Efficiencies of singlet oxygen production and rate constants for oxygen quenching in the S₁ state of dicyanonaphthalenes and related compounds

Abstract

The quantum yield of singlet oxygen (¹O₂ (¹Δ_g)) production (Φ_Δ) in the oxygen quenching of photoexcited states for 1,2-dicyanonaphthalene (1,2-DCNN), 1,4-dicyanonaphthalene (1,4-DCNN) and 2,3-dicyanonaphthalene (2,3-DCNN) in cyclohexane, benzene, and acetonitrile was measured using a time-resolved thermal lens (TRTL) technique, in order to determine the efficiency of singlet oxygen (¹Δ_g) production in the first excited singlet state (S₁), (f_Δ^S). The efficiencies of singlet oxygen (¹Δ_g) production from the lowest triplet state (T₁), (f_Δ^T), were nearly unity for all DCNNs in all the solvents. The values of f_Δ^S were fairly large for 1,2-DCNN (0.33–0.57) and 1,4-DCNN (0.33–0.66), but were close to zero for 2,3-DCNN. Rate constants for oxygen quenching in the S₁ state (k_q^S) obtained for these compounds were significantly smaller than diffusion-controlled rate constants. The kinetics for processes leading to production and no production of singlet oxygen is discussed on the basis of the values of f_Δ^S and k_q^S. The results obtained regarding phenanthrene (PH), 9-cyanophenanthrene (9-CNPH), pyrene (PY) and 1-cyanopyrene (1-CNPY) are also discussed.

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Introduction

The quenching of photoexcited molecules by oxygen and resultant production of singlet oxygen have been long investigated since they are common phenomena for almost all molecules and important in relation to applications such as photooxygenation and photodynamic therapy. Over two decades, the kinetics and mechanism for explaining the rate constant of oxygen quenching and the efficiency of singlet oxygen production have been elucidated fairly well.^1–3 However, information on the production of singlet oxygen by the oxygen quenching in the S₁ state is still scarce in comparison with that in the lowest excited triplet state (T₁). The efficient production of singlet oxygen from the S₁ state is rare in contrast to high efficiencies for the T₁ state of most organic molecules and, hence, the approach to the kinetics and mechanism based on data of f_Δ^S and k_q^S is restricted.^4–16

The three possible processes for the oxygen quenching in the S₁ state are given in Scheme 1, in which ³(S₁–³O₂) represents an encounter complex and k_diff and k_-diff are the diffusion-controlled rate constants for encounter and separation of the complex, respectively. The processes (1)–(3) are energy transfer producing T₁ and ¹O₂, intersystem crossing without production of ¹O₂ and internal conversion producing neither T₁ nor ¹O₂, respectively. k_Δ, k_ST and k_SS are the rate constants for the three processes, respectively. The process (1) is energetically possible when the energy gap between S₁ and T₁ is larger than the state energy of ¹O₂ (¹Δ_g) (94 kJ mol^–1). However, the process (1) is in competition with processes (2) and (3) and the efficiency of production of ¹O₂ is determined by the amplitude of k_Δ relative to k_ST and k_SS. According to data obtained so far, f_Δ^S is zero or a low value even for many organic molecules with a larger S₁–T₁ energy gap than 94 kJ mol^–1 and k_q^S for these molecules is nearly a diffusion-controlled rate constant (k_diff).^9–12 It was found that 9,10-dicyanoanthracene, 9-cyanoanthracene, 9,10-dichloroanthracene and other molecules which had an electron-withdrawing group, i.e., a high oxidation potential, yielded a significant value of f_Δ^S, and k_q^S for these molecules was distinctly smaller than k_diff.^{4–9,13–17} This inverse correlation between f_Δ^S and k_q^S can be qualitatively explained in terms of a proposition that the process (2) is enhanced by charge transfer interaction between the S₁ molecule and ³O₂.^5,9 It is also known that the process (3) becomes open in acetonitrile for molecules with a low oxidation potential owing to enhancement of charge transfer interaction in the polar solvent, whereas this process can be neglected in nonpolar solvents.^10–12,18

Scheme 1 The three possible processes for the oxygen quenching in the S₁ state.

