Theoretical study of hydrogen bonding excited states of fluorenone with formaldehyde

Accepted Manuscript Theoretical Study of Hydrogen Bonding Excited States of Fluorenone with Formaldehyde Juan Yang, An Yong Li PII: DOI: Reference:

S2210-271X(16)30525-4 http://dx.doi.org/10.1016/j.comptc.2016.12.031 COMPTC 2348

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Computational & Theoretical Chemistry

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3 December 2016 20 December 2016 20 December 2016

Please cite this article as: J. Yang, A. Yong Li, Theoretical Study of Hydrogen Bonding Excited States of Fluorenone with Formaldehyde, Computational & Theoretical Chemistry (2016), doi: http://dx.doi.org/10.1016/j.comptc. 2016.12.031

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Theoretical Study of Hydrogen Bonding Excited States of Fluorenone with Formaldehyde Juan Yang

An Yong Li*

School of Chemistry and Chemical Engineering, Southwest University, Tiansheng Road No.1, Chongqing 400715, People’s Republic of China

Abstract: Time-dependent density functional method was performed to investigate the intermolecular hydrogen bond between fluorenone and formaldehyde in the electronically excited states. The geometric structures of the hydrogen bonding complex in the ground state and the first singlet and triplet excited states S1 and T1 are optimized respectively by the DFT and TD DFT methods, the vibrational spectra, electronic absorption spectra and fluorescence spectra are calculated. The two intermolecular hydrogen bonds C=O⋅⋅⋅H−C formed between fluorenone and CH2O in the complex are strengthened in the S1 and T1 states relative to the ground state. The excited states S1 and T1 are locally excited, where fluorenone is excited but CH2O remains in the ground state. The hydrogen bonds cause a frequency blue shift of the involved CH bond in formaldehyde in the ground state, the electron excitations S0 →S1 and S0 →T1 mainly lead to large frequency red shift of the C=O bond in fluorenone. Keywords: Time-dependent density functional method(TD DFT), excited states, vibrational spectra, electronic spectra, blue shifted hydrogen bonds

1. Introduction In recent years noncovalent interactions are interesting and valuable topics attracting theoretical and experimental chemists. Hydrogen bonding as the most important noncovalent interaction [1-3] is critical to understand microscopic structures, properties and functions of many molecular and supermolecular systems, proteins and DNA [4-10]. The nature of hydrogen bonds in the ground state has been explored by various theoretical methods and experimental techniques. The investigation of hydrogen bonding in electronically excited states expands the contemporary hydrogen bond research interests [4-13]. Upon photoexcited to electronically excited states, the hydrogen bonding complex is significantly changed with charge density redistributing and the hydrogen donor and acceptor reorganizing. This process is called electronic excited-state hydrogen bonding dynamics (ESHBD) [3,10,14,15]. There are three different ways for ESHBD, one of the hydrogen acceptor or donor is photoexcited whereas the other remains in the ground state, or both the acceptor and donor are photoexcited [5].

