Conformational analysis of 2-anthryl-ethylene derivatives: Photochemical and computational investigation

Article abstract of DOI:10.1134/S0022476612050058

2-Anthrylethylene derivatives 1 E-5 E and 1 Z are synthesized to study the cis-trans photoisomerization. Interestingly, unlike 9-anthrylethylene derivatives, 2-anthrylethylene derivatives 1 E to 5 E do not exhibit E(trans) to Z (cis) photoisomerization upon direct and triplet sensitization. One-way Z (cis) to E (trans) photoisomerization of 1 Z is found to be very efficient under direct and triplet sensitization conditions, demonstrating the involvement of both singlet and triplet states. 1 E-5 E exhibits excitation wavelength dependent fluorescence indicating the existence of conformers (rotamers) at room temperature, which is confirmed by fluorescence lifetimes measurements of compounds 1 E and 2 E. Theoretical studies are carried out at DFT and ab initio methodology and the calculated relative energy difference of the conformers is very small; it ranges between 2.9 kJ·mol^-1 to 6.3 kJ·mol^-1 for both ground and excited states.

Full text of DOI:10.1134/S0022476612050058

Journal of Structural Chemistry. Vol. 53, No. 5, pp. 851-865, 2012
Original Russian Text Copyright © 2012 by U. Srinivas, P. Arun Kumar, Kolupula Srinivas, K. Bhanuprakash, V. Jayathirtha Rao
CONFORMATIONAL ANALYSIS OF 2-ANTHRYL-
ETHYLENE DERIVATIVES:
PHOTOCHEMICAL AND COMPUTATIONAL INVESTIGATION
1 1
© U. Srinivas, P. Arun Kumar, Kolupula Srinivas,
2,3
UDC 541.6:547.53
2
K. Bhanuprakash, and V. Jayathirtha Rao
1
2-Anthrylethylene derivatives 1E–5E and 1Z are synthesized to study the cis–trans photoisomerization.
Interestingly, unlike 9-anthrylethylene derivatives, 2-anthrylethylene derivatives 1E to 5E do not exhibit E
(trans) to Z (cis) photoisomerization upon direct and triplet sensitization. One-way Z (cis) to E (trans)
photoisomerization of 1Z is found to be very efficient under direct and triplet sensitization conditions,
demonstrating the involvement of both singlet and triplet states. 1E–5E exhibits excitation wavelength
dependent fluorescence indicating the existence of conformers (rotamers) at room temperature, which is
confirmed by fluorescence lifetimes measurements of compounds 1E and 2E. Theoretical studies are
carried out at DFT and ab initio methodology and the calculated relative energy difference of the
–1
–1
conformers is very small; it ranges between 2.9 kJ⋅mol to 6.3 kJ⋅mol for both ground and excited states.
Keywords: 2-anthryethylene derivatives, photochemical cis–trans isomerization, fluorescence, charge
transfer, conformational analysis, theoretical studies.
INTRODUCTION
The photochemical ZꢀE (cisꢀtrans) isomerization of olefins has been extensively investigated. Much work has
been carried out for stilbene and its derivatives [1-18], because of its importance in academia [19, 20], industry [21-24], and
biological processes [25-27]. The light-driven E (trans) to Z (cis) isomerization of stilbene and other diarylethylenes has been
a subject of active investigation because of fundamental [3, 20] and practical applications [8, 28]. Stilbene undergoes
conventional efficient two-way isomerization with a weak fluorescence. Strong fluorescence was observed in trans-
diarylethenes containing large polyaromatic group such as anthracene, which display only one-way cis-to-trans isomerization
[29-32]. Saltiel et al. studied the rotamer dependent E (trans) to Z (cis) isomerization in 2-anthrylethylene derivatives [33-36].
Conformational analysis of the vinyl naphthalenes, naphthylacrylic acid and amide derivatives were carried out by Lewis
et al. [37] by means of NOE and Infrared spectroscopy. The rotational isomerization of 2-anthrylethene in the ground and
excited states is a subject of interest in photochemistry [38-44]. Rotamers of (E)-2-anthrylethylenes, particularly one-way
Z(cis)→E(trans) isomerization has been studied using steady-state fluorescence [38, 39], time-correlated single-photon
counting (SPC) [39-42], time-gated fluorescence spectroscopy [45, 46], principal component analysis with self-modeling
(PCA-SM) [40, 41] and T–T absorption spectroscopy [47, 48]. Despite the literature is abundant with articles related
1
2
Organic Division-II, Indian Institute of Chemical Technology, Hyderabad, India. Inorganic and Physical
Chemistry Division, Indian Institute of Chemical Technology, Hyderabad, India; skolupula@niperhyd.ac.in,
3
ksrinivas07@yahoo.com. National Institute of Pharmaceutical Education and Research (NIPER), Balanagar, Hyderabad,
India. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 53, No. 5, pp. 872-886, September-
October, 2012. Original article submitted November 5, 2011.
0022-4766/12/5305-0851
851

