Photochemical Reaction Between 1,2-Naphthoquinone and Adenine in Binary Water-Acetonitrile Solutions

Article abstract of DOI:10.1111/php.12808

The photochemical reaction between 1,2-naphthoquinone (NQ) and adenine was investigated using nanosecond time-resolved laser flash photolysis. With photolysis at 355?nm, the lowest triplet state T₁ of NQ was produced via intersystem crossing from its singlet excited state. The triplet-triplet absorption of the state contributes three bands of transient spectra at 374, 596 and 650?nm, respectively, in pure acetonitrile and binary water-acetonitrile solutions. In the presence of adenine, the observation of A ^.+ (at 363?nm) and NQ_+H^. radical (at 343 and 485?nm) indicates a multistep mechanism of electron transfer process followed by a proton transfer between ³NQ* and adenine. By fitting with the Stern-Volmer relationship, the quenching rate constant k_q of ³NQ* by adenine in binary water-acetonitrile solutions (4/1, volume ratio, v/v) is determined as 1.66?×?10⁹?m^?1?s^?1. Additionally, no spectral evidence confirms the existence of electron transfer between ³NQ* with thymine, cytosine and uracil.

Full text of DOI:10.1111/php.12808

Article type : Research Article
Photochemical Reaction Between 1,2-Naphthoquinone and
Adenine in Binary Water-Acetonitrile Solutions
Qiaohui Zhou1, Yaxiong Wei 1, Xiang Liu 1, Lin Chen1, Xiaoguo Zhou1,2,*,
Shilin Liu1,2
1 Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical
Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2 Synergetic Innovation Center of Quantum Information & Quantum Physics, University of
Science and Technology of China, Hefei, Anhui 230026, China
* Corresponding author email: xzhou@ustc.edu.cn (Xiaoguo Zhou)
ABSTRACT
The photochemical reaction between 1,2-naphthoquinone (NQ) and adenine was investigated
using nanosecond time-resolved laser flash photolysis. With photolysis at 355 nm, the lowest
triplet state T1 of NQ was produced via intersystem crossing from its singlet excited state. The
triplet-triplet absorption of the state contributes three bands of transient spectra at374, 596
and 650 nm, respectively, in pure acetonitrile and binary water-acetonitrile solutions. In the
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doi: 10.1111/php.12808
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presence of adenine, the observation of A+ (at 363nm) and NQ+H radical (at 343 and 485 nm)
indicates a multi-step mechanism of electron transfer process followed by a proton transfer
3
between NQ* and adenine. By fitting with the Stern-Volmer relationship, the quenching rate
constant kq of 3NQ* by adenine in binary water-acetonitrile solutions (4/1, volume ratio, v/v) is
determined as 1.66×109 M-1·s-1. Additionally, no spectral evidence confirms the existence of
electron transfer between 3NQ* with thymine, cytosine and uracil.
INTRODUCTION
Electron transfer (ET) is one of the most common photochemical processes. Usually it is
initiated by an oxidation process in the presence of photosensitizers with photo-irradiation
(1,2). As ET in double-stranded DNA can lead to genetic information error reading and damage
(1-3), ET from DNA nucleobases to the excited photosensitizer has attracted extensive interest.
The typical bases, adenine (A), thymine (T), cytosine (C), guanine (G) and uracil (U) are usually
the primary targets of ET to the excited photosensitizers (4,5). The oxidation energies (Eox) of
the five bases follow a sequence: G<A<T<C<U (6), and generally lower EOX suggests more
favorable ET probability.
Quinones are ubiquitous micromolecule compounds, and their derivatives often act as the key
intermediates of dyes or disinfectants in pesticide and traditional medicine. According to their
various biological activities, quinones play important roles in aerobic respiration and energy-
producing photosynthesis (7-9), and participate in transport of electrons in cell membrane as a
high-efficient electron acceptor (10,11). Due to the high redox potentials, quinones are
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commonly used as the photosensitizer in many photochemical processes. Therefore, several
experimental and theoretical investigations have been performed on the ET between bases and
quinones in the past decades (3,12,13).
Typical chemical reaction processes between photo-excited quinones and nucleobases involve
ET and hydrogen-atom abstraction (HA) (14,15). The mechanisms of ET and HA were identified
in the photochemical reaction between tetrachloroquinone (TCBQ) with T and U (16). A series
of experimental investigations on the reactions between nucleobases and several quinones, e.g.
