1
|
Nair S, Bysewski OA, Klosterhalfen N, Sittig M, Winter A, Schubert US, Dietzek-Ivanšić B. Intramolecular Energy Transfer Competing with Light-Driven Intermolecular Proton Transfer in an Iron(II)-NHC Complex? A Query into the Role of Photobasic Ligands and MLCT States. ACS OMEGA 2024; 9:13427-13439. [PMID: 38524456 PMCID: PMC10955593 DOI: 10.1021/acsomega.3c06196] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 08/21/2023] [Revised: 11/15/2023] [Accepted: 11/20/2023] [Indexed: 03/26/2024]
Abstract
Inorganic photoacids and photobases comprising of photoactive transition metal complexes (TMCs) offer the ability to modulate proton transfer reactions through light irradiation, while utilizing the excellent optical properties of the latter. This provides a powerful tool for precise control over chemical reactions and processes, with implications for both fundamental science and practical applications. In this contribution, we present a novel molecular architecture amending an Fe-NHC complex with a pendant quinoline, as a prototypical photobase, as a representative earth-abundant TMC based inorganic photobase. We characterize the excited-state properties and proton-transfer dynamics using steady-state absorption and emission spectroscopy as well as pump wavelength dependent transient absorption spectroscopy in various protic solvents. The kinetics and thermodynamics of proton transfer in the quinoline moiety are influenced by both the presence of the metal center and the choice of the solvent. Furthermore, we see indications of intramolecular energy transfer from the quinoline to the MLCT state as a limiting factor for panchromatic photobasicity of the complex.
Collapse
Affiliation(s)
- Shruthi
S. Nair
- Department
Functional Interfaces, Leibniz Institute
of Photonic Technology (Leibniz-IPHT), Albert-Einstein-Strasse 9, 07745 Jena, Germany
- Institute
of Physical Chemistry, Friedrich-Schiller
University Jena, Helmholtzweg 4, 07743 Jena, Germany
| | - Oliver A. Bysewski
- Laboratory
of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany
- Center for
Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, Jena 07743, Germany
| | - Niklas Klosterhalfen
- Department
Functional Interfaces, Leibniz Institute
of Photonic Technology (Leibniz-IPHT), Albert-Einstein-Strasse 9, 07745 Jena, Germany
- Institute
of Physical Chemistry, Friedrich-Schiller
University Jena, Helmholtzweg 4, 07743 Jena, Germany
| | - Maria Sittig
- Department
Functional Interfaces, Leibniz Institute
of Photonic Technology (Leibniz-IPHT), Albert-Einstein-Strasse 9, 07745 Jena, Germany
- Institute
of Physical Chemistry, Friedrich-Schiller
University Jena, Helmholtzweg 4, 07743 Jena, Germany
| | - Andreas Winter
- Laboratory
of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany
- Center for
Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, Jena 07743, Germany
| | - Ulrich S. Schubert
- Laboratory
of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany
- Center for
Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, Jena 07743, Germany
| | - Benjamin Dietzek-Ivanšić
- Department
Functional Interfaces, Leibniz Institute
of Photonic Technology (Leibniz-IPHT), Albert-Einstein-Strasse 9, 07745 Jena, Germany
- Institute
of Physical Chemistry, Friedrich-Schiller
University Jena, Helmholtzweg 4, 07743 Jena, Germany
| |
Collapse
|
2
|
Yucknovsky A, Amdursky N. Controlling pH-Sensitive Chemical Reactions Pathways with Light - a Tale of Two Photobases: an Arrhenius and a Brønsted. Chemistry 2023:e202303767. [PMID: 38084008 DOI: 10.1002/chem.202303767] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2023] [Indexed: 12/22/2023]
Abstract
Light-gated chemical reactions allow spatial and temporal control of chemical processes. Here, we suggest a new system for controlling pH-sensitive processes with light using two photobases of Arrhenius and Brønsted types. Only after light excitation do Arrhenius photobases undergo hydroxide ion dissociation, while Brønsted photobases capture a proton. However, none can be used alone to reversibly control pH due to the limitations arising from excessively fast or overly slow photoreaction timescales. We show here that combining the two types of photobases allows light-triggered and reversible pH control. We show an application of this method in directing the pH-dependent reaction pathways of the organic dye Alizarin Red S simply by switching between different wavelengths of light, i. e., irradiating each photobase separately. The concept of a light-controlled system shown here of a sophisticated interplay between two photobases can be integrated into various smart functional and dynamic systems.