to conformational analysis and or rotamerism [49-58], less attention has been paid towards understanding the photochemical
E(trans)–Z(cis) isomerization of 2-anthrylethylenes carrying electron withdrawing end groups. Our continuing efforts in
studying the photochemical ZꢀE isomerization of various olefins [59-62], led us to investigate the photochemical ZꢀE
isomerization of 2-anthrylethylenes derivatives. So we report herein the synthesis, photoisomerization, fluorescence,
conformational analysis and computational studies on 2-anthrylethylenes carrying electron withdrawing end groups. 2-
Anthrylethylene derivatives 1E to 5E did not exhibit E(trans)ꢀZ(cis) photoisomerization upon direct and triplet sensitization
conditions, in contrast to the 9-anthrylethylene derivatives [60-62]. However, we found that isomerization can take place
exclusively Z→E one-way, and without producing any photocyclized product. Steady state fluorescence and fluorescence
lifetime experiments indicated the existance of two rotamers. Furthermore, theoretical studies were conducted to estimate the
stability of the rotamers in the ground and excited states.
EXPERIMENTAL
A Perkin-Elmer Lambda-2 UV-Visible spectrometer was used to obtain the absorption spectra. A SPEX Fluorolog
0.22 m fluorimeter was used for fluorescence measurements. Mass spectra were recorded on a VG Micro Mass 7070H
1
instrument. Proton nuclear magnetic resonance ( H NMR) spectra were recorded on a FT-200 MHz and FT-300 MHz
instrument in CDCl solvent. A Shimadzu LC-8 HPLC system with a CR-8 integrator was used for HPLC analysis. An
3
Applied Photo Physics QYR-20 Quantum Yield Reactor coupled with a 200 W Hg lamp was used to determine the quantum
yields. All other chemicals were purchased from Aldrich/Fluka and locally.
7 (-2-anthracenecarbaldehyde): to a cooled solution of 2-cyanoanthracene 6 (2.03 g, 0.01 mol) in 30 ml of
dichloromethane was added DIBALH (14 ml, 0.02 mol) at –78°C. The mixture was stirred for 30 min and at ambient
temperature for 5 h. Unreacted DIBALH was quenched with a pinch of NaF/H O at 0°C. The supernatant liquid was filtered,
2
extracted with chloroform (3×15 ml). The combined extracts were dried and evaporated under reduced pressure and purified by
column chromatography on silica using AcOEt/Hexane (3:97) as eluent to give anthracene-2-carbaldehyde 7 as yellow solid.
1
Yield: 90%. Rf = 0.75 (AcOEt/Hexane 5:95); m.p.: 204-206°C; H NMR (200 MHz; CDCl ): δ 10.21 (s, 1H, CHO), 8.71-8.42
3
(m, 3H, Ar), 8.20-7.91 (m, 4H, Ar), 7.58-7.41 (m, 2H, Ar); EI-MS (m/z) (%): 206 (M+) (35), 176 (14), 149 (100), 76 (8).
1E (methyl(E)-3-(2-anthryl)-2-propenoate): methyl diethylphosphonoacetate (2 g, 11 mmol) in dry DMF (15 ml)
was slowly added to a suspension of NaH (264 mg, 11 mmol) in dry DMF (20 ml) at room temperature and it was slowly treated
with 2-anthracene carbaldehyde 7 (2.06 g, 10 mmol) in 10 ml DMF. After 4 h of stirring the reaction mixture was quenched with
water and extracted with ether, (4×20 ml). The combined extracts were dried, evaporated and the residue was purified by silica
gel column chromatography to give pure 1E as yellow solid. Yield: 80%. Rf = 0.77 (AcOEt/Hexane: 5/95); m.p.: 194-196°C; IR
–
(KBr): ν = 1713 cm , 1620 cm , 1435 cm , 1245 cm , 1050 cm , 986 cm ; H NMR (200 MHz; CDCl ): δ 8.40-8.35 (d,
–1
–1
–1
–1
–1
–1 1
3
2H, J = 10.4 Hz, Ar); 8.04-7.93 (m, 4H, Ar), 7.89-7.81 (d, 1H, J = 16.0 Hz, CHCHCO Me), 7.65-7.6 (d, 1H, J = 8.0 Hz, Ar),
2
13
7.48-7.44 (m, 2H, Ar), 6.56-6.48 (d, 1H, J = 16 Hz, CHCO Me), 3.83 (s, 3 H, CO CH ), C NMR (75 MHz; CDCl ): δ 167.5,
2
2
3
3
+
144.8, 132.4, 132.1, 131.7, 131.3, 129.0, 128.1, 127.5, 126.2, 126.1, 125.8, 122.2, 117.7, 51.7; EI-MS (m/z) (%): 262 (M ) (100),
–1
232 (20), 202 (31), 141 (18), 101 (5). UV/Vis (n-hexane): λ (ε): 366 nm (9600 mol ⋅dm ⋅cm ); Emission maxima λ (n-
3
–1
max
emi
hexane): 434 nm; Quantum yield of fluorescence Φ (n-hexane): 0.72. Elemental analysis Calcd (%) for C H O (262.3):
flu 18 14 2
C, 82.42; H, 5.38; O, 12.20%. Found: C, 82.87; H, 5.62.
2E ((E)-3-(2-anthryl)-2-propenenitrile): a similar procedure, as mentioned above, was adopted using
diethylphosphonoacetonitrile to synthesize 2E, which gave yellow solid. Yield: 80%. Rf = 0.5 (AcOEt/Hexane: 5/95); m.p.:
–
–1
–1
–1
–1
–1
–1 1
268-270°C; IR (KBr): ν = 2213 cm , 1610 cm , 1433 cm , 1329 cm , 1267 cm , 946 cm ; H NMR (200 MHz, CDCl ):
3
13
δ 8.64-8.40 (m, 2H, Ar), 8.0-7.97 (m, 3H, Ar), 7.64-7.47 (m, 5H, Ar), 6.15-6.07 (d, 1H, J = 16.4 Hz, CHCN); C NMR
(75 MHz, CDCl ): δ 149.1, 133.8, 133.6, 131.1, 129.3, 128.1, 126.9, 125.6, 124.3, 117.9, 117.6, 105.2; EI-MS (m/z) (%): 229
3
+
(M ) (45), 171 (25), 141 (12), 95 (80), 59 (76), 43 (100); UV/Vis (n-hexane): λ
–1
(ε) = 366 nm (3871 mol ⋅dm ⋅cm );
3
–1
max
852

