Photophysics and Photochemistry of Imipramine, Desimipramine, and

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Photophysics and Photochemistry of Imipramine, Desimipramine, and Clomipramine in Several Solvents: A Fluorescence, 266 nm Laser Flash, and Theoretical Study Carmelo Garcı´a,*,† Rolando Oyola,† Luis Pin˜ ero,† Dionne Herna´ ndez,† and Rafael Arce‡ UniVersity of Puerto Rico at Humacao, Department of Chemistry, Humacao, Puerto Rico 00791, and UniVersity of Puerto Rico at Rı´o Piedras, Department of Chemistry, San Juan, Puerto Rico 00936 ReceiVed: February 7, 2007; In Final Form: September 11, 2007

Imipramine (IPA) and its derivatives are used widely for the treatment of depression and other mental disorders. Although there are more than 20 FDA-approved antidepressant drugs, the search continues for better compounds with fewer deleterious side effects and higher efficacy. Over the past decade, several classes of antipsychotic drugs have been developed, whichsin spite of their structural diversitysshare an ability to modulate neurotransmission and to produce undesirable side effects. Phototoxicity is one of the most important side effects noted in treatment with tricyclic antidepressants (TCAs), but its mechanism has not yet been elucidated. To develop new knowledge regarding the relationship between the structure and the photophysics of these TCAs, we measured the photophysical properties of IPA, desimipramine (DIPA), and clomipramine (CIPA) in different solvents. The electronic configurations for the ground and the first excited singlet states were calculated using the AM1/RHF/CI and the AM1/RHF/HE semiempirical quantum theoretical methods, respectively. The ground-state properties are solvent-independent, while the emission maxima are red-shifted with increasing solvent polarity/polarizability. However, the fluorescence quantum yield is relatively low in all of the tested solvents (φf < 0.02). The primary transient intermediates produced by 266 nm high-intensity laser photolysis are the solvated electron and the corresponding radical cation, with a negligible contribution of triplet-triplet absorption. The properties determined for the primary transients generated with a 266 nm laser flash are consistent with the photodamaging effects generated through a limited radical mechanism.

1. Introduction The tricyclic antidepressant drugs (TCA)* imipramine (IPA, 1d), desimipramine (DIPA, 1e), and clomipramine (CIPA, 1f) belong to the dibenzazepine type (Figure 1). They are actually used in the management of neurogenic pain, attention-deficit hyperactivity disorder in children over age 6, depression, eating disorders, and panic or phobic disorders. These substances share a basic chemical structure comprising a three-ring core and an alkylamino side chain. Within the IPA family, as in the phenothiazines, the chlorinated derivative CIPA has the highest potency and the most pronounced side effects.1,2 It produces, among others effects, allergic skin reactions (skin rash, urticaria), photosensitization, pruritus, edema, and drug fever.3 The patients on CIPA treatment develop severe photodermatitis with associated liver involvement, which might be the result of photoallergy and contact allergy to the drug. For the rest of the members of this family, light-induced effects are not remarkable and take much longer to appear. IPA, for example, produces only a slategray discoloration, which can last for years after cessation of the therapy. DIPA is the active in vivo metabolite of IPA and, as such, shares many of its pharmacological effects. DIPA produces a blue-gray cutaneous pigmentation after a period of 8 years of treatment.4 These and other observations on several members of the TCA family lead to the conclusion that changes in the structure of these drugs do not only change their * Author to whom correspondence should be addressed. Address: University of Puerto Rico-Humacao, UPRH - Chemistry, 100 Road 908, Humacao, Puerto Rico 00791-4300. Phone: (787)-850-9387. Fax: (787)850-9422. E-mail: [email protected]. † University of Puerto Rico at Humacao. ‡ University of Puerto Rico at Rı´o Piedras.

Figure 1. Structure of the imipramine derivatives and the parent molecules.

neuroleptic activity but they also change the spectrum and intensity of their side effects. Obata and Egashira5 reported that IPA enhances the hydroxy radical formation induced by 1-methyl-4-phenylpyridium ion during enhanced dopamine overflow. It is also known that IPA and other TCAs inhibit monoamine oxidase (MAO) in vitro.6 In dopamine nerve cells, free radicals are generated mainly by MAO through the deamination of dopamine and nonenzymatically by the autoxidation of dopamine.7 Although the results of Obata and Egashira are related to dark reactions, these studies demonstrate the participation of IPA derivatives in electron-

10.1021/jp0710739 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/18/2007

Photophysics and Photochemistry transfer processes. According to these authors, the enhanced hydroxy radical formation may be part of the actual mechanism of free-radical formation in the pathogenesis of neurodegenerative brain disorders including Parkinson’s disease, Alzheimer’s disease, and traumatic brain injuries. Moreover, the TCA derivatives IPA and DIPA are consistently associated with the cytochrome P-450 interaction, which is known to participate in electron-transfer processes.8 Thus, TCA radical formation can induce cellular-damaging effects. In a photochemical related study, Dall’Acqua and co-workers reported the phototoxic potential of IPA and amitriptyline.9 According to this group, a negligible production of singlet oxygen was observed, along with a significant production of superoxide anion. Also, both drugs photosensitized the peroxidation of linoleic acid, suggesting a predominant involvement of radical species. The IPA-radical (IPA•) formation was originally observed by Borg using anodic electrolysis and paramagnetic resonance, who assigned to it absorption maxima at 261 and 640 nm.10 The participation of IPA• in the photodestruction mechanism of CIPA was proposed by Canudas and Contreras.11 These authors also reported the isolation and identification of the photodegradation products of CIPA in PBS/ 7.4, PBS/6.0, and methanol under aerobic conditions. They proposed an homolytic cleavage of the carbon-chlorine bond and photooxidation of the amine group as the key steps in the radical mechanism for the photodegradation of CIPA. Besides, it is known that the “surfactant-like” behavior of TCAs in aqueous media12,13 can play an important role in a biological matrix.14-16 The TCA partitioning in different microenvironments enhances localized photochemical damage in the cell. In this study, we measured the photophysical and photochemical properties and characterized the intermediates of several IPA derivatives produced during the 266 nm photolysis with the purpose of correlating these properties with the phototoxic side effect of these drugs. Absorption, fluorescence, and nanosecond laser flash photolysis were used to characterized the ground state, the singlet excited state, and the primary 266 nm laser-induced transient intermediates, respectively. In addition, two TCA derivatives, N-methyl iminodibenzyl (NMIDB, 1c) and N-methyl-pentyl-iminodibenzyl (NPIDB, 1g), were synthesized and submitted to the same studies to investigate the effects of the length of the alkyl chain on the photochemistry and photophysics of the TCA. This factor is known to play a relevant role in the photolysis mechanism of these drugs, as proposed previously by Epling and co-workers.17 Finally, quantum theoretical calculations were performed to determine the atomic orbital contributions to the ground and excited states of all TCA derivatives. This work is part of our ongoing research on the photophysical and photochemical properties of different TCA families commonly used in mental health care.18-21 2. Materials and Methods 2.1. Chemicals. 5-[3-(Dimethylamino)propyl]-10,11-dihydro5H-dibenz[b,f]azepine hydrochloride (IPA 1d, Jaminine), and 5H Dibenz[b,f] azepine-5-propanamine, 10,11-dihydro-N-methyl-hydrochloride (DIPA 1e, Norpramine) were from SigmaAldrich (St. Louis, MO). 3-Chloro-5-[3-(dimethylamino)propyl]10,11-dihydro-5H-dibenz[b,f]azepine hydrochloride (CIPA 1f, Anafranil) was obtained from RBI -Sigma-Aldrich (St Louis, MO). Among other reactants, iodomethane, 4-methyl-1-bromo pentane, and dimethyl sulfoxide were obtained from Aldrich; methylene chloride and potassium hydroxide were from Fisher. The reference compound IDB 1a was from Fluka. Organic solvents were high-quality spectrophotometric grade, and other

