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Laser flash photolysis study of the photochemistry of 4,5-diaza-9-fluorenone
Journal of Photochemistry and Photobiology A: Chemistry 299 (2015) 166–171
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Laser flash photolysis study of the photochemistry of 4,5-diaza9-fluorenone Ada R. Bertoti a , Alexandre K. Guimarães a , José Carlos Netto-Ferreira a,b, * a b
Departamento de Química Geral e Inorgânica – Universidade Federal da Bahia, Campus de Ondina, Salvador, 40170-290 BA, Brazil Departamento de Química – Universidade Federal Rural do Rio de Janeiro, Antiga Rodovia Rio-São Paulo km 47, Seropédica, 23970-000 RJ, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 August 2014 Received in revised form 19 October 2014 Accepted 27 October 2014 Available online 24 November 2014
The triplet excited state of 4,5-diaza-9-fluorenone (1) shows absorption maxima at 410 and 470 nm and a lifetime of 3 ms, in acetonitrile. Its intersystem crossing quantum yield was determined using 9-fluorenone as a secondary standard and a value of 0.41 0.01 was obtained. The reactivity of the triplet excited state of 1 towards several quenchers, in acetonitrile, was investigated employing the laser flash photolysis technique quenching rate constants ranging from 7.9 104 M1 s1 (2-propanol) to 1.0 1010 M1 s1 (triethylamine) were obtained. From the quenching rate constants obtained one can conclude that 4,5-diaza-9-fluorenone has a pp* triplet excited state. A Hammett plot for the quenching rate constants of triplet 1 by phenols containing polar substituents against s + gave a reaction constant r of –1.54 0.10, which demonstrates the electrophilic character of the 4,5-diaza-9-fluorenone triplet excited state. ã 2014 Elsevier B.V. All rights reserved.
Keywords: 4,5-Diaza-9-fluorenone Triplet excited state Laser flash photolysis Electron transfer Hydrogen abstraction Hammett equation
1. Introduction The photochemical reactivity of the triplet excited state of aromatic ketones is one of the most studied processes in organic photochemistry not only in organic solvents but also in organized media [1–7]. There is an appreciable difference in reactivity between np* and pp* states for a given triplet carbonyl. One of more dramatic examples of this difference in reactivity is shown by xanthone. The n,p* triplet excited state of xanthone is highly reactive in non-polar media, such as alkanes. On the other hand, in polar solvents the lowest energy triplet excited state has p,p* character, and shows little tendency of hydrogen abstraction [8,9]. The introduction of a nitrogen atom in the aromatic ring of xanthone leads to 1-azaxanthone, which has been proved to be efficiently photoreduced in both polar and non-polar media [10–12]. In the case of benzophenone, the replacement of a phenyl by a pyridyl group leads to an enhancement of its reactivity toward hydrogen abstraction reactions, with the photophysics and
* Corresponding author at: Departamento de Química Geral e Inorgânica – Universidade Federal da Bahia, Campus de Ondina, Salvador, 40170-290 BA, Brazil. Tel.: +55 21 2145 3286. E-mail address: [email protected] (J.C. Netto-Ferreira). http://dx.doi.org/10.1016/j.jphotochem.2014.10.021 1010-6030/ ã 2014 Elsevier B.V. All rights reserved.
photochemistry of dipyridyl ketones depending on the position of the nitrogen atom in the aromatic ring as well as on the solvent polarity, with both effects cooperating in the determination of their excited state properties [6,13–16]. Recent work from our group showed that the laser flash photolysis of 1,4-diaza-9-fluorenone and 1,4-diaza-benz[b]-9fluorenone in acetonitrile leads to formation of the corresponding triplets, which have absorption maxima in the 400–500 nm region and lifetimes in the order of few microseconds, with energies around 50 kcal/mol. The triplet excited state of both 1,4diazafluorenones was effectively quenched by electron donors such as DABCO (kq 109 M1 s1). However, for quenchers in which the primary process corresponds to hydrogen transfer, such as 2-propanol or 1,4-cyclohexadiene, quenching rate constants of around 105 M1 s1 were measured. This behavior indicates that the photochemistry of 1,4-diaza-9-fluorenone and 1,4-diaza-benz [b]-9-fluorenone is dominated by a triplet excited state of pp* character [17] . The present work is aimed to study the reactivity of the triplet excited state of 4,5-diaza-9-fluorenone, in acetonitrile, toward several quenchers that can behave as pure hydrogen donors, such as 1,4-cyclohexadiene, 2-propanol, toluene or cyclohexane, as hydrogen donors through a proton-coupled electron transfer, such as indole or phenol and its derivatives containing polar substituents on the aromatic ring and, finally, toward electron donors,
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such as 1,4- diazabicyclo[2.2.2]octane (DABCO) and triethylamine. Triplet energy acceptors such as 1,3- cyclohexadiene, b-carotene, E- and Z-stilbenes were also used.
O
N
N
2. Materials1 and methods 2.1. Reagents The solvents acetonitrile, benzene and methanol, all spectroscopic grade, were purchased from Aldrich. 4,5-diaza-9-fluorenone (1) was purchased from Aldrich and recrystallized from methanol/water. The quenchers E- stilbene, Z-stilbene, toluene, b-carotene, 2-propanol, cyclohexane, triethylamine, 1,4-diazabicyclo[2.2.2]octane (DABCO), indole, phenol, 4-aminophenol, 4-hydroxyphenol, 4- methoxyphenol, 4-fluorphenol, 4-cyanophenol, 4-tert-butylphenol, 4-bromophenol, 4- chlorophenol, 4-phenylphenol, 3-chlorophenol, 4-methylphenol, 3-methylphenol, 3-methoxyphenol were purchased from Aldrich and used as received. 1,3-Cyclohexadiene and 1,4-cyclohexadiene (from Aldrich) were bulb-to-bulb distilled just prior to use. 2.2. Instruments UV–visible spectra were obtained on a Varian Cary 3E spectrophotometer. The laser flash photolysis experiments were conducted on a Luzchem instrument, model mLFP 122. In all experiments 10 mm 10 mm Suprasil cells containing 3.0 mL solution of the corresponding fluorenone in acetonitrile were used, with the solutions being deaerated with oxygen- free nitrogen for 20 min. The samples were irradiated with the 3rd harmonic (l = 355 nm, 5 ns, 40 mJ/pulse) of a Nd/YAG Surelite laser controlled by a Dell computer, 4700 series, which uses a Labview 4.1 (National Instruments) application. The concentration of fluorenone was chosen to obtain an absorbance of 0.3 at the excitation wavelength (355 nm). Stock solutions were prepared in the same solvent used for sample preparation, so that it was only necessary to add microliter volumes to the cell to obtain the appropriate quencher concentrations.
