Excited-State Dynamics of Thienoguanosine,an Isomorphic Highly Fluorescent Analogue of Guanosine

Thienoguanosine (^thG) is an isomorphic analogue of guanosine with promising potentialities as fluorescent DNA label. As a free probe in protic solvents, ^thG exists in two tautomeric forms, identified as the H1, being the only one observed in nonprotic solvents, and H3 keto–amino tautomers. We herein investigate the photophysics of ^thG in solvents of different polarity, from water to dioxane, by combining time-resolved fluorescence with PCM/TD-DFT and CASSCF calculations. Fluorescence lifetimes of 14.5–20.5 and 7–13 ns were observed for the H1 and H3 tautomers, respectively, in the tested solvents. In methanol and ethanol, an additional fluorescent decay lifetime (≈3 ns) at the blue emission side (λ≈430 nm) as well as a 0.5 ns component with negative amplitude at the red edge of the spectrum, typical of an excited-state reaction, were observed. Our computational analysis explains the solvent effects observed on the tautomeric equilibrium. The main radiative and nonradiative deactivation routes have been mapped by PCM/TD-DFT calculations in solution and CASSCF in the gas phase. The most easily accessible conical intersection, involving an out-of plane motion of the sulfur atom in the five-membered ring of ^thG, is separated by a sizeable energy barrier (≥0.4 eV) from the minimum of the spectroscopic state, which explains the large experimental fluorescence quantum yield.     

Quantum chemical studies on prototautomerism of 1H‐imidazo[4,5‐c]pyridine

The structural features of the 1H‐imidazo[4,5‐c]pyridine (ICPY) tautomers and homodimers of the most stable tautomers have been studied by quantum chemical methods. FTIR and Raman spectra of the ICPY were recorded in the range of 4000–60 cm^?1 and 3500–5 cm^?1. The predominant tautomer among four possible isomers of ICPY were determined. The optimized geometries and vibrational frequencies of possible ICPY tautomers and dimers were computed by B3LYP/DFT method with 6‐311++G(d,p) and 6‐31G(d) basis sets. All vibrational frequencies assigned in detail with the help of total energy distribution (TED) and isotopic shifts. ICPY dimeric forms were also characterized according to their hydrogen bonding interactions, and it has been found that the most stable ICPY homodimer establishes moderate strong N ? H …N type hydrogen bond. ¹H NMR, ¹³C NMR, and ¹⁵N NMR properties have been calculated for all tautomeric forms using the gauge independent atomic orbital (GIAO) method. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Excited state tautomerization of azaindole

Fluorescent tryptophan analogs, like azatryptophan, offer an advantage for exploring protein and peptide structure and dynamics. The chromophoric moieties, azaindole, of the azatryptophan analogs are investigated for their potential as fluorescent probes. The photophysical properties of 4-azaindole (4AI) and 5-azaindole (5AI) and their tautomers are characterized through computational and experimental methods. Both 4AI and 5AI undergo excited state tautomerization in the presence of 1 M NaOH. The protonated forms of 4AI and 5AI have a fluorescence emission of 415 and 410 nm, respectively, while the tautomers of 4AI and 5AI have a fluorescent emission of 480 and 450 nm, respectively. Gas phase computations (B3LYP/6-31+G**) show that the N1H azaindole tautomer is lower in energy in the ground state by as much as 12.5 kcal mol(-1), while the N(n)H azaindole tautomer is lower in energy in the excited state by as much as 18.1 kcal mol(-1). Solvent effects on the tautomer energy differences were computed using the isodensity polarized continuum model (IPCM). The polarity of the solvent helps to reduce the energy difference between the tautomers in the ground state by as much as 5.8 kcal mol(-1), but not enough to reverse the ground state tautomer preference.     