J. Phys. Chem. B, Vol. 112, No. 1, 2008 169 chemicals were from well-known suppliers and used without further purification. Nitrogen, helium, and nitrous oxide were purchased from Air Products (Humacao, PR). Aqueous solutions were prepared with nanopure water. The TCA free base derivatives were prepared by addition of NaOH to an aqueous solution of the protonated drug and then extracting with diethyl ether. 2.2. Synthesis of N-Methyl Iminodibenzyl (NMIDB, 1c) and N-Methyl-pentyl-iminodibenzyl (NPIDB, 1g). These compounds were synthesized by a method based on literature procedures with some minor modifications.22-24 Briefly, a solution of DMSO (25 mL) containing 0.0051 mol of iminodibenzyl and 0.0051 mol of potassium hydroxide was stirred at room temperature, while adding 4.8 mL of iodomethane (0.051 mol) dropwise for NMIDB or 0.084 mL (0.0056 mol) of 1-bromo-4-methylpentane for NPIDB. After 4 h, 30 mL of water were added and the product was extracted by washing the solution several times with methylene chloride, saving the organic phase. This organic phase was then washed with water and brine and dried over magnesium sulfate. The solvent was removed by rotary evaporation. NMIDB 1c is a white solid after recrystallization from methanol with a 93% yield (mp 103106 °C compared with 106-108 °C) with the following properties: NMR: 1H NMR (CDCl3, 400 MHz) δ ) 7.1-7.0 (ddd J ) 15.2 Hz, J ) 7.6 Hz, J ) 1.4 Hz, 2H), 7.01-6.99 (dd, J ) 7.8 Hz, J ) 1.4 Hz, 4H), 6.84-6.80 (ddd J ) 14.6 Hz, J ) 7.3 Hz, J ) 1.4 Hz, 2H), 3.28 (s, 3H), 3.09 (s, 4H). 13C NMR (CDCl , 100 MHz), δ ) 148.67, 133.20, 129.76, 3 126.47, 121.91, 118.83, 40.58, 32.89. The δ values correlate very well with the corresponding values of IDB.25 NPIDB 1g was purified by silica gel column using hexane as eluent giving a 56% yield. It has the following properties: NMR data: 1H NMR (CDCl3, 400 MHz), δ ) 7.06-6.78 (m, 8H, aromaticH), 3.6 (t, 2H, CH2), 3.06 (s, 4H, CH2-CH2), 1.5-1.3 (m, 3H), 1.2-1.1 (m, 4H), 0.71 (d, 6H); 13C NMR (CDCl3, 100 MHz), δ ) 148.6, 134.3, 129.8, 126.4, 122.4, 120.2, 110.8, 51.2, 36.5, 32.4, 25.9, 25.4, 22.8. 2.3. Absorption and Emission Spectroscopy. Absorption spectra were taken with a HP 8453 UV-Vis photodiode array spectrophotometer. Fluorescence spectra and the corresponding emission quantum yields were determined with a Spex Fluorolog Tau 3.11 spectrofluorometer (Spex Industries, NJ). The fluorescence quantum yield (φf) was obtained relative to tryptophan (φf ) 0.13).26 The excitation wavelength was 280 nm, reference and samples were optically matched (A < 0.08), and the monochromator slits were set to 2.5 nm. Corrections were made for differences in the instrument sensitivity as a function of wavelength and for differences in refractive index. Fluorescence lifetimes were determined using the frequency domain method. The frequency-dependent emission was obtained against PPO, POPOP, and/or a scatter solution using 300 or 315 nm excitation and either a 310 or 370 nm long pass filter. Lifetime regression analysis was performed with the software developed by the Center for Fluorescence Spectroscopy, University of Maryland (Baltimore, MD). 2.4. Phosphorescence Studies. For the determination of the triplet state energy, ethanol or methanol/ethanol solutions of the imipramine derivatives with an absorbance of 0.60 at 280 nm in a 1 cm2 cell were prepared and poured in a 4 mm diameter quartz tube, which was then inserted in an optical Dewar flask containing liquid nitrogen. The Dewar flask was inserted into the phosphoroscope adapter of a SLM-4800 Aminco-Bowman spectrofluorometer upgraded with single photon counting detec-

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Figure 2. Absorption spectra of imipramine, its parent molecules, and several related compounds in methanol. Left: IDB 1a, NMIDB 1c, IPA 1d, IPA-free base (1d-FB), and NPIDB 1g. Right: ISB 2a, DBCH 2b, and PTL 2d.

tion capability. The sample compartment was purged with nitrogen before and during data acquisition. 2.5. Nanosecond Laser Flash Spectroscopy. The nanosecond spectrokinetic system has been described elsewhere.18,20,21 Briefly, the 266 nm harmonic of a Nd/YAG Continuum Surelite II Laser was used for sample excitation with the following standard conditions: pulse duration ) 5 ns, repetition rate ) 10 Hz, laser irradiation area ) 0.30 cm2, and maximum fluence ) 80 mJ/cm2). The laser actinometry was measured using the 266 nm laser-induced photoionization of KI (φe- ) 0.36).27 Transient absorption spectra were taken using a 1 cm2 flowthrough cell connected to a relatively large reservoir to minimize photodegradation effects. Kinetics at one wavelength were determined using static samples for which not more than 10 laser pulses were averaged to avoid sample degradation. The laser photolysis of a nitrogen-saturated 1.12 mM aqueous solution of IPA was performed in the presence of 5 mM K2S2O8 to assign the radical cation’s absorption bands. Under such conditions, the majority of the light is absorbed by the S2O8-2 ions generating the one-electron oxidant sulfate radical anion (SO4•-) with o[SO4•-/SO42-] ) 2.4V, as described elsewhere.18 2.6. Theoretical Calculations. The geometry optimization using a combination of molecular mechanics (MM+), molecular dynamics, and semiempirical calculations for closed shells (AM1/RHF) and open shells (PM3/UHF) was performed with HyperChem 7.5 (HyperCube Inc., Florida) and Spartan’02 (Wavefunction, Inc; CA). All ground states were optimized at least three times using different starting structures, as described previously.18,20,21 For the optimization of the excited states, the AM1/RHF/Half electron method was applied for the S1 because AM1 is more accurate for polar systems and transition states.28 PM3/UHF was applied for T1 and radical species. The conformational, thermodynamic, and spectral parameters were obtained from a single point calculation using AM1/RHF/CI for the S0 and S1 states, and PM3/RHF for the other species. The gasphase ionization potential (IPg) was taken either as the negative of the HOMO energy (Koopman’s theorem) or calculated from the formation enthalpies of the molecule and the corresponding cation [IPg[X] ) (∆Hf[X+] - ∆Hf[X])/23.06]. The second method (“adiabatic” process) was used for further calculations of the ionization potential in solution. 3. Results and Discussion 3.1. Ground-State Properties. The absorption spectra of the IPA derivatives were measured to better understand the effects of the substituents (R1 and R2) and the C10-C11 double bond

