Photoreversible Molecular Motion of stpyCN Coordinated to fac-[Re

Article Cite This: J. Phys. Chem. A 2018, 122, 6071−6080

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Photoreversible Molecular Motion of stpyCN Coordinated to fac-[Re(CO)3(NN)]+ Complexes Ronaldo C. Amaral, Lais S. Matos, Kassio P. S. Zanoni, and Neyde Y. Murakami Iha* Laboratory of Photochemistry and Energy Conversion, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo − USP, Avenue Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil

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S Supporting Information *

ABSTRACT: In this work, efficient trans ⇌ cis photoswitchings of 4-(4-cyano)styrylpyridine (stpyCN) coordinated to organometallic bipyridyl tricarbonyl rhenium(I) complexes, fac-[Re(CO)3(NN)(trans-stpyCN)]+, where NN = 2,2′-bipyridine (bpy) or 4,4′-dimethyl-2,2′-bipyridine (dmb), are described. For both complexes, the true trans-to-cis quantum yields determined by 1H NMR spectroscopy are similar at 313, ∼ 0.45), with a 334, and 365 nm irradiations (Φtrue(313−365nm) trans→cis true(404nm) small decrease at 404 nm (Φtrans→cis ∼ 0.37). The investigated complexes also exhibit significant quantum yields for the reversible cis-to-trans photoreactions (Φ(255nm) cis→trans = 0.22). The luminescent properties of these complexes were also analyzed in different media to elucidate a key role of the 3ILstpyCN state in photophysical and photochemical processes, giving new insights on their intriguing photobehavior.

■

Chart 1. Structure of the Investigated Re(I) Complexes

INTRODUCTION Photophysical and photochemical properties of organometallic tricabonyl polypyridyl rhenium(I) complexes have been extensively investigated due to an efficient intersystem crossing from singlet to triplet excited states as a consequence of a strong spin−orbit coupling (SOC), with ξRe ∼ 700−900 cm−1.1 In particular, they find applications in biological probes,2,3 organic light-emitting devices (OLEDs),4,5 photoreduction of CO2 to CO6,7 and photosensitization of 1O2.8,9 Usually, trans-stilbene like ligands (trans-L) exhibit a transto-cis photoisomerization in the UV region via 1π → π*trans‑L excited states.10,11 The coordination of trans-L to Re(I) extends the photoisomerization pathways to the visible, mainly via metal-to-ligand charge transfer (1MLCT/3MLCT) photosensitizations of the intraligand (3ILtrans‑L) 3π → π*trans‑L state.1,11−41 The trans-cis photosensitization of such ligands have been successfully exploited in the photoregulation of DNA hybridizations42 as well as in the conversion of light into mechanical motion43,44 and the search for new efficient trans-cis photosystems is a key element toward the development of molecular machines. In our previous works, syntheses and photocharacterization of fac-[Re(CO)3(NN)(stpyCN)]+ complexes (NN = 1,10phenanthroline (phen) or 7,4-diphenyl-1,10-phenanthroline (ph2phen); stpyCN = 4-(4-cyano)styrylpyridine) were performed for the understanding of molecular-level photocontrolled motions.14,31 In this work, the photochemical and photophysical behavior of new fac-[Re(CO)3(bpy)(transstpyCN)]PF6 (trans-1) and fac-[Re(CO)3(dmb)(trans-stpyCN)]PF6 (trans-2) compounds, Chart 1, were investigated in order to © 2018 American Chemical Society

obtain further insights of the excited state dynamics that govern the photoprocesses of fac-[Re(CO)3(NN)(stpyCN)]+ series.

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EXPERIMENTAL SECTION All chemicals employed in the syntheses (reagent grade) and the solvents for photochemical and photophysical measurements (HPLC grade) were used as supplied. fac-[Re(CO)3(NN)Cl], fac-[Re(CO)3(NN)(tfms)], trans-stpyCN and protonated trans-stpyCN ([trans-HstpyCN]Cl) in the Received: March 19, 2018 Revised: June 25, 2018 Published: June 28, 2018 6071

DOI: 10.1021/acs.jpca.8b02630 J. Phys. Chem. A 2018, 122, 6071−6080

Article

The Journal of Physical Chemistry A

Figure 1. 1H NMR (300 MHz) spectra for (A) trans-1 and (B) trans-2 in CD3CN (T = 298 K).

