Guanine Functionalization of Single-Wall Carbon Nanotubes: A Quantum Chemical Study

The guanine functionalization reaction uses singlet oxygen to covalently link single-wall carbon nanotubes to guanine bases in ssDNA coatings. This creates shallow but densely spaced exciton traps that modulate nanotube band gaps with energetic and spatial control, giving red-shifted electronic transitions. To better understand guanine functionalization, we used quantum chemical computations to compare the stabilities of several candidate addends in multiple orientations on the nanotube surface. Structures of three possible isomers of guanine peroxide (GPO), the reactive intermediate formed through reaction of 9-methyl guanine with singlet O₂, were optimized using the semiempirical PM3 method. To examine effects of nanotube diameter on adduct stability, we then computed the enthalpy changes for bonding of each GPO isomer to a 6 nm segment of (5,4), (6,5), (7,6), and (8,7) single-wall carbon nanotubes (SWCNTs). Six orientations of the addend on the SWCNT surface were considered for each (<i>n</i>,<i>m</i>) species, giving a total of 72 adduct structures. The results showed that for all four SWCNTs, the most energetically stable adduct is the 4,5-GPO isomer bonded in the ortho L_-30 orientation. This adduct can be considered to be a derivative of 1,4-dioxane. Subsequent <i>ab initio</i> DFT and TDDFT computations comparing bonding orientations of one guanine addend on a 12 nm long SWCNT segment found that ortho L_-30 gives a slightly reduced HOMO-LUMO gap, a moderately localized exciton structure, and a slightly red-shifted E₁₁ optical transition as compared to the pristine SWCNT, in agreement with experiment. We conclude that guanine functionalization of near-armchair SWCNTs leads mainly to 4,5-GPO addends bonded in the ortho L_-30 orientation.