Emission maxima λ (n-hexane) = 434 nm; Quantum yield of fluorescence Φ (n-hexane) = 0.59. Elemental analysis Calcd
emi flu
(%) for C H N (229.2): C, 89.06; H, 4.84, N, 6.11; Found: C, 89.27; H, 5.01; N, 6.28%.
17 11
1Z (methyl (Z)-3-(2-anthryl)-2-propenoate): bis-(2,2,2-trichloroethyl)-1-carbometh-oxy methyl phosphonate
(4.17 g, 10 mmol) in dry DMF (15 ml) was added slowly to a suspension of NaH (264 mg, 11 mmol) in dry DMF (10 ml) at
rt. The reaction mixture is cooled to –78°C and anthracene-2-carbaldehyde 7 (2.06 g, 10 mmol) was added slowly. The
reaction mixture is warmed to room temperature, stirred for 3 h and then poured into crushed ice and extracted with ether
(3×20 ml). The combined extracts were dried and evaporated under reduced pressure and the residue was purified by column
chromatography on silica using AcoEt/Hexane as eluent to give 1Z as yellow solid. Yield: 70%. Rf = 0.76 (AcOEt/Hexane:
1
5/95); m.p.: 162-164°C; H NMR (200 MHz; CDCl ): δ 8.43-8.38 (m, 2H, Ar), 8.00-7.93 (m, 4H, Ar), 7.73-7.71 (d, 1H,
3
J = 9.1 Hz), 7.47-7.45 (m, 2H, Ar), 7.13-7.11 (d, 1H, J = 12.2 Hz, CHCHCO CH ), 6.06-6.04 (d, 1H, J = 12.2 Hz,
2 3
–1
3
–1
CHCO CH ), 3.85 (s, 3H, CO CH ); UV/Vis (n-hexane): λ (ε) = 365 nm (8400 mol ⋅dm ⋅cm ); Emission maxima λ
2 3 2 3 max emi
(n-hexane) = 430 nm; Quantum yield of fluorescence Φ (n-hexane) = 0.68.
flu
3E ((E)-4-(2-Anthryl)-3-buten-2-one): to a solution of 2-anthraldehyde 7 in 50 ml of acetone 2 ml of 5% NaOH
solution was slowly added at 0°C and stirred at rt for 2 h. The solvent was removed and the product was extracted with
chloroform (3×20 ml). The combined extracts were dried and evaporated under reduced pressure and residue was purified by
column chromatography on silica using AcoEt/Hexane as eluent to give 3E as yellow solid. Yield: 90%. Rf = 0.6
–
(AcOEt/Hexane: 10/90); m.p.: 221-223°C; IR (KBr): ν = 1725 cm , 1667 cm , 1429 cm , 1358 cm , 1249 cm , 977 cm .
–1
–1
–1
–1
–1
–1
1
H NMR (200 MHz; CDCl ): δ 8.41-8.37 (d, 2H, J = 8.8 Hz, Ar), 8.0-7.95 (m, 4H, Ar), 7.77-7.62 (m, 2H, Ar), 7.72-7.6 (d,
3
13
1H, J = 16.2 Hz, CHCHCOCH ), 7.5-7.45 (m, 2H, Ar), 6.85-6.77 (d, 1H, J = 16.2 Hz, CHCOCH ), 2.42 (s, 3H, COCH );
3 3 3
C
NMR (75 MHz; CDCl ): δ 197.3, 142.3, 133.6, 131.6, 131.1, 130.8, 130.2, 128.3, 127.3, 126.6, 126.3, 125.4, 125.0, 121.5,
3
+
26.8; EI-MS (m/z) (%) 246 (M ) (20), 231 (14), 202 (18), 149 (100), 43 (75). UV/Vis (n-hexane): λ (ε) = 364 nm
max
–1
3
(6410 mol ⋅dm ⋅cm ); Emission maxima λ (n-hexane) = 428 nm; Quantum yield of fluorescence Φ (n-hexane) = 0.11.
–1
emi
flu
Elemental analysis Calcd (%) for C H O (246.3): C, 87.78; H, 5.73; O, 6.50%; Found: C, 88.12; H, 5.86%.
18 14
4E ([(E)-2-Nitro-1-ethenyl] anthracene): to the mixture of 2-anthraldehyde 7 (2.06 g, 0.01 mol) and nitromethane
(1.2 g, 0.19 mol) in 30 ml of dry dichloromethane 2 drops of piperidine were added. The reaction mixture was refluxed for
8 h. After the completion of the reaction, it was poured into cold water and extracted with dichloromethane (3×20 ml). The
combined extracts were dried and evaporated under reduced pressure and the residue was purified by column
chromatography on silica using AcoEt/Hexane as eluent to give 4E (2.24 g, 90%) as red solid. Yield: 90%. Rf = 0.4
–
(AcOEt/Hexane: 10/90); m.p.: 212-214°C; IR (KBr): ν = 1630 cm , 1220 cm , 1026 cm , 771 cm ; H NMR (200 MHz,
–1
–1
–1
–1 1
CDCl ): δ 8.53-8.43 (d, 1H, J = 14.0 Hz, CHCHNO ); 8.3-8.2 (m, 4H, Ar), 8.12-8.0 (m, 2H, Ar), 7.78-7.71 (d, 1H,
3 2
13
J = 14.1 Hz, CHNO ), 7.6-7.5 (m, 3H, Ar); C NMR (75 MHz, CDCl ): δ 139.2, 136.9, 134.2, 133.1, 131.9, 130.8, 129.8,
2
3
+
128.4, 128.2, 127.4, 127.2, 126.8, 126.3,121.7; EI-MS (m/z) (%) = 249 (M ) (5), 223 (8), 149 (100), 57 (20) 41 (10). UV/Vis
–1
(n-hexane): λ (ε) = 380 nm (5510 mol ⋅dm ⋅cm ); Emission maxima λ (n-hexane) = 454 nm; Quantum yield of
3
–1
max
emi
fluorescence Φ (n-hexane) = 0.03. Elemental analysis Calcd (%) for C H NO (249.2) C, 77.10; H, 4.45; N, 5.62; Found:
flu 16 11 2
C, 77.05; H, 4.32; N, 5.21.
5E ((E)-3-(2-anthryl)-2-propen-1-ol): to a cooled solution of compound 1E (1.25 g, 5 mmol) in dichloromethane
(20 ml), DIBALH (5.3 ml, 7.5 mmol) was added at –10°C. The reaction mixture was stirred for 30 min at rt. Unreacted
DIBALH was quenched with a pinch of NaF/H O at 0°C. The supernatant liquid was filtered, extracted with chloroform
2
(3×15 ml). The combined extracts were dried and evaporated under reduced pressure and purified by column chromatography
on silica using AcoEt/Hexane as eluent to give 5E as yellow solid. Yield: 80%. Rf = 0.25 (AcOEt/Hexane: 10/90); m.p.: 232-
–
–1
–1
–1
–1
–1 1
236°C; IR (KBr): ν = 3385 cm , 1617 cm , 1456 cm , 1308 cm , 956 cm ; H NMR (200 MHz; CDCl ): δ 8.5 (s, 2H,
3
13
Ar), 8.1-7.9 (m, 5H, Ar), 7.51-7.42 (m, 4H, Ar), 6.62-6.5 (m, 2H, CHCH), 4.49-4.4 (d, 2H, J = 6.77 Hz, CH OH); C NMR
2
(75 MHz, CDCl ): δ 144.1, 133.3, 132.4, 131.3, 130.4, 129.2, 127.1, 126.4, 124.6, 123.2, 116.4, 63.8; EI-MS (m/z) (%) = 234
3
853

+
(M ) (21), 215(31), 202 (20), 149 (100); UV/Vis (n-hexane): λ (ε) = 364 nm (6345 mol ⋅dm ⋅cm ); Elemental analysis
–1
3
–1
max
Calcd (%) for C H O (234.2): C, 87.15; H, 6.02; O, 6.83%; Found: C, 87.06, H, 5.85.
17 14
General procedure for photolysis. A 450 W medium-pressure Hg arc lamp along with suitable filters [63, 64] were
used for irradiation. All reactions were monitored by HPLC. An amino silica 5μ, 0.5×25 cm column was used for HPLC
analysis. In a typical experiment, 10 ml of a 0.001 M solution of 1E–5E and 2Z, N bubbled, was used for irradiation. After
2
irradiation, the products were characterrized by comparison with authentic materials. Triplet sensitized reactions were carried
out using a mixture of sensitizer (0.01 M) and substrate (1E–5E and 1Z 0.001 M) in MeOH (10 ml), N bubbled, which was
2
irradiated for 1 h using a 450 W Hg lamp with filters.
Fluorescence. A fluorimeter equipped with a 450 W Xe lamp was used for fluorescence studies. Dry solvents were
used, and identical conditions were maintained for all the fluorescence measurements. The slit widths were 2×2×2×2 mm.
The emission spectral range was 350-500 nm. All operations were at room temperature. The quantum yield of fluorescence
was determined relative to that of 9,10-diphenylanthracene (0.9) [65, 66].
RESULTS
Synthesis of 2-anthrylethylene derivatives. 2-Anthrylethylene derivatives 1E–5E and 1Z were synthesized to study
ZꢀE photoisomerization, fluorescence, and rotamerism (conformational isomerization). 2-Anthracene carbaldehyde 7 is the
key intermediate involved in the synthesis of 1E–5E and 1Z compounds (Scheme 1). Friedel–Crafts phthaloylation on
chlorobenzene [67] gave chlorobenzophenone carboxylic acid, which upon treatment with sulfuric acid produced 2-
chloroanthraquinone [68]. 2-Chloroanthraquinone was subsequently converted into 2-chloroanthracene [69] and then to
2-cyanoanthracene [70]. 2-Cyanoanthracene was transformed to the key intermediate 2-anthracene carbaldehyde 7 using
DIBALH [71] (Scheme 1). Compound 7 was subjected to Horner-Wadsworth-Emmons olefination using suitable
phosphonates [72, 73] to obtain 1E and 2E. Condensation of 7 with acetone afforded compound 3E and with nitromethane
yielded 4E. Reduction of 1E using DIBALH provided compound 5E [62]. The 1Z isomer was synthesized by adopting HWE
olefination using a modified phosphonate reagent [74], previously reported by us (Scheme 1).
Photochemical ZꢀE (cisꢀtrans) isomerization. 2-Anthrylethylene derivatives 1E–5E and 1Z were subjected to
direct and triplet sensitized ZꢀE isomerization. Compound 1Z upon direct excitation and triplet sensitization underwent Z
(cis) to E (trans) isomerization exclusively, within 15 min and in quantitative yield. The E (trans) isomers of 1-5 did not
undergo E (trans) to Z (cis) isomerization upon direct excitation in various solvents or upon triplet sensitization, unlike 9-
anthrylethylene derivatives [60-62], but 1Z displayed one-way Z (cis) to E (trans) isomerization (Scheme 2; Table 1). Triplet
TABLE 1. Photochemical Z–E Isomerization of 2-Anthrylethylene Derivatives (1Z and 1E–5E)
b
Photo-isomer composition
a
Compound
Photolysis
E (trans), %
Z (cis), %
Direct and Sensitized
irradiation
98
2
Direct and Sensitized
irradiation
100
0
a
The nitrogen bubbled 0.001 M solutions were irradiated (15 min for 1Z and 3–12 h for 1E–5E). A 450 W Hg arc
–1
lamp (∼400 nm) was used and 0.001 M methanol solutions with 0.01 M sensitizers rose Bengal (163 kJ⋅mol ), erythrosine
–1 –1
(176 kJ⋅mol ), and eosine (180 kJ⋅mol ) were used for sensitized irradiations (>500 nm, 15 min for 1Z and 3-6 h for 1E–5E)
— a 450 W Hg arc lamp with filter solutions was used.
b
Photo isomer composition was analysed by normal phase HPLC and 1H-NMR.
854