As for the oxygen quenching in the T₁ state, dependences of rate constants for three processes leading to ¹O₂ (¹Σ_g), ¹O₂ (¹Δ_g) and ³O₂ (³Σ_g) on the T₁ energy and the charge transfer interaction have been investigated in detail on the basis of data of the efficiencies of ¹O₂ (¹Σ_g, ¹Δ_g) production and the rate constant of oxygen quenching in the T₁ state (k_q^T).^1,2,19 A similar approach based on data of f_Δ^S and k_q^S for the oxygen quenching in the S₁ state was made only for a few cases such as anthracene derivatives.^9,13–16 For deep understanding about the elementary processes of (1)–(3), systematic data of f_Δ^S and k_q^S about more compounds are needed.

The object of this paper is to estimate the rate constants for the three elementary processes of (1)–(3) from values of k_q^S and f_Δ^S obtained for three DCNNs, 9-CNPH and 1-CNPY, in order to elucidate determining factors for the three rate constants of k_Δ, k_ST and k_SS.

Experimental

1,2-DCNN, 1,4-DCNN, 2,3-DCNN, 1,4-dimethylnaphthalene (1,4-DMN) (extra pure grade, Tokyo Kasei), 9-CNPH (guaranteed grade, Tokyo Kasei) and PH (zone refined, Tokyo Kasei) were used as received. PY (Wako Pure Chemicals Ltd.) was purified by column chromatography and recrystallization. 1-CNPY was given by Prof. K. Mizuno of Osaka Prefecture University. Cyclohexane, benzene and acetonitrile of spectroscopic grade (Dojin Kagaku) were used.

Absorption and fluorescence spectra were measured with a spectrophotometer (Jasco UV/VIS 660) and a spectrofluorophotometer (Shimadzu RF-5300), respectively. The measured fluorescence spectra were corrected using fluorescence spectra of sulfuric acid solutions of quinine sulfate and 2-aminopyridine used as standard fluorescence solutions.²⁰ The average fluorescence energy (E_F^av) and the fluorescence quantum yield (Φ_F) were determined using the corrected fluorescence spectra. The value of Φ_F⁰ = 1 for 9-cyanoanthracene in the deaerated solution was used as a standard value of Φ_F in all the solvents used.⁹ Triplet energies were obtained from phosphorescence spectra in methanol–ethanol (1 : 1) at 77 K in the absence or presence of methyl iodide. The fluorescence lifetime (τ_F) was measured with a time-correlated single photon counting instrument (Horiba NAES-700). The value of k_q^S was obtained from the values of τ_F in deaerated and air-saturated solutions and the concentration of O₂ in the air-saturated solution.²¹

The TRTL system used for the determination of Φ_Δ was described in a previous paper.⁹ The quantum yield of triplet state in the absence of oxygen (Φ_T⁰) was also determined by the same TRTL instrument. A nitrogen laser (NDC JH-1000L) and a He–Ne laser (Spectra-Physics 117A) were used as excitation and probe light sources, respectively. The excitation light (337 nm) was focused at the center of a 1 cm square cuvette containing a sample solution. The probe light (632.8 nm) focused in front of the cuvette passed through the sample solution, and the intensity of the probe light passing through a band filter and pinhole (a diameter of 0.5 mm) was detected with a photomultiplier (Hamamatsu R928). The signal from the photomultiplier was fed into the 1 kΩ input of a preamplifier and recorded by a digital oscilloscope (YOKOGAWA DL1520L) with averaging of 256 signals. The concentration of sample solutions was adjusted to the absorbance of 0.30 at 337 nm (path length of 1 cm). In the TRTL measurement, the lens signal composes of a fast-rising component (U_f) and a slow-rising component (U_s), as shown in Fig. 1.

Fig. 1 Time-resolved thermal lens signals of 1,4-dicyanonaphthalene in air-saturated acetonitrile (upper) and in deaerated acetonitrile (lower).

