J. Phys. Chem. A 2010, 114, 9007–9013
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The Effect of Intermolecular Hydrogen Bonding on the Fluorescence of a Bimetallic Platinum Complex Guang-Jiu Zhao,† Brian H. Northrop,‡ Ke-Li Han,*,† and Peter J. Stang*,‡ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112, USA ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: July 29, 2010
The bimetallic platinum complexes are known as unique building blocks and arewidely utilized in the coordination-driven self-assembly of functionalized supramolecular metallacycles. Hence, photophysical study of the bimetallic platinum complexes will be very helpful for the understanding on the optical properties and further applications of coordination-driven self-assembled supramolecular metallacycles. Herein, we report steady-state and time-resolved spectroscopic experiments as well as quantum chemistry calculations to investigate the significant intermolecular hydrogen bonding effects on the intramolecular charge transfer (ICT) fluorescence of a bimetallic platinum compound 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone 3 in solution. We demonstrated that the fluorescent state of compound 3 can be assigned as a metal-to-ligand charge transfer (MLCT) state. Moreover, it was observed that the formation of intermolecular hydrogen bonds can effectively lengthen the fluorescence lifetime of 3 in alcoholic solvents compared with that in hexane solvent. At the same time, the electronically excited states of 3 in solution are definitely changed by intermolecular hydrogen bonding interactions. As a consequence, we propose a new fluorescence modulation mechanism by hydrogen bonding to explain different fluorescence emissions of 3 in hydrogen-bonding solvents and nonhydrogen-bonding solvents. I. Introduction Solute-solvent interactions play a fundamental role in the photophysics and photochemistry of organic and biological chromophores in solution.1-8 In addition to the nonspecific dielectric interactions between solute and solvent, the sitespecific intermolecular hydrogen bonding interaction between hydrogen donor and acceptor molecules is another important type of solute-solvent interaction and is central to the understanding of the microscopic structure and function in many molecular systems.9-24 In the past years, the hydrogen bonding effects on the structures and dynamics of many important molecular systems have been well investigated.25-32 Furthermore, it has been demonstrated that excited-state hydrogen bonding can significantly determine many photophysical processes and photochemical reactions that occur in hydrogenbonded surroundings.33-37 For example, scientists have studied the excited-state hydrogen bonding structures and dynamics for a number of organic and biological chromophores in solution by use of combined experimental and theoretical methods.38-47 It was demonstrated that the intermolecular hydrogen bonding formed between chromophores and solvents can be significantly strengthened or weakened in electronically excited states and the excited-state hydrogen bonding dynamics (ESHBD) has a remarkable influence on the excited-state processes of organic and biological chromophores in solution, such as internal conversion (IC),38-40 intersystem crossing (ISC),41 photoinduced electron transfer (PET),42 site-specific solvation (SSS),43 excited* To whom correspondence should be addressed. Phone: 86-41184379293; 801-581-8329 (USA). Fax: 86-411-84675584; 801-581-6306 (USA). E-mail:
[email protected] (K.-L.H.);
[email protected] (P.J.S.). † Chinese Academy of Sciences. ‡ University of Utah.