2-methyl-1,4-naphthoquinone (MQ), 9,10-anthraquinone (AQ) and benzoquinones (BQ), were
performed by Basu and co-workers (17-20). Radical pairs and radical cations produced in the
ET and HA processes were observed in transient-state photolysis (18,21). Interestingly, the ET
efficiency from A, C, T and U to MQ showed a consistent order with the Eox value (20). However,
the probability of ET from three pyrimidines to AQ did not follow the Eox sequence. The similar
phenomena were observed in the photochemical reaction of 1,8-dihydroxyanthraquinone
(DHAQ) and nucleobases (20,22). Therefore, the substituent group on pyrimidine or purine has
shown a significant effect on the ET efficiency, as well as the Eox values.
It is well known that both nπ* and ππ* excitation are involved in photo-excitation of quinones.
With photolysis, the ET and HA processes of p-quinones occur via the triplet state T1 of nπ*
character (23,24). Compared to p-quinones, only a few photolysis of o-quinones, e.g. NQ and
9,10-phenanthrenequinon (PQ), have been performed (25-27). The time-resolved ESR
confirmed that the dominant character of the T1 state could be changed from ππ* in ethanol to
3
3
nπ* in nonpolar solvents due to the smaller energy difference between nπ* and ππ* states in
these o-quinones (28).
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To our knowledge, there is no experimental investigation on the photochemical reaction
between NQ and bases, although both the reactants are common electron acceptor and donor
respectively. In the present work, the photochemical reaction dynamics of NQ and four bases, A,
T, C and U, are studied with a method of nanosecond time-resolved laser flash photolysis at 355
nm. Transient absorption spectra are measured to identify the major intermediates and
products. The influence of solvent polarity on the absorption spectra and the reaction rates is
also checked from pure acetonitrile to binary water-acetonitrile solutions. Based on the
observed ET and/or HA processes, the interaction mechanisms between 3NQ* and the bases are
clarified, and compared with the Eox sequence of bases. An additional discussion is performed on
influence of the substituent group and/or steric hindrance of bases.
MATERIALS AND METHODS
Materials. 1,2-NQ (97%) and bases (>99%) were purchased from Sigma-Aldrich and
Fluorochem Inc. respectively, and used without any purification. The solvents were acetonitrile
of high performance liquid chromatography (Spectrochem) and triply distilled deionized water.
The typical concentration of NQ was 5×10-4 M and those of DNA bases were changed from 5×10-4
to 5×10-3 M in experiments. Due to too low solubility of bases in pure acetonitrile, binary water-
acetonitrile solutions were utilized to dissolve both NQ and bases efficiently, and two typical
volume ratios of CH3CN/H2O were chosen as 9/1 and 4/1 (v/v). The influence of solvent polarity
on photochemical reactions between NQ and the bases was discussed by comparing the dynamic
behaviors in pure acetonitrile and binary water-acetonitrile solutions.
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Equipment. The present experiments were performed by using a home-built nanosecond laser
flash photolysis system. The details have been introduced previously (22,29,30), and thus only a
brief description is presented here. The pulsed excitation light source was the third harmonic
(355 nm, pulse duration 8 ns, pulse energy <7 mJ/pulse) of a Q-Switched Nd:YAG laser (PRO-
190, Spectra Physics, repetition rate of 10 Hz). A 500 W Xenon lamp was used as the analyzing
light source. The pulsed laser and the analyzing light passed perpendicularly through a flow
quartz cuvette with an optical path length of 10 mm. A monochromator equipped with a
photomultiplier (CR131, Hamamatsu) was used to measure the transient absorption spectra.
The spectral resolution of the system was less than 1 nm. The dynamic decay curve of
intermediate was averaged by 512 laser shots and recorded with an oscilloscope (TDS3052B,
Tektronix). All the solutions were deoxygenated by purging with high purity argon (99.99%) for
about 30 minutes before measurements. The steady-state UV-Vis absorption spectra were
recorded from 300 nm to 650 nm with a UV spectrophotometer (UV-3600, Shimadzu). All the
present photochemical experiments were performed at ambient room temperature (25 ◦ C).