ESHBD is mainly investigated by vibrational spectra of hydrogen-bonding acceptor and donor groups, which occur in femtosecond time scales. The formation of hydrogen bonds can mainly lead to spectral shifts of some unique vibrational modes on the hydrogen bonding acceptor and donor [16-19]. Various femtosecond time-resolved spectroscopic techniques and accurate excited-state quantum chemistry calculations are applied to study ESHBD. Time-dependent density functional theory (TD DFT) method is a feasible tool to calculate vibrational absorption spectra in electronically excited states because of its moderate efficiency and accuracy [2,20,21]. Detailed investigation of hydrogen bonding in electronic excited states involving different molecules in solution using various techniques of theoretical chemistry and experimental methods has been reported. The hydrogen bonding C=O⋅⋅⋅H−O in excited states between carbonyl chromophores such as coumarin and fluorenone dyes, and polar protonic solvents such as water, alcohol and phenol have been extensively studied by experiments and theoretical calculations [2,6,20-23]. It has been demonstrated by experiments and theoretical calculations that the hydrogen bond between coumarin 102 (C102) or fluorenone (FN) chromophores and some hydrogen-donating solvents such as methanol was significantly strengthened in the electronically excited state S1 relative to the ground state S0 [2, 3, 10]. The carbonyl chromophore is a desired choice used to probe the hydrogen bonding interactions by means of absorption, IR and emission spectroscopy [22,23]. Fluorenone as an ideal chromophore has some characteristically spectroscopic properties and has shown the potential to give a good insight into the molecular photophysics and photochemistry [24,25]. It has an active >C=O group, the n→ π* and π→ π* transitions are well known and responsible for important photochemical processes. FN in the S1 and T1 states is dynamically quenched due to formation of hydrogen bond interaction through radiationless deactivation processes [25], and the internal conversion (IC) from the fluorescent state to the ground state and intersystem crossing (ISC) from the singlet excited state to triple excited state are two major deactivation processes. Fluorenone is very weakly fluorescent in solution. In the aprotic solvents, the intersystem crossing (ISC) plays a major role in the deactivation process. However, in the protic solvents, the internal conversion (IC) is the dominant deactivation process that is completed with the fluorescence emission of the fluorenone derivation, which implies that the intermolecular hydrogen bond in the protic solvents can facilitate the IC process [2,10]. Hydrogen bond dynamics of fluorenone with protic solvents such as alcohols has been largely investigated by theoretical calculations and experiments [10], where the OH group is a sensitive vibrational mode to monitor ESHBD since its stretch frequency is strongly red shifted both by hydrogen bond and electron excitation which strengthens hydrogen bond in the electronically excited state. The OH bond is always red shifted in hydrogen bonds. The CH bond in formaldehyde can also participate in a hydrogen bond Y⋅⋅⋅H−C and its stretch frequency is usually blue shifted as the proton

acceptor is not very strong, since in CH2O there is a strong intramoleculoar hyperconjugation n(O) →σ*(CH) coupling with the intermolecular hyperconjugation n(Y) →σ*(CH) [26]. So far there is no theoretical and experimental report on ESHBD of fluorenone with CH2O, which may be interesting since two hydrogen bonds C=O⋅⋅⋅H−C can be formed between fluorenone and CH2O, one of which is red shifted but the other blue shifted. Both FN and CH2O act as the proton acceptor using its C=O in one hydrogen bond and as the proton donor using its CH bond in the other hydrogen bond. The two hydrogen bonds form an eight-membered ring, it is interesting whether there are some cooperative effects in this system. In the present work, we are engaged in studying theoretically the hydrogen bond FN⋅⋅⋅CH2O in the excited states using TDDFT method. The vertical transition energies in different electronic states, the adiabatic transition energies, adiabatic geometry and the corresponding vibrational spectra in the electronically excited states S1 and T1 for the isolated fluorenone, formaldehyde and their complexes were calculated. The hydrogen bonding complexes both in the ground state and the excited states S1 and T1 have planar geometry, as shown in Fig.1. The calculated electronic absorption and fluorescence spectra and the vibrational absorption spectra of hydrogen-bonded groups were presented. The hydrogen bond energies in different electronic states were calculated, we found that the intermolecular hydrogen bonds are strengthened in the excited states S1 and T1 relative to the ground state.

2. Computational details TDDFT is a valuable tool to study electronically excited states and evaluate vibrational spectra in excited states. It becomes a good candidate for studying ESHBD by monitoring frequency shifts of special vibrational motions involved in the hydrogen bonds in different electronic states [21,27,28]. The equilibrium geometry structures of the isolated monomers and the hydrogen bonding complex at the ground state S0 have been optimized using B3LYP/aug-cc-pVDZ [29,30]. The optimized geometry in the ground state has been regarded as the starting point for excited-state calculations. The geometric structures of the monomers and complexes in the excited states S1 and T1 were optimized using TD DFT at the B3LYP/aug-cc-pVDZ level. The vertical and adiabatic transition energies, vibrational spectra, electronic absorption and fluorescence spectra in different excited states were also calculated at the B3LYP/aug-cc-pVDZ level. All of the calculations were done with the Gaussian 09 package in the gas phase [31].