842, 558 (P−F), 977 (C−Htrans). 1H NMR (300 MHz, CD3CN, δ/ppm): 9.03 (d, J3 = 5.7 Hz, 2H); 8.23 (d, J4 = 0.7 Hz, 2H); 8.16 (dd, J3 = 6.7 Hz; J3 = 1.4 Hz, 2H); 7.72 (m, 4H); 7.60 (ddd, J3 = 5.7 Hz; J = 1.7 Hz; J4 = 0.7 Hz, 2H); 7.47 (d, J3 = 16.5 Hz, 1H); 7.40 (dd, J3 = 6.8 Hz, J3 = 1.4 Hz, 2H); 7.20 (d, J3 = 16.5 Hz, 1H); 2.55 (s, 6H). Anal. Calc. for C29H22F6N4O3PRe: C, 43.23; N, 6.95; H, 2.75. Found: C, 43.32; N, 7.04; H, 2.81. General Procedures. Electronic absorption spectra were recorded using a Hewlett-Packard 8453 spectrophotometer with 1.000 or 0.100 cm optical length quartz cuvettes. Hydrogen nuclear magnetic resonance, 1H NMR, spectra were recorded in a Varian Unity Inova (300 MHz) spectrometer at 298 K using CD3CN as solvent, having the 1.94 ppm signal of residual CD3CN as an internal standard. Infrared (IR) spectra were recorded in a KBr pellet using an attenuated total reflectance (ATR) with a Bruker Spectrum Alpha. PMMA-based films were prepared following the procedure previously described.15,19,23 The compounds were dissolved in dichloromethane, added to a PMMA dichloromethane solution and left to dry protected from humidity and light. Photochemical Measurements. Photolyses of solutions at 313, 334, 365, or 404 nm were carried out as previously described,15,24 using 200 W Hg(Xe) or 500 W Hg(Xe) Oriel systems by selecting the wavelength with appropriate interference filters. All irradiations were performed in a cuvette especially designed, as previously described.31 Light intensities were determined by potassium tris(oxalate)ferrate(III) actinometry before and after each photolysis. Apparent trans-to-cis photoisomerization quantum yields (Φapp trans→cis) were obtained following absorbance changes by eq 1; they are apparent, since both trans- and cis-isomers absorb in the same region.11,15,16,18−20,24,25

Supporting Information. fac-[Re(CO)3(phen)(stpyCN)]PF6 and fac-[Re(CO)3(ph2phen)(stpyCN)]PF6 complexes were synthesized according to the procedure described.14,31 Syntheses of Complexes trans-1 and trans-2. These complexes were synthesized according to the procedure previously reported for other fac-[Re(CO)3(NN)(trans-L)]+ complexes.14,19,20,24,25 fac-[Re(CO)3(bpy)(trans-stpyCN)]PF6 (trans-1). The fac[Re(CO)3(bpy)(tfms)] precursor (1.54 g, 2.68 mmol) and trans-stpyCN (1.66 g, 8.05 mmol) were dissolved in methanol (Synth) and heated to reflux for 9 h. The reaction was monitored by UV−vis spectroscopy and TLC. After cooling to room temperature, solid NH4PF6 (1.11 g, 6.81 mmol) (Aldrich) was added to the solution to precipitate a yellow complex. The purification was performed by dissolving the trans-stpyCN ligand in absolute ethanol (Synth). The solid was filtered and washed with ultrapure water, ethyl ether (Synth) and dried under vacuum (10 mmHg, 60 °C) to obtain 0.771 g of pure product (0.991 mmol, 38% yield). IR (KBr/cm−1): 2227 (C N), 1931, 1915 (CO), 842, 558 (P−F), 975 (C−Htrans). 1 H NMR (300 MHz, CD3CN, δ/ppm): 9.23 (ddd, J3 = 5.5 Hz; J4 = 1.5 Hz; J5 = 0.8 Hz, 2H); 8.38 (ddd, J3 = 8.1 Hz; J5 = 0.9 Hz, 2H); 8.26 (td, J3 = 7.8 Hz; J4 = 1.5 Hz, 2H); 8.18 (dd, J3 = 6.7 Hz; J3 = 1.4 Hz, 2H); 7.79 (ddd, J3 = 7.6 Hz; J3 = 5.5 Hz; J4 = 1.5 Hz, 2H); 7.72 (m, 4H); 7.47 (d, J3 = 16.5 Hz, 1H); 7.39 (dd, J3 = 6.7 Hz; J3 = 1.5 Hz, 2H); 7.19 (d, J3 = 16.5 Hz, 1H). Anal. Calc. for C27H18F6N4O3PRe: C, 41.70; N, 7.20; H, 2.33. Found: C, 41.84; N, 7.15; H, 2.39. fac-[Re(CO)3(dmb)(trans-stpyCN)]PF6 (trans-2). The fac[Re(CO)3(dmb)(tfms)] precursor (1.05 g, 1.74 mmol, 1 equiv) and trans-stpyCN (1.07, 4.97 mmol) were dissolved in 50 mL methanol (Synth) and heated to reflux for 9 h. The reaction was monitored by UV−vis spectroscopy and TLC. After cooling to room temperature, solid NH4PF6 (1.42 g, 8.70 mmol) (Aldrich) was added to the solution to precipitate a yellow complex. The purification was performed by dissolving the trans-stpyCN ligand in absolute ethanol (Synth). The solid was filtered and washed with ultrapure water, ethyl ether (Synth) and dried under vacuum (10 mmHg, 60 °C) to obtain 0.592 g of pure product (0.734 mmol, 42% yield). IR (KBr/cm−1): 3078 (C−H), 2228 (CN), 1931, 1915 (CO), 1613 (C = C),