Computation. All density functional theory calculations were performed with the Gaussian-09
program package (31). The geometry of NQ in the ground electronic state were optimized at the
CAM-B3LYP level with the 6-311++G* basis set (32), while those of NQ in the low-lying excited
singlet and triplet states were obtained with the time-dependent B3LYP (TD-B3LYP) level of
theory. The vertical and adiabatic excitation energies of NQ were calculated at the same level.
The solvent effect was taken into account by applying the Polarizable Continuum Model (PCM).
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RESULTS AND DISCUSSION
In the steady-state absorption spectra of NQ and adenine in different solvents (shown
as Figure S1 in the Supporting Information), only two bands were observed in a
wavelength range of 300 ~ 650 nm. In pure acetonitrile, the lower energy band is
located at 398 nm as the S1S0 transition of NQ, and the higher one is the S2S0
absorption at 338 nm. The optimized geometries of singlet and triplet NQ at S0, S1 and
T1 are calculated and shown in Figure 1, as well as the representation of major
molecular orbitals. Obviously, the C=O bond lengths of S1 and T1 states are elongated
from the values in the ground state because of the excited anti-bonding orbital.
The calculated vertical and adiabatic excitation energies of the low-lying electronic states of
NQ in pure acetonitrile are summarized in Table 1, and agree with the experimental data. No
parameters can be accurately predicted and used in PCM model for the mixed solvents. Thus, we only
compared the calculated energies of the excited states and the optimized structures in pure acetonitrile
and water with those in gas phase (the data are not shown in Table 1). Although no direct calculations
on the mixed solvents, the changes of the steady-state absorption spectra confirm the solvent effect in
the expected sequence. It is well known that both n* and * excitations are involved in the S1
S0 absorption with the different weights. With the solvent polarity increase, the * excitation is
gradually stabilized as the lowest singlet state (33,34). Thus the contribution from the * transition is
enhanced in the S1 S0 excitation, and the maximum absorption exhibits a slight red-shift in Figure
S1.
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Transient absorption spectra of NQ in pure acetonitrile and binary water-
acetonitrile solutions
Figure 2(a) shows the transient absorption spectra of NQ in pure acetonitrile at different delay
times, with photolysis at 355 nm. The sharp drop at 355 nm is the scattering of the excitation
laser. Three dominant absorption peaks are observed at 374, 596 and 650 nm, respectively. The
bands are attributed to the triplet-triplet absorptions, because all of them are quenched
drastically in the presence of the solvated oxygen (see Figure S2). As suggested in the previous
experiment (35), the bands at ~370, 600 and 650 nm are assigned as the absorption of the
triplet NQ. In addition, the similar transient absorption spectra are observed with photolysis at
450 nm (see Figure S3), although the Frank-Condon excitation of NQ at 355 nm partially
involves the S2S0 transition. It implies that the internal conversion (IC) from S2 to S1 is much
faster than the other photochemical processes, which agrees with the recent conclusions in
deactivation of the excited states of quinones (36,37).
The dynamic decay curves at 374, 596 and 650 nm were recorded respectively. All of them have
the similar time profiles (see Figure S4), and hence only the curve at 374 nm was plotted in
Figure 2(b). Interestingly, both the excitation energies of the lowest two triplet states, T1 and T2,
are lower than that of S1 state as shown by the present TD-DFT calculations. Thus, with
photolysis at 355 nm, both the T1 and T2 states could be produced from ISC after the S1 state is
prepared. Because the fast internal conversion between T2 and T1 in aromatic compound is
usually in picosecond timescale (36,38,39), we believe a similar decay rate for the internal
conversion from T2 to T1 of NQ. Thus, the dynamic decay in Figure 2(b) observed in experiment
should be the photochemical behavior of 3NQ* in the T1 state.
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The quenching rates of the intermediates in Figure 2(a) can be obtained by fitting the decay time
profile with a pseudo first-order kinetic. Table 2 summarizes the absorption wavelengths and
the quenching rates of the intermediates. Acetonitrile as an inert solvent does not react with the
triplet NQ, and thus the self-quenching by collision is most significant in the overall decay
process. As show in Table 2, the decay rates at 596 and 650 nm are close. The smaller value at
374 nm is thought due to influence of the nearby absorption band at the higher energy.