on their electronic transitions. The absorption spectrum of IPA 1d shows two broad bands at 250 and 275 nm (Figure 2A). This spectrum is almost solvent-independent for CH3CN, CH3OH, and PBS/7.4 (Table 1). Interestingly, the IPA-free base (1d-FB) shows an hyperchromic effect relative to the protonated form 1d. The absence of the nitrogen atom at the end of the N-alkyl group (NPIDB 1g) induces small bathochromic and hyperchromic effects. A major change in the absorption spectra is observed when the whole N-alkyl chain is substituted by a methyl group (NMIDB 1c). These results indicate that the R2substituent affects the electronic transitions of the heterocyclic chromophore. Figure 2B shows the absorption spectra of those TCA derivatives with a double bond at C10-C11 (ISB 2a, DBCH 2b, and PTL 2d). The ISB absorption maximum is blue-shifted relative to those of DBCH and PTL. This blue shift is also accompanied by a large hyperchromic effect. Alternatively, a bathochromic/hyperchromic effect is observed for PTL and DBCH. Altogether, these results show that the absorption properties of the IPA derivatives are sensitive to the substituents (R1, R2) and the presence of the C10-C11 double bond. To determine the contribution of the atomic orbitals to these electronic transitions, we performed quantum theoretical calculations. Table 2 presents the semiempirically calculated thermodynamical and conformational properties for the ground and excited states of the IPA derivatives and several parent compounds. Contrary to the case of the phenothiazines,21 the N-alkylation destabilizes the ground state of the molecules by 2-10 kcal/mol, as shown for IDB 1a and the unprotonated series NMIDB 1c, IPA 1d, DIPA 1e, and CIPA 1f. These values also show that the length of the alkyl chain itself is not as destabilizing as the terminal amino group. Moreover, the presence of a N-heteroatom increases the formation enthalpy by 14 kcal/mol (IDB 1a vs DBS 1b and ISB 2a vs DBCH 2b). Protonation of the alkylamino chain, however, stabilizes the molecule by approximately 27 kcal/mol. The endocyclic C10C11 double bond has a large effect on the thermal stability of nonalkylated derivatives because it forces the tricyclic moiety to flatten. Molecules with the double bond are less stable by more than 20 kcal/mol (IDB 1a vs ISB 2a and DBS 1b vs DBCH 2b). Nevertheless, this extra bond has no effect on molecules with a large amino-alkyl group (IPA 1d vs PTL 2d). The 2-halogen substituent in CIPA, however, has a stabilization effect of about 7 kcal/mol (IPA 1d vs CIPA 1f). In brief, the most stable derivatives are those with a protonated N-alkylamino and a 2-chloro substitutions, such as CIPA 1f. This system has

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TABLE 1: Absorption Properties and Theoretical Parameters of the Protonated Form of the Tricyclic Compounds and Their Parent Molecules λmax (nm) [log ] molecule

PBS

CH3OH

CH3CN

hexanes

λmax (nm) [f] 313 [0.18], 287 [0.26], 277 [0.32], 240 [0.42] 264 [0.06], 262 [0.08], 248 [0.35], 218 [0.45] 296 [0.08], 277 [0.28], 269 [0.39], 245 [0.50] 289 [0.30], 264 [0.20], 261 [0.26], 241 [0.13] b 289 [0.30], 264 [0.18], 262 [0.25], 242 [0.12] c 296 [0.09], 279 [0.30], 264 [0.37], 239 [0.55] d 295 [0.06], 274 [0.30], 268[0.37], 243 [0.49] 306 [0.22], 274 [0.19], 251 [1.20], 234 [0.09] 311 [0.24], 279 [0.06], 250 [0.27] 302 [0.33], 229 [0.46], 218 [0.78], 213 [0.55]

1a IDB

a

290 [4.24]

290 [4.25]

285 [4.16]

1b DBS

a

a

a

1c NMIDB

a

1d IPA

1g NPIDB

275 [3.81], 250 [3.92] 275 [3.81], 250 [3.92] 279 [3.78], 252 [3.86] a

2a ISB

a

2b DBCH

a

278 [3.98], 249 [3.84] 274 [3.83], 251 [3.99] 276 [3.81], 251 [3.98] 276 [3.89], 251 [4.00] 276 [3.85], 255 [3.94] 300 [3.60], 259 [4.75] 285 [4.26]

279 [4.01], 250 [3.86] 276 [3.78], 253 [3.93] 276 [3.76], 252 [3.96] 280[3.83], 252 [3.92] 276 [3.86], 256 [3.94] 300 [3.54], 259 [4.73] 285 [4.17]

265 [1.34], 213 [2.81] 277 [4.08], 250 [3.99] a

2d PTL

292 [4.11], 237 [4.04], 224 [4.34], 208 [4.46]

293 [4.15], 238 [4.08], 227 [4.30], 210 [4.49]

293 [4.15], 239 [4.02], 226 [4.41], 210 [4.56]

1e DIPA 1f CIPA

a a 276 [3.84], 255 [3.97] 300 [3.47], 259 [4.69] 285 [4.15] a

a Insoluble. b Values for the corresponding free base: 285 [0.23], 260 [0.24], 249 [0.17], 215 [0.26]. c Values for the corresponding free base: 290 [0.28], 264 [0.24], 261 [0.26], 239 [0.19]. d Values for the corresponding free base: 295 [0.13], 273 [0.24], 263 [0.37], 233 [0.45].