The value of Φ_Δ for solutions containing oxygen is determined from the ratio of U_s to U_t by the following equation,^9,22

where U_s corresponds to heat released by nonradiative deactivation of singlet oxygen and U_t (= U_f + U_s) corresponds to total heat released by all nonradiative transitions, and E_ex and E_Δ are the energy of excitation light (355 kJ mol^–1) and the energy of singlet oxygen (¹Δ_g) (94 kJ mol^–1), respectively. The value of Φ_T° for solutions in the absence of oxygen is determined by the following equation,where U_s corresponds to heat released by nonradiative deactivation of triplet state and E_T is the energy of triplet state. In order to eliminate the influence of multiphoton processes which may be caused by excitation of high intensity, values of Φ_Δ and Φ_T were determined by using values of U_s/U_t measured in a range of low excitation intensity where the decrease in the intensity of probe light caused by the total heat released was 1–2%. In these conditions, the value of U_s/U_t was almost constant. The influence of population lens and transient absorption in the measurement of Φ_T° with TRTL should also be taken into account because the TRTL signal may include contributions from these effects besides thermal lens.²³ However, these effects are considered to be insignificant in the present measurement since the values of Φ_T° determined are reasonable judging from the result that the sum of Φ_T° and Φ_F⁰ is approximately unity as described below. The accurate values of Φ_T° and Φ_Δ for 2,3-DCNN in cyclohexane could not be obtained owing to low solubility.

Deaeration of sample solutions was performed by freeze–pump–thaw cycles. Oxygen-rich solutions were prepared by replacing air in the sample cuvette for an adequate time by pure oxygen gas. All measurements were carried out at 25 ± 2 °C.

Results and discussion

Efficiencies of singlet oxygen production (f_Δ^S) and rate constants of oxygen quenching (k_q^S) for dicyanonaphthalenes

The S₁ energy (E_S), the T₁ energy (E_T), the fluorescence quantum yield (Φ_F⁰) and the triplet quantum yield (Φ_T⁰) for three DCNNs are given in Table 1. As seen in Table 1, the energy gap of the S₁ and T₁ states (ΔE_S–T) is larger than E_Δ (94 kJ mol^–1) for all the three DCNNs and, hence, the production of singlet oxygen from the S₁ state is energetically possible in the process (1), as well as the production from the T₁ state. The sum of the quantum yields of fluorescence and triplet, (Φ_F⁰ + Φ_T⁰), is approximately unity in all the studied systems of compound and solvent, indicating that the S₁ state is mainly deactivated by two processes of fluorescence and intersystem crossing.

Table 1 Photophysical parameters of dicyanonaphthalenes and phenanthrene, pyrene and their cyano-derivatives in cyclohexane (CH), benzene (BZ) and acetonitrile (AC)

Compound	Solvent	E S/kJ mol^–1	E T/kJ mol^–1	Φ F ^0	Φ T ^0	τ F ^0/ns
a Accurate TRTL measurements on 2,3-DCNN in cyclohexane were impossible owing to low solubility.
1,2-DCNN	CH	347	226	0.36	0.57	7.80
	BZ	340	226	0.49	0.39	9.54
	AC	337	226	0.49	0.35	12.0
1,4-DCNN	CH	355	221	0.26	0.67	3.00
	BZ	344	221	0.63	0.24	9.25
	AC	345	221	0.69	0.23	9.77
2,3-DCNN	CH^a	351	246			37.5
	BZ	348	246	0.69	0.22	28.4
	AC	349	246	0.76	0.17	26.3
PH	CH	345	257^21	0.10	0.89	52.6
	BZ	343	257^21	0.17	0.87	51.6
	AC	344	257^21	0.13	0.86	52.6
9-CNPH	CH	335	242	0.17	0.59	28.0
	BZ	333	242	0.29	0.55	24.4
	AC	334	242	0.24	0.69	23.6
PY	CH	322	203^21	0.61	0.33	417
	BZ	322	203^21	0.69	0.30	318
	AC	321	203^21	0.67	0.28	322
1-CNPY	CH	313	194	0.77	0.13	36.2
	BZ	311	194	0.86	0.10	19.4
	AC	312	194	0.80	0.13	22.0

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The measured value of Φ_Δ and the efficiencies for ¹O₂ production in the S₁ and T₁ states (f_Δ^S and f_Δ^T) are related by the following equation, under the condition that the T₁ state is completely quenched by O₂,^9,13

