ISC Rate Research Articles

Optimizing phototherapy efficiency via dynamic adjusting the contributions of photodynamic and photothermal therapy of diketopyrrolopyrrole-based compounds

Three diketopyrrolopyrrole-based photosensitizers (S1, Se1 and Se2) were designed to optimize the contributions of photodynamic (PDT) and photothermal (PTT) therapy in tumor phototherapy via adjusting the spin-orbit coupling constant and the intersystem crossing efficacy. The photo-physicochemical as well as biological properties indicated that the introduction of Se and methoxy group can increase the ISC rate of the photosensitizers with increased PDT efficiency and decreased PTT effects, which were in accordance with the results of DFT and TD-DFT calculations. Importantly, the in vivo evaluations revealed that supramolecular nanoparticles Se2-NPs showed excellent phototherapeutic performance via synergistic PDT and PTT against cancer. These findings suggest that controlling the relative contributions of PDT and PTT may be an effective way to improve phototherapy efficiency.

The effect of heavy atoms on the deactivation of electronic excited states of dye molecules near the surface of metal nanoparticles.

The influence of a heavy atom and the plasmon field on the efficiency of populating the lowest triplet state (T1) and on the phosphorescence intensity has been studied for fluorescein, 2Br-fluorescein, eosin and erythrosine, which have an increasing number of substituted heavy atoms. We show that the heavy atoms affect not only the rate constant of intersystem crossing (kISC) but also the rate constant of internal conversion (kIC). The calculations show that the C-H bonds in the meso position are the primary acceptors of the excitation energy of the lowest excited electronic singlet state (S1). Substitution of the meso hydrogen atoms with I or Br leads to a smaller kIC rate constant of 1 × 108 s-1 for fluorescein to 8 × 106 s-1 for eosin. Substitution with heavy atoms also leads to a larger ISC rate constant (kISC) between the T2 and S1 states because the spin-orbit coupling matrix element 〈S1|HSO|T2〉 increases by two orders of magnitude from 0.36 cm-1 for fluorescein to 35.0 cm-1 for erythrosine. The phosphorescence rate constant increases by three orders of magnitude from 4.8 × 10 s-1 for fluorescein to 3.3 × 104 s-1 for erythrosine, which is supported by experimental data. The plasmon effect increases the intensity of the xanthene dye emissions. The intensity and the quantum yield of fluorescence increase in the series fluorescein < 2Br-fluorescein < eosin < erythrosine. The intensity of the delayed fluorescence and phosphorescence grows in the same way. The enhancement factor of the phosphorescence intensity increases from 1.8 to 5.6 in the series from fluorescein to erythrosine. The differences in the plasmon effect originate from intensity borrowing to the radiative triplet-singlet transition (T1 → S0) from the singlet-singlet transitions (Sn → S0), which is more efficient when molecules have heavy atoms in the meso position.

Thermally Activated Delayed Fluorescence of a Pyromellitic Diimide Derivative in the Film Environment Investigated by Combined QM/MM and MS-CASPT2 Methods.

Arylene diimide compounds exhibit thermally activated delayed fluorescence (TADF), but its mechanism remains elusive. Herein we studied the TADF mechanism of a carbazole-substituted pyromellitic diimide derivative (CzPhPmDI) in poly(methyl methacrylate) (PMMA) film by using DFT, TD-DFT, and MS-CASPT2 methods within the QM/MM framework. We found that the TADF mechanism involves three electronic states (i.e., S0, S1, and T1), but the T2 state is not involved because its energy is higher than the S1 state by 6.9 kcal/mol. By contrast, the T1 state is only 3.2 kcal/mol lower than the S1 state and such small energy difference benefits the reverse intersystem crossing (rISC) process from T1 to S1 thereto TADF. This point is seconded by relevant radiative and nonradiative rates calculated. At room temperature, the ISC rate from S1 to T1 is calculated to be 6.1 × 106 s-1, which is larger than the fluorescence emission rate, 2.2 × 105 s-1; thus, the dominant S1 population converts to the T1 state. However, in the T1 state, the rISC process (1.8 × 104 s-1) becomes the most important channel because of the negligible phosphorescence emission rate (3.5 × 10-2 s-1). So, the T1 population is still converted back to the S1 state to fluoresce enabling TADF. Unfortunately, the rISC process is blocked in low temperature. Besides, we found that relevant Huang-Rhys factors have dominant contribution from low-frequency vibrational motion related to the torsional motion of functional groups. These gained insights could provide useful information for the design of organic TADF materials with excellent luminescence efficiency.