Φapp trans → cis =

(A trans − Airr )NAVirr 1 × I0tirr εtransb

(1)

where tirr = irradiation time (s); I0 = light intensity (quanta s−1); b = optical path length of the irradiated cuvete (cm); Virr = volume of the irradiated solution; NA = Avogadro number. 6072


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The Journal of Physical Chemistry A Table 1. 1H NMR Spectral Data (300 MHz) for the Investigated Compounds in CD3CN at 298 Ka

a1

H NMR signal assignment for cis complexes is discussed in section “Photochemical behavior in fluid solution”.

true True quantum yields (Φtrans→cis ) were obtained by correcting Φapp to the molar absorptivities of the cis-isomer (εcis (λ)/ trans→cis L mol−1 cm−1) using eq 2.15,16,18,31

Φtrue trans → cis =

(A − A trans )NAVirr 1 × irr I0tirr (εcis − εtrans)b

Φcis → trans =

(A − Airr )NAVirr 1 × PS I0tirr (εcis − εtrans)b

(3)

in which APS = initial absorption of the photostationary solution. The εcis(λ) values employed in both eqs 2 and 3 were obtained using the ratio between the integrals of trans and cis 1 H NMR signals in irradiated solutions, eq 4, as previously described.16

(2)

Quantum yields for the reverse cis-to-trans photoreaction were obtained at 255 nm irradiation of the photostationary solution by using eq 3.13,14,17,31 6073


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The Journal of Physical Chemistry A εcis(λ) = εtrans(λ) ×

HNMR (Airr (λ) − A trans (λ)) × %1trans

A trans (λ) × %1cisHNMR

(4)

in which εtrans(λ) = molar absorptivity for the trans-isomer H NMR (L mol−1 cm−1); %1trans and %1cisH NMR = percentages of trans- and cis-isomers in the irradiated solution obtained by the distinct integrals in the 1H NMR spectrum; εtrans(λ) = absorbance of the trans-isomer solution before irradiation; Airr(λ) = absorbance of the irradiated solution. Photophysical Measurements. Emission experiments were performed at room temperature (298 K) as previously described15,17,20,24 using an ISS photon counting spectrofluorometer, model PC1, with a 1.000 cm optical length quartz cuvette for fluid solution and a front face arrangement for PMMA films. Emission measurements of pure trans-solutions were performed using a quartz flow cell (0.5 mL min−1) in order to avoid any interferences from residual cis-photoproducts formed during the experiments. Low-temperature emission spectra were obtained in glassy EPA (diethyl ether/isopentane/ethanol, 5:5:2) at 77 K using a quartz tube inside a Dewar flask filled with liquid nitrogen. The emission color was evaluated by CIE coordinates, calculated as reported in our previous works.45,46 Emission quantum yields of the investigated complexes in degassed acetonitrile were determined against fac-[Re(CO)3(bpy)Cl] (ϕ = 0.0060)47 using the procedure previously described.45,46

Figure 2. Infrared spectra for (A) trans-1 and (B) trans-2 in KBr.