Φ_Δ(F⁰/F) = (f_Δ^S + f_T^Of_Δ^T)[(F⁰/F) – 1] + f_Δ^TΦ_T⁰ (6)

where (F⁰/F) is the ratio of fluorescence intensities in the absence and the presence of oxygen and f_T^O expresses a fraction of the T₁ state produced in the process of the oxygen quenching of the S₁ state, that is, f_T^O is decreased by the increase of contribution of process (3) in Scheme 1. Plots of Φ_Δ (F⁰/F) versus [(F⁰/F) – 1] according to eqn (6) for solutions at three different concentrations of oxygen (air-saturated, oxygen-saturated and intermediate solutions) give a straight line with intercept of (f_Δ^TΦ_T⁰) and slope of (f_Δ^S + f_T^Of_Δ^T), as shown in Fig. 2. The value of f_Δ^T is determined from values of the intercept and the triplet quantum yield. The value of f_Δ^S is determined from the value of slope and the obtained value of f_Δ^T if the value of f_T^O is known. It is considered that the process (3) in Scheme 1 is negligible in nonpolar solvents and the value of f_T^O is unity^11,13 and, hence, the value of f_Δ^S in cyclohexane and benzene can be obtained using f_T^O = 1. In acetonitrile the value of f_T^O is not necessarily unity and values less than unity are reported for some compounds, indicating involvement of process (3).^10,12,14,18 Then, the value of f_Δ^S is obtained as f_Δ^S ≥ [(value of slope) – f_Δ^T] because values of f_T^O were not measured in this work. In general, the value of f_T^O is decreased through enhancement of process (3) when charge transfer interaction between the S₁ state and O₂ is increased. Therefore, it is expected that the value of f_T^O is not much lower than unity for DCNNs possessing high oxidation potential as discussed later, and minimum values of f_Δ^S determined using f_T^O = 1 would be regarded as approximate values in acetonitrile. The obtained values of f_Δ^S and f_Δ^T are listed in Table 2, with values of k_q^S for DCNNs and 1,4-DMN.

Table 2 Values of Φ_Δ^air, f_T^O, f_Δ^T, f_Δ^S and k_q^S for dicyanonaphthalenes, phenanthrene, pyrene and their cyano-derivatives, and values of k_q^S for 1,4-dimethylnaphthalene in cyclohexane (CH), benzene (BZ) and acetonitrile (AC)

Compound	Solvent	Φ Δ ^air ^b	f T ^O ^c	f Δ ^T	f Δ ^S	10^–9kq^S/M^–1 s^–1
a Accurate TRTL measurements on 2,3-DCNN in cyclohexane were impossible owing to low solubility. b Values of ΦΔ in air-saturated solutions. c The value of unity was assumed for DCNNs in the three solvents and for other compounds in cyclohexane and benzene. d The average of two reference values. e The value in cyclohexane was used.
1,2-DCNN	CH	0.78	1	0.97 ± 0.02	0.57 ± 0.02	14
	BZ	0.59	1	0.99 ± 0.03	0.33 ± 0.04	15
	AC	0.72	1	1.01 ± 0.08	0.37 ± 0.10	21
1,4-DCNN	CH	0.68	1	0.91 ± 0.02	0.63 ± 0.04	12
	BZ	0.52	1	1.00 ± 0.04	0.63 ± 0.05	14
	AC	0.59	1	0.96 ± 0.13	0.66 ± 0.16	19
2,3-DCNN	CH^a					16
	BZ	0.59	1	0.96 ± 0.18	0.01 ± 0.01	16
	AC	0.64	1	0.96 ± 0.18	0.05 ± 0.05	22
1,4-DMN	CH					25
	BZ					30
	AC					36
PH	CH	0.63	1	0.65 ± 0.05	0	24
			0.87 ± 0.1^11	0.54,^11 0.62^11	0.02^11
	BZ	0.42	1	0.46 ± 0.02	0	27
	AC	0.38	0.96	0.47 ± 0.04	0	35
			0.96 ± 0.1,^12 0.47^18	0.50 ± 0.05^12	0.00^12
9-CNPH	CH	0.87	1	1.06 ± 0.12	0	17
	BZ	0.63	1	0.99 ± 0.13	0	21
	AC	0.67	0.85	0.93 ± 0.11	0	24
			0.85 ± 0.1,^12 0.85^18	0.80 ± 0.08^12	0.04 ± 0.04^12
PY	CH	0.96	1	0.93^d	0.06	26
			1.10 ± 0.1^11	0.86,^11 0.99^11	0.00^11
	BZ	0.91	1	0.93^e	0.02	26
	AC	0.66	0.47^d	0.79 ± 0.08^12	0.31	39
			0.49 ± 0.05,^12 0.45^18		0.30 ± 0.03^12
1-CNPY	CH	0.82	1	1.08 ± 0.23	0.08 ± 0.08	21
	BZ	0.55	1	1.00 ± 0.20	0.06 ± 0.06	21
	AC	0.59	≤1	1.00 ± 0.31	≥0.02	31

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Fig. 2 Plots to determine f_Δ^S and f_Δ^T according to eqn (6): 1,4-dicyanonaphthalene in (●) cyclohexane, (■) benzene, (▲) acetonitrile.