TABLE 2: Thermodynamical and Conformational Properties of the Tricyclic Compounds and Their Parent Moleculesa ∆Hf (kcal/mol) molecule 1a IDB 1b DBS 1c NMIDB 1d IPA 1e DIPA 1f CIPA 1g NPIDB 2a ISB 2b DBCH 2d PTL

state S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1 S0 S1 T1

proton

32.3 123.0 68.1 28.0 118.8 66.9 25.2 115.3 59.4

26.1 120.6 72.5

free base 49.7 145.3 100.1 36.9 142.4 97.7 59.1 151.6 101.7 59.3 150.1 95.4 55.1 146.0 97.0 52.0 141.9 86.0 29.9 121.1 76.5 75.2 127.2 114.1 60.2 152.3 100.1 53.1 147.6 101.3

µ (D) proton

2.34 2.86 5.07 2.47 2.96 4.43 2.31 3.91 5.83

3.17 3.09 4.41

torsion angle (degree) free base 0.64 3.99 1.16 0.34 0.54 0.46 0.51 1.19 1.53 1.05 1.77 2.22 1.16 1.96 0.93 1.15 2.86 3.06 0.55 1.38 1.37 1.04 1.38 0.96 0.08 0.32 0.30 1.07 1.06 1.07

proton

72.3 70.8 63.9 72.4 69.7 59.2 63.2 69.7 62.6

0.2 0.1 55.4

free base

hyper.

80.9 85.3 63.0 64.5 76.1 76.8 66.9 69.8 57.1 72.8 71.0 65.2 72.8 71.2 65.2 63.3 69.2 62.0 63.4 71.9 65.6 0.0 0.0 44.4 0.0 0.0 56.6 0.2 0.2 55.4

29.3 -4.6 -9.5 51.8 70.2 69.6 52.8 -4.6 2.1 34.1 -7.9 -17.7 34.1 -7.9 -3.9 55.5 -7.3 -7.8 56.4 -8.5 -19.4 62.4 0.36 59.0 65.5 61.1 69.0 70.9 72.0 75.8

a The absolute value of the torsion angle is measured for the Ph-C10-C11-Ph bonds (see Figure 1). The torsion angle for the Ph-R2-Ph plane (hyperconjugation angle) is taken as the largest angle of the protonated molecules.

a relatively large dipole moment of 2.31 D, making it very soluble in polar solvents. The geometry of the IPA derivatives is highly affected by the C10-C11 extra bond and the spatial conformation of the alkyl amino chain, similar to amitripthyline.18 The tertiary heterocyclic system is twisted by more than 60 degrees because of the sp3-

sp3 hybridization of this C10-C11 bond. The corresponding torsion angles for the Ph-R2-Ph planes are 34.1, 34.1, and 55.5 degrees for IPA, DIPA, and CIPA, respectively. All these data correspond to the most stable conformation of the drugs. Magnetic resonance and theoretical studies of IDB derivatives, including IPA, have demonstrated that these systems have

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Figure 3. Left: (a) Excitation spectrum of IPA in PBS/7.4 measured at 400 nm. Emission spectra of IPA taken with an excitation wavelength of 280 nm in (b) CH3CN, (c) CH3OH, and (d) PBS/7.4. (e) Phosphorescence spectrum of IPA measured in ethanol glass at 77 K and an excitation wavelength of 300 nm. Right: Frequency domain response of CH3CN solutions of IPA (b), DIPA (4), and CIPA (9).

TABLE 3: Emission Properties of the Protonated Imipramine Derivatives and the Parent Compounds Stokes’ shift (cm-1)

λmax (nm) molecule

PBS

CH3OH

CH3CN

1a IDB 1c NMIDB 1d IPA 1e DIPA 1f CIPA 1g NPIDB 2b DBCH 2c PTL i

a b 400 400 400 e f 360

359 348 353 356 353 360 380 358

354 346 348 348 348 352 383 358

PBS a b 11 364 11 364 10 842 e f 6469

τf (ns)

φf

CH3OH

CH3CN

PBS

CH3OH

CH3CN

PBS

CH3OH

CH3CN

6627 7236 8168 8142 7903 8454 8772 6197

6236 6940 7496 7496 6979 7823 8978 6197

a b 0.021 0.012 0.006 e f 0.62

0.131 0.027 0.010 0.010 0.008 0.021 ∼0.016g 0.55

0.200 0.031 0.012 0.009 0.007 0.029 0.82h 0.56

a b 1.12 0.70 0.18 e f 2.56

3.68 0.81c 0.54d 0.32 0.19 0.86e g 2.38j

3.70 0.63c 0.85 0.45 0.22 0.89e 3.72h 2.41

a IDB is insoluble in water. The corresponding values for solutions in hexanes are λ ) 329 nm, ∆V ) 4692 cm-1, φ ) 0.13, and τ ) 1.99 ns. The lifetime value measured in cyclohexane relative to POPOP is τ ) 1.35 ns. b NMIDB is insoluble in PBS. The corresponding values for ethanolic solutions are λ ) 344 nm, ∆V ) 6901 cm-1, and φ ) 0.033. c The emission of NMIDB, measured relative to PPO, has two components with the following lifetime (% contribution) values: 2.91 (35), 0.81 (65) in methanol; 3.24 (18), 0.63 (82) in acetonitrile. d The emission of the free base of IPA in methanol has two components (measured relative to POPOP): 2.98 (33), 0.85 (67). e NPIDB is insoluble in water. The emission lifetime (measured relative to PPO) has two components: 0.86 (48), 3.79 (52) in methanol; 4.00 (54), 0.92 (46) in ethanol; 0.89 (63), 3.99 (37) in acetonitrile. f DBCH is insoluble in water. The corresponding values for solutions in hexanes are λ ) 378 nm, ∆V ) 8633 cm-1, φ ) 0.46, and τ ) 2.8 ns. The values measured in a solvent consisting of 20% isopentane and 80% methylcyclohexane are λ ) 380 nm and τ ) 5.8 ns (ref 46). The τ value measured in CH3CN is 4.6 ns (ref Candle-Lo´pez, M. Chem.sEur. J. 1999, 5, 1192-1201). g Alcohols inhibit the fluorescence of DBCH. h Reported values are φf ) 0.86 and τf ) 5.04 ns (ref 39). i Values for protonated PTL from ref 18. j The corresponding value in ethanol is 2.17 ns.