state intramolecular proton transfer (ESIPT),44 and excited-state double proton transfer (ESDPT), etc.45-47 In recent years, hydrogen bonding effects on the intramolecular charge transfer (ICT) excited states of many functionalized molecular systems have been examined.48-55 The hydrogenbonded twisted intramolecular charge transfer (TICT) excited state was theoretically studied by Zhao et al. by use of the TDDFT method to calculate excited-state vibrational modes.56 Very recently, they also investigated the intermolecular hydrogen bonding and its influence on the structural and spectral properties of aquo palladium(II) complexes.57 It is worthy to note that these molecular systems are organic or mononuclear metallic complexes. However, the hydrogen bonding effects on the ICT fluorescent states of polynuclear metallic complexes are less investigated. Hence, in this study, we investigate the hydrogen bonding effects on the ICT fluorescent states of a bimetallic platinum compound 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone. Stang and co-workers have reported the synthesis of 4,4′bis(trans-Pt(PEt3)2OTf)benzophenone and its applications for the coordination-driven self-assembled metallosupramolecular macrocycles.58-70 Consequently, it is useful to investigate the ICT excited states in order to understand the photophysics and photochemistry of the metallosupramolecular macrocycles built from 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone. At the same time, the bimetallic platinum compound 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone is a good probe to study the intermolecular hydrogen bonding effects on the electronically excited states, since the carbonyl group of 4,4′-bis-benzophenone is an ideal site to form intermolecular hydrogen bonding which may significantly influence the electronic excitation of 4,4′-bis(transPt(PEt3)2OTf)benzophenone in protic solvents. Moreover, Flynn et al. have systematically investigated the charge transfer dynamics and the rate of intersystem crossing in the rectangular
10.1021/jp105009t 2010 American Chemical Society Published on Web 08/11/2010
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SCHEME 1: Synthesis of Bimetallic Platinum Compound 4,4′-Bis(trans-Pt(PEt3)2OTf)benzophenone 3
anthracene-containing metallacycles and triangular phenanthrene-containing metallacyclic systems by use of femtosecond fluorescence upconversion and transient absorption.71 As a result, they observed that the ultrafast charge transfer and intersystem crossing processes vary with the different geometries and dimensions of these metallacyclic systems.71 In the present work, both steady-state and time-resolved spectroscopic techniques have been used to investigate the spectral properties of the bimetallic platinum complex 4,4′bis(trans-Pt(PEt3)2OTf)benzophenone in hexane and alcoholic solvents, respectively. Furthermore, the hydrogen-bonded complexes formed by 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone and alcoholic molecules were used to study the intermolecular hydrogen bonding of the bimetallic platinum complex 4,4′bis(trans-Pt(PEt3)2OTf)benzophenone in protic alcoholic solvents. The ground and excited states of the isolated 4,4′bis(trans-Pt(PEt3)2OTf)benzophenone and its hydrogen-boned complexes have been theoretically investigated by use of density functional theory (DFT) and time-dependent density functional theory (TDDFT) method, respectively. At the same time, the conductor-like screening model (COSMO) has also been used to consider the solvation effects of the bimetallic platinum complex 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone in various solvents. II. Experimental and Theoretical Methods The bimetallic platinum compound 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone (3) was synthesized from AgOSO2CF3 and the Pt iodide compound (2) which is made by 4,4′-diiodobenzophenone (1) and Pt(PEt3)4 according to literature procedures (Scheme 1).69,70 The UV-vis absorption spectra were measured on an HP 8453 spectrophotometer. Steady-state fluorescence and timeresolved fluorescence decays were determined on a Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. Time-resolved fluorescence decays were recorded using the time-correlated single photon counting (TCSPC) method.72-76 Data analysis was carried via commercial software provided by Horiba Instruments. The ground states of both the isolated molecule 3 and its hydrogen-bonded complexes were studied by the density functional theoretical (DFT) method with the BP86 functional.77 Moreover, resolution-of-the-identity (RI) approximation was also used to improve the efficiency without sacrificing the accuracy of the results.78 The triple-ζ valence quality with one set of polarization functions (TZVP) was chosen as basis sets and the corresponding auxiliary basis sets for the RI approximation throughout. In addition, the pseudopotential def-ecp basis set was also used for the platinum atoms. The electronic excited
Figure 1. Steady-state absorption and fluorescence spectra of compound 3 in different solvents. Solid lines denote the absorption spectra. Dashed lines denote the fluorescence spectra at the excitation wavelength of 295 nm. The values of the absorption and fluorescence spectral maxima are also shown.
