Both Intra- and Interstrand Charge-Transfer Excited States in Aqueous B-DNA Are Present at Energies Comparable To, or Just Above, the ¹ππ* Excitonic Bright States

Abstract

Vertical electronic excitations in model systems representing single- and double-stranded B-DNA are characterized using electronic structure theory, including both time-dependent density functional theory (TD-DFT) and correlated wave function techniques. Previous TD-DFT predictions of charge-transfer (CT) states well below the optically bright (1)pipi* states are shown to be artifacts of the improper long-range behavior of standard density-functional exchange approximations, which we rectify here using a long-range correction (LRC) procedure. For nucleobase dimers (hydrogen-bonded or pi-stacked), TD-LRC-DFT affords vertical excitation energies in reasonable agreement with the wave function methods, not only for the (1)npi* and (1)pipi* states but also for the CT states, and qualitatively reproduces well-known base-stacking effects on the absorption spectrum of DNA. The emergence of (1)pipi* Frenkel exciton states, localized on a single strand, is clearly evident, and these states (rather than low-energy CT states) are primarily responsible for the fact that DNA's absorption spectrum exhibits a red tail that is absent in monomer absorption spectra. For B-DNA in aqueous solution, the low-energy tail of the CT band (representing both intra- and interstrand CT states) appears at energies comparable to those of the optically bright (1)pipi* exciton states. In systems with more than one base pair, we also observe the emergence of delocalized, interstrand CT excitations, whose excitation energies may be significantly lower than the lowest CT excitation in a single base pair. Together, these observations suggest that a single Watson-Crick base pair is an inadequate model of the photophysics of B-DNA.

Exciton delocalization, charge transfer, and electronic coupling for singlet excitation energy transfer between stacked nucleobases in DNA: An MS-CASPT2 study

“…Such charge transfer states have been invoked in previous experimental and computational studies on DNA spectroscopy. 17,22,24,26,27,45 In the stacks AT, GC, and GT, there is a low-lying charge transfer state that mixes with the two locally excited diabatic states. In the AC dimer, the charge transfer character is not so prominent (8% for S 2 ).…”

“…Excitonic and charge transfer states have been made responsible for this difference, and the role of these states has been discussed extensively on the basis of experimental 2,3,[15][16][17][19][20][21][22][23][24][25] and theoretical 14,24,[26][27][28] work. Most theoretical ab initio approaches on the excited states of DNA have considered small polymers, for which the excited states are calculated using time-dependent density functional theory 24,[26][27][28] (TD-DFT) or wave function based methods. 14 The excited state studies are often preceded by molecular dynamics calculations to account for the structural fluctuations of the polymer.…”

Advances in molecular quantum chemistry contained in the Q-Chem 4 program package

“…While very difficult to remove fully, SIE can be significantly reduced by including 100% exact (wave function) exchange at large electronelectron distances, and a much smaller fraction at short distances, where DFT exchange functionals are effective. Examples of functionals of this type include the LC-ωPBE family of methods [35][36][37][38], the ωB97 functionals [39][40][41][42], M11 [28], as well as tuned functionals of the BNL type [43,44], where the range-separation parameter can be chosen for the problem at hand based on physical criteria [45,46]. Another option is to include 100% Hartree-Fock (HF) exchange at all inter-electronic distances, as in the M06-HF functional [47].…”

Excitation Energies of Stacked DNA Base Pair

“…。 一方、近年のコンピュータの発達によって、多く の高精度ab initio法が適用可能となってきているこ ともあり、スタックした核酸塩基全系を取り扱った、 ab initio 法 や 密 度 汎 関 数 法 (Density-Functional Theory:DFT)法による量子力学的な計算結果が近年 いくつか報告され始めている [11][12][13][14][15][16][17][18][19][20][21][22][23][24] …”