Collapse
Affiliation(s)
- Anna Yucknovsky
- Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa, 3200003, Israel
| | | |
Collapse
|
3
|
Mathew R, Verma P, Barak A, Adithya Lakshmanna Y. Excited-State Dynamics in 4-[4'(Dimethylamino)styryl]pyridine, a Photobase: Role of Photoinitiated Proton-Coupled Electron Transfer. J Phys Chem A 2023; 127:7419-7428. [PMID: 37647516 DOI: 10.1021/acs.jpca.3c02502] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/01/2023]
Abstract
The photoinitiated proton-coupled electron transfer (PCET) process in photoacid-based adducts is predominantly governed by the evolution of the electron-proton transfer state. However, such a process is underexplored in the case of photobases as the excited states evolve through multiple competitive channels. Here, we elucidate the excited-state dynamics of a photobase, 4-[4'-(dimethylamino)styryl]pyridine (DMASP), in the presence of hexafluoroisopropanol (HFIP) that enables PCET. Transient absorption measurements show the evolution of a protonated species in the excited state with a time constant of ∼2.5 ps. Fluorescence upconversion measurements reveal the signatures of an emissive intramolecular charge transfer state and a protonated state. The role of such states is further confirmed by time-resolved measurements in the presence of trifluoroacetic acid and computational analysis. Furthermore, the proton-abstraction dynamics of DMASP is analyzed in bulk methanol and butanol solvents. The extent of proton abstraction by DMASP is found to be higher in the presence of HFIP when compared with the normal alcohols over a time period of a few picoseconds.
Collapse
Affiliation(s)
- Reshma Mathew
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram 695551, India
| | - Preetika Verma
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram 695551, India
| | - Arvind Barak
- Department of Inorganic and Physical Chemistry, Indian Institute of Science Bangalore, Bangalore 560012, India
| | - Yapamanu Adithya Lakshmanna
- School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram 695551, India
| |
Collapse
|
4
|
Ley C, Siedel A, Bertaux T, Croutxé-Barghorn C, Allonas X. Photochemical Processes of Superbase Generation in Xanthone Carboxylic Salts. Angew Chem Int Ed Engl 2023; 62:e202214784. [PMID: 36533332 DOI: 10.1002/anie.202214784] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2022] [Revised: 12/16/2022] [Accepted: 12/16/2022] [Indexed: 12/23/2022]
Abstract
Photobase generators are species that allow the photocatalysis of various reactions, such as thiol-Michael, thiol-isocyanate, and ring-opening polymerization reactions. However, existing compounds have complex syntheses and low quantum yields. To overcome these problems, photobase generators based on the photodecarboxylation reaction were developed. We synthesized and studied the photochemistry and photophysics of two xanthone photobase, their carboxylic acid precursors, and their photoproducts to understand the photobase generation mechanism. We determined accurate quantum yields of triplet states and photobase generation. The effect of the intermediate radical preceding the base release was demonstrated. We characterized the photophysics of the photobase by femtosecond spectroscopy and showed that the photodecarboxylation process occurred from the second excited triplet state with a rate constant of 2.2×109 s-1 .