states were investigated by the time-dependent density functional theory (TDDFT) method with the B3LYP (Becke, threeparameter, Lee-Yang-Parr) exchange-correlation functional and the TZVP basis set.79-81 The solvent permittivity constants of 1.88 for hexane, 13.3 for hexanol, and 24.5 for ethanol were used in COSMO (conductor-like screening model) solvation model to consider solvation effects of 3 in hexane, hexanol, and ethanol, respectively.82 All quantum chemical calculations were performed using the Turbomole program suite. III. Results and Discussion The steady-state absorption and fluorescence spectra of compound 3 in different solvents are presented in Figure 1. The values of the absorption and fluorescence spectral maxima are also shown. It is evident that the absorption maximum of 3 shifts to the red from 299 nm in hexane to 305 and 308 nm in hexanol and ethanol, respectively. At the same time, one can note that the fluorescence maximum of 3 has also an evident red-shift from 323 nm in hexane to 338 and 355 nm in hexanol and ethanol solvents, respectively. Hence, the formation of intermolecular hydrogen bonds between 3 and protic solvents can be indicated by the absorption and fluorescence spectral redshift of 3 in alcoholic solvents by comparison with that in hexane solvent. Moreover, the Stokes shift of 3 in hexane is 24 nm, whereas the Stokes shifts in hexanol and ethanol increase to 33 and 47 nm, respectively. The increased Stokes shift in polar protic solvents suggests that the fluorescent state of 3 may be of the ICT feature.46,54 Hence, intermolecular hydrogen bonding interactions will have a very significant influence on the fluorescent states of 3 in solutions. To further understand the influence of intermolecular hydrogen bonding interaction on the fluorescent states of 3 in solutions, the time-resolved fluorescence decays of the fluorescence maxima for 3 in different solvents at the excitation wavelength of 295 nm have been measured and are shown in Figure 2. The fluorescence decays are fitted by a triexponential with good quality as indicated by the χ2 values. The triexponential fitting results with three fluorescence lifetime values and their corresponding relative amplitude (RA) as well as the average fluorescence lifetime values are listed in Table 1. The average lifetime values of the fluorescence maxima at 323 nm for 3 in hexane, 338 nm in hexanol, and 355 nm in ethanol are 4.257, 12.54, and 12.85 ns, respectively. It is evident that the lifetime values for the fluorescence of 3 in hydrogen-bonding solvents are much larger than that in nonhydrogen-bonding
Intramolecular Charge Transfer Fluorescence
Figure 2. Time-resolved fluorescence decays of the fluorescence maxima for compound 3 in different solvents at the excitation wavelength of 295 nm. Dotted lines denote the experimental data and solid lines denote the fitted results.
solvents. As a result, the fluorescence lifetime of compound 3 in solution is evidently increased by the intermolecular hydrogen bonding interactions. Moreover, it can be noted that the fluorescence lifetime value of 3 in ethanol is very close to that in hexanol, although the hydrogen donor ability of ethanol is much stronger than that of hexanol. In addition, the corresponding time constants via triexponential function fitting are nearly same for 3 in hexanol and ethanol solvents. The different fluorescence lifetimes of 3 in hydrogen-bonding and nonhydrogen-bonding solvents suggests that the electronically excited states of 3 in solution may be changed by intermolecular hydrogen bonding interactions. Consequently, the mechanism of fluorescence emission for C3 in hydrogen-bonding solvents will be different from that in nonhydrogen-bonding solvents. The geometric and electronic structures for the ground and excited states of the isolated molecule 3 and its hydrogen-bonded complexes are studied by use of the DFT/TDDFT method. Figure 3 presents the optimized geometric structures of 3 and the hydrogen-bonded ethanol/3 and hexanol/3 complexes. Some important parts have been denoted in their geometric structures. It is evident that the conformation of molecule 3 can be divided into two identical parts by the central carbonyl bond. Hence, we will only give the structural information for one-half, including some important coordination bonds. It should be noted that the parent benzophenone is not coplanar. So the dihedral angles formed by the two benzene rings are important structural data. In addition, the structural framework of 3 can be determined by the triangle formed by the two Pt atoms and C5. Thus, the distances between Pt, Pt′, and C5 and the angles formed by them are also important structural data. Moreover, the structural information of two intermolecular hydrogen bonds C5dO2 · · · H1 and C2-H2 · · · O3 formed between 3 and alcohols are presented. All the important geometric parameters mentioned above are listed in Table 2. One can note that all the coordination bond lengths are nearly unchanged in both the hydrogen-bonded hexanol/3 and ethanol/3 complexes by comparison with those in the compound 3 itself. This suggests that the coordination bonds cannot be strongly influenced by the formation of the intermolecular hydrogen bonds between 3 and alcoholic solvents. However, it should be noted that the intermolecular hydrogen bonds can influence the structural framework of 3 determined by the PtC5Pt′ triangle. One can observe that all the distances between Pt, Pt′, and C5 decrease in the presence of the intermolecular hydrogen bonds. At the same time, the PtC5Pt′ angle also decreases in the hydrogen-bonded complexes. The decrease of the distance between the C5 and Pt atoms may