It is well known that the solvent polarity can affect the efficiencies of IC and ISC processes. The
transient absorption spectra of NQ in two binary water-acetonitrile solutions were recorded and
presented in Figure 3. Neither significant shift nor change of intensity was observed with the
three major triplet bands compared to those in Figure 2(a). The decay rates of the three
absorptions in binary water-acetonitrile solutions are summarized in Table 2, and similar to those
in pure acetonitrile. The viscosities of the mixed solutions are increased with the fraction of
water increase, which agrees with the slight decrease of the self-quenching rates at 374 nm in
Table 2.
Additionally, two new weak and wide bands gradually appear at 343 nm and 420 nm,
respectively, with the increased ratio of water. Harada et al. assigned the absorption of the
NQ+H radical at 340 nm (35), and thus a HA process between 3NQ* and water is expected in the
present experiment to produce the NQ+H radical. The further HA leads to formation of 1,2-
dihydroxynaphthalene (NQ+2H). As suggested by Kuhmet. al. (40,41), the absorption at 420 nm is
attributed as the contribution of NQ+2H as the HA product of photo-excited NQ in the absence of
oxygen. As shown in Figure 3, these two absorptions are more and more obvious with the ratio
of water increase. Thus the HA process between 3NQ* and water exists indeed.
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Reaction between 3NQ*and adenine in binary water-acetonitrile solutions
Compared with water, the bases prefer to play an efficient role of electron donor due to its lower
Eox, instead of hydrogen abstraction. Thus an electron transfer is expected to occur between
3NQ*and the bases, as shown in equation (1).
3NQ* + base  NQ- + base+
(1)
Among the bases of G, A, C, T and U, guanine has the lowest Eox value, but it was not studied due
to its low solubility in the solvents used. Except for the low Eox value, lone pair electrons of
amino nitrogen in –NH2 increase the electron density of the ring and make adenine the easier to
lose electrons, thus the electron transfer from A to 3NQ* should be the most feasible.
Figure 4(a) shows the transient absorption spectra of NQ with A (5×10-4 M) in an
acetonitrile/water solvent of 4:1 volume ratio, while the spectra with A (5×10-3 M) is presented
in the Figure 4(b). At the same delay time, the spectral intensities of three 3NQ* absorptions are
apparently decreased due to quenching by adenine. Three new absorption peaks can be
3
observed with the centers at 343, 420 and 485 nm, besides the absorptions of NQ*. In a high
3
concentration of A (5×10-3 M), the absorption bands at 374 nm and 660 nm of NQ* are almost
completely quenched. As the intensities at 343, 420 and 485 nm are increased to the maximums
later than the triplet-triplet absorptions of 3NQ*, these bands should come from the absorptions
of the reaction products.
It is worth noting that the absorption of A+ is located at ~360 nm (20) and overlapped with that
3
of NQ*(T1) at 374 nm. The overlapping causes the band slightly blue-shifted from 374 nm to
3
363 nm in Figure 4(a). The other product (NQ-) of the electron transfer from A to NQ* cannot
be observed due to its absorption is beyond the wavelength range of the detector (42). As shown
in the dynamic decay curves of the major absorptions of Figure 5, the absorptions at 343 and
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485nm reach the maximums with the almost same rates and both of them are slower than that
at 363 nm, thus the bands are attributed to the products in the secondary reaction between NQ-
and A+. The absorption intensity at 420 nm increases gradually with the slowest rate as
expected.
Through fitting the dynamic curves with a bi-exponential function of formula (2), the quenching
rates of all the intermediates were obtained and listed in Table 3.
I(λ) = I0(λ) + A1(λ)·exp(-k1t) + A2(λ)·exp(-k2t)
(2)
where I(λ) is the absorption intensity at the delay time t, k1 and k2 are the faster and slower
quenching rates, respectively, A1(λ) and A2(λ) are the weights of both quenching processes,
I0(λ)A1(λ)A2(λ) represents the absorption intensity at the initial time. Usually, the faster
quenching process is thought as the photochemical reaction, while the slower is the physical
quenching (e.g. collision).
3
Compared with that in the absence of A with same solvent, NQ* is efficiently quenched with a
much faster quenching rate, indicating that A plays an efficient role of quencher. The initial
products are A+ and NQ-. In binary water-acetonitrile solutions, there are three candidates to
+
provide a proton to yield the NQ+H· radical: one is adenine, another is A• , and the other is water.