As shown in Table 2, values of f_Δ^T for three dicyanonaphthalenes were close to unity in all the solvents used, indicating that ¹O₂ is produced with an efficiency of about 100% in the oxygen quenching of triplet state. Wilkinson et al. measured values of f_Δ^T for several naphthalene compounds and showed that f_Δ^T increased with increasing oxidation potential. The f_Δ^T values for 1-cyanonaphthalene with high oxidation potential were 1.03 (cyclohexane), 0.75 (benzene) and 0.74 (acetonitrile).^24,25 Since the oxidation potential for DCNNs is expected to be higher than that for 1-cyanonaphthalene, the f_Δ^T values close to unity obtained for three DCNNs in all the solvents are reasonable results.

The f_Δ^S values for 1,2-DCNN and 1,4-DCNN are 0.33–0.66, indicating that ¹O₂ is produced with considerable efficiencies from the S₁ state in all the solvents. The k_q^S values of these compounds are lower by about one-half than those of 1,4-DMN which are considered as nearly diffusion-controlled rate constants (k_diff). This observation represents that k_q^S is smaller than k_diff when production of ¹O₂ is significant, in agreement with previous studies on anthracene derivatives.⁹ The f_Δ^S value of 1,4-DCNN is not decreased in acetonitrile, whereas the remarkable decrease of f_Δ^S was shown in changing solvent from cyclohexane to acetonitrile for 9-cyanoanthracene, 9,10-dicyanoanthracene and 9,10-dichloroanthracene.^9,13,14 In contrast to 1,2-DCNN and 1,4-DCNN, the f_Δ^S value of 2,3-DCNN is close to zero in both benzene and acetonitrile, whereas the k_q^S values of 2,3-DCNN are about one-half of k_diff analogously to 1,2-DCNN and 1,4-DCNN. The results obtained on f_Δ^S and k_q^S of DCNNs demonstrate that the f_Δ^S value varies largely from 0.01 to 0.66 though the value of (k_q^S/k_diff) varies slightly from 0.48 to 0.66.

Kinetics for oxygen quenching in the S₁ state and rate constants of elementary processes (k_Δ, k_ST, k_SS)

Based on the kinetics for the oxygen quenching of the S₁ state in Scheme 1, rate constants for elementary processes of (1)–(3) are derived from the values of f_Δ^S and k_q^S. By assuming the stationary state for the encounter complex, eqn (7) is obtained,⁹where k_D represents k_Δ + k_ST + k_SS and k_D can be evaluated when values of k_diff and k_-diff are known.

In this work, values of k_q^S obtained for 1,4-DMN in each solvent are used as k_diff in the corresponding solvents. Values of k_-diff are estimated using eqn (8),^3,27,28

where N is the Avogadro constant and r is the internuclear distance in the encounter complex in the unit of cm. The value of r = 5.1 × 10^–8 cm is used on the basis of molecular radii of 1.73 × 10^–8 cm for O₂ and 3.39 × 10^–8 cm for DCNNs which were calculated by the method of Bondi.²⁹ As a result, k_-diff = 3.0 × k_diff is obtained from eqn (8), and values of k_D are obtained by eqn (7). Values of k_Δ, k_ST and k_SS are obtained by eqn (9)–(11).

k_Δ = f_Δ^Sk_D (9)

k_ST = (f_T^O – f_Δ^S)k_D (10)

k_SS = (1 – f_T^O)k_D (11)

The values of k_Δ and k_ST in cyclohexane and benzene are obtained using f_T^O = 1, that is, neglecting k_SS. In acetonitrile the value of f_T^O for DCNNs may be smaller than unity and, indeed, values of f_T^O measured for several molecules in acetonitrile are in the range from 0.25 to 1.0.^12,18 The value of f_T^O is decreased with increasing charge transfer interaction and it is related to the free energy change (Δ_ETG_S) in the electron transfer from the S₁ state to O₂ in acetonitrile. The value of Δ_ETG_S is given by eqn (12),