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as a secondary standard, for which a value of 0.48 can be found in the literature [16,18]. Thus, matched samples (absorbance at 308 of 0.225) of 9-fluorenone and 4,5-diaza-9-fluorenone were excited with a Lumonics Excimer laser TE-860-2 (l = 308 nm, 5 ns, >80 mJ/pulse) and the laser energy was attenuated by the use of neutral density filters with transmittance values of T = 10, 40, 50, 63.1, 79.4 and 100%. From the slope of the plots of DA at time zero after the laser pulse versus %T and employing the Eq. (1), the ’isc for 4,5 -diaza-9-fluorenone was obtained [18]. slope for 4; 5 diaza 9 fluorenone slope for 9 fluorenone f 4; 5diaza 9 fluorenone ¼ ces fces 9fluorenone
(1)
3. Results and discussion Laser flash photolysis (lexc = 355 nm) of a solution of 4,5-diaza9-fluorenone in acetonitrile led to the formation of a transient with absorption maxima at 410 and 470 and a shoulder at 560 nm (Fig. 1), whereas in benzene these maxima are located at 490 and 510 nm (Fig. 1S, Supporting information). In methanol, a wide band with maximum at 430 nm and a shoulder at 570 nm (Fig. 2S, Supporting information) could be observed. In acetonitrile, this transient followed either at 410 or at 470 nm has a lifetime of 3 ms and its decay shows a first-order kinetics with a strong 2nd-order contribution (Fig. 1, inset), which is usually associated with a deactivation process involving triplet-triplet annihilation [19]. It is worth noting that 4,5-diaza-9fluorenone does not show any phosphorescence either at room temperature or at 77 K. The intersystem crossing quantum yield (’isc) for 4,5-diaza-9fluorenone in acetonitrile was calculated using Eq. (1) and employing 9-fluorenone as the secondary standard (’isc = 0.48 0.03), in this same solvent [16–18]. From the slope of the plots of DA at time zero after the laser pulse versus %T (Fig. 3S, Supporting information) and employing the Eq. (1) a value of 0.41 0.01 for the ’isc of 4,5- diaza-9-fluorenone was obtained. This value is slightly lower when compared to that for 9-fluorenone and can be
2.3. Intersystem crossing quantum yield In the laser flash photolysis technique the concentration of triplets formed after the pulse can be determined at low concentrations of sample and low laser power. In this case the initial concentration of triplets (DAo) measured at zero time after the laser pulse has a linear dependence on the excitation energy (E). Thus, the intersystem crossing quantum yield for a given sample can be determined from the slope of a plot of E versus DAo which is then compared to a standard (9-fluorenone in acetonitrile, Fisc = 0.48), assuming that the difference between the molar absorptivity for the ground state and the excited triplet state (eG eT) remain constant. For this measurement, solutions of unknown and standard must have exactly the same absorbance at the excitation wavelength and the molar absorptivity for the triplet excited state of both the unknown (eT1,4-diaza-9-fluorenone) and the standard (eT9-fluorenone) must be known. However, since in this case the pair standard/unknown shows great similarity between their orbitals, one can accept as a reasonable approximation that eT9fluorenone = eT4,5-diaza-9- fluorenone [18]. The intersystem crossing quantum yield for 4,5-diaza-9fluorenone was calculated employing 9-fluorenone in acetonitrile
Fig. 1. Absorption spectrum for the transient generated upon the excitation (l = 355 nm) of 4,5-diaza-9-fluorenone in acetonitrile and recorded 1.8 ms after the laser pulse. Inset: decay for this transient monitored at 470 nm.
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explained by the presence of the two nitrogen atoms in the 4,5diaza-9-fluorenone structure, which should affect the charge density on the carbonyl group and on the carbon atoms of the aromatic ring. Similar behavior was previously reported for 1,4diaza-9-fluorenone and 1,4-diaza-9- benz[b]fluorenone [17]. To confirm the triplet character of the transient generated upon photolysis of 4,5-diaza-9-fluorenone, as well as to study its reactivity, experiments with species normally employed as quenchers for triplet excited states through energy, electron or hydrogen transfer process were performed. The decay of the transient generated by excitation (l = 355 nm) of 4,5-diaza-9fluorenone and monitored at 470 nm when in the presence of these quenchers follows a pseudo-first order rate constant as measured experimentally (kobs) following the Stern–Volmer equation (Eq. (2)) [20] and in all cases linear plots were obtained. kobs ¼ ko þ kq ½Q
(2)
where ko and kq are the decay rate constants for the triplet excited state in the absence and in the presence of a quencher Q, respectively, and [Q] is the quencher concentration in mol L1. The transient generated upon laser excitation of 4,5-diaza-9fluorenone was quenched by 1,3-cyclohexadiene (ET = 52.4 kcal mol1) [21] in acetonitrile, with a rate constant of (8.5 0.6) 109 L mol1 s1 (Fig. 4 Supporting information, Table 1) which indicates that it is in fact the triplet excited state of 4,5-diaza-9fluorenone. This value is lower than the diffusion limit in this solvent, i.e., 1010 L mol1 s1, at 25 C, according to the Smoluchowski theory [21], which indicates that the triplet energy of this diazafluorenone must be lower than that of the quencher, since an exergonic energy transfer process is expected to be diffusion controlled. The triplet character of this transient was confirmed by studies employing b-carotene (ET = 19.0 kcal mol1) [22] as the quencher. This polyene has an intersystem crossing quantum yield of zero [23] and therefore its triplet state can only be formed through an energy transfer process from an appropriate donor. Since b-carotene has a triplet-triplet absorption spectrum with maximum at 520 nm (Fig. 5, Supporting information), the quenching process for 4,5-diaza-9-fluorenone triplet can be easily monitored by following either its triplet decay, at 410 or at 470 nm, or the b-carotene triplet growth at 520 nm (Fig. 5, inset, Supporting information), as a function of the quencher concentration. In both cases, a diffusion controlled quenching rate constant (kq = 1.1 0.1 1010 L mol1 s1) was obtained (Table 1), which allowed us to confirm the nature of this transient as the triplet excited state of 4,5-diaza-9-fluorenone in acetonitrile. A quenching rate constant with a value lower than the diffusion rate constant was also obtained employing Z-stilbene (ET = 54.3 kcal mol1) [21] as quencher (kq = (5.8 0.1) 109 L mol1 s1) (Fig. 4 Supporting information, Table 1). However, for E-stilbene (ET = 49.3 kcal mol1)
Fig. 2. Absorption spectrum generated for the quenching of 4,5-diaza-9-fluorenone triplet by 7.05 103 mol L1 1,4-cyclohexadiene in acetonitrile, recorded 5.9 ms after the laser pulse.
Fig. 3. Absorption spectrum for the transient generated upon the photolysis of 4,5diaza-9-fluorenone with 2.0 103 mol L1 of triethylamine in acetonitrile, recorded 10 ms after the laser pulse.
Table 1 Second-order rate constants for the quenching of 4,5-diaza-9-fluorenone by several quenchers. Quencher 1,3-Cyclohexadiene E-Stilbene Z-Stilbene b-Carotene 1,4-Cyclohexadiene 2-Propanol Cyclohexane Toluene Triethylamine DABCO Phenol 4-Aminophenol 4-Hydroxyphenol
kq (L mol1 s1) (8.5 0.6) (1.1 0.4) (5.8 0.1) (1.1 0.3) (1.0 0.1) (7.9 0.7) (1.2 0.1) (1.2 0.1) (1.0 0.3) (6.3 0.1) (1.8 0.1) (1.3 0.1) (9.0 0.5)
9
x 10 x 1010 x 109 x 1010 x 106 x 104 x 105 x 105 x 1010 x 108 x 108 x 1010 x 1010
Quencher
kq (L mol1 s1)
4-Methoxyphenol 4-Fluorphenol 4-Cyanophenol 4-tert-Butylphenol 4-Bromophenol 4-Chlorophenol 4-Phenylphenol 3-Chlorophenol 4-Methylphenol 3-Methylphenol 3-Methoxyphenol Indole
(5.2 0.1) x 109 (3.5 0.1) x 108 (7.8 0.3) x 108 (1.1 0.1) x 109 (1.6 0.2) x 108 (2.7 0.1) x 108 (5.6 0.5) x 108 (4.9 0.1) x 107 (7.1 0.1) x 108 (5.8 0.1) x 108 (7.5 0.2) x 108 (3.0 0.3) x 109
Fig. 4. Absorption spectrum for transient generated upon photolysis of 4,5-diaza-9fluorenone in the presence of 1.84 103 mol L1 of 4-methoxyphenol in acetonitrile, recorded 5.5 ms after the laser pulse.