several stable conformations because of their molecular flexibility.12,29-31 The three main types of internal conformational mobility are (a) the bridge flexing at the C10-C11 bond, (b) the ring inversion of the benzene system, and (c) the flexibility of the alkyl chain.30 A double bond at the C10-C11 position eliminates option (a) and the system would exist in a smaller number of conformations.31 It was reported30 that, for systems with single bonds at this position, the bridge flexing occurs in the subnanosecond time scale, while the ring inversion takes up to milliseconds to occur. These authors further found that the ring inversion requires about 15 kcal/mol to occur and is always lower for the free base than for the corresponding hydrochloride salt by 0.6 kcal/mol. The AM1/CI-calculated bands at 264 and 289 nm (HOMOfLUMO) for IPA, DIPA, and CIPA are predicted to have π-π* character with a very small n-π* contribution. The largest torsion angle for the Ph-N-Ph hyperconjugation planes corresponds to CIPA (55.5 degrees), which has the longest absorption wavelength with the largest n-π* contribution, which is mainly a contribution of chlorine atomic orbitals. The IDB 1a parent compound, however, shows only a red-shifted absorption band at 290 nm. This band corresponds to the HOMOfLUMO+2 transition and has a significant contribution of the N-atomic orbitals. The large molar absorption coefficient of the π-π* band of ISB 2a at 259 nm is due to the presence of the C10-C11 double bond and the better conjugation of the system (Figure 2B). Compared to the parent molecule IDB 1a, which has no double

bond and is nonplanar (81°), the better C10-C11 planarity of ISB makes its absorption band about 3 times more intense. This is done at the expense of a poorer hyperconjugation (62°), forcing the n-π* contribution for this band to disappear. If the N-heteroatom is replaced by a C group with sp3 geometry (PTL 2d and DBCH 2b), then the π-π* band is replaced by a band at 285-290 nm. Moreover, for the ISB derivatives there is a poor mixing of π-π* and n-π* transitions. In consequence, the maximum absorption bands and their absorption coefficients are very similar to the corresponding values of IDB and its derivatives. In conclusion, the data in Table 2 and Figure 2 corroborate that the main chromophore of the IPA derivatives is localized at the heterocyclic ring and that it is affected by both the C10C11 and Ph-R2-Ph torsion angles. The first angle strongly affects the absorption maxima and the molar absorption coefficient, while the second one determines the participation of the N-atomic orbitals in the transitions. 3.2. Excited Singlet State Properties. The emission spectrum of IPA derivatives consists of a single broad band (Figure 3). The emission maxima are solvent-dependent but do not change with the R1 substituent at the IDB group (Table 3). In addition, the emission spectrum is independent of the excitation wavelength in the range 250-320 nm (data not shown). A large bathochromic effect is observed for IPA, DIPA, and CIPA in PBS/7.4. This relative large Stoke’s shift is probably due to a charge-transfer character of the emission or to an excited-state

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Figure 4. Lippert-Mataga plot for IPA 1d for protic (2, m ) 71091, r 2 ) 0.8638) and non-protic solvents (b, m ) 5021, r 2 ) 0.9211).

solvent interaction because the ground-state absorption process is solvent-independent. The stabilization of the excited state depends on its dipole moment and the excited species and the time required for solvent reorientation; that is, the way excited states interact with the medium depends on its chemical and physical properties.32 Therefore, the interaction between the solvent and the fluorophore also affects the energy difference between the ground and the excited state and can be described by the Von LippertMataga equation33,34

∆µ ) µE - µG )

[ ( )] hca3 ∆ν 2 ∆f

1/2

(1)

where µE the dipole moment of the excited state, µG is the dipole moment of the ground state, h is Plank’s constant in erg‚s, c is the speed of light, a is the radius of the cavity in which the fluorophore resides, and (∆V/∆f) is the slope of the plot of the Stoke’s shift against the dielectric and refractive index function (Figure 4).35,36 For IPA, this plot shows separate excellent linear relationships for protic and for non-protic solvents. Assuming the minimum fluorophore radius as the AM1/CI determined radius of an IPA molecule (6.17 Å), the ∆µ for IPA was then determined from the corresponding slope to be 10.8 D for the locally excited singlet state (LE, non-protic)26 and 40.8 D for the internal charge transfer (ICT, protic). These changes in dipole moment correspond to a charge separation of 2.3 Å for the LE state and 8.5 Å for the ICT. The large charge separation of the ICT is most probably due to the charge delocalization from the lone electron pairs in the heterocyclic nitrogen toward the benzene rings. According to the semiempirical calculations, this charge separation is associated to the LUMO+3 orbital, which corresponds to only one of the benzene rings and contributes to the 241 nm transition (Table 1). The fact that IPA shows the largest shift in PBS indicates that it has a special behavior in this solvent, which depends not only on the solvent properties but also on some other specific interactions not considered in the Von Lippert-Matagas’s equation. The AM1/ CI calculated dipole moments for the ground and excited state of IPA are only 2.34 and 2.86 D, respectively. These correspond to a change of about 0.52 D, indicating that the largest contribution to the excited-state dipole moment does not result from the electron excitation itself but from the solventfluorophore interactions. It further establishes that the fluorescence emission originates from a relaxed state with chargetransfer character, and not from the Franck-Condon state. These

results are in agreement with those reported by Haink and Huber for the singlet excited state of IDB 1a and other related compounds.37,38 The emission spectra were also obtained as a function of pH and in different ethyl acetate/ethanol solvent mixtures to determine specific solvation effects on the excited singlet state (data not shown). The emission intensity and maximum were found to be pH-independent between 2.4 and 8.0. Systems with a C10-C11 double bond and no substitution at the 5 position have been found to have excited-state carbon acid behavior.39-41 These authors found that the emission intensity of these compounds is strongly affected by the exchange of the 5-methylene protons with solvent water on excitation. If this position is substituted by an alkyl amino chain, as in the case of the IPA derivatives, then this proton exchange is not possible and the acid/base behavior is lost. The pH effects found for IPA derivatives are in agreement with those observed on the PTL emission for pH values between 2 and 9.42 The same experiments were performed in different ethyl acetate/ethanol mixtures to ensure a constant solvent polarity/polarizability.43 In this case, the emission maxima are blue-shifted and the spectrum becomes narrower as the ethyl acetate concentration increases. These results indicate that the IPA derivatives have polar excited states with no acid/base behavior, confirming the results of the Von Lipppert-Mataga analysis. The TCAs of this study have low φf values, indicating that fluorescence is not the major S1 deactivation process, as in the case of the promazines (Table 3).21 The parent compound IDB 1a has fluorescence quantum yields of 0.13-0.20 in all solvents. The corresponding values of φf for IPA and DIPA are less than 9-14 times smaller than those of IDB. The 3-chloro substituent at the iminodibenzyl chromophore further decreases φf by a factor of about 1.3 in methanol, 1.7 in acetonitrile, and 3.5 in PBS. This decrease in the fluorescence quantum yields for CIPA is due to the heavy atom and/or electron withdrawing effect of the chlorine atom. No fluorescence was detected for ISB 2a in the solvents considered in this work. This is in agreement with the observations made for this and other derivatives containing different substituents at the 5 and 10 positions.44 The compounds with the lowest fluorescence quantum yield are the 5-acyl ISB derivatives.44,45 If the ring nitrogen is replaced by a CHR-group, as in the case of DBCH and PTL, then the system turns out to be strongly fluorescent and the quantum yield can be as high as 0.86.20,39,46 The analog to DBCH, cis-stilbene, has no fluorescence because radiationless deactivation is induced by the fast free rotation cis/trans-isomerization.47 Alternatively, the large fluorescence quantum yield of DBCH in inert solvents is typical of “locked stilbenes”.39,46,48 In summary, only systems with both a C10-C11 double bond with no possible rotation or twisting and no 5-heteroatom present large fluorescence quantum yields.49 This is confirmed by the φf values of IDB 1a, DBCH 2b, IPA 1d, ISB 2a, and PTL 2d. The lower values of the IPA series are due to the broader conformational distribution introduced by the N-alkyl chain, which also affects the twisting at the C10-C11 single bond, as shown by the quantum theoretical calculations. The first five most stable conformations of IPA predicted by AM1/RHF/CI calculations (Table 2), for example, differ only by ∼3 kcal/mol and have C10-C11 torsion angles in the range of -63.5 to -72.8°, while the alkyl chain switches from one side of the iminodibenzyl head (torsion angle ) -146.3°) to the other side (torsion angle ) 155.9°). This behavior is in agreement with the “free rotor” model for the triggering of radiationless transitions and the effects of the