Table 2. Spectral Parameters of Rhenium(I) Complexes in Acetonitrile at 298 Ka

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λmax/nm (ε/104 L mol−1 cm−1)

compound

RESULTS AND DISCUSSION Chemical Structure Characterization. The investigated compounds, Chart 1, were prepared in their trans-isomers and isolated as PF6− salts by a well-established procedure,11,16 in which a fac-[Re(CO)3(NN)Cl] precursor is obtained, followed by substitution of the Cl− by the trans-stpyCN ligand (see the Experimental Section). For these Cs-symmetry complexes, the 1 H NMR spectra of both compounds exhibit only one set of five hydrogen signals (Ha, He,f, Hb, Hc, and Hd) for transstpyCN and another set of four hydrogen signals (H6, H3, H5, and H4 or H7) for the NN ligand whose total signal integration is equal to half of the number of hydrogens in this ligand, Figure 1 and Table 1. Coordination of NN ligands to the Re(I) center leads to hydrogens deshielding hence 1H NMR signals are shifted to lower fields (higher δ) when compared to noncoordinated NN ligands, Table 1 (Figures S4 and S5 in the Supporting Information). Such an effect is ascribed to the strong electronwithdrawing effect from Re(I), as usually observed for Re(I)− polypyridine complexes.20,23,24 The aromatic signals for dmb in 2 lie at higher fields than for bpy in 1 as a consequence of a higher hydrogen shielding induced by the electron-donating CH3. On the other hand, coordination of trans-stpyCN to Re(I) results in the shifting of hydrogens signals to higher fields (lower δ) because the anisotropic effect exerted by NN is stronger than the metal’s electron-withdrawing. Such an observation is typical of fac-[Re(CO)3(NN)(trans-L)]+ complexes.11,16 The hydrogen coupling constant between Hc and Hd (J3 ≈ 16 Hz, Table 1) is typical of trans-olefin isomers.16,25 Infrared (IR) spectra of both organometallic trans-1 and trans-2 complexes exhibit intense bands at 2031 and 1915 cm−1, Figure 2, assigned to the symmetric CO stretching frequencies, ν(CO), typical of A′(1) and A″/A′(2) modes of fac[Re(CO)3(NN)(L)] complexes with Cs symmetry.21,22,24,26 The change of Cl− in fac-[Re(CO)3(NN)Cl] precursors

trans-1 trans-2 cis-1c cis-2c

238 237 237 235

(2.1), 320 (4.4), 334(3.8)b (2.9) Sh, 317 (4.4), 330(3.8)b (2.9), 281 (2.1) Sh, 310 (2.3) Sh, 320 (3.4), 333 (1.3)b (3.6), 280 (2.6) Sh, 305 (2.8) Sh, 317 (2.8), 330 (1.7)b

a

Sh = Shoulder. bContribution of 1MLCTRe→NN and 1ILstpyCN transitions cThe εcis (λ) of cis complexes were determined by eq 4 described in the Experimental section.

Figure 3. Electronic spectra for (A) trans-1 (orange −), cis-1 (red -·-), (B) trans-2 (cyan −) and cis-2 (blue -·-) in acetonitrile at 298 K. The εcis (λ) of cis-isomers were determined by eq 4, as described in the Experimental Section. 6074


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Figure 4. 1H NMR (300 MHz, CD3CN) and absorption (CH3CN) spectral changes for (A) trans-1 and (B) trans-2 as a function of photolysis time (λirr = 404 nm, Δt = 30 s, T = 298 K) toward a photostationary state (PS).

Table 3. Apparent and True Quantum Yields for trans-to-cis Photoisomerization of trans-1 and trans-2, Non-Coordinated trans-stpyCN, and [trans-HstpyCN]Cl in Acetonitrile at 298 K λirradiation 313 nm compound

Φapp trans→cis

trans-1 trans-2 trans-stpyCNa [trans-hstpyCN]Cla

0.28 ± 0.02 0.27 ± 0.01 − −

334 nm Φtrue trans→cis

Φapp trans→cis

± ± ± ±

0.27 ± 0.04 0.28 ± 0.02 − −

0.44 0.43 0.49 0.47

0.03 0.02 0.04 0.05


0.44 0.47 0.43 0.49

± ± ± ±

0.01 0.02 0.04 0.05

Φapp trans→cis


0.28 ± 0.02 0.45 ± 0.02 0.28 ± 0.02 0.48 ± 0.06 non absorptive − 0.52 ± 0.05

Φapp trans→cis

Φtrue trans→cis

0.23 ± 0.01 0.39 ± 0.03 0.20 ± 0.02 0.36 ± 0.03 non absorptive non absorptive

a

data from ref 14.