Tautomer‐Specific Resonant Photoelectron Imaging of Deprotonated Cytosine Anions

Tautomers of the nucleobases play fundamental roles in spontaneous mutations of DNA. Tautomers of neutral cytosine have been studied in the gas phase, but much less is known about charged species. Here, we report the observation and characterization of three tautomers of deprotonated cytosine anions, [trans‐keto‐amino‐N3H‐H8b] (tKAN3H8b^?), [cis‐keto‐amino‐N3H‐H8a] (cKAN3H8a^?) and [keto‐amino‐H] (KAN1^?), produced by electrospray ionization. Excited dipole‐bound states (DBSs) are uncovered for the three anions by photodetachment spectroscopy. Excitations to selected DBS vibrational levels of cKAN3H8a^? and tKAN3H8b^? yield tautomer‐specific resonant photoelectron spectra. The current study provides further insight into tautomerism of cytosine and suggests a new method to study the tautomers of nucleobases using electrospray ionization and anion spectroscopy.     

Steady‐State Spectroscopy of the 2‐(N‐methylacetimidoyl)‐1‐naphthol Molecule

The steady‐state spectroscopy of 2‐(N‐methylacetimidoyl)‐1‐naphthol (MAN) reveals composite absorption and emission spectra from 298 to 193 K in hexane. The ground electronic state (So) absorption can be assigned to the sum of three molecular structures: the OH normal tautomer, and two NH proton transfer tautomers. The NH‐structures are the most stable ones in equilibrium with the OH tautomer for the S₀ state. On photoexcitation of the OH tautomer the excited state intramolecular proton transfer is undergone, and the corresponding NH emission is monitored at 470 nm. On photoexcitation of the NH tautomers the previous emission is monitored in addition to another emission at 600 nm, which is ascribed to intramolecular hydrogen‐bonded (IHB) nonplanar NH structures generated from the IHB planar NH tautomers. A Jab?oński diagram is introduced which gathers all the experimental evidence as well as the theoretical calculations executed at the DFT‐B3LYP and TD‐DFT levels. The MAN molecule is compared with other analogs such as 1‐hydroxy‐2‐acetonaphthone (HAN), 2‐(1?‐hydroxy‐2?‐naphthyl)benzimidazole and methyl 1‐hydroxy‐2‐naphthoate to validate the theoretical calculations. Photoexcitation of MAN generates two emission bands at longer wavelengths than that of the emission band of HAN. The MAN molecule exhibits a great photostability in hydrocarbon solution which depends on the photophysics of the NH tautomers (keto forms).     

Structure and tautomerism of 4-substituted 3(5)-aminopyrazoles in solution and in the solid state: NMR study and Ab initio calculations

Annular tautomerism of 3(5)-aminopyrazoles containing a cyano, thiocyanato, or aryl substituent in the 4-position has been studied by ¹H and ¹³C NMR in solution, cross-polarization and magic-angle spinning ¹³C NMR in the solid state, and ab initio quantum chemical calculations (B3LYP/6-31G**). The title compounds in the solid state exist as 3-amino tautomers. A rare case of slow (on the NMR time scale) annular prototropic tautomerism has been observed in DMSO-d ₆: signals of particular tautomers (3- and 5-aminopyrazoles) have been detected in the NMR spectra. 4-Cyano and 4-thiocyanato derivatives exist preferentially as 5-amino tautomers, whereas 4-methoxy analog is represented mainly by the 3-amino tautomers. Ab initio calculations (B3LYP/6-31G**) for the gas phase and DMSO solution (in terms of the polarizable continuum model) have shown increase of the relative stability of more polar 5-amino tautomer in going to DMSO.     