J. Phys. Chem. A, Vol. 114, No. 34, 2010 9009 be due to the shortening of the C5-C3 bond induced by intermolecular hydrogen bonding interactions in the hydrogenbonded complexes. However, one can observe that the carbonyl bond is lengthened due to intermolecular hydrogen bonding. Moreover, the intermolecular hydrogen bond length of C5dO2 · · · H1 is 1.867 and 1.863 Å in the hydrogen-bonded hexanol/3 and ethanol/3 complexes, respectively. It is evident that the intermolecular hydrogen bond C5dO2 · · · H1 in the hydrogen-bonded ethanol/3 is stronger than that in the hydrogenbonded hexanol/3. This is consistent with the hydrogen donor ability of these alcoholic solvents. In addition, the intermolecular hydrogen bond length of C2-H2 · · · O3 is 2.472 and 2.449 Å in the hydrogen-bonded hexanol/3 and ethanol/3 complexes, respectively. One can note that the intermolecular hydrogen bond C2-H2 · · · O3 is much weaker than the intermolecular hydrogen bond C5dO2 · · · H1. Similarly, the intermolecular hydrogen bond C2-H2 · · · O3 in the hydrogen-bonded ethanol/3 complex is also stronger than that in the hydrogen-bonded hexanol/3 complex. At the same time, the formation of intermolecular hydrogen bonds C5dO2 · · · H1 and C2-H2 · · · O3 induced dihedral angles of C2C3C5O2 and C4C3C5C6 to become larger in the hydrogenbonded hexanol/3 and ethanol/3 complexes. The calculated electronic excitation energies and the corresponding oscillator strengths of the low-lying electronically excited states with consideration of the COSMO solvation model are listed in Table 3. As seen the oscillator strength of the S1 state (f ) 0.004) for 3 in hexane solvent is very close to zero, whereas the S2 state has the maximum oscillator strength (f ) 0.763). Consequently, it is demonstrated that compound 3 in hexane can be directly photoexcited to the S2 state. At the same time, there is a dark state (S1 state) of the lowest energy level below the fluorescent state of 3 in hexane. On the other hand, the calculated electronic excitation energy of the S2 state for 3 in hexane is 317 nm, which is in agreement with the experimental spectral results. It should be noted that the S1 state of both the hydrogen-bonded hexanol/3 and ethanol/3 complexes in the corresponding solvents have the maximum oscillator strength. Although the oscillator strengths of the S2 state for both the hydrogen-bonded hexanol/3 and ethanol/3 complexes become very small. Therefore, compound 3 in alcoholic solvents can be directly photoexcited to the S1 state. That means that there is no dark state of the lowest energy level below the fluorescent state of 3 in alcoholic solvents. Hence, it is confirmed by the theoretical results that the intermolecular hydrogen bonding interaction changes the electronically excited states of compound 3 in solution. Furthermore, the calculated electronic excitation energy of the S1 state for the hydrogen-bonded hexanol/3 and ethanol/3 complexes in these solvents is 334 and 335 nm, respectively. It is evident that the electronic excitation energies of the hydrogen-bonded hexanol/3 and ethanol/3 complexes have a red-shift in comparison with that of pure 3, which is also consistent with the experimental results. The main orbital transition contribution and corresponding CI coefficients of the low-lying electronically excited states of pure 3 and its hydrogen-bonded hexanol/3 and ethanol/3 complexes are also listed in Table 2. It is noted that the S2 state of compound 3 corresponds to the orbital transition from the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital). At the same time, the S1 state of both the hydrogen-bonded complexes also corresponds to the HOMO f LUMO orbital transition. Hence, all the fluorescent states of the maximum oscillator strengths are corresponding to the HOMO f LUMO orbital transition. Herein, we will only discuss the HOMO and LUMO orbitals
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TABLE 1: Tri-Exponential Fit Results with Three Lifetime Values (τ/ns) And The Corresponding Relative Amplitude (RA) As Well As The Average Lifetime Values (τa/ns) for The Fluorescence Decays At the Fluorescence Maxima of Compound 3: 323 nm in Hexane, 338 nm in Hexanol, and 355 nm in Ethanola triexponential fit solvents
τ1
RA1
τ2
RA2
τ3
RA3
τa
χ2
hexane hexanol ethanol
0.518 ( 0.09 6.306 ( 0.19 5.031 ( 0.39
7.08 38.41 12.66
2.140 ( 0.10 1.633 ( 0.04 1.083 ( 0.13
26.88 11.29 2.18
5.519 ( 0.03 19.75 ( 0.08 14.31 ( 0.04
66.04 50.29 85.16
4.257 12.54 12.85
1.137 1.105 1.067
a
The Quality of The Tri-Exponential Fit Is Measured by the χ2 Values.