Phosphorus-containing amorphous pure organic room-temperature phosphorescent materials

Amorphous organic room temperature phosphorescent (RTP) materials have attracted widespread attention due to their wonderful properties such as facile processability and good repeatability. Based on the existing design principles, embedding heteroatom-contained phosphors into rigid polymer matrices is a convenient way to construct high-efficiency amorphous RTP materials. In addition, phosphorus (P)-containing amorphous RTP materials are seldom seen to be reported. In this work, four amorphous polymers (P1-P4) containing P atoms have been successfully developed, which all could emit room-temperature phosphorescence but vary from each other under ultraviolet radiation. The phosphorescence was attributed to the lone-pair-electron-contained P atom which is favorable for n–π* transitions that leads to effective spin–orbit coupling and high ISC rate (kISC) thereby enhances RTP in this system. Meanwhile, the hydrogen bonding effect is also necessary because it could enhance the conformation rigidification result in the reducing of the non-radiative decay. This study would enrich the amorphous pure organic RTP systems, and could be useful to provide thoughts for designing superior materials.

Engineering the Charge‐Transfer State to Facilitate Spin–Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones

AbstractSpiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin–orbit charge transfer intersystem crossing (SOCT‐ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine‐tune the charge transfer state. Its feasibility for SOCT‐ISC was theoretically predicted, then experimentally evaluated. Through fine‐tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×108 s−1 to 1.0×1011 s−1, which is the highest ISC rate reported for SOCT‐ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT‐ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers.

Engineering the Charge-Transfer State to Facilitate Spin-Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones.

Spiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine-tune the charge transfer state. Its feasibility for SOCT-ISC was theoretically predicted, then experimentally evaluated. Through fine-tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×108 s-1 to 1.0×1011 s-1 , which is the highest ISC rate reported for SOCT-ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT-ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers.

Mechanism study of TADF and phosphorescence in dinuclear copper (I) molecular crystal using QM/MM combined with an optimally tuned range-separated hybrid functional

Abstract Investigations of the detailed photophysical processes are of great significance for future material improvements and novel designing strategies. Herein, the interconversion and decay rates of the first excited singlet state (S1) and triplet state (T1) for the Cu2I2(P^N)3 complex are calculated using the thermal vibration correlation function (TVCF) theory, combined with the optimally tuned range-separated hybrid functional (OT-ωB97XD) method at different temperature. A methodology with the building different ONIOM models, was carried over into simulation of crystal environment. All calculated results perfectly match the experimentally available data, demonstrating the validity of our applied theoretical approach. It has been found that the reverse intersystem crossing (RISC) rate kRISC from T1 to S1 is 3.11 × 1010 s−1 at 300 K, about 7 order of magnitude larger than the phosphorescence rate kr(T) = 3.71 × 103 s−1, and far more than ISC rate kISC(T1-S0) of 6.38 s−1. The S1 state can be an efficient thermal population from the T1 state by the RISC pathway, leading to an occurrence of thermally activated delayed fluorescence (TADF), and the estimated delayed time of τ(TADF) = 10 μs. On the other hand, the T1 state also exhibits stronger admixtures with higher lying singlet states due to stronger SOC, having a larger ZFS of 17 cm−1, thus, a relatively fast phosphorescence decay rate of kr(T) = 8.542 × 103 s−1 estimated by Einstein emission formula is observed. Further calculation results show that the emission intensities are stemming by 91% from the S1 state as TADF and by 9% as phosphorescence from the T1 state at 300 K, which indicates the ambient temperature emission represents the combined luminescence of TADF and phosphorescence. Our work would be useful for improving and designing the luminescent material combined high-efficiency TADF with phosphorescence.

Thermally activated delayed fluorescence processes for Cu(i) complexes in solid-state: a computational study using quantitative prediction.

The photophysical properties of four representative Cu(i) complex crystals have been investigated using the combination of an optimally tuned one- and two-dimensional range-separated hybrid functional with the polarizable continuum model, and the thermal vibration correlation function (TVCF) approach. The calculated excited singlet–triplet energy gap, radiative rates and lifetimes match the experimentally available data perfectly. At 300 K, the reverse intersystem crossing (RISC) proceeds at a rate of kdir.RISC ≈ 106–8 s−1, which is 4–5 orders of magnitude larger than the mean phosphorescence rate, kP ≈ 102–3 s−1. At the same time, the ISC rate kdir.ISC ≈ 109 s−1 is again 2 orders of magnitude larger than the fluorescence rate kF ≈ 107 s−1. In the case of kdir.RISC ≫ kF and kdir.RISC ≫ kP, thermally activated delayed fluorescence should occur. Vibronic spin–orbit coupling can remarkably enhance the ISC rates by the vital “promoting” modes, which can provide crucial pathways to decay. This can be helpful for designing novel excellent TADF Cu(i) complex materials.