bending of trans-ethylene’s C−H (δ(C−H)) is observed around 975 cm−1, as previously reported for trans-4-styrylpyridines with cyano and halogen substituents.48 Assuming a Oh symmetry for the PF6− counterions, two bands assigned to 2F1u modes can be observed:49 a stretching mode, ν(P−F), lying at higher frequencies (842 cm−1) and a bending mode, δ(P−F), at lower frequencies (558 cm−1), within the trans-stpyCN fingerprint region.

(Figures S11 and S12 in the Supporting Information) to transstpyCN in trans-1 and trans-2 leads to a decrease of the Re-toCO π-backbonding, resulting in a shift of CO modes to higher frequencies (Δν(CO) ∼ 15 cm−1). The band around 2225 cm−1 is assigned to the ν(CN) stretching mode, similarly observed for the noncoordinated trans-stpyCN (Figure S13 in the Supporting Information). The characteristic out-of-plane 6075


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The Journal of Physical Chemistry A Electronic Absorption Spectra. Acetonitrile solutions of trans and cis isomers of complexes 1 and 2 at 298 K exhibit an intense absorption between 200 and 300 nm, Table 2 and Figure 3, ascribed to π → π* intraligand transitions in the bipyridyl ligand, i.e.1ILNN (for comparison, electronic spectra of noncoordinated NN ligands are shown in Figure S15 in the Supporting Information). The intense band from 330 to 420 is assignedby comparison with precursor’s spectra, Figures S16 and S17 in the Supporting Informationto an overlap between 1MLCTRe→NN and 1ILstpyCN transitions, similarly observed for other fac-[Re(CO)3(NN)(trans-L)]+ complexes.11,12,14,18,21,31,33,36−38,41 The absorption spectra of the investigated complexes are blue-shifted in comparison to other fac-[Re(CO)3(NN)(transstpyCN)]+ complexes with NN = phen14 or ph2phen31 (i.e., 1 MLCT Re→dmb > 1 MLCT Re→bpy > 1 MLCT Re→phen > 1 MLCTRe→ph2phen) as the electron-withdrawing stabilization of π*NN orbitals is less pronounced for NN ligands. Furthermore, bipyridyl ligands have lower oscillator strengths than phenanthroline ligands,14,31 leading to overall lower molar absorptivities for complexes 1 and 2. The absorption profiles are very similar to fac-[Re(CO)3(NN)(stpy)]+ complexes, in which stpy =4-styrylpyridine;22,37,40,41 however, the CN group’s electron-withdrawing effect leads to a redshift of 1ILstpyCN in comparison to 1ILstpy states. Photochemical Behavior in Fluid Solution. trans-tocis. Irradiations of acetonitrile solutions of trans-1 and trans-2 at 313, 334, 365, and 404 nm lead to UV−vis and 1H NMR spectral changes as a function of photolysis time, Figure 4, ascribed to the trans-to-cis photoisomerization of the stpyCN ligand coordinated to Re(I). The continuous irradiation of trans-1 and trans-2 solutions leads to a photostationary state (PS), where the concentrations of the cis- and trans- species remain constant. UV−vis spectral changes lead to well-defined isosbestic points (at 282 nm for 1 and 284 nm for 2), which indicate no competitive photoreactions within the experiment time. The absence of side photoproducts is also corroborated by the 1H NMR spectra of PS solutions, Figure 4. The decrease of the absorption intensity in the 280−400 nm region after light irradiation is assigned to a lower contribution of the 1ILstpyCN transition in the absorption of the cis-isomers due to the difference in the dihedral angles of trans and cis species:14,16,38 in trans-complexes, the Py and Ph rings of the ethylene group (Py−CH = CH−Ph) are planar, allowing a strong π interaction; in cis-complexes, however, such a π conjugation is decreased due to an angle distortion of the rings.38 Upon irradiation, the 1H NMR signals for the trans-isomer decrease gradually, while new signals ascribed to the cis-isomer build up in intensity (Figure 4). In special, the hydrogen coupling constants for Hc′ and Hd′ (J3 = 12 Hz, Table 1) are typical for cis-olefinic hydrogens11 and confirm a successful photoisomerization. A similar behavior is also observed for the PS-solution of the noncoordinated stpyCN ligand. For cis-complexes, the planarity loss around the Py−CH CH−Ph group leads to a stronger shielding of the cis-1H nucleus and hence to a displacement of cis-1H signals to lower δ; such an effect is more pronounced for stpyCN hydrogens than for NN ones. The ratio of 1H NMR signal integrals for cis and trans isomers leads to PS solutions with 54% and 57% cis-fractions for 1 and 2, respectively. The trans-to-cis photoisomerization quantum yields deterapp mined by absorption changes are apparent (Φtrans→cis ) since the