TABLE 2: Calculated Important Bond Lengths (L/Å), Bond Angles (A/°), Dihedral Angles (DA/°) of the Optimized 3 and Hydrogen-Bonded Hexanol/3 and Ethanol/3 Complexes L(Pt-P1) L(Pt-P2) L(Pt-C1) L(Pt-O1) L(Pt-Pt′) L(Pt-C5) L(Pt′-C5) L(C5-C3) L(C3-C1) L(C5dO2) L(O2 · · · H1) L(H1-O3) L(O3 · · · H2) L(H2-C2) A(C2H2O3) A(O3H1O2) A(C5O2H1) A(PtC5Pt′) DA(C2C3C5O2) DA(C8C6C5O2) DA(C4C3C5C6) DA(C7C6C5C3) Figure 3. Optimized geometric structures of compound 3 and the hydrogen-bonded ethanol/3 and hexanol/3 complexes. Some important atoms and intermolecular hydrogen bonds are labeled.
of pure 3 and the hydrogen-bonded complexes and show them in Figure 4. One can note that the electron density of the HOMO orbital for both the 3 and hydrogen-bonded complexes is populated at the two metallic Pt and benzene rings. Moreover, the electron density populations of the LUMO orbital for both 3 and hydrogen-bonded complexes are changed and transferred to the carbonyl group and benzene rings. This means that the charge transfer from the two metallic Pt to the carbonyl group will take place during the HOMO f LUMO transition. As a consequence, both the S2 state of pure 3 and the S1 state of the hydrogen-bonded complexes, which correspond to the HOMO f LUMO orbital transition, can be assigned as a metal-to-ligand charge transfer (MLCT) state.83 It is also confirmed that the fluorescent state of 3 has definite ICT feature. In addition, one can observe that the electron density at the site of carbonyl group will significantly increase after the HOMO f LUMO orbital transition. This resembles some of the previously studied carbonyl compounds, such as Coumarin 102 and Fluorenone, etc.,38,40 whose intermolecular hydrogen bonds in solution have been demonstrated to be significantly strengthened in the electronically excited states upon photoexcitation. Consequently, it can also be proposed that intermolecular hydrogen bonds between compound 3 and alcoholic solvents will be strengthened in the MLCT excited state following the ICT from the two metallic Pt to the carbonyl group. The strengthened intermo-
3
hexanol/3
ethanol/3
2.375 2.372 2.014 2.253 11.36 6.362 6.364 1.498 2.854 1.237
2.377 2.373 2.011 2.250 11.29 6.352 6.353 1.491 2.853 1.244 1.867 0.985 2.472 1.093 151.2 171.3 137.1 125.3 26.64 24.64 30.46 29.38
2.376 2.372 2.011 2.252 11.29 6.352 6.352 1.491 2.853 1.244 1.863 0.985 2.449 1.093 152.4 171.4 136.6 125.4 26.57 24.67 30.43 29.47
1.092
126.4 22.92 24.79 26.96 29.07
lecular hydrogen bonds in the electronically excited states will influence the spectral properties more strongly than the hydrogen bonds in the ground state. Therefore, as shown in Figure 1, the intermolecular hydrogen bonding can induce a larger red-shift of the fluorescence spectra than that of the corresponding absorption spectra. Figure 5 shows the orbital energy levels of the HOMOs and LUMOs and their energy gap for the pure 3 and the hydrogenbonded complexes. It is clear that the energy level of the HOMO increases from -6.37 eV for pure 3 to -6.29 and -6.28 eV for the hydrogen-bonded hexanol/3 and ethanol/3 complexes. Hence, intermolecular hydrogen bonding raises the energy level of the HOMO. At the same time, one can note that the energy level of the LUMO decreases from -1.88 eV for 3 to -2.01 eV for the hydrogen-bonded complexes. This indicates that intermolecular hydrogen bonding can lower the energy level of the LUMO. As a result, the energy gap between the HOMOs and LUMOs becomes drastically reduced from 4.49 eV for 3 to 4.28 and 4.27 eV for the hydrogen-bonded hexanol/3 and ethanol/3 complexes. The reduced energy gap between the HOMOs and LUMOs of the hydrogen-bonded complexes is consistent with the absorption and fluorescence spectral redshift of 3 in alcoholic solvents. Furthermore, it should be noted that both energy levels of the HOMOs and LUMOs as well as their energy gap are nearly the same for the two hydrogenbonded complexes, which is accordant with the nearly same fluorescence lifetime of compound 3 in ethanol and hexanol solvents. Figure 6 presents a schematic diagram on the different mechanisms of fluorescence emission for compound 3 in the
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TABLE 3: Calculated Electronic Excitation Energies (E) and Corresponding Oscillator Strength (f), Main Orbital Transition Contribution (Contrib.), and CI Coefficients of the Low-Lying Electronically Excited States of Compound 3 and Hydrogen-Bonded Hexanol/3 and Ethanol/3 Complexes with Consideration of the COSMO Solvation Modela compound 3 in hexane ( ) 1.88) S0 f S1 S0 f S2 S0 f S3 S0 f S4 S0 f S5 S0 f S6
hexanol/3 complex in hexanol ( ) 13.3)
E(nm/eV)
f
contrib.
