Currently, achieving carbon neutrality is a historical global concern, including China 2060 and USA 2050, which refers to a dynamic balance between carbon emissions and carbon fixation. To address these concerns, human society is not only promoting the scale of artificial carbon sinks but also reducing carbon emissions. The latter plays a vital position in achieving carbon neutrality, necessitating high-efficiency energy consumption. Among the key drivers of energy consumption, buildings consume approximately 40%, more energy than the industry and transportation sectors. Half of building energy is used in ventilation and temperature control (heating and cooling for buildings), making it the largest unrealized cost-effective emission savings. The main reason is that traditional windows allow heat transfer in an undesirable direction that helps buildings gain heat in summer and release heat in winter. Moreover, the traditional daylighting control solutions, i.e., blinds, lead to the commonplace occurrence of closing blinds and lights on, with poor daylight utilization and more electric energy consumption. Smart windows, which can regulate illumination in buildings and heat exchange with the environment, are regarded as the ideal solution of this problem. The most widely used and investigated smart windows are based on electrochromism, with few researchers focusing on the unconventional ones based on mechanochromism. Electrochromism refers to the reversible and persistent changes of colour, transmittance, or other optical properties through electrochemical redox reactions under applied voltages. A typical electrochromic device is essentially a battery with electrochromic materials as electrodes. Our first two research chapters report electrochromic effects in traditional inorganic anode and cathode materials, namely LTO and TiS2, respectively. For LTO anode, we performed a detailed analysis of LTO’s optical properties during charging/discharging via a robust study of the DFT. Our study suggests that the absorption of infrared light is highly xiv sensitive to lithium intercalation in the LTO lattice, in contrast to that of visible wavelengths. The electrochemical intercalation introduced donor states which gradually expanded and moved to deeper levels in the forbidden band, resulting in better conductivity and lower transmittance. For TiS2 cathode, we theoretically explored structural, electronic, and electrochromic properties of bulk and monolayer TiS2 nanosheets before and after lithium intercalation, using the most accurate available theoretical methods: SCAN+rVV10 for structures, GW for band gaps, COHP for bonding strength analysis, and IPA for optical properties in the solar spectrum. Our results suggest that during lithium intercalation/deintercalation, changes in lattice parameters, bandgap between S-p and Ti-d, and electrochromic properties are interlayer distance-dependent. For example, the modulation ranges of transmitted visible light for bulk and monolayer TiS2 are 24-48% and 34-75%, respectively. These two work point out two factors affecting the electrochromic performance, namely donor states in the forbidden band and interlayer distances, which provide insights into the mechanism of electrochromism. Besides traditional inorganic materials, CONASHs are an emerging group of 2D materials with unique physical and chemical properties in a wide range of applications, such as superconductivity, hydrogen evolution catalyst, chemical sensing, spintronic and thermoelectric devices. In the third research chapter, we screen out electrochromic materials in a series of TM-BHT via first-principles calculations. Meanwhile, during lithiation/delithiation, changes in lattice structures, atomic charges, bond strength, and electronic properties are explored in-depth. The incurred changes are then correlated with critical electrochromic properties. Our results show that Cu-BHT and Ag-BHT are the most promising broadband electrochromic materials for optical and thermal management in the wavelength range from xv visible to mid-infrared. The theoretical guidance from this work paves a new path towards electrochromic applications of CONASHs that exploit the versatility of these 2D materials. Further studies on Cu-BHT reveal the mechanochromic effect, due to the fragile balance of large-scale intralayer delocalization. In the fourth research chapter, we theoretically investigate atomic charges, bond length and strength, and electronic structures of Cu-BHT under uniaxial and biaxial strain. The mechanism of mechanochromic phenomenon is based on band-structure engineering, which is due to limited unoccupied states near the Fermi level for compressive strain and the broken 𝜋-d conjugated system for tensile strain. Both strains negatively affect electron conductivity. In terms of mechanochromic performance, Cu-BHT film achieves remarkable tunability in visible and near-infrared spectra under only 3% uniaxial and biaxial compressive strain, respectively. Corresponding modulation ranges are 2.7-74.6% and 0-87.6%, respectively. The strain-tuneable electronic properties of Cu-BHT provide it with great potential in smart windows, thermal camouflage, infrared radiative cooling, and various nanoscale electronic and optoelectronic applications. In summary, we theoretically discover electrochromic and mechanochromic effect for smart windows in traditional inorganic materials and emerging CONASHs. Corresponding mechanism and induced changes in response to applied voltages or mechanical strain have been investigated in detail. The in-depth understanding of related eye-catching phenomena and newly discovered materials could not only promote field development, but also inspire more researchers to get involved.