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to the formation of a long-lived transient with lifetime in excess of 50 ms and absorption in the 400 nm region, which can be assigned to the corresponding aryloxyl radical (Scheme 3). A representative spectrum employing 4- methoxyphenol (1.84 103 mol L1) as the hydrogen donor, recorded 5.5 ms after the laser pulse, is shown in Fig. 4, in which the absorption maximum for the 4methoxyphenoxyl radical is centered at 405 nm, fully in accordance with literature data [27,28]. It is worth noting that this absorption can have some contribution of the ketyl radical derived from the diazafluorenone since, as described above, this species also has absorption in this region of the spectrum. For the phenols containing polar substituents, hydrogen abstraction rate constants ranging from (4.9 0.1) 107 L mol1 s1, for 3-chlorophenol, to (1.3 0, 5) 1010 L mol1 s1, for 4-aminophenol, were obtained, as shown in Table 1. Representative plots for the quenching of 4,5diaza-9-fluorenone by 4-methoxy-; 3-methoxy-; 4-chloro- and 3chlorophenol are shown in Fig. 8S, Supporting information. Phenols react with carbonyl triplets faster than alcohols, alkanes and even 1,4-cyclohexadiene, a very efficient hydrogen donor [29], with this enhanced reactivity being attributed to the low bond dissociation energy for O—H and the ease of oxidation of phenols [30–34]. A linear Hammett plot (Fig. 5) employing the phenolic hydrogen abstraction rate constants listed in Table 1 and analyzed using Eq. (3) [35,36] resulted in a value for the reaction constant r of 1.54 0.10 (r = 0.976), similar to those obtained for other aromatic monoketones (acenaphthenone [34] and thiochromanone [37]) or a-diketones (acenaphthenoquinone [34], 1,2aceanthrylenedione [33] and 1,2-diketopyracene [32]).
Fig. 5. Hammett plot of phenolic hydrogen abstraction rate constants (kq) by 4,5diaza-9-fluorenone triplet against s +.
[21] the quenching rate constant approaches the diffusion limit (kq = (1.1 0.4) 1010 L mol1 s1) (Fig. 4 Supporting information, Table 1). From this set of data on the kinetics of the energy transfer process we can conclude that the triplet excited state energy of 4,5diaza-9-fluorenone is located between 49.3 and 52.4 kcal mol1. It is well known that several parameters influence the rate constant for hydrogen abstraction in photoreduction reactions, namely the triplet state energy, the nature of excited state (if np* or pp*), the C—H bonding energy of the hydrogen atom being transferred, the number of equivalent atoms available for the reaction, the solvent and the reduction potential of the ground state ketone. The low values for the quenching rate constant of 4,5diaza-9-fluorenone triplet by 1,4-cyclohexadiene (kq = (1.1 0.1) 106 L mol1 s1) (Fig. 6 Supporting information, Table 1) and other hydrogen donors, such as 2-propanol, cyclohexane and toluene (Table 1), seem to indicate that this triplet excited state has pp* character. A comparison of the quenching rate constants for 4,5-diaza-9-fluorenone triplet by these hydrogen donors with those obtained for other fluorenones that are known to have p p* triplet configuration, such as 9-fluorenone [24–26], 1,4-diaza-9fluorenone [17] and 1,4-diaza-9-benz[b]fluorenone [17] clearly shows that these values are very close together, which confirms the pp* character of the triplet excited state of 4,5-diaza-9fluorenone. With 1,4- cyclohexadiene the quenching process leads to the formation of a new transient which was assigned to the ketyl radical derived from the diazafluorenone, which exhibits absorption maximum at 390 nm (Scheme 1) (Fig. 2). When electron donors such as triethylamine (kq = (1.0 0.3) 1010 L mol1 s1) and DABCO (kq = (6.3 0.1) 108 L mol1 s1) (Fig. 7S, Supporting information, Table 1) were employed as quenchers of the 4,5-diaza-9-fluorenone triplet, the formation of a new transient was also observed (Fig. 3). This species has absorption maximum at 560 nm and was assigned to the radical anion derived from this fluorenone, following Scheme 2. Laser photolysis of 4,5-diaza-9-fluorenone in the presence of phenol and its derivatives containing polar substituents also leads
O
N
H
N
+
169
X
kq log H ¼ s þ r kq
(3)
where kqX is the quenching rate constant for a substituted phenol, kqH is the quenching rate constant for phenol, s + is a Hammett constant, which depends solely on the nature and position of the substituent, and r is the reaction constant, a function of the reaction under investigation and the conditions under which it takes place. The Hammett equation expresses a general quantitative relation between the nature of a given substituent and the reactivity of the reaction center, with the validity of Eq. (2) being restricted to substituents in the meta- and para-positions of the benzene ring. The reaction constant (r) measures the susceptibility of the reaction to the influence of the substituent. Based on r values, the transition state characteristic with respect to a developing charge can be obtained. For r > 0, a negative charge is built in the transition state, whereas for r < 0 the reaction develops a positive charge in the transition state. The negative value for the Hammett reaction constant r in the phenolic hydrogen abstraction by 4,5-diaza-9-fluorenone is in accordance with the proposed mechanism for this type of reaction, in which a positive charge on the phenol is developed, as shown in Scheme 3. In this mechanism, an initial electron transfer from the phenol to the excited ketone through an intermediate exciplex is proposed, which is followed by an ultrafast proton transfer, ultimately resulting in the formation of the radical pair ketyl/phenoxyl [32–34,37–42].
OH
H
.
hν ACN Scheme 1.
H
N
+ N
.
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O
N
O
..N N ..
+
N
_
.
hν ACN
+
N
N
N
+. N
Scheme 2.
*3
O
H
hν
+ N
.
ACN
N
*3
δ+ O
OH
O
δ-
N
. N
exciplex e- transfer OH
O
.
N
N
+
.
O
ultrafast proton transfer
_
.
+ OH
.
N
+ N
Scheme 3.
A similar mechanism can also be applied to the hydrogen abstraction by the triplet excited state of 4,5-diaza-9-fluorenone from indole (kq = (3.0 0.3) 109 L mol1 s1). The transient absorption spectrum recorded upon laser excitation of an acetonitrile solution of 1 with 2.5 103 mol L1 indole is shown in Fig. 6 and formation of the indolyl radical is clearly observed at
520 nm [43], besides an intense absorption at 400 nm that can be associated to the ketyl radical derived from 1, as described above. 4. Conclusion In conclusion, studies by laser flash photolysis indicated that excitation of 4,5-diaza-9-fluorenone in acetonitrile leads to the formation of its triplet excited state (triplet energy between 49.3 and 52.4 kcal/mol) with an intersystem crossing quantum yield of 0.41. The 4,5-diaza-9-fluorenone triplet was efficiently quenched by electron donors such as triethylamine and DABCO (kq > 108 L mol1 s1). For quenchers where the primary process corresponds to hydrogen transfer such as 1,4-cyclohexadiene, cyclohexane, 2-propanol and toluene low quenching rate constants (kq 105 L mol1 s1) were obtained. This is a clear indication that the photochemistry of 4,5-diaza-9-fluorenone is dominated by a triplet excited state showing pp* character. A Hammett plot for the quenching of the 4,5-diaza-9-fluorenone triplet by phenols containing polar substituents led to a reaction constant r of 1.54 0.10, with this negative value for r being consistent with the electrophilic character of the 4,5-diaza-9fluorenone triplet. Acknowledgements
Fig. 6. Absorption spectrum for transient generated upon photolysis of 4,5-diaza-9fluorenone in the presence of 2.5 103 mol L1 indole in acetonitrile, recorded 5.0 ms after the laser pulse.