CI
E(nm/eV)
f
342 (3.63) 317 (3.91) 297 (4.17) 296 (4.18) 290 (4.28) 289 (4.30)
0.004 0.763 0.003 0.001 0.058 0.014
H-4 f L HfL H-2 f L H-3 f L H-1 f L H-2 f L+2
0.405 0.950 0.900 0.956 0.635 0.468
334 (3.72) 328 (3.78) 314 (3.95) 312 (3.97) 299 (4.14) 291 (4.25)
0.762 0.017 0.000 0.009 0.065 0.006
contrib. HfL H-1 f L H-3 f L H-2 f L H-1 f L H-4 f L
ethanol/3 complex in ethanol ( ) 24.5)
CI
E(nm/eV)
f
0.971 0.305 0.979 0.779 0.560 0.726
335 (3.71) 328 (3.78) 315 (3.94) 313 (3.96) 299 (4.14) 292 (4.25)
0.765 0.017 0.000 0.010 0.060 0.006
contrib. HfL H-1 f L H-3 f L H-2 f L H-1 f L H-4 f L
CI 0.973 0.304 0.977 0.749 0.541 0.727
a Solvents are chosen to be consistent with the spectral experiments. The permittivity constants () of various solvents used in the COSMO solvation model are given in the corresponding parentheses. The CI coefficients are in absolute values. H stands for the HOMO (highest occupied molecular orbital), and L stands for LUMO (lowest unoccupied molecular orbital).
Figure 6. Schematic diagram on the different mechanisms of fluorescence emission for compound 3 in hydrogen-bonding solvents and nonhydrogen-bonding solvents.
Figure 4. Calculated HOMOs and LUMOs for compound 3 (a) and the hydrogen-bonded complexes ethanol/3 (b) and hexanol/3 (c).
Figure 5. Calculated energy levels of the HOMOs and LUMOs.
hydrogen-bonding solvents and the nonhydrogen-bonding solvent. In hexane, compound 3 can be photoexcited directly to the fluorescent state (S2 state) of the ICT feature that is located above a nonfluorescent state (S1 state). On the one hand, compound 3 can relax from the fluorescent state to the ground state via fluorescence emission, which is a radiative transition process. In contrast, compound 3 in the fluorescent state can also decay to the nonfluorescent state with a lower energy level by use of the nonradiative transition pathway. That is to say the nonradiative transition will compete with the fluorescence emission of 3 in hexane due to the presence of the nonfluorescent state below the fluorescent state. Therefore, the fluorescence
lifetime of 3 in hexane is relatively short because of the competition by the nonradiative transition. In Table 3, one can note that the nonfluorescent state of pure 3 corresponds to the orbital transition from HOMO - 4 to LUMO. It is also observed that the electron density populations of the HOMO - 4 and LUMO orbitals are nearly identical from Figure 4a. That means that the nonfluorescent state of 3 has no evident ICT feature. The energy level of the ICT fluorescent state can be more easily lowered by intermolecular hydrogen bonding in alcoholic solvents than that of the nonfluorescent state having no evident ICT feature. As a consequence, the fluorescent state of 3 in hexanol and ethanol solvents is significantly lowered and under the nonfluorescent state by intermolecular hydrogen bonding interactions. Therefore, compound 3 in alcoholic solvents can be photoexcited directly to the fluorescent state of the lowest energy level. In this case, the nonradiative transition from the fluorescent state to the nonfluorescent state cannot take place anymore. As a result, the fluorescence lifetimes of 3 in hexanol and ethanol solvents are significantly lengthened by comparison with that of 3 in hexane due to the absence of competition by nonradiative transition from the fluorescent state to the nonfluorescent state. IV. Conclusion In the present work, various spectral properties of a bimetallic platinum complex 4,4′-bis(trans-Pt(PEt3)2OTf)benzophenone 3 in different solvents were investigated by use of both steadystate and time-resolved spectroscopic techniques. The formation of intermolecular hydrogen bonds between compound 3 and protic solvents is indicated by the absorption and fluorescence spectral red-shift of 3 in alcoholic solvents by comparison with that in hexane. Moreover, the increased Stokes shift in polar protic solvents suggests that the fluorescent state of 3 may be of the intramolecular charge transfer (ICT) feature. To further