cis-photoproducts absorb in the same region of the transcomplexes. The true quantum yields (Φtrue trans→cis) is determined by 1H NMR since chemical shifts (δ) differ for trans- and cisisomers, Figure 4 and Table 1. The average apparent true (Φapp trans→cis) and true (Φtrans→cis) quantum yields for trans-to-cis photoisomerization of trans-1 and trans-2 in acetonitrile are listed in Table 3 (individual triplicate data are summarized in Tables S2−S5 and S7−S10 in the Supporting Information). true for trans-1 and trans-2 are dependent on The Φtrans→cis irradiation wavelengths. Under UV irradiations, the values are

Figure 5. Absorption spectral changes for photostationary solutions of (A) cis-1 and (B) cis-2 in CH3CN as a function of photolysis time (λirr = 255 nm; Δt = 1 min; T = 298 K).

Figure 6. Absorption spectral changes for (A) trans-1 and (B) trans-2 in PMMA films (λirr = 404 nm; Δt = 10 s; T = 298 K). 6076


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Figure 7. Emission spectra for (A) trans-1 (cyan), (B) cis-1 (blue), (C) trans-2 (green), (D) cis-2 (dark green), (E) fac-[Re(CO)3(phen)(transstpyCN)]PF6 (yellow), (F) fac-[Re(CO)3(phen)(cis-stpyCN)]PF6 (dark green), (G) fac-[Re(CO)3(ph2phen)(trans-stpyCN)]PF6 (brown), (H) fac-[Re(CO)3(ph2phen)(cis-stpyCN)]PF6 (brown), (I) trans-stpyCN (gray), and (H) cis-stpyCN (black) in fluid acetonitrile () and in rigid PMMA (−•−•−) (λexc = 300 nm, T = 298 K). 3

ILstpyCN counterpart through 1MLCTRe→NN/3MLCTRe→NN states. A similar behavior has been observed for other fac[Re(CO)3(Rphen)(trans-L)]+ complexes.14−16,18 cis-to-trans. Irradiation at 255 nm of photostationary solutions of cis-1 and cis-2 complexes also lead to spectral changes ascribed to the reverse cis-to-trans photoisomerization, Figure 5. The isosbestic points for the cis-to-trans photoprocess is observed in the same wavelength (at 282 nm for 1 and 284 nm for 2) of the trans-to-cis isomerization, demonstrating a high photochemical stability of 1 and 2.

similar to those determined for noncoordinated trans-stpyCN and [trans-HstpyCN]Cl. The Φtrue trans→cis for irradiations at 404 nm− spectral region where the noncoordinated ligands do not absorb light−are slightly lower than those at the 313−365 nm region. The occurrence of trans-to-cis photoisomerizations for trans-1 and trans-2 complexes under excitation at higher energies (313−365 nm) can be ascribed to the stpyCN-localized 1ILstpyCN excited state, accessible only under UV irradiation. On the other hand, the occurrence of the photoisomerization in the visible is ascertained by an efficient sensitization of the lowest-lying 6077


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Table 4. Photophysical Parameters for fac-[Re(CO)3(NN)(stpyCN)]+ and Non-Coordinated stpyCN in Acetonitrile at 298 K (λexc = 300 nm) λem/nm

compound trans-1 cis-1 trans-2 cis-2 trans-stpyCN cis-stpyCN

505 505 505 505 340 340

(Sh), (Sh), (Sh), (Sh), (Sh), (Sh),

535 535 530 530 355, 490 (Sh), 530 355, 490 (Sh), 530

ϕema 0.019 ± 0.022 ± 0.026 ± 0.027 ±

Photoreversible Molecular Motion of stpyCN Coordinated to fac-[Re

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