The authors very much appreciate grants from Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB),Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP). A.R. Bertoti thanks CNPq for
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a graduate fellowship. J.C. Netto-Ferreira thanks CNPq for a Visiting Professor fellowship at Universidade Federal da Bahia (Brazil). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2014.10.021. References [1] J.C. Scaiano, Intermolecular photoreduction of ketones, J. Photochem. 2 (1973/ 74) 81–118. [2] P.J. Wagner, B.-S. Park, Photoinduced hydrogen atom abstraction by carbonyl compounds, Org. Photochem. 11 (1991) 227–366. [3] P.J. Wagner, Chemistry of excited triplet organic carbonyl compounds, Topics Curr. Chem. 66 (1976) 1–52. [4] N.W.A. Geraghty, M.T. Lohan, CRC Handbook of Organic Photochemistry and Photobiology, in: F. Ghetti, A.G. Griesbeck, M. Oelgemöller (Eds.), 3rd ed., CRC Press, Boca Raton, FL, 2012, pp. 445–448. [5] N.J. Turro, Photochemistry of ketones adsorbed on porous silica, Tetrahedron 43 (1987) 1589. [6] N.J. Turro, Photochemistry of organic molecules in microscopic reactors, Pure Appl. Chem. 58 (1986) 1219–1228. [7] C. Bohne, M.C. Barra, R. Boch, E. Abuin, J.C. Scaiano, Excited triplet states as probes in organized systems. An overview of recent results, J. Photochem. Photobiol. A: Chem. 65 (1992) 249–265. [8] A. Garner, F. Wilkinson, Laser photolysis studies of triplet-state of xanthone and its ketyl radical in fluid solution, J. Chem. Soc. Faraday Trans. 2 (72) (1976) 1010–1020. [9] J.C. Scaiano, Solvent effect in the photochemistry of xanthone, J. Am. Chem. Soc. 102 (1980) 7747–7753. [10] C. Coenjarts, J.C. Scaiano, Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes, J. Am. Chem. Soc. 122 (2000) 3635–3641. [11] L.J. Martinez, J.C. Scaiano, The photochemistry of 1-azaxanthone in aqueous solutions and in micellar environments, J. Phys. Chem. A 103 (1999) 203–208. [12] J.C. Scaiano, D. Weldon, C.N. Pliva, L.J. Martínez, Photochemistry and photophysics of 1-azaxanthone in organic solvents, J. Phys. Chem. A 102 (1998) 6898–6903. [13] F. Elisei, G. Favaro, F. Ortica, Effects of protolytic interactions on the photophysics of phenyl pyridyl ketones, J. Chem. Soc. Faraday Trans. 90 (1994) 279–285. [14] A. Romani, F. Elisei, F. Masetti, G. Favaro, pH-induced effects on the photophysics of dipyridyl ketones, J. Chem. Soc. Faraday Trans. 88 (1992) 2147–2154. [15] G. Favaro, F. Masetti, A. Romani, Effect of the nitrogen position on the excited state properties of the six isomeric di-pyridyl ketones, J. Photochem. Photobiol. A: Chem 53 (1990) 41–49. [16] F. Elisei, G. Favaro, A. Romani, A laser flash photolysis study of di-pyridyl ketones, Chem. Phys. 144 (1990) 107–115. [17] A.A.C. Takaizumi, F.R. Santos, M.T. Silva, J.C. Netto-Ferreira, The reactivity of the triplet excited state of 1,4-diaza-9-fluorenones towards hydrogen and electron donors, Quim. Nova 32 (2009) 1799–1804. [18] J. Andrews, A. Deroulede, H. Linschitz, Photophysical processes in fluorenones, J. Phys. Chem. 82 (1978) 2304–2309. [19] N.J. Turro, V. Ramamurthy, J.C. Scaiano, Principles of Molecular Photochemistry, University Science Books, Sausalito, California, 2009. [20] O. Stern, M. Volmer, The fading time of fluorescence, Physik Z. 20 (1919) 183–188. [21] S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, Marcel Dekker Inc, New York, 1993. [22] R. Bensasson, E.J. Land, B. Mavdinas, Triplet-states of carotenoids from photosynthetic bacteria studied by nanosecond ultraviolet and electron pulse irradiation, Photochem. Photobiol. 23 (1976) 189–193.
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[23] C.V. Kumar, S. Chattopadhay, P.K. Das, Triplet excitation transfer to carotenoids from biradical intermediates in Norrish type-II photoreactions of ortho-alkylsubstituted aromatic carbonyl compounds, J. Am. Chem. Soc. 105 (1983) 5143–5144. [24] R.S. Murphy, C.P. Moorlag, W.H. Green, C. Bohne, Photophysical characterization of fluorenone derivatives, J. Photochem. Photobiol. A: Chem. 110 (1997) 123–129. [25] S.V. Jovanovic, D.G. Morris, C.N. Pliva, J.C. Scaiano, Laser flash photolysis of dinaphthyl ketones, J. Photochem. Photobiol. A: Chem. 107 (1997) 153–158. [26] L. Biczók, T. Bérces, Temperature dependence of the rates of photophysical processes of fluorenone, J. Phys. Chem. 92 (1988) 3842–3845. [27] P.K. Das, M.V. Encinas, S. Steenken, J.C. Scaiano, Reaction of tert-butoxy radicals with phenols – comparison with the reaction of carbonyl triplets, J. Am. Chem. Soc. 103 (1981) 4162–4166. [28] D. Shukla, N.P. Schepp, N. Mathivan, L.J. Johnston, Generation and spectroscopic and kinetic characterization of methoxy-substituted phenoxyl radicals in solution and on paper, Can. J. Chem. 75 (1997) 1820–1829. [29] M.V. Encinas, J.C. Scaiano, Reaction of benzophenone triplets with allylic hydrogens – a laser flash photolysis study, J. Am. Chem. Soc. 103 (1981) 6393–6397. [30] P.K. Das, M.V. Encinas, J.C. Scaiano, Photolysis study of the reactions of carbonyl triplets with phenols and photochemistry of p-hydroxypropiophenone, J. Am. Chem. Soc. 103 (1981) 4154–4162. [31] N.C. de Lucas, R.J. Correa, A.C.C. Albuquerque, C.L. Firme, S.J. Garden, A.R. Bertoti, J.C. Netto-Ferreira, Laser flash photolysis of 1.2-diketopyracene and a theoretical study of the phenolic hydrogen abstraction by the triplet state of cyclic alpha-diketones, J. Phys. Chem. A 111 (2007) 1117–1122. [32] N.C. de Lucas, H.S. Fraga, C.P. Cardoso, R.J. Correa, S.J. Garden, J.C. NettoFerreira, A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet a-naphthoflavone, Phys. Chem. Chem. Phys. 12 (2010) 10746–10753. [33] A.C. Serra, N.C. de Lucas, J.C. Netto- Ferreira, Laser flash photolysis study of the phenolic hydrogen abstraction by 1,2-aceanthrylenedione triplet, J. Braz. Chem. Soc. 15 (2004) 481–486. [34] N.C. de Lucas, J.C. Netto- Ferreira, Laser flash photolysis study of the hydrogen abstraction properties of acenaphtenequinone and 1-acenaphthenone, J. Photochem. Photobiol. A: Chem. 116 (1998) 203–208. [35] L.P. Hammett, The effect of structure upon the reactions of organic compounds benzene derivatives, J. Am. Chem. Soc. 59 (1937) 96–103. [36] L.P. Hammett, Linear free energy relationships in rate and equilibrium phenomena, Trans. Faraday Soc. 34 (1938) 