174 J. Phys. Chem. B, Vol. 112, No. 1, 2008 double bond and the alkyl chain on the chromophore properties discussed in the previous section. The singlet excited state of the IPA derivatives parent compound, IDB 1a, presents a monoexponential decay with a lifetime value of ∼3.7 ns in methanol and acetonitrile. Curiously, its derivatives NMIDB 1c and NPIDB 1g have two exponential decays in both solvents; and the free base of IPA (1d-FB) also presents a two exponential decay in methanol. However, the average lifetime for these derivatives ranges between 1.10 and 2.38 ns. This fluorophore heterogeneity appears to be consequence of the two deactivation mechanisms described previously, the twisting around the IDB C10-C11 bond, and the relaxation of the alkyl amino chain. Additionally, the single decay observed for the protonated IPA derivatives is further affected by the restrain induced by the protonation and consequent ionic interactions. The chloro substitution in CIPA, as expected, diminishes the lifetime by a factor of ∼3.0-4.0. The corresponding short-lived components account for more than 60% of the emission and have τf values in the range of 0.37-0.85 ns. In this range are also the values of all alkylated derivatives, including the protonated IPA series. In the case of ISB 2a, DBCH 2b, and their derivatives, the rigidity of the double bond eliminates the twisting and should increase the overall singlet quantum yield. This is observed for DBCH and all derivatives with no heterocyclic nitrogen. All of these derivatives, including PTL 2d, show only one relatively longlived component with τf values in the range of 2.4-7.0 ns and relative high quantum yields. The S1 state of those having nitrogen, like ISB, somehow deactivates without showing any fluorescence (see above). The τf values of protonated IPA are 0.85 ns, 0.54 ns, and 1.12 ns in acetonitrile, methanol, and PBS/7.4, respectively (Table 3), similar to the promazine series.21 Nevertheless, the φf values of the phenothiazine derivatives are closer to the corresponding values of the imipramines than those of PTL. In the case of CIPA, which has the lowest φf and τf values, the Cl-substituent must be responsible for only 20% of the decrease in φf and for more than 60% of the decrease in τf (IPA 1d vs CIPA 1f), similar to the reported values for promazine (τf ) 2.6 ns) and chlorpromazine (τf ) 0.85 ns).19 In the imipramine series of this work, the largest solvent effect is also observed in PBS/7.4, in which τf for CIPA drops to only 16% of the IPA corresponding value. The fluorescence decay rate constants (kf) for IPA and DIPA calculated from τf and φf are practically constant (kf0 ) 1.4 × 107 and 1.8 × 107 s-1 in acetonitrile and methanol/PBS, respectively). However, the kf0 of CIPA is 3.0 × 107 s-1. For comparison purposes, the φf of IDB is 0.11 with a lifetime of 3 ns,50 yielding a kf0 of 3.7 × 107 s-1. Theoretical determination of the kf0 using the Strickler-Berg equation and the integrated absorption band yield a value of 1.6 × 106 s-1, which is 10 times lower than the experimentally determined one. 3.3. Nanosecond Laser Flash. The 266 nm nanosecond laserinduced transient absorption spectra of a nitrogen-saturated aqueous solution of IPA 1d are shown in Figure 5. The spectra consist of three main broad absorption bands produced within the laser pulse: a low-energy band extending from 400 to 700 nm, a low-intensity band between 360 and 450 nm, and a UV band peaking at 310-320 nm. Similar results were obtained for DIPA and CIPA, although CIPA always presents transient absorption bands with lower intensities (data not shown). The kinetic decay analysis of the 700 nm band can be described by a two exponential function with a fast component (τ ) 0.7 µs) and a slower one (τ ) 14 µs). The addition of

Garcı´a et al.

Figure 5. Time-resolved absorption spectra of a 50 µM IPA in N2saturated PBS/4.5 taken at 0.20 µs (O), 0.60 µs (b), 1.2 µs (4), and 16 µs (2) after the 266 nm laser pulse with ∼8 mJ/cm2.

Figure 6. (A) Laser intensity dependence of the maximum transient absorption at 700 nm (λexc ) 266 nm, 10 ns pulse duration, repetition rate of 10 Hz) of a 50 µM nitrogen-saturated PBS/7.4 solution of IPA (4), DIPA (9), CIPA ([), and 15 mM KI (O). (B) Plot of the decay rate constant of the solvated electron at 680 nm after 266 nm laser photolysis (∼8 mJ/cm2) for different ground-state molar concentrations of IPA in nitrogen-saturated PBS/7.4 solution.

N2O in the presence of 0.20 M t-BuOH, used to scavenge the hydroxyl radicals formed by the reaction between N2O and the solvated electron, results in the disappearance of the short-lived component in the 400-700 nm region. This effect of N2O indicates that the fast decaying species is the solvated electron. Alternatively, the UV bands were not affected by N2O. Thus, after 266 nm laser irradiation, photoionization of IPA 1d, DIPA 1e, and CIPA 1f occurs with concomitant formation of the corresponding radical cation. The electron photoejection photonicity was determined by measuring the absorbance at 700 nm as function of laser intensity using KI as reference, which photoionizes monophotonically with a quantum yield of 0.3627 (Figure 6A). For all three TCA derivatives, a nonlinear dependence with laser intensity was observed, characteristic of multiphotonic processes.