3. Results and Discussion 3.1 Geometric structures in the S0, S1 and T1 states

It is well known that free CH2O is planar in the ground state, but a bent C2v structure in the excited states S1 and T1. Our calculation shows that the free fluorenone (C2v) and the hydrogen bonding complexes FN⋅⋅⋅CH2O (Cs) are planar structures in all the three states S0, S1 and T1. This implies that in the excited states S1 and T1 of the hydrogen bonding complexes the CH2O moiety is not excited but remains in the ground state and only the FN moiety is excited, thus the excited states S1 and T1 of the hydrogen bonding complexes are locally excited (LE). In the complexes there are two hydrogen bonds (HBs), HB1: (FN)C=O⋅⋅⋅HCHO and HB2: H2C=O⋅⋅⋅H−C(FN), they form an eight-membered ring planar structure, in all the three states S0, S1 and T1. The calculated structural parameters of the HBs are shown in Fig.1. The intermolecular hydrogen bonds affect the geometrical structures of the monomers involved in the HBs. In the ground state, the formation of HBs elongates the C=O bonds both in FN and CH2O by 0.003Å and 0.006Å respectively, and increases the CH bond in FN involved in the HBs by 0.001Å, but shortens the CH bond in CH2O involved in the HBs by 0.005Å, compared to the free monomers.

Fig. 1 Optimized geometric structures and bond lengths (Å) related to the HBs of the hydrogen-bonded complex FN⋅⋅⋅CH2O, the black figures are for the monomers, the red, blue and green are for the complex in the S0, S1 and T1 states, respectively

In the excited states S1 and T1 of the complex, the C=O bond of FN has a large increment 0.04 and 0.025Å respectively, compared to the ground state. But the structure of CH2O in the complex is almost not influenced by electron excitation,

except for a small C=O bond elongation by 0.001~0.002Å (for the free CH2O the C=O bond is largely elongated 0.09~0.1Å by electron excitation to the S1 and T1 states related to the ground state, see Table S1 in supporting materials). This is another evidence of the LE character of the complex in the excited states S1 and T1. The large elongation of C=O in FN is mainly due to electron excitation, but the small elongation of C=O in CH2O is due to the strengthening of the HBs in the excited states S1 and T1, compared to the ground state S0. Thus the formation of the HBs largely influences the CH2O moiety, but the electron excitations S0 →S1 and S0 →T1 only largely influence the FN moiety.

3.2 Electronic Spectra We calculated the vertical transition energies of several singlet and triplet excited states of the free FN and the complexes FN⋅⋅⋅CH2O by the TDDFT method at the B3LYP/aug-cc-pVDZ level, based on the optimized S0 structures at the same level. The electronic excitation energies and corresponding oscillator strengths of the hydrogen-bonded complex FN⋅⋅⋅CH2O and the FN monomer are presented in Table 1. An inspection of Table 1 clearly shows that the strongest absorption for the free fluorenone and the hydrogen-bonded complex occurs at 255/256nm corresponding to the S0 →S6/S8 excitations respectively. Both for the free fluorenone and the complex the oscillator strength of the lowest singlet excited state S1 is very small. The S0 →S1 transition is the typical π−π* excitation, its absorption peak is calculated to be 401nm (longer than the experimental value 379nm [10]) and 411nm for the fluorenone monomer and the FN⋅⋅⋅CH2O complex, respectively. In order to gain insight into the transition nature of the absorption spectrum, we have drawn the calculated absorption spectra of fluorenone (FN) and the complex in the range of 180nm-600nm in Fig.2. In this wavelength region, we can see that the maximum absorption peaks for the isolated FN and the complex are at around 255nm. Moreover, there is a slightly weak S1 absorption band at around 400nm. This also implies that upon photoexcitation by 400nm laser pulse used in the experiment, the fluorenone moiety is excited to its excited state but the CH2O moiety remains in its ground state. Table 1 Vertical excitation energies (nm) and the corresponding oscillator strengths for the singlet and triplet excited states of the hydrogen-bonded complex FN⋅⋅⋅CH2O as well as the free fluorenone based on the geometry of the ground state at the B3LYP/aug-cc-pVDZ level States