156–164. [37] A.M. Ribeiro, A.R. Bertoti, J.C. Netto- Ferreira, Phenolic hydrogen abstraction by the triplet excited state of thiochromanone: a laser flash photolysis study, J. Braz. Chem. Soc. 21 (2010) 1071–1076. [38] M.T. Silva, J.C. Netto- Ferreira, Laser flash photolysis study of the photochemistry of isatin and N-methylisatin, J. Photochem. Photobiol. A: Chem. 162 (2004) 225–229. [39] J.C. Netto-Ferreira, B. Bernardes, A.B.B. Ferreira, M.A. Miranda, Laser flash photolysis study of the triplet reactivity of beta-lapachones, Photochem. Photobiol. Sci. 7 (2008) 467–473. [40] J.C. Netto-Ferreira, V. Lhiaubet- Vallet, B. Bernardes, A.B.B. Ferreira, M.A. Miranda, Characterization, reactivity and photosensitizing properties of the triplet excited state of alpha-lapachone, Phys. Chem. Chem. Phys. 10 (2008) 6645–6652. [41] J.C. Netto-Ferreira, V. Lhiaubet- Vallet, B. Bernardes, A.B.B. Ferreira, M.A. Miranda, Photosensitizing properties of triplet beta-lapachones in acetonitrile solution, Photochem. Photobiol. 85 (2009) 153–159. [42] N.C. de Lucas, M.M. Elias, C.L. Firme, R.J. Correa, S.J. Garden, J.C. Netto-Ferreira, D.E. Nicodem, A combined laser flash photolysis density functional theory and atoms in molecules study of the photochemical hydrogen abstraction by pyrene-4,5-dione, J. Photochem. Photobiol. A: Chem. 201 (2009) 1–7. [43] S.V. Jovanovic, S. Steenken, Substituent effects on the spectral, acid-base, and redox properties of indolyl radicals – a pulse radiolysis study, J. Phys. Chem. 96 (1992) 6674–6679.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Laser flash photolysis study of the photochemistry of 4,5-diaza9-fluorenone Ada R. Bertoti a , Alexandre K. Guimarães a , José Carlos Netto-Ferreira a,b, * a b
Departamento de Química Geral e Inorgânica – Universidade Federal da Bahia, Campus de Ondina, Salvador, 40170-290 BA, Brazil Departamento de Química – Universidade Federal Rural do Rio de Janeiro, Antiga Rodovia Rio-São Paulo km 47, Seropédica, 23970-000 RJ, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 August 2014 Received in revised form 19 October 2014 Accepted 27 October 2014 Available online 24 November 2014
The triplet excited state of 4,5-diaza-9-fluorenone (1) shows absorption maxima at 410 and 470 nm and a lifetime of 3 ms, in acetonitrile. Its intersystem crossing quantum yield was determined using 9-fluorenone as a secondary standard and a value of 0.41 0.01 was obtained. The reactivity of the triplet excited state of 1 towards several quenchers, in acetonitrile, was investigated employing the laser flash photolysis technique quenching rate constants ranging from 7.9 104 M1 s1 (2-propanol) to 1.0 1010 M1 s1 (triethylamine) were obtained. From the quenching rate constants obtained one can conclude that 4,5-diaza-9-fluorenone has a pp* triplet excited state. A Hammett plot for the quenching rate constants of triplet 1 by phenols containing polar substituents against s + gave a reaction constant r of –1.54 0.10, which demonstrates the electrophilic character of the 4,5-diaza-9-fluorenone triplet excited state. ã 2014 Elsevier B.V. All rights reserved.
Keywords: 4,5-Diaza-9-fluorenone Triplet excited state Laser flash photolysis Electron transfer Hydrogen abstraction Hammett equation
1. Introduction The photochemical reactivity of the triplet excited state of aromatic ketones is one of the most studied processes in organic photochemistry not only in organic solvents but also in organized media [1–7]. There is an appreciable difference in reactivity between np* and pp* states for a given triplet carbonyl. One of more dramatic examples of this difference in reactivity is shown by xanthone. The n,p* triplet excited state of xanthone is highly reactive in non-polar media, such as alkanes. On the other hand, in polar solvents the lowest energy triplet excited state has p,p* character, and shows little tendency of hydrogen abstraction [8,9]. The introduction of a nitrogen atom in the aromatic ring of xanthone leads to 1-azaxanthone, which has been proved to be efficiently photoreduced in both polar and non-polar media [10–12]. In the case of benzophenone, the replacement of a phenyl by a pyridyl group leads to an enhancement of its reactivity toward hydrogen abstraction reactions, with the photophysics and
* Corresponding author at: Departamento de Química Geral e Inorgânica – Universidade Federal da Bahia, Campus de Ondina, Salvador, 40170-290 BA, Brazil. Tel.: +55 21 2145 3286. E-mail address: [email protected] (J.C. Netto-Ferreira). http://dx.doi.org/10.1016/j.jphotochem.2014.10.021 1010-6030/ ã 2014 Elsevier B.V. All rights reserved.
photochemistry of dipyridyl ketones depending on the position of the nitrogen atom in the aromatic ring as well as on the solvent polarity, with both effects cooperating in the determination of their excited state properties [6,13–16]. Recent work from our group showed that the laser flash photolysis of 1,4-diaza-9-fluorenone and 1,4-diaza-benz[b]-9fluorenone in acetonitrile leads to formation of the corresponding triplets, which have absorption maxima in the 400–500 nm region and lifetimes in the order of few microseconds, with energies around 50 kcal/mol. The triplet excited state of both 1,4diazafluorenones was effectively quenched by electron donors such as DABCO (kq 109 M1 s1). However, for quenchers in which the primary process corresponds to hydrogen transfer, such as 2-propanol or 1,4-cyclohexadiene, quenching rate constants of around 105 M1 s1 were measured. This behavior indicates that the photochemistry of 1,4-diaza-9-fluorenone and 1,4-diaza-benz [b]-9-fluorenone is dominated by a triplet excited state of pp* character [17] . The present work is aimed to study the reactivity of the triplet excited state of 4,5-diaza-9-fluorenone, in acetonitrile, toward several quenchers that can behave as pure hydrogen donors, such as 1,4-cyclohexadiene, 2-propanol, toluene or cyclohexane, as hydrogen donors through a proton-coupled electron transfer, such as indole or phenol and its derivatives containing polar substituents on the aromatic ring and, finally, toward electron donors,
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such as 1,4- diazabicyclo[2.2.2]octane (DABCO) and triethylamine. Triplet energy acceptors such as 1,3- cyclohexadiene, b-carotene, E- and Z-stilbenes were also used.