尿酸分子互变异构体平面构象的理论研究

使用半经验量子化学中的AM1方法、从头计算Hartree-Fock理论(在3-21G*水平)和密度泛函理论中的B3LYP方法(使用6-31G(d)基组),研究了尿酸分子的所有35种互变异构体。计算结果表明,三羰基互变异构体是所有异构体中能量最低的,其次为单羟基异构体和双羟基异构体,而含有三羟基的互变异构体相对能量最高。随着羟基数的增加, C-N键的平均键长从1.395逐渐缩短到1.351,而CC键的平均键长基本保持不变(1.400~1.406)。     

Solid‐State and Solution Structural Studies of 4‐{[C(E)]‐1H‐Azol‐1‐ylimino)methyl}pyridin‐3‐ols

The new N‐salicylideneheteroarenamines 1 – 4 were prepared by reacting the biologically relevant 3‐hydroxy‐4‐pyridinecarboxaldehyde ( 5 ) with 1H‐imidazol‐1‐amine ( 6 ), 1H‐pyrazol‐1‐amine ( 7 ), 1H‐1,2,4‐triazol‐1‐amine ( 8 ), and 1H‐1,3,4‐triazol‐1‐amine ( 9 ). Solution ¹H‐, ¹³C‐, and ¹⁵N‐NMR were used to establish that the hydroxyimino form A is the predominant tautomer. A combination of ¹³C‐ and ¹⁵N‐CPMAS‐NMR with X‐ray crystallographic studies confirms that the same form is present in the solid state. The stabilities and H‐bond geometries of the different forms, tautomers and rotamers, are discussed by using B3LYP/6‐31G** calculations.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

Is the DPT tautomerization of the long A·G Watson–Crick DNA base mispair a source of the adenine and guanine mutagenic tautomers? A QM and QTAIM response to the biologically important question

Herein, we first address the question posed in the title by establishing the tautomerization trajectory via the double proton transfer of the adenine·guanine (A·G) DNA base mispair formed by the canonical tautomers of the A and G bases into the A*·G* DNA base mispair, involving mutagenic tautomers, with the use of the quantum‐mechanical calculations and quantum theory of atoms in molecules (QTAIM). It was detected that the A·G ? A*·G* tautomerization proceeds through the asynchronous concerted mechanism. It was revealed that the A·G base mispair is stabilized by the N6H···O6 (5.68) and N1H···N1 (6.51) hydrogen bonds (H‐bonds) and the N2H···HC2 dihydrogen bond (DH‐bond) (0.68 kcal·mol^?1), whereas the A*·G* base mispair—by the O6H···N6 (10.88), N1H···N1 (7.01) and C2H···N2 H‐bonds (0.42 kcal·mol^?1). The N2H···HC2 DH‐bond smoothly and without bifurcation transforms into the C2H···N2 H‐bond at the IRC = ?10.07 Bohr in the course of the A·G ? A*·G* tautomerization. Using the sweeps of the energies of the intermolecular H‐bonds, it was observed that the N6H···O6 H‐bond is anticooperative to the two others—N1H···N1 and N2H···HC2 in the A·G base mispair, while the latters are significantly cooperative, mutually strengthening each other. In opposite, all three O6H···N6, N1H···N1, and C2H···N2 H‐bonds are cooperative in the A*·G* base mispair. All in all, we established the dynamical instability of the А*·G* base mispair with a short lifetime (4.83·10^?14 s), enabling it not to be deemed feasible source of the A* and G* mutagenic tautomers of the DNA bases. The small lifetime of the А*·G* base mispair is predetermined by the negative value of the Gibbs free energy for the A*·G* → A·G transition. Moreover, all of the six low‐frequency intermolecular vibrations cannot develop during this lifetime that additionally confirms the aforementioned results. Thus, the A*·G* base mispair cannot be considered as a source of the mutagenic tautomers of the DNA bases, as the A·G base mispair dissociates during DNA replication exceptionally into the A and G monomers in the canonical tautomeric form. © 2013 Wiley Periodicals, Inc.     