Scheme 1. Reagents and conditions: (a) AlCl , 60°C, 8 h (b) conc. H SO , 135°C, 6 h (c) Zn dust/AcOH/
3 2 4
pyridine, 110°C, 8 h; (d) CuCN, pyridine, 220°C, 24 h; (e) DIBALH, –78°C, 4 h. (f )
(EtO) P(O)CH COOMe, NaH, DMF, rt and 4 h; (g) (EtO) P(O)CH CN, NaH, DMF, rt, 4 h; (h) Acetone,
2
2
2
2
5% NaOH, rt, 2 h; (i) nitromethane, piperidine, dichloromethane, 40°C; (j) DIBALH, 0°C, 6 h. (k) NaH,
DMF, rt, cool to –78°C, then add 2-anthraldehyde
Scheme 2. Photochemical ZꢀE (cisꢀtrans) isomerization of compounds 1Z
and 1E–5E
–1
energy of the sensitizers [63-66] used varied between 160-184 kJ⋅mol . Prolonged irradiation (∼400 nm, 3 h to 12 h) of
1
compounds 1E–5E and 1Z did not yield any new products other than the E (trans) isomer, as judged by H NMR.
Absorption and fluorescence studies. Absorption and fluorescence studies were carried out in various organic
solvents to understand the ground and excited state properties of 2-anthrylethylene derivatives and the data was compiled in
Table 2. The absorption spectra of these derivatives are structured with a vibronic progression characteristic of the anthryl
chromophore. This suggests that the excitation energy is largely localized in the anthryl chromophore, whereas
855

TABLE 2. UV-Visible Absorption and Fluorescence of Compounds 1E–5E
a
Compound
Solvent
λ , nm
abs
λ , nm
flu
Φ
flu
1E
Cyclohexane
Hexane
356, 365, 384, 400
356, 366, 384, 400
354, 364, 400
406, 434, 458, 490 (s)
0.75
0.72
0.62
0.41
0.66
0.61
0.59
0.40
0.41
0.57
0.14
0.11
0.22
0.18
0.12
0.031
0.03
0.14
0.096
0.56
0.42
0.68
0.40
406, 432, 458, 486 (s)
Chloroform
Acetonitrile
Methanol
454
354, 364, 400
460
354, 364, 382, 402
354, 366, 382, 404
354, 366, 382, 404
354, 366, 382,406
354, 366, 382, 406
354, 366, 382,406
366, 382, 404
480
2E
3E
4E
Cyclohexane
Hexane
408, 434, 458, 492 (s)
410, 436, 462, 490 (s)
Chloroform
Acetonitrile
Methanol
455
456
462
Cyclohexane
Hexane
386, 410, 430, 453 (s)
364, 382, 404
385, 406, 428, 452 (s)
Chloroform
Acetonitrile
Methanol
366, 384, 408
476
356, 386, 410
483
356, 386,406
536
Cyclohexane
Hexane
362, 372, 402, 426
360, 376, 426
451
454
595
Chloroform
Acetonitrile
Hexane
362, 380, 434
360, 376, 424
615
5E
1Z
356, 364, 382, 404
356, 365, 400
386, 397, 422, 450 (s)
399, 416, 450 (s)
406, 430, 457, 486 (s)
460
Acetonitrile
Hexane
356, 365, 384, 400
355, 364, 400
Acetonitrile
a
Nitrogen bubbled 0.00001 M solutions were used for measuring the fluorescence at room temperature; quantum
46
yields of fluorescence were determined using 9,10-Diphenyl anthracene (Φ = 0.9) as standard ; experimental error is
flu
10%.
–5
Fig. 1. UV-Visible absorption spectra of 4E (10 M) in various
solvents.
9-anthrylethylenes, carrying electron-withdrawing end groups, exhibited broad absorption in the 300-450 nm region [60-62].
The UV-visible absorption of 1E, 2E, 3E, and 5E shows a slight bathochromic shift (2-10 nm) upon increasing the solvent
polarity. A prominent bathochromic shift is observed for compound 4E in the UV-vis absorption maxima with an increase in
solvent polarity (Fig. 1).
The steady state fluorescence spectra of all the compounds were recorded at room temperature in various organic
solvents. The fluorescence emission maxima and quantum yield of fluorescence data are given in Table 2; the corresponding
856

Fig. 2. Normalized fluorescence excitation and emission spectra 1E
–5
and 1Z in hexane (10 M).
–5
Fig. 3. Effect of solvent on the fluorescence spectrum of 1E (10 M).
–5
Fig. 4. Fluorescence emission spectra of 1E in hexane (10 ) as a
function of the excitation wavelength.
fluorescence spectra for four compounds are displayed in Figs. 2-5. The excitation wavelength strongly affects the
fluorescence spectra of derivatives 1E–5E. Compounds 1E to 3E and 1Z display structured fluorescence in nonpolar solvents,
such as hexane and cyclohexane, whereas broad fluorescence emission was observed in polar solvents. Compound 4E did not
exhibit structured fluorescence both in nonpolar and polar solvents due to the strong electron-withdrawing nature of the nitro
group. On the other hand, compound 5E displayed structured fluorescence irrespective of the solvent polarity. The
fluorescence emission of all these derivatives, except that of 5E, were found to be sensitive towards the solvent polarity.
857

