolecular free radical remains 90% after 1 h at 25°C. Moreover, the supramolecular free radical also has a good stability even at 60°C. Considering that the photothermal conversion process is usually completed within 30 min, such stability of the supramolecular free radical should be enough for the photothermal conversion.

The detailed derivation and calculation of photothermal conversion efficiency for BPDI radical anion and BPDI/(CB[7]) 2 radical anion.
The photothermal conversion efficiency could be calculated from the energy balance during the irradiation for BPDI or BPDI/(CB[7]) 2 solution. The total energy balance for the system is: where m and C p are the mass and heat capacity of the solvent (water) and T is the solution temperature. Q in,radical is the photothermal energy input from the radical anions. Define the photothermal conversion efficiency η as the fraction of the total light energy that is converted to heat, that is: where I is the laser power. Q in,surr is the heat generation from the BPDI or BPDI/(CB[7]) 2 solution, while Q out is the heat loss to the surroundings, following the theory of convection heat transfer: Where h is the heat transfer coefficient, A is the irradiated area of the container and T surr is the temperature of surrounding (can be regarded as constant). When the system reaches at the steady state, the rate of photothermal heating is equal to the rate of heat dissipation to the surrounding, thus achieving the energy balance and the maximal temperature, so that: From the equation (2) and (3) the equation (1) will become to be: A characteristic rate constant has been defined as τ out = mC p /hA, the above differential equation can be solved: Integrating using the initial condition, we can get: θ, a dimensionless driving force of temperature is defined as follow: = --By this way the equation has been simplified as this: From the data of photothermal experiments, we can achieve the characteristic rate constant τ out with the linear fitting method, as shown in Figure S7. The characteristic rate constant τ out of BPDI radical anion or BPDI/(CB[7]) 2 radical anion can be attained from the fitting slopes, and then hA can also be determined, thus calculating the total heat generation: Similarly, Q in,surr can be calculated by dealing with the data for BPDI and BPDI/(CB[7]) 2 solution without reduction. So far the photothermal conversion efficiency could be calculated by:

Adaptivity of supramolecular free radicals.
1-Adamantanamine hydrochloride (AD), can be used to disassociate the complexation between BPDI and CB [7], which has been confirmed by a variety of methods. As shown in Figure  S8a, the evolution of 1H NMR induced by the inclusion of CB[7] recovered after the addition of AD. In addition, the separation of CB[7] heads led to the close aggregation of BPDI once again, resulting in the decrease of UV-Vis absorbance and the quenching of fluorescence emission ( Figure S8b and S8c).
Moreover, the spectra of BPDI/(CB[7]) 2 /AD 2.5 were almost the same as those of BPDI, indicating the perfect reversibility and adaptivity of the supra-amphiphile. More importantly, the above dynamic properties have also been introduced into the supramolecular free radical. The production of PDI radical anions can be adjusted to the initial level ( Figure S8d).