However, the proton-transfer from adenine and water are unworkable in thermodynamics, because
-
+
H2O H+ + OH- and A  A-H + H are very endothermal. In addition, the decay rate of A• produced
from the electron-transfer between 3NQ* and adenine is very fast as shown in Figure 3, indicating that
its quenching is efficient. Due to the strong electrostatic interaction, the proton transfer reaction
+
-
+
between NO• and A• naturally occurs as equation (3). As shown in Figure 4, the apparent decay
3
rate at 363 nm is reduced in some extent due to the overlap from the absorption of NQ* at 374
nm, and thus we cannot derive the reaction rate directly. The absorption of NQ+H radicali
located at 343 nm (35), and its formation process can be directly observed in Figure 5. The final
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product, 1,2-dihydroxynaphthalene (NQ+2H), is gradually formed as equation (4) and its
absorption reaches the maximum at the delay time of ~2 μs in Figure 5.
NQ- + A+ NQ+H + A-H
(3)
NQ+H + Hydrogen-donor  NQ+2H + Product
(4)
Moreover, the dynamic behavior of the absorption at 485 nm shows very similar to that at 343
nm in Figure 5, where the normalized curves at 343 and 485 nm are almost identical with the
same decay rates of 1.1×106 s-1. Thus, the candidates of the absorption at 485 nm are the A-H
radical or another band of the NQ+H radical. An additional experiment was performed to
identify the intermediate with absorption at 485 nm. With photolysis at 355 nm, the bands at
343 and 485 nm also exist in the system of NQ and diphenylamine (DPA) in pure acetonitrile. It
is well known that an electron transfer occurs between NQ and DPA, and the absorption of
DPA•+ is located at 780 nm (29). Thus, the band at 485 nm is believed to form by the absorption
of NQ+H radical as well as that at 343 nm.
Therefore, a multi-step process is confirmed in the reaction between 3NQ* and A: a fast electron
transfer is followed by a subsequent proton transfer, and 1,2-dihydroxynaphthalene as the final
product is formed via an additional hydrogen-abstraction process. Moreover, the quenching
rates of 3NQ* are increased with the fraction of water, which is contrary to the case without A in
3
Table 2. Thus, the hydrogen bond looks beneficial to the electron transfer between NQ* and A.
Through fitting with the equation (5) of Stern-Volmer relationship, the quenching rate constant
kq of 3NQ* by A can be determined as 1.66×109 M-1 s-1 as shown in Figure 6, which is much lower
than the theoretical diffusion-controlling rate limit in CH3CN/H2O (4/1, v/v) (~ 1.45×1010 M-1 s-
1).
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1/ = 1/0 + kq (A)
(5)
3
where 1/0 and 1/ are the quenching rate of NQ* in solution of the absence and presenceof A,
respectively. With the ratio of water increase, there is a little bit increase on the quenching rate
constant kq.
Reaction between 3NQ* and T, C and U in binary water-acetonitrile solutions
Figure 7 shows the transient absorption spectra of NQ with T in a solvent of 4:1 volume ratio of
acetonitrile/water. Compared with those in the absence of bases (Figure 2 and 3), the spectra
look very similar and no additional peak is observed. All the three 3NQ* absorption bands have a
negligible wavelength-shift, and two weak bands at 343 and 420 nm are attributed to hydrogen
abstraction from water. Because the absorption of the T•+ was at 500 nm (18) and no such peak
was observed in Figure 7, the electron transfer did not apparently occur. In addition, the decay
3
rate of NQ* at 374 nm is almost as the same as that in Figure 3(b). Thus, we can conclude that
no photochemical reaction between NQ and T exists with photolysis at 355 nm. The similar
phenomena are also observed for the cases with C and U.
It is well known that the electron transfer can occur when the Gibbs free energy ΔG predicted by
the Rehm-Weller equation (6) is negative. In general, Eox is the most significant parameter.