DFT studies on tautomerism of C5-substituted 1,2,4-triazoles

DFT (B3LYP/6-311++G**, B3PW91/6-311++G**) Gibbs free energy and single point CCSD(T)/6-311++G**//DFT total energy calculations were performed to investigate stability and tautomerism of C5-substituted 1,2,4-triazoles. Three different tautomers are possible for the substituted 1,2,4-triazoles: N1–H, N2–H, and N4–H. Unlike for the 1,2,3-triazoles, where the most stable is the N2–H tautomer regardless of substituent applied, for the 1,2,4-triazoles, the electron donating substituents (–OH, –F, –CN, –NH₂, and –Cl) and the C5-cation stablize the N2–H tautomer, whereas the electron withdrawing substituents (–CONH₂, –COOH, –CHO, –BH₂, and –CFO) and the C5-anion stablize the N1–H tautomer. Except for the C5-anion and C5-cation, the N4–H form is the least stable tautomer. The relative stability of the C5-substituted 1,2,4-triazole tautomers is strongly influenced by attractive and/or repulsive intramolecular interactions between substituent and electron donor or electron acceptor centres of the triazole ring.     

An X‐ray crystallographic and density functional theory study of (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one

The Schiff base enaminones (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₃H₁₇NO₄S, (I), and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₅H₂₁NO₂, (II), were studied by X‐ray crystallography and density functional theory (DFT). Although the keto tautomer of these compounds is dominant, the O=C—C=C—N bond lengths are consistent with some electron delocalization and partial enol character. Both (I) and (II) are nonplanar, with the amino–phenol group canted relative to the rest of the molecule; the twist about the N(enamine)—C(aryl) bond leads to dihedral angles of 40.5 (2) and −116.7 (1)° for (I) and (II), respectively. Compound (I) has a bifurcated intramolecular hydrogen bond between the N—H group and the flanking carbonyl and hydroxy O atoms, as well as an intermolecular hydrogen bond, leading to an infinite one‐dimensional hydrogen‐bonded chain. Compound (II) has one intramolecular hydrogen bond and one intermolecular C=O...H—O hydrogen bond, and consequently also forms a one‐dimensional hydrogen‐bonded chain. The DFT‐calculated structures [in vacuo, B3LYP/6‐311G(d,p) level] for the keto tautomers compare favourably with the X‐ray crystal structures of (I) and (II), confirming the dominance of the keto tautomer. The simulations indicate that the keto tautomers are 20.55 and 18.86 kJ mol⁻¹ lower in energy than the enol tautomers for (I) and (II), respectively.     

Photoelectron spectrum of valence anions of uracil and first-principles calculations of excess electron binding energies

The photoelectron spectrum (PES) of the uracil anion is reported and discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of the anions of various tautomers of uracil. The PES peak maximum is found at an electron binding energy of 2.4 eV, and the width of the main feature suggests that the parent anions are in a valence rather than a dipole-bound state. The canonical tautomer as well as four tautomers that result from proton transfer from an NH group to a C atom were investigated computationally. At the Hartree-Fock and second-order Moller-Plesset perturbation theory levels, the adiabatic electron affinity (AEA) and the VDE have been converged to the limit of a complete basis set to within +/-1 meV. Post-MP2 electron-correlation effects have been determined at the coupled-cluster level of theory including single, double, and noniterative triple excitations. The quantum chemical calculations suggest that the most stable valence anion of uracil is the anion of a tautomer that results from a proton transfer from N1H to C5. It is characterized by an AEA of 135 meV and a VDE of 1.38 eV. The peak maximum is as much as 1 eV larger, however, and the photoelectron intensity is only very weak at 1.38 eV. The PES does not lend support either to the valence anion of the canonical tautomer, which is the second most stable anion, and whose VDE is computed at about 0.60 eV. Agreement between the peak maximum and the computed VDE is only found for the third most stable tautomer, which shows an AEA of approximately -0.1 eV and a VDE of 2.58 eV. This tautomer results from a proton transfer from N3H to C5. The results illustrate that the characteristics of biomolecular anions are highly dependent on their tautomeric form. If indeed the third most stable anion is observed in the experiment, then it remains an open question why and how this species is formed under the given conditions.     