FN

FN⋅⋅⋅CH2O

States

FN

FN⋅⋅⋅CH2O

S1

401(0.003)

411(0.002)

T1

506

517

S2 S3 S4 S5 S6 S7 S8 S9 S10

398(0) 311(0.013) 280(0.032) 261(0) 255(0.850) 246(0.040) 238(0.004) 231(0.004) 225(0)

391(0) 319(0.000) 315(0.022) 308(0) 283(0.034) 269(0.001) 256(0.831) 255(0) 249(0.049)

T2 T3 T4 T5 T6 T7 T8 T9 T10

469 387 366 350 303 287 285 262 262

456 389 389 372 352 310 307 287 284

The electronic excitation energies both for the singlet and triplet excited states S1 and T1 of the intermolecular hydrogen-bonded complex are lowered in comparison with the isolated fluorenone. These two excited states of the complex are of local π → π* excitation, and the hydrogen bonds in the excited states S1 and T1 are stronger than in the ground state. The electron excitation π →π* strengthens the hydrogen bonds and on the other hand the hydrogen bonds lower the energy of the π* MO. Since the excitation energy has a positive correlation with the π →π* energy difference, so the hydrogen bonding complex has larger excitation energy than the FN monomer. In order to better understand the nature of the electronic excitation, we also show the emission spectra of the isolated FN and the complex in Fig.2. Normal phosphorescence has not been observed from fluorenone triplet, and hence only the calculated fluorescence spectra recorded here. The fluorescence maxima of the isolated fluorenone and the complex FN⋅⋅⋅CH2O calculated at the adiabatic geometry of the S1 excited state, corresponding to the transition S1 →S0, appears at 530nm and 550nm, respectively, the experiment fluorescence peak of the free FN appears at a lower wavelength 514nm. Fluorescence peak red shifts by 20nm due to the formation of the hydrogen bonds, the reason is similar to the excitation energy.

Fig.2 Calculated absorption and fluorescence spectra of isolated fluorenone and the hydrogen-bonded complex FN⋅⋅⋅CH2O

3.3 Natural bond orbital and AIM analyses To assess the influence of the HBs and electron excitation on the two moieties, MO, NBO [32,33] and AIM [34-36] analyses were performed in the states S0, S1 and T1 at the B3LYP/aug-cc-pVDZ level to provide insight into the nature of the excited states. Fig.3 shows HOMO and LUMO of the complex. The π character of the HOMO and the π* character of the LUMO indicate that the S1 and T1 states are due to a distinct π−π* feature. Electron densities of HOMO and LUMO are only distributing on the FN moiety, which again implies that the excited states S1 and T1 are LE states, where only the carbonyl chromophore is excited, but the CH2O moiety remains in its ground state. The LE character of the complex in the S1 and T1 excited states makes easy to calculate the hydrogen binding energy. It is obvious that the electron density at the site of the carbonyl group in the FN moiety increases in LUMO compared to HOMO, thus the electron excitations S0 → S1/T1 will transfer electron density from the aromatic parts to the carbonyl group, which is in favour of the formation of the hydrogen bonds. So the C=O group of FN in the excited states forms hydrogen bonds more readily than in the ground state. It

suggests that the intermolecular hydrogen bonds would be strengthened in the S1 and T1 states of the complex, compared with the ground state.