ΔG = Eox − Ered – q2/(ε·r) – ET
(6)
3
where Eox and Ered are the oxidation potential of bases and the reduction potential of NQ*,
respectively, q2/(ε·r) is the Coulombic work term of the product ions at the encounter distance r,
and ET is the excited energy of the triplet NQ. The oxidation energies (Eox, vs NHE) of the four
bases are 1.96 V for A, 2.11 V for T, 2.14 V for C and 2.39 V for U (6), respectively. The reduction
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3
potential of NQ* is -0.338 V (vs NHE) (43). For the ET between the triplet NQ and adenine, the
electron is transferred from the amino group, while it is from the ring of pyrimidine for T, C, and
U. Thus, the effective ET distance in the encounter pair for adenine is smaller than that of pyrimidine,
and consistent with the calculation of ion pair. Using the calculated values of the encounter
distances between quinone and nucleobases, r(A) = 2.0 Å and r(T/C/U) = 2.5 Å, the Coulombic
work is slightly decreased from 3.86 kcal·mol-1 for A to 3.09 kcal·mol-1 for T/C/U. The data are
larger than the common value, 1.4 kcal·mol-1 or 0.06 eV,(44) implying that the encounter
distances in solutions should be larger than the calculated values. However, the difference of the
Coulombic work for the bases can be estimated as 0.77 kcal·mol-1, and such small difference
suggests that the Coulombic work has very little effect on the total Gibbs free energy. Taking ET of
3NQ* as 2.11 eV in Table 1, the ΔG values are calculated to be 0.48 kcal·mol-1 for A, 4.70 kcal·mol-
1
for T, 5.40 kcal·mol-1 for C, and 11.16 kcal·mol-1 for U, respectively. Thus, the ΔG differences
between A and the other three bases are more than 4.22 kcal·mol-1, and only ΔG of the reaction
3
between NQ* and A is close to zero (or negative) when taking into account the uncertainty. It
agrees with the present experimental conclusions. Additionally, the ET possibility sometime
doesn’t follow the Eox order in the previous studies, e.g. for 1,8-dihydroxyanthraquinone (22) and
9,10-anthraquinone (20), where the steric hindrance plays a significant role as well as Eox. However,
the similar phenomena are not observed for the present reaction. Based on the weak dependency of
3
the quenching rate on solvent polarity, the quenching of NQ* by adenine more likely takes place
along the inner-sphere mechanism.
CONCLUSION
Using nanosecond time-resolved laser flash photolysis, photoreaction between 1,2-NQ and four
bases has been investigated. With photo-excitation at 355 nm, the transient absorption spectra
3
of NQ* and the other products have been recorded in pure acetonitrile or acetonitrile/water
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(9/1, 4/1,v/v). All the three bands observed at 374, 596 and 650 nm are attributed to the
absorption of the triplet states of NQ. By fitting the decay curve at 374 nm, the relaxation rate of
3NQ* is determined as 8.19×106 s-1 in pure acetonitrile and slightly decreased with the ratio of
water increase. Moreover, two weak absorptions at ~340 and 420 nm are observed in the
transient absorption spectra in binary water-acetonitrile solutions, which are assigned as the
contributions of the NQ+H radical (340 nm) and 1,2-dihydroxynaphthalene (NQ+2H, at 420 nm).
Thus a HA process between 3NQ* and water is confirmed.
3
When adenine exists in solution, a multistep photochemical reaction occurs between NQ* and
A: the fast electron transfer is followed by the subsequent proton transfer, and 1,2-
dihydroxynaphthalene as the final product is formed via an additional hydrogen-abstraction
3
process. Besides the absorption bands of NQ*, three new bands are observed in the transient
spectra at 343, 363 and 485 nm, respectively. The peaks at 343 and 485 nm are both
contributed to the NQ+Hradical, while that at 363 nm is attributed to A+. All the spectral
assignments agree very well with the previous data or conclusion of the additional experiment
of NQ and DPA. By fitting with the Stern-Volmer relationship, the quenching rate constant kq of
3NQ* by A in binary water-acetonitrile solution (4/1, v/v) is determined as 1.66×109 M-1s-1.
In addition, no distinct spectral evidence is obtained for the electron-transfer or proton-transfer
processes between NQ with the other three bases, T, C and U. Thus, ΔG is speculated to change
from a negative value for adenine to a positive data for T/C/U. Based on the Rehm-Weller
3
equation, the ΔG values of the electron transfer between NQ* and the bases are estimated to
increase at least 4.22 kcal·mol-1 from A to T/C/U.
ACKNOWLEDGEMENTS: This work was supported by the National Natural Science
Foundation of China (No.21373194 and 21573210) and the National Key Basic Research Special
Foundation (No.2013CB834602).