Photophysics and Photochemistry

J. Phys. Chem. B, Vol. 112, No. 1, 2008 175

TABLE 4: Ionization Potentials of Some IPA Derivatives and Their Parent Compounds in Water (E ) 78.5 D), Methanol (E ) 32.6 D), and Acetonitrile (E ) 37.5 D) IPgas (eV)

IPsolution (eV) c

IP+ (eV)

molecule

vol. (Å3)

a

b

H2O

CH3OH

CH3CN

H2O

CH3OH

CH3CN

IDB 1a NMIDB 1c IPA-FB 1dFB IPA 1d CIPA 1f NPIDB 1g

630.0 670.8 908.0 984.5 1046.5 921.9

8.31 8.45 8.52 8.57 8.51 8.40

7.34 7.06 6.71 6.63 6.89 7.10

1.34 1.31 1.18 1.15 1.13 1.18

1.31 1.29 1.16 1.13 1.11 1.16

1.32 1.29 1.17 1.14 1.11 1.16

4.70 4.45 4.23 4.18 4.46 4.62

5.03 4.77 4.55 4.50 4.78 4.94

4.52 4.27 4.04 3.99 4.28 4.44

a Obtained from Koopman’s Theorem. b Calculated with the formation enthalpy of the molecule and the corresponding cation. c Calculated with the “adiabatic” gas ionization potential.

The log/log analysis of the photonicity function ∆OD700 ) AI n gave n values of 0.97 for KI, 2.3 for DIPA, 1.8 for IPA, and 2.1 for CIPA. Therefore, it is concluded that the 266 nm laserinduced photoionization of these TCAs occurs through a biphotonic process. Similar results were reported for DBCH.51 The calculated18,21 ionization potentials (IPs) of the TCAs and their parent compound (Table 4) are more or less equal to the energy delivered by 266 nm photons (4.63 eV). Thus, under normal light intensity, the photoionization process must follow a multiphoton mechanism. This nonlinear processes can occur by either a simultaneous or a consecutive absorption, where a singlet and/or triplet excited state can be the intermediate in the second case. The IPA derivatives’ singlet lifetimes in PBS/ 7.4, except for CIPA, are long enough for a second photon to be absorbed by this transient. In the case of CIPA, its short fluorescence lifetime must result in a lower photoionization yield, as corroborated by comparing the electron absorption signals at 700 nm in nitrogen-saturated solutions. However, more detailed experiments are required to completely characterize the photoionization mechanism of these TCAs. In summary, in vivo phototoxicity of these drugs should not be related to these multiphoton processes because the required high intensity is not available under normal light conditions. The fast decay at 700 nm was also found to depend on the drug ground-state concentration. The bimolecular rate constant for the reaction between the solvated electron and the groundstate molecules was determined with a Stern-Volmer analysis using the equation kobs ) k1 + k2[TCA], where k1 and k2 are the first- and second-order decay rate constants of the electron (Figure 6B). The k2 determined from the slope of these plots for IPA, DIPA, and CIPA are 3.9 × 109, 3.1 × 109, and 3.4 × 109 (M‚s)-1, respectively. The electron-withdrawing Cl substituent in CIPA was expected to induce a higher rate constant for the reaction between the electron and the drug, but this was not observed. In the case of promazine and chlorpromazine, however, these rate constants have been reported as 5.3 × 109 (M‚s)-1 and 2.2 × 1010 (M‚s)-1, respectively.21,52 In general, this indicates that the reactivity of the solvated electron is lower for the IPA derivatives than for the promazines. This lower reactivity might be related to the presence of the sulfur atom in the promazine derivatives, which increases their reduction potential. The 266 nm laser-induced photoionization also results in the concomitant formation of the corresponding radical cation. The bands with maxima at 320 and 670 nm at times longer than 10 µs in the nitrogen-saturated transient absorption spectrum can be assigned to the drug’s radical cation or secondary radicals derived from it.53 To corroborate this assignment, irradiation of IPA was done in the presence of K2S2O8. The peroxydisulfate anion absorbs the 266 nm laser light producing the high oxidizing agent radical peroxydisulfate (SO4•-). This radical oxidizes the IPA ground state, producing then the IPA radical

cation (IPA•+). Indeed, the results showed an absorbance increase at short times after the laser pulse in the 320 and 670 nm region (data not shown). Moreover, the 320 nm band increases with a rate constant of 1.58 × 106 s-1, while the 670 nm band increases at a rate of 1.77 × 106 s-1, corroborating the bands assignment. According to Frank and co-workers,54 this radical cation is not observed in electrochemical oxidations because its formation is followed by a very fast coupling reaction. Thus, it is possible that IPA•+ reacts very fast by deprotonation or secondary reactions and the transient absorption spectra is actually due to derived secondary radicals. Furthermore, the reaction between the solvated electron and the IPA derivatives ground state should result in the formation of the radical anion (IPA•-). Nevertheless, this transient seems to have a low molar absorption coefficient because no growth of signal was observed with increasing ground-state concentration in the presence of N2. The low-intensity transient absorption band between 360 and 450 nm is present in all nitrogen- and nitrous oxide-saturated aqueous solutions. Although with lower intensity, the band is also present in oxygen-saturated aqueous solutions. Because oxygen is an excellent triplet quencher, it cannot be assigned to a triplet-triplet transition with certainty. Therefore, this band is proposed to have contributions from secondary amine-derived radicals. This secondary radicals could be the aminyl radical and deprotonated amine radical cation.53 These authors found that the bimolecular oxidation of aromatic amines in aqueous solution with classical one-electron oxidants generates products of not fully characterized identity, like amine radical cations, aminyl radicals, R-N-alkyl, and cyclohexadienyl-type radicals. The characterization of the actual amine transients resulting from the very similar optical absorption behavior of the radical cations and the aminyl radicals derived from the aromatic amines requires more detailed experiments. The 308 nm laser transient spectra of the parent compound IDB in acetonitrile showed a triplet-triplet absorption band between 400 and 600 nm with a maximum at approximately 530 nm.24 This group also assigned two additional bands to the IDB radical cation (IDB•+ with λmax ) 670 nm, τ ) 2.2 µs) and the neutral radical (IDB• with λmax ) 340 nm, τ > 20 µs). The neutral radical was postulated to form by the deprotonation of the heterocyclic nitrogen. They also corroborated the neutral radical formation using NMIDB 1c. This compound is not expected to show any neutral radical transient absorption because it does not have a weak nitrogen-hydrogen bond as IDB does. In fact, the 308 nm laser-induced transient absorption spectra of a nitrogen-saturated cyclohexane solution of NMIDB only showed the triplet-triplet band with a weak maximum at 500 nm and a strong shoulder at 320 nm (data not shown). Therefore, the heterocyclic moiety in IPA, DIPA, and CIPA should also produce the corresponding triplet-triplet absorption, albeit with a lower quantum yield than IDB or NMIDB.

176 J. Phys. Chem. B, Vol. 112, No. 1, 2008

Garcı´a et al.

Figure 7. Laser flash (266 nm) photolysis (10 mJ/cm2) transient spectra of a nitrogen-saturated acetonitrile solution containing 0.30 M acetone and 1.12 mM DIPA [left: after 0.49 µs (O), 1.98 µs (b), 15.8 µs (4), and 43.0 µs (2)] and the corresponding decay kinetics [right: for 660 nm (a), 330 nm (b), 560 nm (c), 430 nm (d), 470 nm (e), and 380 nm (f)].