Collapse
Affiliation(s)
- Christian Ley
- Laboratoire de Photochimie et d'Ingénierie Macromoléculaire, Université de Haute Alsace, 3b rue A. Werner, 68200, Mulhouse, France
| | - Antoine Siedel
- Laboratoire de Photochimie et d'Ingénierie Macromoléculaire, Université de Haute Alsace, 3b rue A. Werner, 68200, Mulhouse, France
| | - Tony Bertaux
- Laboratoire de Photochimie et d'Ingénierie Macromoléculaire, Université de Haute Alsace, 3b rue A. Werner, 68200, Mulhouse, France
| | - Céline Croutxé-Barghorn
- Laboratoire de Photochimie et d'Ingénierie Macromoléculaire, Université de Haute Alsace, 3b rue A. Werner, 68200, Mulhouse, France
| | - Xavier Allonas
- Laboratoire de Photochimie et d'Ingénierie Macromoléculaire, Université de Haute Alsace, 3b rue A. Werner, 68200, Mulhouse, France
| |
Collapse
|
5
|
Abstract
Dative bonding or Lewis acid-base chemistry underpins a large number of chemical phenomena in a variety of fields, such as catalysis, metal-ligand interactions, and surface chemistry. Developing light-controlled Lewis acid-base interactions could offer a new way of controlling and understanding such phenomena. Photoinduced proton transfer, that is, excited-state Brønsted acidity and basicity, has been extensively studied and applied. Here, in direct analogy to excited-state Brønsted basicity, we show that exciting a photobasic molecule with light generates a thermodynamic drive for the transfer of a Lewis acid from a donor to a photobasic molecule. We have used the archetypal BF3 as our Lewis acid and our photoactive Lewis bases are a family of quinolines, which are known Brønsted photobases as well. We have constructed the experimental Förster cycle for this system and have verified it computationally to demonstrate that a significant drive (0.2-0.7 eV) exists for the transfer of BF3 to a photoexcited quinoline. The magnitude of this drive is similar to those reported for Brønsted photobasicity in quinolines. Computational results from TDDFT and energy decomposition analysis show that the origin of such an effect is similar to the Brønsted photoactivity of these molecules, in that they follow the Hammett parameter of substituent groups. These results suggest that photobases may be capable of controlling the chemical phenomena beyond proton transfer and may open opportunities for a new handle in photocatalysis.
Collapse
Affiliation(s)
- Matthew J Voegtle
- Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
| | - Jahan M Dawlaty
- Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
| |
Collapse
|
6
|
Alamudun SF, Tanovitz K, Espinosa L, Fajardo A, Galvan J, Petit AS. Structure-Photochemical Function Relationships in the Photobasicity of Aromatic Heterocycles Containing Multiple Ring Nitrogen Atoms. J Phys Chem A 2021; 125:13-24. [PMID: 33373234 DOI: 10.1021/acs.jpca.0c07013] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Photobases are compounds that become more basic when promoted to an excited electronic state. Previous experimental and computational studies have demonstrated that several quinoline and quinoline-derived compounds are strong photobases (pKa* > 14). Moreover, the strength of photobasicity was shown to depend strongly on the identity and position of the substituent group(s), with the strongest photobases having multiple electron-donating substituents on a fused benzene ring as opposed to the ring containing the photobasic nitrogen atom. These electron-donating substituents build up electron density on one side of the molecule that shifts onto the nitrogen-containing ring in the electronic transition. This shift in electron density produces an increase in negative charge on the ring nitrogen atom responsible for the photobasicity. In this paper, we expand on our previous investigation to study the effect of an additional ring nitrogen atom on photobasicity in aromatic heterocycles. In particular, we consider how the thermodynamic driving force for excited-state protonation can be tuned by changing the relative placement of the ring nitrogen atoms and varying the position and number of electron-donating substituents. In the set of 112 molecules screened, we identified 42 strong photobases with generally comparable pKa* but lower vertical excitation energies than the quinoline derivatives with only a single ring nitrogen atom. We additionally explored photobasicity in substituted azaindole and carboline derivatives, identifying 76 strongly photobasic compounds with pKa* as large as 22.6 out of the 155 compounds that we considered. Overall, this work provides new insights into the design principles necessary to develop next-generation photocatalysts that employ photobasicity.