O
N
N
2. Materials1 and methods 2.1. Reagents The solvents acetonitrile, benzene and methanol, all spectroscopic grade, were purchased from Aldrich. 4,5-diaza-9-fluorenone (1) was purchased from Aldrich and recrystallized from methanol/water. The quenchers E- stilbene, Z-stilbene, toluene, b-carotene, 2-propanol, cyclohexane, triethylamine, 1,4-diazabicyclo[2.2.2]octane (DABCO), indole, phenol, 4-aminophenol, 4-hydroxyphenol, 4- methoxyphenol, 4-fluorphenol, 4-cyanophenol, 4-tert-butylphenol, 4-bromophenol, 4- chlorophenol, 4-phenylphenol, 3-chlorophenol, 4-methylphenol, 3-methylphenol, 3-methoxyphenol were purchased from Aldrich and used as received. 1,3-Cyclohexadiene and 1,4-cyclohexadiene (from Aldrich) were bulb-to-bulb distilled just prior to use. 2.2. Instruments UV–visible spectra were obtained on a Varian Cary 3E spectrophotometer. The laser flash photolysis experiments were conducted on a Luzchem instrument, model mLFP 122. In all experiments 10 mm 10 mm Suprasil cells containing 3.0 mL solution of the corresponding fluorenone in acetonitrile were used, with the solutions being deaerated with oxygen- free nitrogen for 20 min. The samples were irradiated with the 3rd harmonic (l = 355 nm, 5 ns, 40 mJ/pulse) of a Nd/YAG Surelite laser controlled by a Dell computer, 4700 series, which uses a Labview 4.1 (National Instruments) application. The concentration of fluorenone was chosen to obtain an absorbance of 0.3 at the excitation wavelength (355 nm). Stock solutions were prepared in the same solvent used for sample preparation, so that it was only necessary to add microliter volumes to the cell to obtain the appropriate quencher concentrations.
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as a secondary standard, for which a value of 0.48 can be found in the literature [16,18]. Thus, matched samples (absorbance at 308 of 0.225) of 9-fluorenone and 4,5-diaza-9-fluorenone were excited with a Lumonics Excimer laser TE-860-2 (l = 308 nm, 5 ns, >80 mJ/pulse) and the laser energy was attenuated by the use of neutral density filters with transmittance values of T = 10, 40, 50, 63.1, 79.4 and 100%. From the slope of the plots of DA at time zero after the laser pulse versus %T and employing the Eq. (1), the ’isc for 4,5 -diaza-9-fluorenone was obtained [18]. slope for 4; 5 diaza 9 fluorenone slope for 9 fluorenone f 4; 5diaza 9 fluorenone ¼ ces fces 9fluorenone
(1)
3. Results and discussion Laser flash photolysis (lexc = 355 nm) of a solution of 4,5-diaza9-fluorenone in acetonitrile led to the formation of a transient with absorption maxima at 410 and 470 and a shoulder at 560 nm (Fig. 1), whereas in benzene these maxima are located at 490 and 510 nm (Fig. 1S, Supporting information). In methanol, a wide band with maximum at 430 nm and a shoulder at 570 nm (Fig. 2S, Supporting information) could be observed. In acetonitrile, this transient followed either at 410 or at 470 nm has a lifetime of 3 ms and its decay shows a first-order kinetics with a strong 2nd-order contribution (Fig. 1, inset), which is usually associated with a deactivation process involving triplet-triplet annihilation [19]. It is worth noting that 4,5-diaza-9fluorenone does not show any phosphorescence either at room temperature or at 77 K. The intersystem crossing quantum yield (’isc) for 4,5-diaza-9fluorenone in acetonitrile was calculated using Eq. (1) and employing 9-fluorenone as the secondary standard (’isc = 0.48 0.03), in this same solvent [16–18]. From the slope of the plots of DA at time zero after the laser pulse versus %T (Fig. 3S, Supporting information) and employing the Eq. (1) a value of 0.41 0.01 for the ’isc of 4,5- diaza-9-fluorenone was obtained. This value is slightly lower when compared to that for 9-fluorenone and can be
2.3. Intersystem crossing quantum yield In the laser flash photolysis technique the concentration of triplets formed after the pulse can be determined at low concentrations of sample and low laser power. In this case the initial concentration of triplets (DAo) measured at zero time after the laser pulse has a linear dependence on the excitation energy (E). Thus, the intersystem crossing quantum yield for a given sample can be determined from the slope of a plot of E versus DAo which is then compared to a standard (9-fluorenone in acetonitrile, Fisc = 0.48), assuming that the difference between the molar absorptivity for the ground state and the excited triplet state (eG eT) remain constant. For this measurement, solutions of unknown and standard must have exactly the same absorbance at the excitation wavelength and the molar absorptivity for the triplet excited state of both the unknown (eT1,4-diaza-9-fluorenone) and the standard (eT9-fluorenone) must be known. However, since in this case the pair standard/unknown shows great similarity between their orbitals, one can accept as a reasonable approximation that eT9fluorenone = eT4,5-diaza-9- fluorenone [18]. The intersystem crossing quantum yield for 4,5-diaza-9fluorenone was calculated employing 9-fluorenone in acetonitrile
Fig. 1. Absorption spectrum for the transient generated upon the excitation (l = 355 nm) of 4,5-diaza-9-fluorenone in acetonitrile and recorded 1.8 ms after the laser pulse. Inset: decay for this transient monitored at 470 nm.
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explained by the presence of the two nitrogen atoms in the 4,5diaza-9-fluorenone structure, which should affect the charge density on the carbonyl group and on the carbon atoms of the aromatic ring. Similar behavior was previously reported for 1,4diaza-9-fluorenone and 1,4-diaza-9- benz[b]fluorenone [17]. To confirm the triplet character of the transient generated upon photolysis of 4,5-diaza-9-fluorenone, as well as to study its reactivity, experiments with species normally employed as quenchers for triplet excited states through energy, electron or hydrogen transfer process were performed. The decay of the transient generated by excitation (l = 355 nm) of 4,5-diaza-9fluorenone and monitored at 470 nm when in the presence of these quenchers follows a pseudo-first order rate constant as measured experimentally (kobs) following the Stern–Volmer equation (Eq. (2)) [20] and in all cases linear plots were obtained. kobs ¼ ko þ kq ½Q
(2)
where ko and kq are the decay rate constants for the triplet excited state in the absence and in the presence of a quencher Q, respectively, and [Q] is the quencher concentration in mol L1. The transient generated upon laser excitation of 4,5-diaza-9fluorenone was quenched by 1,3-cyclohexadiene (ET = 52.4 kcal mol1) [21] in acetonitrile, with a rate constant of (8.5 0.6) 109 L mol1 s1 (Fig. 4 Supporting information, Table 1) which indicates that it is in fact the triplet excited state of 4,5-diaza-9fluorenone. This value is lower than the diffusion limit in this solvent, i.e., 1010 L mol1 s1, at 25 C, according to the Smoluchowski theory [21], which indicates that the triplet energy of this diazafluorenone must be lower than that of the quencher, since an exergonic energy transfer process is expected to be diffusion controlled. The triplet character of this transient was confirmed by studies employing b-carotene (ET = 19.0 kcal mol1) [22] as the quencher. This polyene has an intersystem crossing quantum yield of zero [23] and therefore its triplet state can only be formed through an energy transfer process from an appropriate donor. Since b-carotene has a triplet-triplet absorption spectrum with maximum at 520 nm (Fig. 5, Supporting information), the quenching process for 4,5-diaza-9-fluorenone triplet can be easily monitored by following either its triplet decay, at 410 or at 470 nm, or the b-carotene triplet growth at 520 nm (Fig. 5, inset, Supporting information), as a function of the quencher concentration. In both cases, a diffusion controlled quenching rate constant (kq = 1.1 0.1 1010 L mol1 s1) was obtained (Table 1), which allowed us to confirm the nature of this transient as the triplet excited state of 4,5-diaza-9-fluorenone in acetonitrile. A quenching rate constant with a value lower than the diffusion rate constant was also obtained employing Z-stilbene (ET = 54.3 kcal mol1) [21] as quencher (kq = (5.8 0.1) 109 L mol1 s1) (Fig. 4 Supporting information, Table 1). However, for E-stilbene (ET = 49.3 kcal mol1)
Fig. 2. Absorption spectrum generated for the quenching of 4,5-diaza-9-fluorenone triplet by 7.05 103 mol L1 1,4-cyclohexadiene in acetonitrile, recorded 5.9 ms after the laser pulse.