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SUPPORTING INFORMATION
Additional Supporting Information can be found in the online version of this article:
Figure S1. Steady-state absorption spectra of 1,2-NQ and adenine in pure acetonitrile and
binary water-acetonitrile solutions.
3
Figure S2. Decay curves of NQ* at 374 nm in pure acetonitrile in the absence/presence of
oxygen.
Figure S3. Transient absorption spectra of NQ in pure acetonitrile at different delay times
under photolysis at 450 nm.
Figure S4. Normalized decay curves of 3NQ* at 374, 596 and 650 nm in pure acetonitrile.
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TABLES
Table 1. The calculated vertical and adiabatic excitation energies of NQ at CAM-B3LYP/6-
311++G(d,p) level.a
Vertical energy
∆E/eV
λ /nm
Adiabatic energy
f b
composition
E/hartree
S1 -534.87817
S2 -534.84575
T1 -534.89383
T2 -534.88283
Cal. Exp.
E/hartree
-534.89228
-534.85505
-534.91372
-534.89692
∆E/eV
3.08
3.96
2.65
2.95
403
313
468
420
398
338
--
2.69
3.42
2.11
2.57
0.1197
HOMOLUMO
0.2149 HOMO-1LUMO
0.0000 HOMOLUMO
0.0000 HOMO-1LUMO
--
a. The energy of the ground electronic state of NQ is calculated as -534.99122 hartree.
b. Oscillator strengths.
Table 2. Absorption wavelengths and quenching rates of various intermediates in solvents.
Band
NQ -CH3CN
Ⅰ
Ⅱ
Ⅲ
λ(nm)
k(×106 s-1)
374
596
650
8.19±0.11
10.15±0.20
9.96±0.19
NQ -CH3CN/H2O 9/1
λ(nm)
374
600
655
k(×106 s-1)
7.52±0.11
9.72±0.18
9.57±0.18
NQ -CH3CN/H2O 4/1
λ(nm)
374
596
660
k(×106 s-1)
7.04±0.14
9.42±0.15
9.60±0.19
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Table 3. Decay rates and absorption wavelengths of intermediates in CH3CN/H2O (v/v).
Band
NQ +A(5×10-4 M) in CH3CN/H2O 4/1
VII
Ⅰ
Ⅱ
Ⅲ
Ⅳ
Ⅴ
Ⅵ
Carrier
λ (nm)
NQ+H
A+
3NQ*
374
NQ+2H
420
NQ+H
485
3NQ*
596
3NQ*
660
343
363
k(×106 s-1)
1.10±0.05 11.23±0.12 15.62±0.04 1.16 ± 0.03
1.12±0.01
14.04±0.14 14.49±0.21
NQ +A(5×10-4 M) in CH3CN/H2O 9/1
Carrier
λ (nm)
NQ+H
A+
3NQ*
374
NQ+2H
420
NQ+H
485
3NQ*
600
3NQ*
660
343
363
k(×106 s-1)
1.13±0.05 10.21±0.10 11.91±0.06 1.08 ± 0.03
1.42±0.04
11.49±0.13 12.50±0.16
FIGURE CAPTIONS
Figure 1. Optimized geometries of the ground, lowest singlet and triplet states of NQ, as well as
the representation of major molecular orbitals.
Figure 2. (a) Transient absorption spectra of NQ in pure acetonitrile at different delay times
with photolysis at 355 nm. (b) Dynamic decay curveof the intermediate at 374 nm, where the
fitted time profile is shown as well.
Figure 3. Transient absorption spectra of NQ in (a) CH3CN/H2O (9/1, v/v) and (b) CH3CN/H2O
(4/1, v/v) at different delay times.
Figure 4. Transient absorption spectra of NQ at different delay times in the presence of A, (a) [A]
=5×10-4 M and (b) [A] =5×10-3 M.
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Figure 5. Dynamic decay curves of major absorptions in the reaction system of NQ (5×10-4 M)
and A (5×10-4 M) in the solvent of CH3CN/H2O (4/1, v/v).
3
Figure 6. Dependence of the observed quenching rates of NQ*on concentration of A in the
solvent of CH3CN/H2O (4/1, v/v).
Figure 7. Transient absorption spectra of NQ and three bases at different delay times in CH3CN/H2O
(4/1, v/v) (a) T, (b) C, (c) U.
Scheme 1. Molecular geometries of 1,2-NQ and four bases.
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