A theoretical study on 2′-deoxyguanosine and its mono-hydration: Intermolecular hydrogen bonding weakening induces the fluorescence strengthening

► Intramolecular hydrogen bonds of keto and enol are different in excited states. ► Intermolecular and hydrogen bonds are both weakened in the S1 state. ► Intermolecular hydrogen bond weakening induces fluorescence strengthening.