Fig. 3. Absorption spectrum for the transient generated upon the photolysis of 4,5diaza-9-fluorenone with 2.0 103 mol L1 of triethylamine in acetonitrile, recorded 10 ms after the laser pulse.
Table 1 Second-order rate constants for the quenching of 4,5-diaza-9-fluorenone by several quenchers. Quencher 1,3-Cyclohexadiene E-Stilbene Z-Stilbene b-Carotene 1,4-Cyclohexadiene 2-Propanol Cyclohexane Toluene Triethylamine DABCO Phenol 4-Aminophenol 4-Hydroxyphenol
kq (L mol1 s1) (8.5 0.6) (1.1 0.4) (5.8 0.1) (1.1 0.3) (1.0 0.1) (7.9 0.7) (1.2 0.1) (1.2 0.1) (1.0 0.3) (6.3 0.1) (1.8 0.1) (1.3 0.1) (9.0 0.5)
9
x 10 x 1010 x 109 x 1010 x 106 x 104 x 105 x 105 x 1010 x 108 x 108 x 1010 x 1010
Quencher
kq (L mol1 s1)
4-Methoxyphenol 4-Fluorphenol 4-Cyanophenol 4-tert-Butylphenol 4-Bromophenol 4-Chlorophenol 4-Phenylphenol 3-Chlorophenol 4-Methylphenol 3-Methylphenol 3-Methoxyphenol Indole
(5.2 0.1) x 109 (3.5 0.1) x 108 (7.8 0.3) x 108 (1.1 0.1) x 109 (1.6 0.2) x 108 (2.7 0.1) x 108 (5.6 0.5) x 108 (4.9 0.1) x 107 (7.1 0.1) x 108 (5.8 0.1) x 108 (7.5 0.2) x 108 (3.0 0.3) x 109
Fig. 4. Absorption spectrum for transient generated upon photolysis of 4,5-diaza-9fluorenone in the presence of 1.84 103 mol L1 of 4-methoxyphenol in acetonitrile, recorded 5.5 ms after the laser pulse.
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to the formation of a long-lived transient with lifetime in excess of 50 ms and absorption in the 400 nm region, which can be assigned to the corresponding aryloxyl radical (Scheme 3). A representative spectrum employing 4- methoxyphenol (1.84 103 mol L1) as the hydrogen donor, recorded 5.5 ms after the laser pulse, is shown in Fig. 4, in which the absorption maximum for the 4methoxyphenoxyl radical is centered at 405 nm, fully in accordance with literature data [27,28]. It is worth noting that this absorption can have some contribution of the ketyl radical derived from the diazafluorenone since, as described above, this species also has absorption in this region of the spectrum. For the phenols containing polar substituents, hydrogen abstraction rate constants ranging from (4.9 0.1) 107 L mol1 s1, for 3-chlorophenol, to (1.3 0, 5) 1010 L mol1 s1, for 4-aminophenol, were obtained, as shown in Table 1. Representative plots for the quenching of 4,5diaza-9-fluorenone by 4-methoxy-; 3-methoxy-; 4-chloro- and 3chlorophenol are shown in Fig. 8S, Supporting information. Phenols react with carbonyl triplets faster than alcohols, alkanes and even 1,4-cyclohexadiene, a very efficient hydrogen donor [29], with this enhanced reactivity being attributed to the low bond dissociation energy for O—H and the ease of oxidation of phenols [30–34]. A linear Hammett plot (Fig. 5) employing the phenolic hydrogen abstraction rate constants listed in Table 1 and analyzed using Eq. (3) [35,36] resulted in a value for the reaction constant r of 1.54 0.10 (r = 0.976), similar to those obtained for other aromatic monoketones (acenaphthenone [34] and thiochromanone [37]) or a-diketones (acenaphthenoquinone [34], 1,2aceanthrylenedione [33] and 1,2-diketopyracene [32]).
Fig. 5. Hammett plot of phenolic hydrogen abstraction rate constants (kq) by 4,5diaza-9-fluorenone triplet against s +.
[21] the quenching rate constant approaches the diffusion limit (kq = (1.1 0.4) 1010 L mol1 s1) (Fig. 4 Supporting information, Table 1). From this set of data on the kinetics of the energy transfer process we can conclude that the triplet excited state energy of 4,5diaza-9-fluorenone is located between 49.3 and 52.4 kcal mol1. It is well known that several parameters influence the rate constant for hydrogen abstraction in photoreduction reactions, namely the triplet state energy, the nature of excited state (if np* or pp*), the C—H bonding energy of the hydrogen atom being transferred, the number of equivalent atoms available for the reaction, the solvent and the reduction potential of the ground state ketone. The low values for the quenching rate constant of 4,5diaza-9-fluorenone triplet by 1,4-cyclohexadiene (kq = (1.1 0.1) 106 L mol1 s1) (Fig. 6 Supporting information, Table 1) and other hydrogen donors, such as 2-propanol, cyclohexane and toluene (Table 1), seem to indicate that this triplet excited state has pp* character. A comparison of the quenching rate constants for 4,5-diaza-9-fluorenone triplet by these hydrogen donors with those obtained for other fluorenones that are known to have p p* triplet configuration, such as 9-fluorenone [24–26], 1,4-diaza-9fluorenone [17] and 1,4-diaza-9-benz[b]fluorenone [17] clearly shows that these values are very close together, which confirms the pp* character of the triplet excited state of 4,5-diaza-9fluorenone. With 1,4- cyclohexadiene the quenching process leads to the formation of a new transient which was assigned to the ketyl radical derived from the diazafluorenone, which exhibits absorption maximum at 390 nm (Scheme 1) (Fig. 2). When electron donors such as triethylamine (kq = (1.0 0.3) 1010 L mol1 s1) and DABCO (kq = (6.3 0.1) 108 L mol1 s1) (Fig. 7S, Supporting information, Table 1) were employed as quenchers of the 4,5-diaza-9-fluorenone triplet, the formation of a new transient was also observed (Fig. 3). This species has absorption maximum at 560 nm and was assigned to the radical anion derived from this fluorenone, following Scheme 2. Laser photolysis of 4,5-diaza-9-fluorenone in the presence of phenol and its derivatives containing polar substituents also leads
O
N
H
N
+
169
X
kq log H ¼ s þ r kq
(3)
where kqX is the quenching rate constant for a substituted phenol, kqH is the quenching rate constant for phenol, s + is a Hammett constant, which depends solely on the nature and position of the substituent, and r is the reaction constant, a function of the reaction under investigation and the conditions under which it takes place. The Hammett equation expresses a general quantitative relation between the nature of a given substituent and the reactivity of the reaction center, with the validity of Eq. (2) being restricted to substituents in the meta- and para-positions of the benzene ring. The reaction constant (r) measures the susceptibility of the reaction to the influence of the substituent. Based on r values, the transition state characteristic with respect to a developing charge can be obtained. For r > 0, a negative charge is built in the transition state, whereas for r < 0 the reaction develops a positive charge in the transition state. The negative value for the Hammett reaction constant r in the phenolic hydrogen abstraction by 4,5-diaza-9-fluorenone is in accordance with the proposed mechanism for this type of reaction, in which a positive charge on the phenol is developed, as shown in Scheme 3. In this mechanism, an initial electron transfer from the phenol to the excited ketone through an intermediate exciplex is proposed, which is followed by an ultrafast proton transfer, ultimately resulting in the formation of the radical pair ketyl/phenoxyl [32–34,37–42].