Δ_ETG_S = F[E(M⁺˙/M) – E(O₂/O₂^–˙)] – E_S (12)

where F is the Faraday constant, and E(M⁺˙/M) and E(O₂/O₂^–˙) are the standard electrode potential of the sensitizer and of O₂ (–0.78 V),¹⁰ respectively. The value of E(M⁺˙/M) for 1-cyanonaphthalene is 2.01 V¹⁹ and, hence, the values of E(M⁺˙/M) for DCCNs are considered to be larger than 2.01 V. Then, the values of Δ_ETG_S are estimated as >–68 kJ mol^–1, >–76 kJ mol^–1 and >–80 kJ mol^–1 for 1,2-DCNN, 1,4-DCNN and 2,3-DCNN, respectively. The reported values of f_T^O are 0.85 for 9-CNPH^12,18 with Δ_ETG_S of –74 kJ mol^–1 and 1.0 for 3,9-dicyanophenanthrene¹⁸ with Δ_ETG_S of –39 kJ mol^–1, respectively. Therefore, it would be reasonable to obtain approximate values of k_Δ and k_ST by using f_T^O = 1 in acetonitrile. The values of k_Δ and k_ST in the three solvents are listed with the values of k_-diff and k_D in Table 3.

Table 3 Values of k_diff, k_-diff, k_D, k_Δ, k_ST, k_SS estimated for dicyanonaphthalenes, 9-cyanophenanthrene and 1-cyanopyrene in cyclohexane (CH), benzene (BZ) and acetonitrile (AC)

Compound	Solvent	10^–9kdiff^a/M^–1 s^–1	10^–9k-diff^b/s^–1	10^–9kD^c/s^–1	10^–9kΔ^d/s^–1	10^–9kST^e/s^–1	10^–9kSS^f/s^–1
a Values of kq^S for 1,4-DMN, PH and PY were regarded as those of kdiff for corresponding compounds in respective solvents. b Values of k-diff were estimated by eqn (8). c Eqn (7). d Eqn (9). e Eqn (10). f Eqn (11).
1,2-DCNN	CH	25	75	95	54 ± 1	41 ± 1	—
	BZ	30	90	90	30 ± 3	60 ± 3	—
	AC	36	108	151	56 ± 15	95 ± 15	—
1,4-DCNN	CH	25	75	69	43 ± 3	26 ± 3	—
	BZ	30	90	79	50 ± 4	29 ± 4	—
	AC	36	108	121	80 ± 19	41 ± 19	—
2,3-DCNN	CH	25	75	133
	BZ	30	90	103	—	102 ± 1	—
	AC	36	108	170	—	161 ± 9	—
9-CNPH	CH	24	62	150	—	150	—
	BZ	27	70	250	—	250	—
	AC	35	91	200	—	170	30
1-CNPY	CH	26	68	286	23 ± 23	263 ± 23	—
	BZ	26	68	286	17 ± 17	269 ± 17	—
	AC	39	101	391	≥8	≤380

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As shown in Table 3, the values of k_ST for 2,3-DCNN are larger than those for 1,2-DCNN and 1,4-DCNN, and the value of k_ST is increased in acetonitrile for all the DCNNs in comparison with cyclohexane and benzene. This result is attributed to the effect of charge transfer interaction on k_ST. The oxidation potential of 2,3-DCNN is considered to be lower than those of 1,2-DCNN and 1,4-DCNN, because the reduction potential of 2,3-DCNN (–1.70 V) is lower than those of 1,2-DCNN (–1.36 V) and 1,4-DCNN (–1.31 V).³⁰ Indeed, 2,3-DCNN is a weaker electron acceptor than 1,4-DCNN in the photo-induced electron transfer reaction in the S₁ state,³¹ meaning that 2,3-DCNN is a stronger electron donor than 1,4-DCNN. As a result, the value of Δ_ETG_S for 2,3-DCNN is smaller than for 1,2-DCNN and 1,4-DCNN because values of E_s are almost same and, hence, the increased charge transfer interaction leads to larger k_ST values of 2,3-DCNN, resulting in small values of f_Δ^S. The larger k_ST values in acetonitrile for all the DCNNs also are explained by increase of charge transfer interaction in a polar solvent.