–5
Fig. 5. Fluorescence emission spectra of 2E in hexane (10 ) as a
function of excitation wavelength.
The fluorescence quantum yield calculated for compounds 1E, 2E, and 5E is high (Table 2) compared to that of compounds
3E and 4E, and this may be due to the intersystem crossing leading to a more populated triplet excited state. Fig. 2 shows the
emission and excitation spectra of 1E and 1Z, and the fluorescence emission spectrum of cis isomer 1Z is almost similar to
the emission spectrum of trans isomer 1E. Fig. 3 shows fluorescence of 1E in various solvents. The fluorescence spectrum is
broadened and the maximum is shifted to longer wavelength on increasing the solvent polarity. Interestingly, fluorescence
emission for these compounds is found to be excitation wavelength dependent suggesting the involvement of the conformers
(rotamers) at room temperature. The fluorescence spectra of 1E and 2E in hexane are displayed in Figs. 4 and 5. The
fluorescence spectra were recorded by exciting at different wavelengths ranging from the lower wavelength region (330-
400 nm; Fig. 4) to a higher wavelength region (400-420 nm; Fig. 4) for 1E. The fluorescence maxima (λ ) were observed at
flu
406 nm, 432 nm, and 458 nm when the excitation was performed at 330-400 nm. Interestingly, upon excitation at 410 nm the
fluorescence maxima were red shifted to 422 nm, 448 nm, and 478 nm. Similarly, for compound 2E upon excitation at a lower
wavelength region (330-400 nm) the fluorescence maxima (λ ) were observed at 410 nm, 436 nm, and 462 nm (Fig. 5) and
flu
upon excitation at 420 nm the fluorescence maxima were found at 424 nm, 448 nm, and 472 nm (Fig. 5). All the other
derivatives exhibited similar behavior.
Fluorescence lifetimes. The fluorescence lifetime profiles were obtained for 2-anthrylethylene derivatives (1E–2E)
using a single photon counting nano-LED spectrofluorimeter (IBH) with a time resolution of around 1 ns. The excitation
wavelengths were 374 nm and 439 nm and the fluorescence decay was monitored near the emission maximum (Table 3).
Satisfactory fits to a single exponential were obtained for 374 nm excitation in all the solvents for these two compounds, and
double exponential fits were found to be the most appropriate for 439 nm excitation in hexane and cyclohexane solvents
(Table 3).
Computational methods and conformational analysis. It is very important in terms of materials based on the
isomerization to consider the reasons why s-trans is more stable than s-cis in the ground as well as the excited state. For this
purpose we applied DFT and ab initio tools to evaluate the relative energy difference of the conformers of 1E–5E compounds
[75]. Geometry optimizations of 1E–5E molecules (Chart) were performed at HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels
of theory for the S state and the CIS/6-31G(d,p) level for the S state without restricting the symmetry. Two potential minima
0
1
(s-trans and s-cis rotamers – rotational axis C –C bond) are obtained for each molecule. In the s-trans rotamer, C and C
2 11 1 12
carbon atoms are trans to each other, whereas in s-cis, they are cis to each other. In both S and S states, irrespective of the
0 1
acceptor, s-trans is found to be stable compared to the s-cis rotamer (Table 4). The energy difference between these rotamers
–1 –1
is very small and it varies from 3.3 kJ⋅mol to 6.7 kJ⋅mol for different substituents at various levels of theory. This minor
difference was expected due to the steric effects in the single bond rotation. The dipole moment difference of these rotamers
858

TABLE 3. Excitation Wavelength Dependent Fluorescence Lifetimes of Compounds 1E and 2E
a
a
Solvent
Compound 1E
Compound 2E
λ , nm
ex
τ , ns
1
τ , ns
2
λ , nm
ex
τ , ns
1
τ , ns
2
Cyclohexane
Hexane
374
439
374
439
374
374
374
9.38
4.13
8.01
4.02
5.40
6.30
6.39
—
0.080
—
374
439
374
439
374
374
374
9.90
5.72
8.54
5.15
6.24
6.75
6.62
—
0.285
—
0.0004
—
0.112
—
Chloroform
Acetonitrile
Methanol
—
—
—
—
a
–5
Nitrogen bubbled solutions 1.0×10 M were used; λ at 480 nm.
em
a
TABLE 4. Relative Energies (ΔE) and Dipole Moment (μ) of 1E–5E
HF/6-31G(d,p) B3LYP/6-31G(d,p)
CIS/6-31G(d,p)
Compound
–1
–1
–1
ΔE, kJ⋅mol
μ, D
ΔE, kJ⋅mol
μ, D
ΔE, kJ⋅mol
μ, D
1E
2E
3E
4E
5E
5.3
5.2
5.2
6.1
3.2
2.7 (2.5)
6.4 (6.5)
4.3 (4.1)
7.8 (8.2)
2.1 (2.0)
5.6
5.1
4.1
5.4
4.2
2.7 (2.5)
6.1 (6.3)
3.9 (3.6)
7.0 (7.2)
1.6 (1.5)
5.6
5.1
5.5
6.5
4.0
3.2 (2.6)
6.4 (6.5)
4.8 (4.3)
8.9 (9.1)
2.2 (2.1)
a
Dipole moment (Debye) of s-trans is in parenthesis.
TABLE 5. Bond Lengths (Å) of 1E–5E in S and S -State
0 1
R (C –C )
1 2 11
R (C –C )
2 11 12
R (C –C )
3 12 13
Compound
S
S
S
S
S
S
1
0
1
0
1
0
1E
2E
3E
4E
5E
s-trans
1.465
1.467
1.465
1.462
1.458
1.461
1.459
1.464
1.473
1.470
1.447
1.438
1.446
1.439
1.449
1.441
1.431
1.426
1.464
1.457
1.333
1.332
1.333
1.333
1.336
1.338
1.328
1.327
1.327
1.326
1.343
1.348
1.345
1.351
1.343
1.349
1.349
1.354
1.331
1.335
1.463
1.464
1.475
1.476
1.427
1.429
1.431
1.432
1.492
1.493
1.454
1.452
1.465
1.462
1.422
1.420
1.404
1.401
1.491
1.490
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
s-trans
s-cis
is also very small in both states. The computational studies show that none of the molecules are planar in the ground state,
whereas in the first excited state, they are nearly planar. The twist angle between the anthracene plane and the olefinic double
bond ranges from 5° to 30°, which is attributed to the interaction of the ortho protons of anthracene with the olefinic protons.
Selected bond lengths C –C (R ), C –C (R ), and C –C (R ) of the optimized geometries of 1E–5E in the ground state
2
11
1
11
12
2
12
13
3
(HF/6-31G (d,p)) and the excited state (CIS/6-31G(d,p)) are presented in Table 5. Bond length alternation is observed from
the ground state to the excited state in all the molecules. The C –C bond length of s-trans of the 1E molecule is 1.465 Å in
2
11
the S state and it is reduced to 1.447 Å in the S state. Similar behavior is observed for the C –C bond in S and S states,
0 1 12 0 1
13
measuring 1.463 Å and 1.454 Å respectively. In contrast, the C –C bond in the S state is 1.333 Å and it is elongated
11 12 0
859

