To investigate the photothermal mechanism, firstly we take up Co-DHBDC synthesis introducing a 660 nm LED, which corresponds to the maximum absorption of the molecular precursor (Fig. ). As evident from the electron microscope images, introducing a 660 nm LED in the Co-DHBDC synthesis yielded MOF rods with a length of 2800 nm and a width of approximately 1200 nm (Fig. , and Supplementary Fig. ). In contrast, as previously discussed, under the same reaction conditions, a photothermal reaction utilizing a 520 nm LED resulted in the assembly of the MOF (Fig. ). To delve deeper, we thereby started by assessing the photothermal response of the different components at the relevant wavelengths in order to identify the dominant photothermal source. Thus, we prepared four separate batches of MOF precursors solution, first two consisted of a DMF solution of cobalt chloride, and the second two 2,5-dihydroxyterephthalic acid (DHBDC) solutions of DMF with same concentrations as used for Co-DHBDC synthesis. The photothermal response was evaluated using these formulations under 520 and 660 nm light irradiation (Fig. ). Unsurprisingly, the ligand solutions couldn't generate heat upon irradiation with either of the green (≈520 nm) or red LEDs (≈660 nm) (Fig. and). On the other hand, the metal precursor (CoCl2) solution readily heated up as evident from the temperature profile under green or red-light illumination (Fig and). The temperature profiles under green and red-light irradiation demonstrated distinct behaviours of the cobalt precursor heating. Under green light, the cobalt chloride solution gradually heated up to 140 °C, while under red light, it rapidly reached a maximum set temperature of 150 °C under identical conditions (Fig. and). To provide further clarity, temperature profiles were generated within a set range of 100 °C to 120 °C (Fig. and). These profiles revealed that under green light, the cobalt chloride precursor completed approximately 65 heating cycles lasting about 61 seconds each, whereas under red light, it completed around 97 cycles lasting approximately 25 seconds each during 2 h reactions (Supplementary Figs. ). These experiments unequivocally establish cobalt chloride as a stable photothermal molecular heater throughout the reaction duration (Supplementary Fig. ). The photothermal response is due to absorption of red and green light by the cobalt precursor that could be attributed to the formation of a blue-coloured tetrahedral cobalt chloride-solvent complex (Fig. ). UV-Vis absorption spectra demonstrate that this complex exhibited a stronger visible light absorption due to d-d transition of Co (II) which heated up through non-radiative relaxation. Then again, for Co-DHBDC, exhibiting typical broad absorption in visible region due to periodic arrays of self-assembled molecules with discrete molecular absorption modes that decreases consistently at longer wavelength (Fig. ). The photothermal response of Co-DHBDC under green and red light was evaluated under similar conditions as for the molecular precursor (Fig. ). The temperature profiles illustrated the photothermal heating ability of Co-DHBDC consistent to the light absorption, and also probed using Raman (Supplementary Fig. ). Under 660 nm LED, Co-DHBDC gradually heated up to 120 °C, albeit at a slower rate compared to green light irradiation with 520 nm LED, contrary to observations for the molecular precursor. The heating cycles for Co-DHBDC under green light indicated approximately 28 cycles lasting about 67 seconds each, while under red light, around 8 cycles lasting approximately 363 seconds each were observed during 1 h reactions (Supplementary Figs. ). These observations under different light irradiations showcase the distinctive photothermal activation mechanism, and the temperature profiles point to a comparative and dominant photothermal responses in the solution (Fig. ). The dominant photothermal heating indicates that only one of the components within the reaction is primarily responsible for driving the synthetic outcome. Under the photothermal synthesis condition using 520 nm green LED, the photothermal response of Co-DHBDC nanorods generated insitu, leads to further nucleation-growth of the MOF in a spatially confined manner yielding the superstructure. Similar findings could also be noted in other MOF systems we synthesized through photothermal activation (Supplementary Figs. ).