In the present result, the k_Δ value is also an important determining factor for f_Δ^S. The k_Δ values of (3.0–8.0) × 10¹⁰ s^–1 for 1,2-DCNN and 1,4-DCNN are fairly large compared with values of (0.9–1.5) × 10¹⁰ s^–1 obtained for 9-cyanoanthracene in a previous paper.⁹ These large k_Δ values, together with small k_ST values, lead to considerably high values of f_Δ^S for 1,2-DCNN and 1,4-DCNN. It is also owing to the increased k_Δ value that f_Δ^S value of 1,4-DCNN is not decreased in acetonitrile regardless of the increase of k_ST. On the other hand, the small f_Δ^S values for 2,3-DCNN are attributed not only to large k_ST values but also to small k_Δ-values. If k_Δ values for 2,3-DCNN were similar to those for 1,2-DCNN and 1,4-DCNN, f_Δ^S values of 2,3-DCNN would result in significant values of 0.23–0.33 regardless of large k_ST values. In practice, however, f_Δ^S values of 2,3-DCNN are nearly zero because of the small k_Δ values. Why are k_Δ values of 2,3-DCNN smaller than those of 1,2-DCNN and 1,4-DCNN? For 2,3-DCNN, the energy difference between S₁ and T₁ is larger than 94 kJ mol^–1 and the process (1) in Scheme 1 is energetically possible. At present, there is no distinct explanation about the difference of k_Δ value between two classes of DCNNs. In the simplified mechanism of Scheme 1, k_Δ and k_ST include rate constants for all kinetic processes which follow the formation of encounter complex, such as the formation of exciplex. Therefore, the process associated with the exciplex might be a cause for the difference in the value of k_Δ.

f _Δ ^S, k_q^S, k_Δ, k_ST and k_SS for phenanthrene, pyrene and their cyano-derivatives

In relation to the present results regarding DCNNs, measurements of f_Δ^S and k_q^S were carried out for 9-CNPH and 1-CNPY in the same three solvents, together with PH and PY. The photophysical parameters are listed in Table 1. The energy gaps between S₁ and T₁ for PH and 9-CNPH are smaller than the energy of ¹O₂ (94 kJ mol^–1), and, hence, the production of ¹O₂ in process (1) of Scheme 1 is energetically impossible. For PH, 9-CNPH and 1-CNPY, values of f_Δ^T were obtained from the intercept of plot of eqn (6). The determination of f_Δ^T for PY by eqn (6) was impossible owing to the long fluorescence lifetime of PY, because the plot based on eqn (6) needs measurements at lower concentrations of oxygen but their measurements make obscure the discrimination between a fast rise and a slow rise in the TRTL signal. Therefore, the literature values were used as f_Δ^T for PY. For the determination of f_Δ^S, the value of f_T^O for the four compounds in cyclohexane and benzene was reasonably taken as unity. The literature values of f_T^O were used for PH, 9-CNPH, PY in acetonitrile, and the minimum value of f_Δ^S for 1-CNPY in acetonitorile was obtained using f_T^O = 1 since the value of f_T^O is unknown.

As seen in Table 2, the values of f_Δ^T for PH in cyclohexane and acetonitrile and for 9-CNPH in acetonitrile are nearly equal to those of Abdel et al.^11,12 The low f_Δ^T values of 0.46–0.65 for PH are attributed to both high energy of T₁ and low oxidation potential.¹ The high f_Δ^T values close to unity for 9-CNPH and 1-CNPY are attributed to the high oxidation potential, analogously to DCNNs.