TABLE 6. Computed Electronic Absorption and Emission Spectra of 1E–5E
Major
Compound
Rotamer
State
f
f
λ , nm
abs
λ , nm
em
Transitions (CI)
1E
s-trans
S
409
0.129
438
0.163
H→L (0.63);
1
H-1→L (–0.14)
S
359
300
431
0.008
1.388
0.093
—
368
303
462
0.017
0.291
0.136
2
S
3
—
s-cis
s-trans
s-cis
S
H→L (0.63);
1
H-1→L (–0.11)
S
359
307
411
0.012
1.041
0.123
—
366
312
436
0.008
0.04
2
S
3
—
2E
3E
4E
5E
S
0.143
H→L (0.62);
1
H-1→L (–0.16)
S
359
297
437
0.009
1.432
0.087
—
366
297
463
0.014
1.325
0.012
2
S
—
3
S
H→L (0.63);
1
H-1→L (–0.12)
S
358
305
414
0.021
1.03
—
364
306
443
0.01
0.985
0.175
2
S
—
3
s-trans
S
0.139
H→L (0.63);
1
H-1→L (-0.12)
S
362
306
437
0.011
1.237
0.1
—
373
310
468
0.018
1.166
0.145
2
S
—
3
s-cis
S
H→L (0.63);
1
H-1→L (–0.12)
S
362
313
451
0.016
0.986
0.151
—
371
317
489
0.012
0.982
0.211
2
S
—
3
s-trans
S
H→L (0.64);
1
H-1→L (–0.15)
S
366
326
477
0.024
0.805
0.108
—
378
331
519
0.03
0.844
0.164
2
S
—
3
s-cis
S
H→L (0.65);
1
H-1→L (–0.12)
S
368
331
390
347
282
403
345
286
0.021
0.738
0.082
0.001
0.466
0.06
—
379
337
422
350
289
438
351
294
0.028
0.804
0.087
0.002
0.143
0.078
0.007
0.017
2
S
—
3
s-trans
S
H→L (0.63)
1
S
—
2
S
—
3
s-cis
S
H→L (0.64)
1
S
0.01
—
—
2
S
0.129
3
Coefficients indicated in parentheses; H indicates HOMO and L indicates LUMO.
to 1.343 Å in the S state. A similar kind of alteration is observed in both rotamers of all the molecules, i.e. in the S state
1 1
compared to the S state, R is reduced, but R is elongated and again R is reduced.
0 1 2 3
Computed absorption and emission analysis. UV-Visible absorption data indicate that there is not much ground
state and medium interaction in almost all of the substrates (Table 2). The absorption spectrum of these derivatives is
860

Fig. 6. Frontier molecular orbital pictures of 1E & 5E.
structured with a vibronic progression characteristic of the anthryl chromophore confirming that the excitation energy is
largely localized on the anthryl chromophore, whereas 9-anthrylethylenes, carrying electron-withdrawing end groups, exhibit
broad absorption in the 300-450 nm region [62]. In order to further analyze the nature of charge transfer, we have carried out
theoretical calculations of vertical excitations at HF/6-31G (d,p) optimized geometries of all the conformations for each
molecule using ZINDO-SCI; the results are shown in Table 6. The selected HOMO and LUMO orbitals of both rotamers in
1E and 5E molecules are shown in Fig. 6. In the HOMO of 1E and 5E, the electron density is located mainly on the
anthracene moiety. In HOMO-1 of 1E, it is spreaded over both anthracene as well as olefinic double bond. But in the case of
the LUMO of 1E, the electron density is located all over the molecule, i.e. it is also extended to the acceptor. In contrast, for
5E the reorganization of the electron density is observed within the anthracene and olefinic bond. Computed electronic
excitation values are in good agreement with the experimental results. Computed electronic absorption of the 1E molecule is
409 nm and 431 nm for s-trans and s-cis rotamers respectively. Absorption analysis is carried out to know the transitions
responsible for the absorption maxima presented in the same table. Table 6 illustrates that the mixture of transitions (HOMO
to LUMO, HOMO-1 to LUMO) is responsible for the observed absorption maxima in both s-trans and s-cis of the 1E
molecule. This reveals that charge transfer in 1E takes place from the donor groups (anthracene) to the acceptor (end) groups.
The calculated absorption maxima are in good agreement with experimentally observed 400 nm in hexane. Owing to the
interconversion between two rotamers at room temperature average (single) absorption maxima are observed experimentally.
The second and third transitions are 359 nm, 300 nm and 359 nm, 307 nm for s-trans and s-cis rotamers respectively, which
is slightly overestimated compared to the experimental results i.e. 380 nm and 360 nm for S and S transitions. This is
2
3
attributed to the calculations being carried out in the gas phase, whereas the experimental results are obtained in the solvent
environment (condensed media). For the 3E molecule, the calculated S , S , S transitions are 414 nm, 362 nm, and 306 nm
1 2 3
for s-trans and 437 nm, 362 nm, 313 nm for the s-cis rotamer, which coincides with experimental 404 nm, 382 nm, and
366 nm. Similarly, the calculated absorption values for s-trans and s-cis of 4E–5E molecules are also in good agreement with
the experimental results. For all the molecules except for 5E the major (HOMO to LUMO) and minor transitions (HOMO-1
to LUMO) are combinedly responsible for the observed absorption maxima. In the case of the 5E molecule, only one
transition, i.e. HOMO to LUMO, is responsible for the absorption maxima of both rotamers. This discrepancy can be
explained by considering their end groups, which are acceptors in 1E–4E molecules, but donors in the 5E molecule. Hence,
861

the absorption maxima of the 5E molecule is slightly blue shifted compared to the other molecules (1E–4E) and the HOMO-1
to LUMO transition disappears.
DISCUSSION
The red shift observed in the emission maxima (fluorescence solvatochromism) [76, 77] for compounds 1E–4E by
changing the solvent polarity indicates that the singlet excited state has some amount of charge transfer or polar character
[78]. Frontier Molecular Orbital analysis (Fig. 6) reveals that the partial charge transfer occurs from the donor to the acceptor
when it is excited for 1E–4E molecules. This is also observed in the geometry of S vs. S where single and double bond
1
0
alteration occurred. From the solvent effects it can also be concluded that the molecule is polarized in the excited state, and
hence, it exhibits different interactions with nonpolar and polar solvents. In contrast, compound 5E did not exhibit
fluorescence solvatochromism and this infers the non-involvement of the charge transfer excited state, which is due to the
lack of electron-withdrawing acceptor groups, and the same is noticed in the frontier molecular orbital of 5E (Fig. 6).
The fluorescence maxima (λ ), for compound 1E were observed at 406 nm, 432 nm, and 458 nm upon excitation at 330-
flu
400 nm. When the same compound was subjected to excitation at 410 nm, different fluorescence maxima were observed at
422 nm, 448 nm, and 478 nm (Fig. 4). The two different fluorescence emission spectra correspond to the two rotamers,
because of different absorptivities at the given λ of excitation. Similar excitation wavelength dependent fluorescence
emission is observed for the other molecules (Fig. 5). The computed emission spectra of the 1E molecule (438 nm for s-trans
and 462 nm for the s-cis rotamer) are in good agreement with the experimental results (Fig.4; Table 6). This is further supported
by the fluorescence lifetime data recorded (Table 3) where biexponential decay becomes prominent at λ of 439 nm. The
exci
excitation at 374 nm provides single exponential decay (Table 3) for compounds 1E and 2E, and this indicates the selective
population of the excited rotamer (Scheme 3). Interconversion of the excited rotamers (Scheme 3) is predicted as not a feasible
process, according to the NEER principle. These excited rotamers do not undergo E–Z isomerization, but exhibit only
fluorescence (Scheme 3). A comparison of both experimental and theoretical results reveals that two rotamers (Chart 1) exist in
the excited state. These rotamers have different first lowest excited states, and hence, it is reflected in fluorescence emission.
Scheme 3. Excited state profile of compound 1E
Chart 1. List of molecules and corresponding rotamers
862