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understand the influence of intermolecular hydrogen bonding interactions on the fluorescent states of 3 in solution, timeresolved fluorescence decays of the fluorescence maxima for 3 in different solvents at the excitation wavelength of 295 nm have been measured. As a result, it is observed that the fluorescence lifetime of 3 in alcoholic solvents is remarkably increased due to the presence of intermolecular hydrogen bonding interactions. Furthermore, it is noted that the fluorescence lifetime of 3 in ethanol is very close to that in hexanol, although the hydrogen donor ability of ethanol is much stronger than that of hexanol. Consequently, the different fluorescence lifetimes suggest that the electronically excited states of 3 in solution may be changed by intermolecular hydrogen bonding and the fluorescence mechanism of 3 in the hydrogen-bonding solvents will be different from that in the nonhydrogen-bonding solvents. The geometric and electronic structures for the ground and excited states of compound 3 and its hydrogen-bonded complexes have been theoretically studied using the DFT/TDDFT method. The COSMO model has also been used to consider the solvation effects of 3 in various solvents. From the optimized geometric structures, it is observed that both the distances between Pt, Pt′, and C5 atoms and the PtC5Pt′ angle decrease in the presence of the intermolecular hydrogen bonds. At the same time, the formation of intermolecular hydrogen bonds C5dO2 · · · H1 and C2-H2 · · · O3 induced dihedral angles of C2C3C5O2 and C4C3C5C6 to become larger in the hydrogenbonded complexes. In addition, it is confirmed by the calculated electronic excitation energies and corresponding oscillator strengths that the intermolecular hydrogen bonding changes the electronically excited states of 3 in solution. One can note that compound 3 in hexane is directly photoexcited to the S2 state. At the same time, there is a dark state of the lowest energy level below the fluorescent state of 3 in hexane. However, there is no dark state of the lowest energy level below the fluorescent state of 3 in alcoholic solvents, since compound 3 in alcoholic solvents is directly photoexcited to the S1 state. On the other hand, it has also been demonstrated that both the S2 state of pure 3 and the S1 state of the hydrogen-bonded complexes correspond to the HOMO f LUMO orbital transition and can be assigned as the metal-to-ligand charge transfer (MLCT) state. Hence, the ICT feature of compound 3 is theoretically confirmed. Moreover, it is proposed that the intermolecular hydrogen bonds between 3 and alcoholic solvents will be strengthened in the MLCT excited state, since the electron density at the site of the carbonyl group will significantly increase following the ICT from the two metallic Pt atoms to the carbonyl group. The strengthened intermolecular hydrogen bonding in the ICT fluorescent state accounts for the larger redshift of the fluorescence spectra than the corresponding absorption spectra. A new fluorescence modulation mechanism by hydrogen bonding has been proposed. Compound 3 in nonhydrogenbonding solvents can be photoexcited directly to the ICT fluorescent state which is located above a nonfluorescent (dark) state. Hence, nonradiative transition from the ICT fluorescent state to nonfluorescent state competes significantly with fluorescence emission in the ICT fluorescent state. As a result, the fluorescence lifetimes of 3 in nonhydrogen-bonding solvents are relatively short. However, compound 3 in hydrogen-bonding solvents can be photoexcited directly to the ICT fluorescent state of the lowest energy level, because intermolecular hydrogen bonding can remarkably lower the energy level of the ICT fluorescent state to be under the nonfluorescent state of no
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