Collapse
Affiliation(s)
- Sophya F Alamudun
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| | - Kyle Tanovitz
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| | - Lanette Espinosa
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| | - April Fajardo
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| | - John Galvan
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| | - Andrew S Petit
- Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834-6866, United States
| |
Collapse
|
7
|
Empel C, Jana S, Pei C, Nguyen TV, Koenigs RM. Photochemical O–H Functionalization of Aryldiazoacetates with Phenols via Proton Transfer. Org Lett 2020; 22:7225-7229. [DOI: 10.1021/acs.orglett.0c02564] [Citation(s) in RCA: 21] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Affiliation(s)
- Claire Empel
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
- School of Chemistry, University of New South Waley, Sydney 2052, Australia
| | - Sripati Jana
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
| | - Chao Pei
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
| | - Thanh Vinh Nguyen
- School of Chemistry, University of New South Waley, Sydney 2052, Australia
| | - Rene M. Koenigs
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
- School of Chemistry, University of New South Waley, Sydney 2052, Australia
| |
Collapse
|
8
|
Alamudun SF, Tanovitz K, Fajardo A, Johnson K, Pham A, Jamshidi Araghi T, Petit AS. Structure-Photochemical Function Relationships in Nitrogen-Containing Heterocyclic Aromatic Photobases Derived from Quinoline. J Phys Chem A 2020; 124:2537-2546. [PMID: 32196344 DOI: 10.1021/acs.jpca.9b11375] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Photobases are compounds that become strong bases after electronic excitation. Recent experimental studies have highlighted the photobasicity of the 5-R quinoline compounds, demonstrating a strong substituent dependence to the pKa*. In this paper, we describe our systematic study of how the thermodynamic driving force for photobasicity is tuned through substituents in four families of nitrogen-containing heterocyclic aromatics. We show that substituent position and identity both significantly impact the pKa*. We demonstrate that the substituent effects are additive and identify many disubstituted compounds with substantially greater photobasicity than the most photobasic 5-R quinoline compound identified previously. We show that the addition of a second fused benzene ring to quinoline, along with two electron-donating substituents, lowers the S0 → SPBS vertical excitation energy into the visible region while still maintaining a pKa* > 14. Overall, the structure-function relationships developed in this study provide new insights to guide the development of new photocatalysts that employ photobasicity.
Collapse
Affiliation(s)
- Sophya F Alamudun
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - Kyle Tanovitz
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - April Fajardo
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - Kaitlind Johnson
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - Andy Pham
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - Tina Jamshidi Araghi
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| | - Andrew S Petit
- Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, California 92834-6866, United States
| |
Collapse
|
9
|
Photoinduzierte Protonentransferreaktionen für milde O‐H‐Funktionalisierungsreaktionen unreaktiver Alkohole. Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.201915161] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
|
10
|
Jana S, Yang Z, Li F, Empel C, Ho J, Koenigs RM. Photoinduced Proton-Transfer Reactions for Mild O-H Functionalization of Unreactive Alcohols. Angew Chem Int Ed Engl 2020; 59:5562-5566. [PMID: 31880397 PMCID: PMC7154649 DOI: 10.1002/anie.201915161] [Citation(s) in RCA: 69] [Impact Index Per Article: 17.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2019] [Revised: 12/26/2019] [Indexed: 12/16/2022]
Abstract
Hexafluoroisopropanol is typically considered as an unreactive solvent and not as a reagent in organic synthesis. Herein, we report on a mild and efficient photochemical reaction of aryl diazoacetates with hexafluoroisopropanol that enables, under stoichiometric reaction conditions, the synthesis of fluorinated ethers in excellent yield. Mechanistic studies indicate there is a preorganization of hexafluoroisopropanol and the diazoalkane acts as an unreactive hydrogen-bonding complex. Only after photoexcitation does this complex undergo a protonation-substitution reaction to the reaction product. Investigations on the applicability of this photochemical transformation show that a broad variety of acidic alcohols can be subjected to this transformation and thus demonstrate the feasibility of this concept for O-H functionalization reactions (54 examples, up to 98 % yield).
