We then heated the films to 160 °C, which is greater than the temperature at which boric acid dehydrates. This was evidenced in the thermogravimetric analysis (TGA), where dehydration temperature of boric acid was found to occur around 110 °C (Figure ). As expected, the formed supramolecular hydrogen-bonded network is more rigid due to the formation of boroxine rings, associated with a higher decomposition temperature of 196 °C measured by TGA. This led to an enhancement of the RTP from TPA-CO2H (Figure ). The change in matrix structure is reflected in the FTIR measurements, where the boroxine out-of-plane vibration at ～705 cm -1 emerges after heat treatment (Figure ), while the PXRD patterns of the heat-treated TPA-CO2H/boric acid composite shows a broad peak characteristic of amorphous regions of the sample in addition to the peaks associated with crystalline boric acid (Figure ). The steady-state PL spectrum of the heat-treated sample is red-shifted (Figure ). The steady-state and time-gated emission recorded at both 298 and 77 K are nearly identical (Figures and). This indicates that the majority of the emission is due to RTP and that TPA-CO2H likely adopts a slightly different conformation to account for the redshifted emission. The tPh at 298 K is longer at 225 ms and the tPL is 4.1 ns (Figure ), together with a much improved FPh of 43%. Thus, the rigidified environment formed during heat treatment enhances the RTP from the guest emitter. The heat-treaded 5 wt% film qualitatively also showed the same redshifted, brighter RTP as that of the 1 wt% film. When the heat-treated sample was dissolved in methanol/water and this solution was drop-cast, the PXRD pattern of the re-prepared TPA-CO2H/boric acid returned to the same pattern to that of the initially drop-cast film, which also matches that of boric acid (Figure ). Coincidentally, the photophysics of the re-prepared film largely mirrors that of the originally prepared drop-cast film (Figure ); the FPh decreased to 8% and the tPh shortened to 184 ms (Figure ).