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The self-trapping nano-loop structures of [1]rotaxanes exhibited multiple Förster resonance energy transfer (FRET) OFF/ON patternsviadual and sequential locking/unlocking upon UV exposure. 

The Watson–Crick A·T and G·C base pairs are not only electronically complementary, but also photochemically complementary. Upon UV irradiation, DNA base pairs undergo efficient excited-state deactivation through electron driven proton transfer (EDPT), also known as proton-coupled electron transfer (PCET), at a rate too fast for other reactions to take place. Why this process occurs so efficiently is typically reasoned based on the oxidation and reduction potentials of the bases in their electronic ground states. Here, we show that the occurrence of EDPT can be traced to a reversal in the aromatic/antiaromatic character of the base upon photoexcitation. The Watson–Crick A·T and G·C base pairs are aromatic in the ground state, but the purines become highly antiaromatic and reactive in the first 1 ππ* state, and transferring an electron and a proton to the pyrimidine relieves this excited-state antiaromaticity. Even though proton transfer proceeds along the coordinate of breaking a N–H σ-bond, the chromophore is the π-system of the base, and EDPT is driven by the strive to alleviate antiaromaticity in the π-system of the photoexcited base. The presence and absence of alternative excited-state EDPT routes in base pairs also can be explained by sudden changes in their aromatic and antiaromatic character upon photoexcitation. 

Photoacids like substituted naphthalenes (X = OH, NH 3 + , COOH) are aromatic in the S 0 state and antiaromatic in the S 1 state. Nucleus independent chemical shifts analyses reveal that deprotonation relieves antiaromaticity in the excited conjugate base, and that the degree of “antiaromaticity relief” explains why some photoacids are stronger than others. 

Computed association strengths for 43 purine and pteridine quartets (38 to 100 kcal mol −1 ) show excellent linear correlation with π-conjugation gain in the assembled monomers ( r 2 = 0.965). Even quartets having the same secondary electrostatic interactions can display very different association strengths depending on the π-conjugation patterns of the monomeric units. 

Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4 n + 2] π-aromatic in the ground state, become [4 n + 2] π-antiaromatic in the first 1 ππ* states, and proton transfer (either inter- or intramolecularly) helps relieve excited-state antiaromaticity. Computed nucleus-independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. o- Salicylic acid undergoes ESPT only in the “antiaromatic” S 1 ( 1 ππ*) state, but not in the “aromatic” S 2 ( 1 ππ*) state. Stokes’ shifts of structurally related compounds [e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates] vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation. 

Density functional theory computations and block-localized wavefunction analyses for 57 hydrogen-bonded base pairs document excellent linear correlation between the gas-phase association energies and the degree of aromaticity gain of paired bases ( r = 0.949), challenging prevailing views of factors that underlie the proposed electronic complementarity of A·T(U) and G·C base pairs. Base pairing interactions can polarize the π-electrons of interacting bases to increase (or decrease) cyclic 4 n + 2π electron delocalization, resulting in aromaticity gain (or loss) in the paired bases, and become strengthened (or weakened). The potential implications of this reciprocal relationship for improving nucleic acid force-fields and for designing robust unnatural base pairs are discussed. 

Textbook explanations for the associations of multipoint hydrogen-bonded arrays have long hinged on the secondary electrostatic interaction (SEI) model, which suggests that array association strengths depend on the proton donor (D) and acceptor (A) patterns of the interacting units. Here, computational results based on the block-localized wavefunction (BLW) method reveal limitations of the SEI model, demonstrating instead that, in the gas-phase (and in implicit chloroform solvation), the inherent free-energies of associations of multipoint hydrogen-bonded arrays correlate with the degree of “aromaticity gain” ( i.e. , the amount of increased cyclic π-electron delocalization) in arrays upon complexation. Excellent correlations for 46 triply ( r = 0.940) and quadruply ( r = 0.959) hydrogen-bonded arrays are presented.