Values of f_Δ^S for PH and 9-CNPH, which were obtained by eqn (6), were approximately zero, except for PH in acetonitrile, and the value of f_Δ^S for PH in acetonitrile is slightly negative for the f_T^O value of 0.96¹² and is 0.16 ± 0.02 for the f_T^O value of 0.47.¹⁸ The value of f_Δ^S obtained by Abdel et al., who determined f_T^O = 0.96, was zero.⁷ The value of 0.80 ± 0.06 as f_T^O for PH in acetonitrile is obtained from the present results when f_Δ^S = 0 is assumed in eqn (6). The f_T^O value of 0.47¹⁸ seems to be too low, as long as f_Δ^S = 0 is accepted. As a result, it is concluded that values of f_Δ^S for PH and 9-CNPH are zero in the three solvents, in agreement with the prediction from the S₁–T₁ energy gap. The f_Δ^S values were approximately zero for PY in cyclohexane and benzene, and the f_Δ^S value for PY in acetonitrile was 0.29 ± 0.03. These values are in good agreement with those of Abdel et al.^10–12 For 1-CNPY, values of f_Δ^S were close to zero in cyclohexane and benzene, and f_Δ^S ≥ 0.02 was estimated in acetonitrile. The value of f_Δ^S was 0.17 ± 0.08 when f_T^O was assumed as 0.85 equal to the value for 9-CNPH because Δ_ETG_S of 1-CNPY is considered to be similar to that of 9-CNPH.³² Taking these values of f_Δ^S into account, it is probable that the value of f_Δ^S for 1-CNPY in acetonitrile is not larger than that for PY. Analogously to DCNNs, values of k_q^S for 9-CNPH and 1-CNPY are significantly smaller than those for PH and PY which are nearly equal to k_diff. The value of f_Δ^S ≈ 0.3 for PY in acetonitrile is noticeable because this is inconsistent with a general tendency that ¹O₂ is produced in the system having a smaller k_q^S value than k_diff.

Using the values of k_q^S for PY and PH as those of k_diff, respectively, values of k_Δ, k_ST and k_SS for 9-CNPH and 1-CNPY are obtained by eqn (7)–(11) and shown in Table 3. The values of k_ST obtained for 9-CNPH and 1-CNPY are large in comparison with DCNNs, suggesting that Δ_ETG_S for these compounds is smaller than that for DCNNs. Values of f_Δ^S close to zero for 1-CNPY in cyclohexane and benzene are due to these large values of k_ST. Values of Δ_ETG_S are estimated to be –74, –116 and –128 kJ mol^–1 for 9-CNPH, PH and PY, respectively.¹² The increase of k_ST, as well as k_SS in polar solvents, by larger charge transfer interaction in comparison with 9-CNPH and 1-CNPY, results in k_q^S of a nearly diffusion-limit for PH and PY. Taking into account k_ST values of (1.5–2.7) ×10¹¹ s^–1 for 9-CNPH and 1-CNPY, it is generally predicted that values of k_ST for many compounds having large negative values of Δ_ETG_S and large k_q^S values equal to k_diff, such as PH and PY, are larger than 2 × 10 ¹¹ s^–1.

Finally, the unusual relation between f_Δ^S and k_q^S for PY in acetonitrile is mentioned because it cannot be explained only by the three processes in Scheme 1. As described above, the f_Δ^S value of PY in acetonitrile is ca. 0.3 though the k_q^S value is in a diffusion limit. A similar result is also reported for perylene in acetonitrile,^10,12i.e.f_Δ^S = 0.27 ± 0.03 and k_q^S = 3.8 × 10¹⁰ M^–1 s^–1, and the production of ¹O₂ by oxygen quenching in the S₁ state was explained from a viewpoint of the energy level of the second triplet state (T₂). However, the f_Δ^S value of PY is zero in cyclohexane and benzene. Therefore, the explanation based on the T₂ level cannot be accepted since the ordering of S₁, T₁ and T₂ levels is considered to be same in all the solvents. A different mechanism from the process (1), for example the process leading to S₀ and ¹O₂ through the charge transfer state, might exist for production of ¹O₂ in acetonitrile for sensitizers with large negative values of Δ_ETG_S.

Conclusions

The significant production of ¹O₂ in the oxygen quenching of the S₁ state was confirmed for 1,2-DCNN and 1,4-DCNN, whereas ¹O₂ was scarcely produced for 2,3-DCNN. The value of k_q^S was smaller than a diffusion-controlled rate constant (k_diff) for the three DCNNs. The value of k_ST increased with increasing charge transfer interaction. It was shown that the dependence of k_Δ on the compound and solvent, as well as those of k_ST and k_SS, is important as a determining factor for f_Δ^S and k_q^S. The unusual relation between f_Δ^S and k_q^S for PY in acetonitrile, i.e. the significant production of ¹O₂ (f_Δ^S ≈ 0.3) and a large value of k_q^S close to k_diff, was pointed out.

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