HOMO LUMO Fig.3 HOMO and LUMO of the hydrogen-bonded complex FN⋅⋅⋅CH2O in the ground state Table 2 Wiberg bond order b, electron density properties ρ, ∇2ρ and H at the BCPs for HB1: (FN)C=O⋅⋅⋅HCHO and HB2: H2C=O⋅⋅⋅H−C(FN) in the hydrogen bonding complexes, the local electron energy density H at the O⋅⋅⋅H BCP is almost zero. C=O

C−H 2

O⋅⋅⋅H 2

2

HB1

b

ρ

∇ρ

H

b

ρ

∇ρ

H

b

ρ

∇ρ

S0(C)

1.734

0.400

0.187

−0.697

0.907

0.278

−1.077

−0.298

0.008

0.012

0.033

S1(C)

1.683

0.370

−0.253

−0.646

0.902

0.279

−1.092

−0.301

0.015

0.017

0.048

T1(C)

1.697

0.380

−0.093

−0.665

0.904

0.279

−1.088

−0.300

0.012

0.015

0.043

2

2

2

HB2

b

ρ

∇ρ

H

b

ρ

∇ρ

H

b

ρ

∇ρ

S0(C)

1.885

0.402

0.463

−0.690

0.891

0.281

−1.088

−0.305

0.011

0.011

0.03

S1(C)

1.879

0.400

0.430

−0.686

0.888

0.282

−1.093

−0.306

0.013

0.012

0.033

T1(C)

1.881

0.400

0.441

−0.687

0.887

0.281

−1.090

−0.306

0.013

0.014

0.032

In Table 2 we list Wiberg bond order b, electron density ρ, Laplacian ∇2ρ and local electron energy density H at the bond critical points (BCPs) for the two HBs in the complex FN⋅⋅⋅CH2O. Table S2 in supporting materials lists the corresponding data for the isolated CH2O and FN. It is notable that in the free monomers FN and CH2O the electron excitations S0 →S1/T1 reduce the bond orders of the C=O bonds, moreover the Laplacian ∇2ρ of the C=O bond is changed in the sign from a positive value in the ground state to a negative value in the excited states S1 and T1, which means that the π electrons have been excited. As the intermolecular hydrogen bonds are formed in the ground state, the C=O

bonds in FN and CH2O are weakened by the HBs, they have smaller b and ρ in the complex than in the free monomers. As the hydrogen bonding complex is excited to the states S1 and T1, the bond order b and electron density ρof the FN C=O bond are further reduced, even its Laplacian ∇2ρ changes in the sign from positive in the ground state to negative in the excited states, thus the FN C=O has been electronically excited. However the bond order b and electron density properties ρ, ∇2ρ and H of the C=O bond in the CH2O moiety have almost no changes as the complex is excited from the ground state to the excited states, thus the CH2O moiety still remains in the ground state in the excited states of the complex. Thus the LE character of the S1 and T1 states is also confirmed by the electron density properties.

3.4 Vibrational Absorption Spectra In order to have a better understanding of the hydrogen bond response upon electronic excitation, we calculated infrared spectra of the hydrogen-bonded complex as well as the isolated monomers in the ground state and the excited states S1 and T1. The calculated infrared absorption spectra and vibrational frequencies of the C=O and CH groups in different electronic states at the spectral rang from 1500 to 3500cm-1 are shown in Fig.4., and for convenience we also listed these spectral frequencies in S1/T1 shifts supporting Table S3. For free FN, one can find the electronic excitation S0→ the C=O stretching frequency from 1771cm−1 to 1625/1669 cm−1, it means a frequency red shift of 146/102 cm-1. Also the C=O stretch frequency of the free CH2O is largely S1/T1 from 1802cm-1 to 1357/1322 cm-1 with a red-shifted by electronic excitation S0→ red shift of 445/480 cm-1. These large red shifts of the C=O modes in free FN and CH2O mean the πelectron excitation in the C=O double bond.