CONCLUSIONS
In conclusion, 2-anthrylethylene derivatives (1E–5E and 1Z) were synthesized to study photochemical Z (cis)–
E (trans) isomerization. All the derivatives did not undergo E (trans) to Z (cis) isomerization upon direct and triplet
sensitization. But compound 1Z exhibited efficient one-way Z (cis) to E (trans) isomerization upon direct and sensitized
irradiation. The UV-Visible absorption spectra of 1E, 2E, 3E, and 5E displayed a slight bathochromic shift (2-10 nm) upon
changing the solvent polarity from non-polar hexane to polar methanol. A prominent bathochromic shift of the UV-Visible
absorption maximum was observed for compound 4E with an increase in the solvent polarity. Compounds 1E–4E displayed
fluorescence solvatochromism due to the polar nature of the singlet excited state. Compounds 1E–5E exhibited strong
dependence of fluorescence on the excitation wavelength due to the presence of two rotamers. Fluorescence decay profiles
also indicate the presence of two rotamers in 2-anthrylethylene derivatives 1E and 2E. Fluorescence life time studies carried
out at room temperature revealed that 2-anthrylethylene derivatives 1E–5E exhibited s-trans and s-cis rotational isomers.
Steady state fluorescence experiments indicated that compounds 1E–5E exhibited intense emission corresponding to the s-
f
trans rotational isomer with shorter λ
upon excitation at a lower wavelength region (320-390 nm), and weak emission
max
f
corresponding to the s-cis rotational isomer with longer λ
is noticed upon excitation at a higher wavelength region (400-
max
420 nm). Theoretical studies performed also supported the existence of rotational isomers, and the s-trans rotamer is more
stable than the s-cis rotamer.
We thank Director, IICT, Project Director, NIPER, and the Head of the Division for the encouraging support.
P. Arun Kumar and Kolupula Srinivas thank CSIR-New Delhi and U. Srinivas thanks UGS-New Delhi for fellowships. We
thank Prof. A. Samantha for providing life time measurement data.
REFERENCES
1. G. S. Hammond, J. Saltiel, A. A. Lamola, N. J. Turro, J. S. Bradshaw, D. O. Cowan, R. C. Counsell, V. Vogt, and
C. Dalton, J. Am. Chem. Soc., 86, 3197-3217 (1964).
2. H. A. Hammond, D. E. DeMeyer, and J. L. R. Williams, J. Am. Chem. Soc., 91, 5180/5181 (1969).
3. D. H. Waldeck, Chem. Rev., 91, 415-436 (1991).
4. H. Gorner and H. Kuhn, J. Adv. Photochem., 19, 1-117 (1995).
5. J. Saltiel and Y. P. Sun, Eds. Photochromism, Molecular Systems, Elsevier, Amsterdam (1990).
6. A. J. Merer and R. S. Mulliken, Chem. Rev., 69, 639-656 (1969).
7. R. S. Mulliken, J. Chem. Phys., 66, 2448-2451 (1977).
8. H. Meier, Angew. Chem., Int. Ed. Eng., 31, 1399-1420 (1992).
9. A. Simeonov, M. Matsuhita, E. A. Juban, E. H. Z. Thompson, T. Z. Hoffmann, A. E. Beuscher IV, M. J. Taylor, Peter
Wirsching, Wolfgang Rettig, J. K. Mc Cusker, R. C. Stevens, D. P. Miller, P. G. Schultz, R. A. Lerner, and K. D. Janda,
Science, 290, 307-313 (2000).
10. V. Papper and G. I. Likhtenshtein, J. Photochem. Photobiol A, 140, 39-52 (2001).
11. J. Saltiel, T. S. R. Krishna, and A. M. Turek, J. Am. Chem. Soc., 127, 6938/6939 (2005).
12. K. Akiu, M. Saijo, M. Ikegami, and T. Arai, Bull. Chem. Soc. Jpn., 78, 1132-1137 (2005).
13. A. Momotake and T. Arai, J. Photochem. Photobiol. C, 5, 1-25 (2004).
14. J. C. Roberts and J. A. Pincock, J. Org. Chem., 71, 1480-1492 (2006).
15. J. S. Yang, K. L. Liau, C. W. Tu, and C. Y. Hwang, J. Phys. Chem. A, 109, 6450-6456 (2005).
16. M. Uda, A. Momotake, and T. Arai, Tetrahedron Lett., 46, 3021-3024 (2005).
17. D. I. Schuster, B. Nuber, S. A. Vail, S. MacMahon, C. Lin, S. R. Wilson, and A. Khong, Photochem. Photobiol. Sci., 2,
315-321 (2003).
18. H. Tatewaki, N. Baden, A. Momotake, T. Arai, and M. Terazima, J. Phys. Chem. B, 108, 12783-12789 (2004).
863