An attempt was undertaken to produce the IPA triplet via T-T energy transfer. The triplet energies determined for IPA, DIPA, and CIPA from phosphorescence spectra are 64.4, 64.7, and 64.6 kcal/mol, respectively. This relatively high triplet energy of the imipramines precludes the use of sensitizers with triplet energies lower than ∼70 kcal/mol. Thus, acetone (ET ) 79.3 kcal/mol) was found to be the best candidate, which has been used successfully in the photosensitization of purines and pyrimidines bases.55 The 266 nm laser transient spectrum of a nitrogen-saturated acetonitrile solution containing 0.30 M acetone and 1.12 mM DIPA consists of several bands across the visible region (Figure 7). All bands initially grow and then decay within 40 µs, although with different lifetimes. The band between 380 and 560 nm corresponds to the acetone triplet and has a decay lifetime of around 7.0-8.0 µs. Decay lifetimes of 12 and 25 µs were observed for the 330 and 680 nm bands, respectively. These results can only be explained in terms of an electron transfer between acetone and DIPA. Similar experiments were done using 1.12 mM IPA in CH3CN with benzophenone (BP, ET ) 69.0 kcal/mol) and thioxanthone (TX, ET ) 65.5 kcal/mol) as photosensitizers (data not shown). Under these conditions, the laser light (355 nm) was absorbed only by BP or TX, producing the corresponding triplet state (3BP*, φisc ) 1.00 or 3TX* φisc ) 0.78).56 For the 3BP*-IPA couple, the transient absorption spectrum at short times consists of a band with a maximum at 520 nm, due to the 3BP*. The absorption maximum shifted with time toward the red with a concomitant absorption increase at 330 nm. The 560 nm band is due to the BP radical, and the 330 nm band is due to the imipramine cation radical (IPA•+). The growth constant determined for 1.6 mM BP and 5 mM imipramine at 330 nm was 1.8 × 107 s-1 (τ ) 55 ns), which gives a bimolecular constant of 3.6 × 109 (M‚s)-1. This is in excellent agreement with other BP-amine systems.57 The most reactive amine group in quenching *BP3 should be the side chain amino terminal. Secondary and tertiary amines have been found to be very reactive toward the benzophenone ketyl radical.58-60 In the case of TX, the TCAs actually react with 3TX*, in agreement with previous publications.60,61 The photochemistry of the parent compound IDB 1a has been reported24 only once, while analog compounds with no ring nitrogen (suberenes, 1b) have been investigated extensively by Wan et al.40 and Johnston et al.51 The first authors found that, upon photolysis, this system undergoes a rapid proton exchange at the benzylic 5-position producing a carbanion. A similar compound with an extra double bond (DBCH 2b) does not undergo this reaction under the same conditions. Steady-state fluorescence studies of acetonitrile solutions also showed a very efficient quenching of the fluorescence of the first compound by water, but not for the second. Moreover, if the 5 position

has no hydrogen atoms, then the fluorescence does not change significantly from pure CH3CN to aqueous CH3CN. Johnston and co-workers,51 however, studied the laser flash photolysis of DBS 1b and several 5-substituted analogs. They reported the formation of the cation radical (λmax ) 470 nm) via a twophoton process. In oxygenated solutions, this transient has a lifetime of τ > 1.5 µs and reacts with a variety of nucleophiles. In nitrogen-purged solutions, excitation of DBS produces a triplet state with λmax ) 420 nm and an energy of 66 kcal/mol. Interestingly, their results did not provide evidence for the anionic intermediates of suberene suggested by Wan et al.41 This group further studied the photochemistry of several structural isomers of suberene. Steady-state photochemistry of these compounds gave products from both the singlet and triplet excited states. Azarini and co-workers62 studied the electrontransfer reactions between the cation radical of suberene and aromatic donors, as proposed previously by Johnston et al.51,63 They found a very efficient reaction between the cation radical and aromatic donors with k > 109 M-1s-1. The reaction with benzyl trimethylsilane, for example, produces 5-benzyl iminostilbene. Either a charge transfer or an electron-transfer mechanism was proposed for this reaction. If the ring nitrogen is present, as in the cases of IDB 1a and ISB 2a, then the photochemistry of the corresponding aromatic amines in polar solvent should be dominated by ionization and, in nonpolar media, by homolytic bond cleavage of the N-H bond. Wang and co-workers24 showed the production of the triplet state of 1a upon UV irradiation (3IDB*, λmax ) 530 nm). This state generates then the aminyl radical via a homolytic N-H bond cleavage. Because there is a difference between the bond dissociation energy (87 kcal/mol) and the T1 energy (75 kcal/mol), the bond cleavage must proceed from a triplet manifold, as demonstrated with two-photon experiments. In acetonitrile, a species assigned to the cation radical was also observed. The inclusion of the C10-C11 double bond decreases the triplet energy in ISB and PTL compared to IPA derivatives (∼48 vs 65 kcal/mol). All of these results confirm the relevant role of the heterocyclic nitrogen and the C10-C11 bond in the photophysics and photochemistry of the imipramine derivatives. 4. Conclusions The photophysical and photochemical properties of imipramine derivatives were determined to better understand the phototoxicity of these drugs. All IPA derivatives present very low fluorescence quantum yields (Table 3), indicating that fluorescence is not the major process responsible for S1 deactivation. No evidence of the drug’s triplet excited-state was observed after 266 nm laser excitation. Thus, the major energy deactivation mechanisms are nonradiative processes.

Photophysics and Photochemistry As a general rule, tricyclics with the tertiary amine terminals (such as IPA, CIPA, amitriptyline, and doxepin) show more extensive unwanted side effects than the secondary amines (such as DIPA, nortriptyline, and amoxapine).64 Besides, the chlorinated derivatives have more profound therapeutic and deleterious effects than the parent compounds. CIPA 1f is a drug clearly and significantly superior to IPA 1d, but it is also more phototoxic.2 In terms of these light-triggered side effects, the imipramine antidepressants are less phototoxic than the related promazine derivatives,1,65 although they also show photosensitivity as a secondary effect.1,2 The low absorption in the UVA region and, especially, the negligible amounts of triplet state formed correlate very well with their low phototoxicity. Abbreviations BP CIPA DBCH DBS DIPA IP IPA IDB ISB MAO NMIDB NPIDB PBS/pH POPOP PPO PTL TCA TX

benzophenone clomipramine hydrochloride dibenzocycloheptene dibensosubarene desimipramine hydrochloride ionization potential imipramine hydrochloride iminodibenzyl iminostilbene monoamine oxidase N-methyl iminodibenzyl N-(4-methyl)pentyl iminodibenzyl phosphate saline buffer at the specified pH 1,4-bis(5-phenyloxazol-2-yl)benzene 2,5-diphenyl-1,3,4-oxadiazole protriptyline tricyclic antidepressive drugs thioxanthone.

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