−

Fig.4 The calculated vibrational absorption spectra and frequencies (cm 1) of free CH2O and the complex

The formation of the intermolecular hydrogen bonds between FN and CH2O mainly influences the C=O and CH stretching modes involved in the HBs. In the ground state, the C=O stretch frequency in the FN moiety is shifted by the HBs from 1771cm-1 in the free monomer to 1758cm-1 in the complex with a small red shift 13cm-1; the C=O frequency in the CH2O moiety is also red shifted from 1802cm-1 to 1783cm-1. However the CH stretch modes in the CH2O moiety are blue shifted by the HBs, the symmetry stretching mode is small blue shifted from 2889cm-1 to 2899cm-1, the asymmetry stretching mode has a larger blue shift 54cm-1 from 2958cm-1 to 3012 cm-1, as shown in Fig.4. As the hydrogen bonding complex is excited to the states S1 and T1, the C=O and CH stretching frequencies in the CH2O moiety have only small changes, which means that the CH2O moiety still remains in the ground state. However the FN moiety has large changes, its C=O stretching mode has a large frequency red shift (137/102cm-1) from 1758cm-1 in the ground state to 1621/1656cm-1 in the S1/T1 state. The C=O stretching frequency in the FN moiety of the complex in the S1/T1 state is very similar to that of the free FN in the S1/T1 state, so the FN moiety has been excited to its excited states. Thus the LE character of the hydrogen bonding complex in the S1/T1 states is also confirmed by the vibrational spectra. So the FN moiety in the complex is sensitive to electron excitation, but the CH2O moiety is sensitive to the formation of the hydrogen bonds, the two moieties are not obviously influenced by the hydrogen bonds’ strengthening in the excited states of the complex. The excited-state hydrogen bonding dynamics of the H2C=O⋅⋅⋅FN system is monitored by both the proton

acceptor FN C=O and the proton donor CH in CH2O. Since both in FN and CH2O there are two identical hydrogen atoms which can be involved in the HBs to form the same hydrogen bonding complex, the two equivalent CH vibrations in the FN moiety or in the CH2O moiety couple with each other. In order to distinguish these two CH vibrations, we replace the hydrogen involved in the HBs by the isotope deuterium, and calculated the vibrational spectra of the isotopereplacing systems. The calculated vibrational frequencies and absorption spectra for the isotope-replacing system are shown in Fig.S1 and also listed in Table S4 in supporting materials. The isotope-replacing system reflects similar informations to the system without isotope replacing. Table 3 The O⋅⋅⋅H distance (Å), the O⋅⋅⋅HC/C=O⋅⋅⋅H angles (°) and the interactional energies ΔE (kJ/mol) of the HBs calculated by (TD) DFT method at the B3LYP/aug-cc-pVDZ level. HB1: (FN)C=O⋅⋅⋅HCHO, HB2: H2C=O⋅⋅⋅H−C(FN) HB1 S0 S1 T1