Controlling the excited-state dynamics of low band gap, near-infrared absorbers via proquinoidal unit electronic structural modulation.

While the influence of proquinoidal character upon the linear absorption spectrum of low optical bandgap π-conjugated polymers and molecules is well understood, its impact upon excited-state relaxation pathways and dynamics remains obscure. We report the syntheses, electronic structural properties, and excited-state dynamics of a series of model highly conjugated near-infrared (NIR)-absorbing chromophores based on a (porphinato)metal(ii)-proquinoidal spacer-(porphinato)metal(ii) (PM-Sp-PM) structural motif. A combination of excited-state dynamical studies and time-dependent density functional theory calculations: (i) points to the cardinal role that excited-state configuration interaction (CI) plays in determining the magnitudes of S1 → S0 radiative (kr), S1 → T1 intersystem crossing (kISC), and S1 → S0 internal conversion (kIC) rate constants in these PM-Sp-PM chromophores, and (ii) suggests that a primary determinant of CI magnitude derives from the energetic alignment of the PM and Sp fragment LUMOs (ΔEL). These insights not only enable steering of excited-state relaxation dynamics of high oscillator strength NIR absorbers to realize either substantial fluorescence or long-lived triplets (τT1 > μs) generated at unit quantum yield (ΦISC = 100%), but also crafting of those having counter-intuitive properties: for example, while (porphinato)platinum compounds are well known to generate non-emissive triplet states (ΦISC = 100%) upon optical excitation at ambient temperature, diminishing the extent of excited-state CI in these systems realizes long-wavelength absorbing heavy-metal fluorophores. This work highlights approaches to: (i) modulate low-lying singlet excited-state lifetime over the picosecond-to-nanosecond time domain, (ii) achieve NIR fluorescence with quantum yields up to 25%, (iii) tune the magnitude of S1-T1 ISC rate constant from 109 to 1012 s-1 and (iv) realize T1-state lifetimes that range from ∼0.1 to several μs, for these model PM-Sp-PM chromophores, and renders new insights to evolve bespoke photophysical properties for low optical bandgap π-conjugated polymers and molecules based on proquinoidal conjugation motifs.

Ultrafast Infrared and UV–Vis Studies of the Photochemistry of Methoxycarbonylphenyl Azides in Solution

The photochemistry of 4-methoxycarbonylphenyl azide (2a), 2-methoxycarbonylphenyl azide (3a), and 2-methoxy-6-methoxycarbonylphenyl azide (4a) were studied by ultrafast time-resolved infrared (IR) and UV-vis spectroscopies in solution. Singlet nitrenes and ketenimines were observed and characterized for all three azides. Isoxazole species 3g and 4g are generated after photolysis of 3a and 4a, respectively, in acetonitrile. Triplet nitrene 4e formation correlated with the decay of singlet nitrene 4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes 2b and 3b, but leads to protonation of 4b to produce nitrenium ion 4f. Singlet nitrenes 2b and 3b have lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene 4b in acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 10(8) L·mol(-1)·s(-1) at room temperature. These results indicate that a methoxycarbonyl group at either the para or ortho positions has little influence on the ISC rate, but that the presence of a 2-methoxy group dramatically accelerates the ISC rate relative to the unsubstituted phenylnitrene. An ortho-methoxy group highly stabilizes the corresponding nitrenium ion and favors its formation in aqueous solvents. This substituent has little influence on the ring-expansion rate. These results are consistent with theoretical calculations for the various intermediates and their transition states. Cyclization from the nitrene to the azirine intermediate is favored to proceed toward the electron-deficient ester group; however, the higher energy barrier is the ring-opening process, that is, azirine to ketenimine formation, rendering the formation of the ester-ketenimine (4d') to be less favorable than the isomeric MeO-ketenimine (4d).