Collapse
Affiliation(s)
- Sripati Jana
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Zhen Yang
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Fang Li
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Claire Empel
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| | - Junming Ho
- School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia
| | - Rene M Koenigs
- Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074, Aachen, Germany
| |
Collapse
|
11
|
Hunt JR, Dawlaty JM. Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase. J Phys Chem A 2019; 123:10372-10380. [PMID: 31710812 DOI: 10.1021/acs.jpca.9b08970] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Photobases are molecules that convert light to proton transfer drive and therefore have potential applications in many areas of chemistry. Previously, we studied the photobasicity of quinolines and explored their applications. While it is possible to tether a photobase near a target proton donor, for the sake of versatility it is desirable to explore their capability to deprotonate molecules dispersed in a solution. Previous evidence suggested that in this scenario at least two proton donors were necessary for successful excited state proton transfer: one to donate a proton and the second to stabilize the photogenerated donor anion. Here we report kinetic evidence from transient absorption (TA) and time-correlated single photon counting (TCSPC) in support of this hypothesis. We used 5-methoxyquinoline as the photobase and 2,2,2-trifluoroethanol (TFE), a low pKa alcohol, as the proton donor. A constant concentration of the photobase was used for a range of proton-donor dilutions spanning several orders of magnitude in an aprotic background solvent. Absorption spectra confirm that over most of the studied range the majority of the photobase population is hydrogen bonded to at least one donor. Short-pulse TA was used to measure the faster (2-500 ps) dynamics, while TSCPC was used to measure the slower (>500 ps) dynamics. The measured proton transfer time constants varied as a function of donor concentration over a wide range. A log-log plot of the proton transfer rate constant as a function of proton-donor concentration shows two regimes: nondiffusive at high donor concentrations where multiple proton donors are near the photobase and diffusive at low donor concentrations where proton donors are more dilute. The nondiffusive regime has a slope of approximately one, suggesting that the proton transfer process is dependent on one donor molecule in addition to the donor molecule already hydrogen bonded with the photobase. The diffusive regime reasonably follows diffusion kinetics. We propose a model for how the second proton-donor molecule may interact with the photogenerated alkoxide to stabilize it. This work highlights the importance of inducing irreversible changes, in this case solvation of the alkoxide, after proton transfer. Understanding of such details is likely to be important in applications of photobases.
Collapse
Affiliation(s)
- Jonathan Ryan Hunt
- Department of Chemistry , University of Southern California , 920 Bloom Walk SSC 702 , Los Angeles , California 90089-0482 , United States
| | - Jahan M Dawlaty
- Department of Chemistry , University of Southern California , 920 Bloom Walk SSC 702 , Los Angeles , California 90089-0482 , United States
| |
Collapse
|
12
|
Hunt JR, Tseng C, Dawlaty JM. Donor-acceptor preassociation, excited state solvation threshold, and optical energy cost as challenges in chemical applications of photobases. Faraday Discuss 2019; 216:252-268. [PMID: 31025987 DOI: 10.1039/c8fd00215k] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Photobases are molecules with increased pKa in the excited state that can serve to transduce light energy into proton removal capability. They can be used to control chemical reactions using light, such as removing protons from a catalytic site in reactions that are rate-limited by proton transfer. We identify and explore several major challenges toward their practical applications. Two important challenges are the need for pre-association (or ground state hydrogen bonding) between the proton donor and the photobase, and the need for excited state solvation of the photogenerated products. We investigate these two challenges with the photobase 5-methoxyquinoline as the proton acceptor and a low-pKa alcohol, 2,2,2-trifluoroethanol, as the proton donor. We vary the concentration of the donor in a background non-hydrogen-bonding solvent. Using absorption spectroscopy, we have identified that the donor-acceptor concentration ratio must exceed 100 : 1 to achieve appreciable ground state hydrogen bonding. Interestingly, emission spectroscopy reveals that the onset of ground state hydrogen bonding does not guarantee successful excited state proton transfer. It takes an additional order of magnitude increase in donor-acceptor ratio to achieve that goal, revealing that it is necessary to have excess donor molecules to reach the solvation threshold for the photogenerated products. The next challenge is reducing the large ground-excited state energy gap, which often requires UV photons to drive proton transfer. We show experimental and computational data comparing the photobasicity and optical energy gap for a few N-aromatic heterocyclic photobases. In general, we find that reducing the energy gap by increasing the conjugation size necessarily reduces photobasicity, while adding substituents of varying electron-withdrawing strength allows some fine-tuning of this effect. The combination of these two factors provide a preliminary design space for creating new photobasic molecules.
Collapse
Affiliation(s)
| | - Cindy Tseng
- University of Southern California, 920 Bloom Walk, Los Angeles, USA.
| | - Jahan M Dawlaty
- University of Southern California, 920 Bloom Walk, Los Angeles, USA.
| |
Collapse
|