HB2

O⋅⋅⋅H

O⋅⋅⋅HC/C=O⋅⋅⋅H

O⋅⋅⋅H

O⋅⋅⋅HC/C=O⋅⋅⋅H

ΔE

2.3620 2.1795 2.2316

158.0/136.3 165.8/132.1 162.8/133.9

2.4103 2.3617 2.3752

179.6/105.7 178.3/101.9 178.9/103.3

−14.70 −22.47 −20.08

3.5 Hydrogen Bond Strengthening In the FN⋅⋅⋅CH2O hydrogen bonding complex, the two formed hydrogen bonds, HB1: (FN)C=O⋅⋅⋅HCHO and HB2: H2C=O⋅⋅⋅H−C(FN), are both of the type C=O⋅⋅⋅ H−C, so they are relatively weak. The hydrogen bonding energy and interacting distances and angles are listed in Table 3. The interaction energies between FN and CH2O can be easily computed in the ground state and the S1 and T1 excited states. The interaction energy ΔE in the ground state S0 is calculated according to the equation ΔE(S0) = E(FN⋅⋅⋅CH2O, S0) −E(FN, S0) −E(CH2O, S0). Since the two excited states S1 and T1 are local excited, only the FN moiety jumps onto its electronically excited states but the CH2O moiety remains in the ground state, so the interaction energy ΔE in these two LE states are approximately calculated according to the equation: ΔE(S1/T1) = E(FN⋅⋅⋅CH2O, S1/T1) −E(FN, S1/T1) −E(CH2O, S0). The hydrogen bonding distances in the three states S0, S1 and T1, in the range of 2.1~2.4Å, are typical HB lengths. The two HB angles are closed to 180°. HB1 has a

shorter O⋅⋅⋅H distance but a smaller O⋅⋅⋅HC angle than HB2 in all the three electronic states S0, S1 and T1, HB2 is almost linear. Thus HB1 is a little stronger than HB2. These two HBs in the excited states S1 and T1 have a smaller interacting distances O⋅⋅⋅H and a bigger interacting energy than in the ground state. In the S1 state, the HBs are the strongest and their interacting distances are the shortest among the three states. Summarily, the HBs in the ground and excited states are weak or moderately strong (≤ 5kcal/mol). Since the hydrogen bonds of the type C=O⋅⋅⋅H−C are weak, and the hydrogen bond strengthening in the excited states S1 and T1 is also small, this hydrogen bonding system FN⋅⋅⋅CH2O is not an ideal system for monitoring hydrogen bonding excited-state dynamics. In addition, although here there are two HBs formed, there is no cooperative effect in this system since the two HBs are separated by the aromatic rings in FN.

4. Conclusion In the present work, we theoretically studied the hydrogen-bonded complex between FN and CH2O in the ground state and electronically excited states using the TDDFT method. We found that the two factors ⎯hydrogen bonding and electron excitation have different influences on the two moieties FN and CH2O. The FN moiety is strongly influenced by electron excitation but not sensitive to the HBs, the C=O stretching frequency is largely red shifted as the system is electronically excited. The CH2O moiety remains in the ground state as the complex is excited to the S1 and T1 states, the formation of hydrogen bonds leads to a relatively large blue shift of the asymmetry CH2 vibration. The intermolecular hydrogen bonds in this system have a weak strengthening in the S1 and T1 states relative to the ground state, which has very small influences on the FN and CH2O moieties.

Author Information Corresponding Author: [email protected]; [email protected]

Supporting Information Available Table S1 lists the lengths of the C=O and CH bonds involved in the hydrogen bonds of the free FN and CH2O in the excited states S1 and T1. Table S2 lists Wiberg bond order b, electron density properties ρ, ∇2ρand H at the BCPs for the C=O and H−C bonds in the free FN and CH2O monomers calculated at the B3LYP/aug-cc- pVDZ level. Table S3 lists vibrational frequencies of the CH and C=O stretch modes in

H2C=O and FN for the no-isotope-replacing system. Table S4 lists vibrational frequencies of the CH and C=O stretch modes in H2C=O and FN for the isotope-replacing system. Fig.S1 shows the calculated vibrational absorption spectra of the free CH2O in the ground state and the complex in the states S0, S1 and T1 with isotope replacing.

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Graphiccall Ab bsttraact

Highlights 1. The intermolecular hydrogen bonds are strengthened in the excited state S1 and T1 compared to the ground state. 2. As the hydrogen bonding complex is excited to the states S1 and T1, the FN C=O has been electronically excited, while the CH2O moiety remains in the ground state. 3. The FN moiety is strongly influenced by electron excitation, the C=O stretching frequency is largely red shifted. The hydrogen bond leads to a relatively large blue shift of the asymmetry CH2 vibration of CH2O.