OH
H
.
hν ACN Scheme 1.
H
N
+ N
.
170
A.R. Bertoti et al. / Journal of Photochemistry and Photobiology A: Chemistry 299 (2015) 166–171
O
N
O
..N N ..
+
N
_
.
hν ACN
+
N
N
N
+. N
Scheme 2.
*3
O
H
hν
+ N
.
ACN
N
*3
δ+ O
OH
O
δ-
N
. N
exciplex e- transfer OH
O
.
N
N
+
.
O
ultrafast proton transfer
_
.
+ OH
.
N
+ N
Scheme 3.
A similar mechanism can also be applied to the hydrogen abstraction by the triplet excited state of 4,5-diaza-9-fluorenone from indole (kq = (3.0 0.3) 109 L mol1 s1). The transient absorption spectrum recorded upon laser excitation of an acetonitrile solution of 1 with 2.5 103 mol L1 indole is shown in Fig. 6 and formation of the indolyl radical is clearly observed at
520 nm [43], besides an intense absorption at 400 nm that can be associated to the ketyl radical derived from 1, as described above. 4. Conclusion In conclusion, studies by laser flash photolysis indicated that excitation of 4,5-diaza-9-fluorenone in acetonitrile leads to the formation of its triplet excited state (triplet energy between 49.3 and 52.4 kcal/mol) with an intersystem crossing quantum yield of 0.41. The 4,5-diaza-9-fluorenone triplet was efficiently quenched by electron donors such as triethylamine and DABCO (kq > 108 L mol1 s1). For quenchers where the primary process corresponds to hydrogen transfer such as 1,4-cyclohexadiene, cyclohexane, 2-propanol and toluene low quenching rate constants (kq 105 L mol1 s1) were obtained. This is a clear indication that the photochemistry of 4,5-diaza-9-fluorenone is dominated by a triplet excited state showing pp* character. A Hammett plot for the quenching of the 4,5-diaza-9-fluorenone triplet by phenols containing polar substituents led to a reaction constant r of 1.54 0.10, with this negative value for r being consistent with the electrophilic character of the 4,5-diaza-9fluorenone triplet. Acknowledgements
Fig. 6. Absorption spectrum for transient generated upon photolysis of 4,5-diaza-9fluorenone in the presence of 2.5 103 mol L1 indole in acetonitrile, recorded 5.0 ms after the laser pulse.
The authors very much appreciate grants from Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB),Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP). A.R. Bertoti thanks CNPq for
A.R. Bertoti et al. / Journal of Photochemistry and Photobiology A: Chemistry 299 (2015) 166–171
a graduate fellowship. J.C. Netto-Ferreira thanks CNPq for a Visiting Professor fellowship at Universidade Federal da Bahia (Brazil). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2014.10.021. References [1] J.C. Scaiano, Intermolecular photoreduction of ketones, J. Photochem. 2 (1973/ 74) 81–118. [2] P.J. Wagner, B.-S. Park, Photoinduced hydrogen atom abstraction by carbonyl compounds, Org. Photochem. 11 (1991) 227–366. [3] P.J. Wagner, Chemistry of excited triplet organic carbonyl compounds, Topics Curr. Chem. 66 (1976) 1–52. [4] N.W.A. Geraghty, M.T. Lohan, CRC Handbook of Organic Photochemistry and Photobiology, in: F. Ghetti, A.G. Griesbeck, M. Oelgemöller (Eds.), 3rd ed., CRC Press, Boca Raton, FL, 2012, pp. 445–448. [5] N.J. Turro, Photochemistry of ketones adsorbed on porous silica, Tetrahedron 43 (1987) 1589. [6] N.J. Turro, Photochemistry of organic molecules in microscopic reactors, Pure Appl. Chem. 58 (1986) 1219–1228. [7] C. Bohne, M.C. Barra, R. Boch, E. Abuin, J.C. Scaiano, Excited triplet states as probes in organized systems. An overview of recent results, J. Photochem. Photobiol. A: Chem. 65 (1992) 249–265. [8] A. Garner, F. Wilkinson, Laser photolysis studies of triplet-state of xanthone and its ketyl radical in fluid solution, J. Chem. Soc. Faraday Trans. 2 (72) (1976) 1010–1020. [9] J.C. Scaiano, Solvent effect in the photochemistry of xanthone, J. Am. Chem. Soc. 102 (1980) 7747–7753. [10] C. Coenjarts, J.C. Scaiano, Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes, J. Am. Chem. Soc. 122 (2000) 3635–3641. [11] L.J. Martinez, J.C. Scaiano, The photochemistry of 1-azaxanthone in aqueous solutions and in micellar environments, J. Phys. Chem. A 103 (1999) 203–208. [12] J.C. Scaiano, D. Weldon, C.N. Pliva, L.J. Martínez, Photochemistry and photophysics of 1-azaxanthone in organic solvents, J. Phys. Chem. A 102 (1998) 6898–6903. [13] F. Elisei, G. Favaro, F. Ortica, Effects of protolytic interactions on the photophysics of phenyl pyridyl ketones, J. Chem. Soc. Faraday Trans. 90 (1994) 279–285. [14] A. Romani, F. Elisei, F. Masetti, G. Favaro, pH-induced effects on the photophysics of dipyridyl ketones, J. Chem. Soc. Faraday Trans. 88 (1992) 2147–2154. [15] G. Favaro, F. Masetti, A. Romani, Effect of the nitrogen position on the excited state properties of the six isomeric di-pyridyl ketones, J. Photochem. Photobiol. A: Chem 53 (1990) 41–49. [16] F. Elisei, G. Favaro, A. Romani, A laser flash photolysis study of di-pyridyl ketones, Chem. Phys. 144 (1990) 107–115. [17] A.A.C. Takaizumi, F.R. Santos, M.T. Silva, J.C. Netto-Ferreira, The reactivity of the triplet excited state of 1,4-diaza-9-fluorenones towards hydrogen and electron donors, Quim. Nova 32 (2009) 1799–1804. [18] J. Andrews, A. Deroulede, H. Linschitz, Photophysical processes in fluorenones, J. Phys. Chem. 82 (1978) 2304–2309. [19] N.J. Turro, V. Ramamurthy, J.C. Scaiano, Principles of Molecular Photochemistry, University Science Books, Sausalito, California, 2009. [20] O. Stern, M. Volmer, The fading time of fluorescence, Physik Z. 20 (1919) 183–188. [21] S.L. Murov, I. Carmichael, G.L. Hug, Handbook of Photochemistry, Marcel Dekker Inc, New York, 1993. [22] R. Bensasson, E.J. Land, B. Mavdinas, Triplet-states of carotenoids from photosynthetic bacteria studied by nanosecond ultraviolet and electron pulse irradiation, Photochem. Photobiol. 23 (1976) 189–193.
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