Electronic transitions of guanine tautomers, their stacked dimers, trimers and sodium complexes

Planar and nonplanar geometries of the keto-N9H and keto-N7H tautomers of the guanine base of DNA as well as the hydrogen bonded complexes of these species with three water molecules each were optimized using the density functional theory at the B3LYP/6-31G** level. Geometries of the isolated bases were also optimized using the ab initio approach at the MP2/6-31G** level. The isolated keto-N9H and keto-N7H tautomers as well as their hydrogen bonded complexes with three water molecules each were solvated in bulk water employing the polarized continuum model (PCM) of the self-consistent reaction field theory (SCRF). Stacked dimers and trimers of both the tautomers of guanine were generated by placing the planar forms of the species at a fixed distance of 3.5 A from the neighboring one and rotating one molecule with respect to the other by 110 degrees for the keto-N9H form and 90 degrees for the keto-N7H form which corresponded to total energy minima at the B3LYP/6-31G** level. Geometry optimization for the cation of the monomer of guanine was performed at the same level of theory, and its solvation in bulk water was treated using the PCM model of the SCRF theory. The geometries of complexes of the two tautomers of guanine with a Na+ ion each were optimized at the B3LYP/6-31G** level, and the Na+ ion is predicted to bind with the keto-N9H tautomer preferentially. While the complex of the keto-N7H form of guanine with three water molecules in gas phase is slightly more stable than the corresponding complex of the keto-N9H form of guanine, the reverse is true in bulk water. Stacking interactions enhance the relative stability of the keto-N9H tautomer over that of the keto-N7H tautomer, suggesting that in bulk solutions, the former would be dominant. Electronic spectra of the isolated tautomers of guanine, those of their complexes with three water molecules each, the (keto-n9h and keto-n7h) cation of guanine, the complexes of the tautomers with a Na+ ion each, the stacked dimers and trimers of the two tautomers were calculated using configuration interaction involving single electron excitations (CIS). The relative absorption intensities of the two tautomers of guanine near 275 and 248 nm in the monomer, dimer, and trimer are predicated to be in the opposite order. Thus the absorption intensity oscillation observed using a guanine aqueous solution can be explained in terms of oscillation of relative populations of the two tautomers of the molecule. The 248 nm absorption peak would be appreciably red-shifted on formation of the cation of guanine. Binding of the Na+ ion with the two tautomers of guanine reduces intensities of their transitions appreciably and also it causes large red-shifts in the same.     

Theoretical investigation of solvent effects on tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol

The density functional theory has been used to study the tautomeric equilibrium of 2‐diazo‐4,6‐dinitrophenol(DDNP) in the gas phase and in 14 solvents at the B3LYP/6‐31G* level. The solvent effects on the tautomeric equilibria were investigated by the self‐consistent reaction field theory (SCRF) based on conductor polarized continuum model (CPCM) in apolar and polar solvents and by the hybrid continuum‐discrete model in protic solvent, respectively. Solvent effects on the computed molecular properties, such as molecular geometries, dipole moments, E_LUMO, E_HOMO, total energies for DDNP tautomers and transition state, tautomerization energies and solvation energies have been found to be evident. The tautomeric equilibrium of DDNP is solvent‐dependent to a certain extent. The tautomer I (cyclic azoxy form) is preferred in the gas phase, while in nonpolar solvents tautomer I and II (quinold form) exist in comparable amounts, and in highly polar solvents, the tautomeric equilibrium is shifted in favor of the more polar tautomer II . © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A thermochemistry and kinetic study on the thermal decomposition of ethoxyquinoline and ethoxyisoquinoline