Photophysics of phenalenone: quantum-mechanical investigation of singlet–triplet intersystem crossing

We have examined the electronic and molecular structure of 1H-phenalen-1-one (phenalenone) in the electronic ground state and in the lowest excited states, as well as intersystem crossing. The electronic structure was calculated using a combination of density functional theory and multi-reference configuration interaction. Intersystem crossing rates were determined using Fermi's golden rule and taking direct and vibronic spin-orbit coupling into account. The required spin-orbit matrix elements were obtained applying a non-empirical spin-orbit mean-field approximation. Our calculated electronic energies are in good agreement with experimental data. We find the lowest excited singlet states to be of the npi* (S1) and pipi* (S2) type. Energetically accessible from S1 are two triplet states of the pipi* (T1) and npi* (T2) type, the latter being nearly degenerate to S1. This ordering of states is retained when the molecular structure in the electronically excited states is relaxed. We expect very efficient intersystem crossing between S1 and T1. Our calculated intersystem crossing rate is approximately 2 x 10(10) s(-1), which is in excellent agreement with the experimental value of 3.45 x 10(10) s(-1). Our estimated phosphorescence and fluorescence rates are many orders of magnitude smaller. Our results are in agreement with the experimentally observed behavior of phenalenone, including the high efficiency of 1O2 production.

Photophysical Properties of a Tetraphenoxy‐Substitued Perylene Bisimide Derivative Characterized by Single‐Molecule Spectroscopy

However, the figures detailed in the article result from an analysis where erroneously 〈δI(t)〉2 rather than 〈I(t)〉2 was inserted in the denominator of Equation (4). Using the correct normalization yields λ1=1/20 μs=50000 s−1, λ2=1/300 μs=3300 s−1 and C1=0.034, C2=0.009 for the fit parameters of Equation (6). As a consequence, the deduced photophysical parameters are changed as well and the entries of Tables 1 and 2 of the original article have to be replaced as given below. The main conclusions of the article, that is, the observation of one “fast” and one “slow” triplet sublevel, a low intersystem-crossing yield, and a high fluorescence quantum yield are not affected by these changes. fast slow population rate [s-1] 4500 110 fractional ISC yield 0.98 0.02 decay rate [s-1] 35000 2800 relative steady-state population 0.77 0.23 Parameter fluorescence decay rate[a] [s-1] 1.7⋅108 effective triplet decay rate [s-1] 28 000 ISC rate [s-1] 4610 triplet quantum yield ΦT 2.7⋅10−5 fluorescence quantum yield ΦF 1.3⋅10−5≈1 fully saturated emission rate R∞ [s-1] 7.8⋅107 S1–S0 transition-dipole moment [Debye] 1.5 α=1+kisc 1.1 The authors apologize for any misunderstanding that may have arisen as a result of this mistake.

Ultrafast Time-Resolved Study of Photophysical Processes Involved in the Photodeprotection of p-Hydroxyphenacyl Caged Phototrigger Compounds

A combined femtosecond Kerr gated time-resolved fluorescence (fs-KTRF) and picosecond Kerr gated time-resolved resonance Raman (ps-KTR(3)) study is reported for two p-hydroxyphenacyl (pHP) caged phototriggers, HPDP and HPA, in neat acetonitrile and water/acetonitrile (1:1 by volume) solvents. Fs-KTRF spectroscopy was employed to characterize the spectral properties and dynamics of the singlet excited states, and the ps-KTR(3) was used to monitor the formation and subsequent reaction of triplet state. These results provide important evidence for elucidation of the initial steps for the pHP deprotection mechanism. An improved fs-KTRF setup was developed to extend its detectable spectral range down to the 270 nm UV region while still covering the visible region up to 600 nm. This combined with the advantage of KTRF in directly monitoring the temporal evolution of the overall fluorescence profile enables the first time-resolved observation of dual fluorescence for pHP phototriggers upon 267 nm excitation. The two emitting components were assigned to originate from the (1)pipi (S(3)) and (1)npi (S(1)) states, respectively. This was based on the lifetime, the spectral location, and how these varied with the type of solvent. By correlating the dynamics of the singlet decay with the triplet formation, a direct (1)npi --> (3)pipi ISC mechanism was found for these compounds with the ISC rate estimated to be approximately 5 x 10(11) s(-)(1) in both solvent systems. These photophysical processes were found to be little affected by the kind of leaving group indicating the common local pHP chromophore is largely responsible for the fluorescence and relevant deactivation processes. The triplet lifetime was found to be approximately 420 and 2130 ps for HPDP and HPA, respectively, in the mixed solvent compared to 150 and 137 ns, respectively, in neat MeCN. The solvent and leaving group dependent quenching of the triplet is believed to be associated with the pHP deprotection photochemistry and indicates that the triplet is the reactive precursor for pHP photorelease reactions for the compounds examined in this study.

Photochemical alpha-cleavage of ketones: revisiting acetone.

The photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal x mol(-1) above the origin of the S(0)-S(1) transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). Direct reaction from the S(1) is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk (approximately 1-2 ns) arises from collision-induced processes that populate low-lying levels of S(1). The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.