19. N. J. Turro, Modern Molecular Photochemistry, The Benzamin, Cummings Publishing Co., Inc., Menlo Park (C.A)-
London, Amsterdam (1978).
20. J. Saltiel and J. L. Charlton, in Rearrangement in Ground, Excited States, P. deMayo (ed.), Academic Press, New York
(1980), 3, p. 25.
21. Kirk-Othner Encyclopedia of Chemical Technology. 4th ed. Willey (1996), 3, p. 25.
22. B. L. Feringa, Tetrahedron, 94, 8267-8310 (1993).
23. A. M. Braun, M. J. Maurette, and E. Oliveros, Photochemical Technology, Wiley, New York (1991).
24. B. L. Feringa, R. A. van Delden, N. Koumura, and E. M. Geertsema, Chem. Rev., 100, 1789-1816 (2000).
25. V. Balogh-Nair and K. Nakanishi, in: Chemistry, Biology of Synthetic Retenoids, M. Dawson and W. Okamura (eds.),
CRC Press, Boca-Ranton (1990), FL, 147.
26. V. J. Rao, F. Derguini, K. Nakanishi, T. Taguchi, A. Hosada, Y. Hanzawa, T. Kobayashi, C. M. Pande, and
R. R. Callender, J. Am. Chem. Soc., 108, 6077/6078 (1986).
27. R. S. H. Liu and Y. Shichida, in: Photochemistry in Organized, Constrained Media, V. Ramamurthy (ed.), VCH
Publishers, Chapter 18, New York (1991).
28. M. Irie, Chem. Rev., 100, 1685-1716 (2000).
29. T. Arai, T. Karatsu, H. Sakuragi, and K. Tokumari, Tetrahedron Lett., 24, 2873-2876 (1983).
30. H. Hamaguchi, M. T. Karatsu, T. Arai, and K. Tokumaru, J. Am. Chem. Soc., 108, 1698/1699 (1986).
31. T. Arai and K. Tokumaru, Chem. Rev., 93, 23-39 (1993).
32. H. D. Becker, Chem. Rev., 93, 145-172 (1993).
33. J. Saltiel, N. Tarkalanov, and D. F. Sears Jr., J. Am. Chem. Soc., 117, 5586/5587 (1995).
34. J. Saltiel, Y. Zhang, and D. F. Sears Jr., J. Am. Chem. Soc., 118, 2811-2817 (1996).
35. J. Saltiel, Y. Zhang, and D. F. Sears Jr., J. Am. Chem. Soc., 119, 11202-11210 (1997).
36. J. Saltiel, G. Krishnamoorthy, and D. F. Sears Jr., Photochem. Photobiol. Sci., 2, 1162-1168 (2003).
37. F. D. Lewis and X. Liu, J. Am. Chem. Soc., 121, 11928/11929 (1999).
38. U. Mazzucato and F. Momicchioli, Chem. Rev., 91, 1679-1719 (1991).
39. G. Fischer and E. Fischer, J. Phys. Chem., 85, 2611-2613 (1981).
40. A. Spalletti, G. Bartocci, F. Masetti, U. Mazzucato, and G. Cruciani, Chem. Phys., 160, 131-144 (1992).
41. G. Bartocci, U. Mazzucato, and A. Spalletti, Chem. Phys., 202, 367-376 (1996).
42. K. P. Ghiggino, P. F. Skilton, and E. Fischer, J. Am. Chem. Soc., 108, 1146-1149 (1986).
43. T. Wismontski-Knittel, P. K. Das, and E. Fischer, J. Phys. Chem., 88, 1163-1168 (1984).
44. T. Arai, T. Karatsu, H. Sakuragi, K. Tokumaru, N. Tamai, and I. Yamazaki, Chem. Phys. Lett., 158, 429-434 (1989).
45. T. J. Kang, T. Etheridge, W. Jarzeba, and P. F. Barbara, J. Phys. Chem., 93, 1876-1881 (1989).
46. A. M. Brearley, A. J. G. Strandjord, S. R. Flom, and P. F. Barbara, Chem. Phys. Lett., 113, 43-48 (1985).
47. T. Karatsu, N. Yoshikawa, A. Kitamura, and K. Tokumaru, Chem. Lett., 381-384 (1994).
48. V. Krongauz, N. Castel, and E. Fischer, J. Photochem., 39, 285-300 (1987).
49. T. Karatsu, H. Itoh, A. Nishigaki, K. Fukui, A. Kitamura, S. Matsuo, and H. Misawa, J. Phys. Chem. A, 104, 6993-7003
(2000).
50. H. J. Jacobs and E. Havinga, Adv. Photochem., 11, 305 (1979).
51. J. B. Birks, G. Bartocci, G. G. Aloisi, S. Dellonte, and F. Barigelletti, Chem. Phys., 51, 113-120 (1980).
52. G. Bartocci, G. Ginocchietti, U. Mazzucato, and A. Spalletti, Chem. Phys., 328, 275-283 (2006).
53. N. Marcotte and S. Fery-Forgues, J. Photochem. Photobiol. A: Chemistry, 130, 133-138 (2000).
54. G. Bartocci and A. Spalletti, J. Phys. Chem. A, 106, 7068 (2002).
55. S. Ciorba, F. Fontana, G. Ciancaleoni, T. Caronna, U. Mazzucato, and A. Spalletti, J. Fluorescence, 19, 759 (2009).
56. G. Bartocci, G. Galiazzo, G. Ginocchietti, U. Mazzucato, and A. Spalletti, Photochem. Photobiol. Sci., 3, 870 (2004).
57. G. Ginocchietti, G. Galiazzo, U. Mazzucato, and A. Spalletti, Photochem. Photobiol. Sci., 4, 547 (2005).
864

58. I. Barldi, E. Benassi, and A. Spalletti, Spectrochim. Acta Part A: Molecular, Biomolecular Spectroscopy, 71, 543
(2008).
59. K. M. Bushan, G. V. Rao, T. Soujanya, V. J. Rao, S. Saha, and A. Samanta, J. Org. Chem., 66, 681-688 (2001).
60. M. J. R. Reddy, G. V. Rao, K. Manibhushan, M. J. R. Reddy, V. R. Gopal, and V. J. Rao, Chem. Lett., 3, 186/187
(2001).
61. M. J. R. Reddy, U. Srinivas, K. Srinivas, V. V. Reddy, and V. J. Rao, Bull. Chem. Soc. Jpn., 75, 2487-2495 (2002).
62. V. R. Gopal, A. M. Reddy, and V. J. Rao, J. Org. Chem., 60, 7966-7973 (1995).
63. P. F. Barbara and W. Jarzeba, Adv. Photochem., 15, 1 (1990).
64. E. Z. Lippert, W. Rettig, V. Banacic-Koutecky, F. Heisel, and J. Meihe, Adv. Chem. Phys., 68, 1 (1987).
65. J. C. Scaiano, Handbook of Organic Photochemistry; CRC press, Boca Raton, FL, 1 (1989).
66. J. G. Calvert and J. N. Pitts Jr., in: Photochemistry, Eds, Wiley, Chapter VII, New York (1966).
67. K. Tanaka, T. Miura, N. Umezawa, Y. Urano, K. Kikuchi, T. Higuchi, and T. Nagano, J. Am. Chem. Soc., 123, 2530-
2536 (2001).
68. A. Fairbourne, J. Chem. Soc., 119, 1573-1582 (1921).
69. J. T. Traxler, Syn. Comm., 7, 161-166 (1977).
70. P. H. Gore, J. Chem. Soc., 1616-1618 (1959).
71. J. A. Marshall, N. H. Anderson, and J. W. Schlicher, J. Org. Chem., 35, 858-861 (1970).
72. W. S. Wadsworth, Org. React., 25, 73-145 (1977).
73. B. E. Maryanoff and A. B. Reitz, Chem. Rev., 89, 863-927 (1989).
74. A. M. Reddy, V. R. Gopal, and V. J. Rao, Ind. J. Chem., 35B, 312-317 (1996).
75. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian 03, Revision B.01, Gaussian, Inc., Wallingford CT
(2004).
76. G. Bartocci, U. Masetti, U. Mazzucato, A. Spalletti, G. Orlandi, and G. Poggi, J. Chem. Soc., Faraday Trans., 84, 385-
399 (1988).
77. F. Momicchioli, I. Baraldi, and E. Fischer, J. Photochem. Photobiol A, 48, 95-107 (1989).
78. C. Reichhardt, Solvents, Solvent Effects in Organic Chemistry, VCH, Weinhein (1988).
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