Quantum chemical calculations were used to study the production of ethylene and keto/enol tautomers from ethoxyquinoline (2‐EQ) and ethoxyisoquinoline (1‐EisoQ and 3‐EisoQ) in the gas phase and ethanol at the MP2/6‐311++G(2d,2p)//BMK/6‐31+G(d,p) level. The obtained data indicate that the elimination of ethylene from 1‐EisoQ and 2‐EQ is slightly more favorable than from 3‐EisoQ. Formation of quinolone and isoquinolone (2‐EQO, 1‐EisoQO, and 3‐EisoQO) is kinetically favored compared to their enols. Decomposition of 2‐EQ and 1‐EisoQ to ethylene and keto forms is thermodynamically and kinetically preferable more stable than the corresponding enols. However, the hydroxy form of 3‐EisoQ is more stable than its keto tautomer in the gas phase and ethanol. The enol tautomers cost less energy when formed from their keto forms rather than from the parent ethoxyquinolone and ethoxyisoquinoline.     

On the Phosphorescence of 1H‐Phenalen‐1‐one

Combining the effects of heavy atom and low temperature, the phosphorescence spectrum from 1H‐phenalen‐1‐one has been unveiled. The 0‐0 band is located at 649 nm in methylcyclohexane and shifted to 646 nm in ethanol, which sets the triplet‐energy level to 185 and 186 kJ⋅mol⁻¹, respectively. The emission is unambiguously identified as phosphorescence originating from 1H‐phenalen‐1‐one through complementary transient absorption and emission studies. The quantum yield for triplet formation is confirmed to be unity.     

Spectroscopic and quantum chemical study of the structure of 4-aminopyrimidinoanthrones

4-Amino derivatives of pyrimidinoanthrone exist in the form of an aminoketone isomer in the crystalline state in neutral organic solutions, but in acid and alkaline media the tautomeric equilibrium is shifted toward formation of ionic forms of the iminohydroxyl tautomer. We present the characteristics of the IR and electronic absorption spectra and quantum chemical calculations for the neutral and ionic forms of the tautomers indicated above.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1109–1116, August, 1994.     

Tautomerism and infrared spectra of 2-thiopurine: an experimental matrix isolation and theoretical ab initio and density functional theory study

Infrared spectra of 2-thiopurine (2-mercaptopurine, 2-purinethiol ) isolated in low-temperature Ar and N2 matrixes are reported. These spectra indicate that the compound adopts exclusively the thiol N9H tautomeric form. The theoretical calculations of relative energies of 2-thiopurine tautomers have been carried out at the MP4(SDTQ)//HF level using the 6-31G(d,p) basis set. The thiol N9H tautomer was predicted to be the most stable of all isomers of 2-thiopurine. The infrared spectra of the tautomers of 2-thiopurine have been calculated at the DFT(B3LYP)/6-31G(d,p) level. Good agreement between the experimental spectra and the spectra calculated for thiol N9H tautomer supported the identification of the dominant tautomer. It has also allowed for the reliable assignment of the bands observed in the experimental IR spectrum.     

Quantum Defects as a Toolbox for the Covalent Functionalization of Carbon Nanotubes with Peptides and Proteins

Single‐walled carbon nanotubes (SWCNTs) are a 1D nanomaterial that shows fluorescence in the near‐infrared (NIR, >800 nm). In the past, covalent chemistry was less explored to functionalize SWCNTs as it impairs NIR emission. However, certain sp³ defects (quantum defects) in the carbon lattice have emerged that preserve NIR fluorescence and even introduce a new, red‐shifted emission peak. Here, we report on quantum defects, introduced using light‐driven diazonium chemistry, that serve as anchor points for peptides and proteins. We show that maleimide anchors allow conjugation of cysteine‐containing proteins such as a GFP‐binding nanobody. In addition, an Fmoc‐protected phenylalanine defect serves as a starting point for conjugation of visible fluorophores to create multicolor SWCNTs and in situ peptide synthesis directly on the nanotube. Therefore, these quantum defects are a versatile platform to tailor both the nanotube's photophysical properties as well as their surface chemistry.