Temperature dependence of bianthryl dual fluorescence

The fluorescence of bianthryl in ethyl acetate and acetone has been investigated in dependence on the temperature. The variation of temperature changes the ratio of the locally excited ( 1LE) and the charge transfer ( 1CT) fluorescence quantum yields, which parallels the dielectric properties of the solvent. Thermodynamic functions ( ΔG CT, ΔS CT) of the excited state charge transfer process of bianthryl in ethyl acetate and acetone have been determined from equilibrium constants ( 1 LE⇔ 1 CT) accessible from the fluorescence data. Charge-transfer induced intersystem crossing occurring in BA is slightly temperature dependent in both solvents, which indicates that the ISC rate constant is mainly governed by the spin inversion in the anthracene subunits of bianthryl.

Computational prediction of the ISC rate for triplet norbornene

The radiationless decay of T 1 norbornene to the singlet ground state is studied using density-functional and ab initio CASSCF calculations of the potential energy surface crossing and of the spin-orbit coupling. The rate of decay is predicted using two approximate multi-dimensional non-adiabatic methods, one of which is based on Fermi's Golden Rule, and the other is a version of RRKM theory adapted for non-adiabatic processes. Unlike a previous Landau–Zener treatment of this process by some of us [Chem. Phys. Lett. 287 (1998) 601–607], both methods correctly predict a short lifetime for the triplet excited state, in reasonable agreement with experimental data. This underlines the importance of tunnelling in non-radiative relaxation processes.

The non-radiative processes from the S 1 state of aminoanthraquinones: a steady state and time-resolved study

The non-radiative processes of deactivation from the lowest singlet excited state of aminoanthraquinones have been studied using steady state and time-resolved methods. The fluorescence decay rate constant k f correlates well with the solvent polarity parameter E T(30) in non-hydrogen-bonding solvents. Large deuterium isotope effects in fluorescence lifetimes τ f and quantum yields φ f are observed in the case of 1-amino and 1-methylamino anthraquinones, where the S 1 state is mainly deactivated through internal conversion to the ground state. The temperature dependence of the fluorescence quantum yields of various aminoanthraquinones was also investigated. φ f and τ f exhibited strong temperature dependences in the case of 1-acetylaminoanthraquinone. In the case of 1-acetylaminoan-thraquinone, intersystem crossing to the triplet state is a major deactivation channel from the S 1 and in this derivative a close-lying T 2 state seems to be responsible for the high k isc rate.

Study of dark states in naphthalene, pyrimidine and pyrazine by detection of phosphorescence after UV laser excitation

The high-resolution fluorescence and phosphorescence detected spectra after excitation of the (3a g) 1 and (4a g) 1 vibronic states of the S 1 ( 1B 3u) in naphthalene have been studied in a molecular beam. The residual Doppler linewidth of 20 MHz allowed rotational resolution. All lines were assigned and the rotational constants of the (3a g) 1 and (4a g) 1 states were determined. It has been shown that the intersystem crossing rates are independent of the rotational quantum numbers J and K up to J=10 in both excited vibronic states. The strong ISC rate of the (4a g) 1 state is confirmed. No evidence was found for a promoting second triplet T 2 state. Detection of phosphorescence after excitation of the vibrationless S 1 ( 1B 3u) state in pyrazine and the S 1 ( 1B 1) s pyrimidine yielded no signal. This observation confirms the assumption that after excitation of the S 1 state the energy is transferred to high vibrational states of the S 0 state.

Effect of high pressure on the triplet yield for anthracene compounds in solution

Transient T-T absorption spectra for anthracene and 2- and 9-methyl derivatives in n-hexane, methylcyclohexane, and toluene were measured at pressures up to 300 MPa and at 25°C. The pressure dependence of the triplet yield was obtained from the variation in the peak intensity. The effect of pressure on the intersystem crossing rate was established by comparison of the present result with the pressure dependence of the fluorescence yield measured previously. The effects of the medium on the ISC rate are explained on the basis of the change of Franck-Condon factor due to the variation of the S 1-T 2 energy gap.

Intersystem crossing processes and molecular geometry in aromatic amines

The effect of a N-methyl substituent on the fluorescence k o f, phosphorescence k o p, and intersystem crossing k ISC(S 1—T 1) rate constants of the aromatic amines diphenylamine, acridan, and carbazole has been investigated in solution. Experimental results and analysis of the T 1—S 0 transition probability based on the CNDO CI/S method suggest that the relatively strong enhancement of the S 1—T 1 and T 1—S 0 intersystem crossing efficiencies between acridan and methylacridan is due to the change of the molecular geometry upon methylation.